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1

The dark energy equation of state

NASA Astrophysics Data System (ADS)

We perform a study of cosmic evolution with an equation of state parameter by selecting a phenomenological ? model of the form, . This simple proposition explains both linearly expanding and inflationary universes with a single set of equation. We note that the inflation leads to a scaling in the equation of state parameter, ?(t), and hence in equation of state. In this approach, one of its two parameters has been pin pointed and the other has been delineated. It has been possible to show a connection between dark energy and Higgs-Boson.

Usmani, A. A.; Ghosh, P. P.; Mukhopadhyay, Utpal; Ray, P. C.; Ray, Saibal

2008-05-01

2

A Nonuniform Dark Energy Fluid: Perturbation Equations

NASA Astrophysics Data System (ADS)

We propose that galactic dark matter can be described by a nonuniform dark energy fluid. The underlying field is a decaying vector field, which might corresponds to a photon-like but massive particle of 4 degrees of dynamical freedom. We propose a very general Lagrangian for this vector field. The model includes a continuous spectrum of plausible gravity theories, for example, quintessence, f(R), Einstein-Aether, MOND, TeVeS, BSTV, V-? theories, and the inflaton scalar field as special cases. We study in detail a special class of models with a fixed norm of the timelike vector field in the physical metric, which includes a nonlinear K4 term and a Ricci scalar term. We derive the Einstein equations in the perturbed form, which are needed for simulating structure growth in an FRW universe to test such theories. A special case of the model V-? shows promise of resembling the ?CDM cosmology. We show that the vector field has the effect of a nonuniform dark fluid, which resembles dark matter in galaxies and dark energy in the late universe.

Halle, Anaelle; Zhao, HongSheng; Li, Baojiu

2008-07-01

3

CONSTRAINING THE DARK ENERGY EQUATION OF STATE WITH COSMIC VOIDS

Our universe is observed to be accelerating due to the dominant dark energy with negative pressure. The dark energy equation of state (w) holds a key to understanding the ultimate fate of the universe. The cosmic voids behave like bubbles in the universe so that its shapes must be quite sensitive to the background cosmology. Assuming a flat universe and using the priors on the matter density parameter ({omega} {sub m}) and the dimensionless Hubble parameter (h), we demonstrate analytically that the ellipticity evolution of cosmic voids may be a sensitive probe of the dark energy equation of state. We also discuss the parameter degeneracy between w and {omega} {sub m}.

Lee, Jounghun; Park, Daeseong [Department of Physics and Astronomy, FPRD, Seoul National University, Seoul 151-747 (Korea, Republic of)

2009-05-01

4

Nonparametric reconstruction of the dark energy equation of state

The major aim of ongoing and upcoming cosmological surveys is to unravel the nature of dark energy. In the absence of a compelling theory to test, a natural approach is to first attempt to characterize the nature of dark energy in detail, the hope being that this will lead to clues about the underlying fundamental theory. A major target in this characterization is the determination of the dynamical properties of the dark energy equation of state w. The discovery of a time variation in w(z) could then lead to insights about the dynamical origin of dark energy. This approach requires a robust and bias-free method for reconstructing w(z) from data, which does not rely on restrictive expansion schemes or assumed functional forms for w(z). We present a new non parametric reconstruction method for the dark energy equation of state based on Gaussian Process models. This method reliably captures nontrivial behavior of w(z) and provides controlled error bounds. We demollstrate the power of the method on different sets of simulated supernova data. The GP model approach is very easily extended to include diverse cosmological probes.

Heitmann, Katrin [Los Alamos National Laboratory; Holsclaw, Tracy [Los Alamos National Laboratory; Alam, Ujjaini [Los Alamos National Laboratory; Habib, Salman [Los Alamos National Laboratory; Higdon, David [Los Alamos National Laboratory; Sanso, Bruno [UC SANTA CRUZ; Lee, Herbie [UC SANTA CRUZ

2009-01-01

5

Observational constraints on dark energy with generalized equations of state

We investigate the effects of viscosity terms depending on the Hubble parameter and its derivatives in the dark energy equation of state. Such terms are possible if dark energy is a fictitious fluid originating from corrections to the Einstein general relativity as is the case for some braneworld inspired models or fourth order gravity. We consider two classes of models whose singularities in the early and late time universe have been studied by testing the models against the dimensionless coordinate distance to Type Ia Supernovae and radio galaxies also including priors on the shift and the acoustic peak parameters. It turns out that both models are able to explain the observed cosmic speed up without the need of phantom (w<-1) dark energy.

Capozziello, S. [Dipartimento di Scienze Fisiche, Universita di Napoli 'Federico II' (Italy) and INFN, Sez. di Napoli, Compl. Univ. Monte S. Angelo, Edificio N, Via Cinthia, I-80126, Naples (Italy); Cardone, V.F. [Dipartimento di Fisica 'E.R. Caianiello', Universita di Salerno (Italy) and INFN, Sez. di Napoli, Via S. Allende, I-84081, Baronissi (Salerno) (Italy); Elizalde, E.; Odintsov, S.D. [Institucio Catalana de Recerca i Estudis Avancats (ICREA) (Spain) and Institut d'Estudis Espacials de Catalunya (IEEC/ICE), Edifici Nexus, Gran Capita 2-4, 08034 Barcelona (Spain); Nojiri, S. [Department of Applied Physics, National Defence Academy, Hashirimizu Yokosuka 239-8686 (Japan)

2006-02-15

6

ESSENCE: Measuring the Dark Energy Equation of State

NASA Astrophysics Data System (ADS)

The accelerating universe appears to be dominated by a dark energy with a significant negative pressure. The ratio of the pressure to density of this mysterious energy {its equation of state} is an observable which can differentiate between the proliferating candidate theories. We propose to estimate the dark energy equation of state by observing Type Ia supernovae at redshifts near z=0.7 with HST in concert with the on-going ESSENCE NOAO Survey program that is discovering and studying supernovae between 0.3

Garnavich, Peter

2003-07-01

7

The state of the dark energy equation of state

By combining data from seven cosmic microwave background experiments (including the latest WMAP results) with the Hubble parameter measurement from the Hubble space telescope and luminosity measurements of type Ia supernovae, we demonstrate the bounds on the dark energy equation of state wQ to be -1.45

Alessandro Melchiorri; Laura Mersini; Carolina J. Ödman; Mark Trodden

2003-01-01

8

The Unified Equation of State for Dark Matter and Dark Energy

NASA Astrophysics Data System (ADS)

We assume that dark matter and dark energy satisfy the unified equation of state: p = B(z)?, with p = pdE, ? = ?dm+?dE, where the pressure of dark matter pdm = 0 has been taken into account. A special function B=-(A)/((1+z)? ) is presented, which can well describe the evolution of the universe. In this model, the universe will end up with a Big Rip. By further simple analysis, we know other choices of the function B can also describe the universe but lead to a different doomsday.

Wang, Wei; Gui, Yuanxing; Zhang, Suhong; Guo, Guanghai; Shao, Ying

9

Nonparametric reconstruction of the dark energy equation of state

A basic aim of ongoing and upcoming cosmological surveys is to unravel the mystery of dark energy. In the absence of a compelling theory to test, a natural approach is to better characterize the properties of dark energy in search of clues that can lead to a more fundamental understanding. One way to view this characterization is the improved determination of the redshift-dependence of the dark energy equation of state parameter, w(z). To do this requires a robust and bias-free method for reconstructing w(z) from data that does not rely on restrictive expansion schemes or assumed functional forms for w(z). We present a new nonparametric reconstruction method that solves for w(z) as a statistical inverse problem, based on a Gaussian process representation. This method reliably captures nontrivial behavior of w(z) and provides controlled error bounds. We demonstrate the power of the method on different sets of simulated supernova data; the approach can be easily extended to include diverse cosmological probes.

Holsclaw, Tracy; Sanso, Bruno; Lee, Herbert [Department of Applied Mathematics and Statistics, University of California, Santa Cruz, California 95064 (United States); Alam, Ujjaini; Heitmann, Katrin [ISR-1, Mailstop D466, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Habib, Salman [T-2, Mailstop B285, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Higdon, David [CCS-6, Mailstop F600, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2010-11-15

10

Dark energy as a modification of the Friedmann equation

Dark energy could actually be the manifestation of a modification to the Friendmann equation arising from new physics (e.g., extra dimensions). Writing the correction as (1 - {Omega}{sub M})H{sup {alpha}}/H{sub 0}{sup {alpha}-2}, they explore the phenomenology and detectability of such. They show that: (1) {alpha} must be {approx}< 1; (2) such a correction behaves like dark energy with equation-of-state w{sub eff} = -1 + {alpha}/2 in the recent past (10{sup 4} > z >> 1) and w = -1 in the distant future and can mimic w < -1 without violating the weak-energy condition; (3) w{sub eff} changes, dz/dw|{sub z {approx} 0.5} {approx} {Omicron}(0.2), which is likely detectable; and (4) a future supernova experiment like SNAP that can determine w with precision {sigma}{sub w}, could determine {alpha} to precision {sigma}{sub {alpha}} {approx} 2{sigma}{sub w}.

Dvali, Gia; /New York U.; Turner, Michael S.; /Chicago U., Astron. Astrophys. Ctr. /KICP, Chicago /Chicago U., EFI /Fermilab

2003-01-01

11

Transition of the dark energy equation of state in an interacting holographic dark energy model

A model of holographic dark energy with an interaction with matter fields has been investigated. Choosing the future event horizon as an IR cutoff, we have shown that the ratio of energy densities can vary with time. With the interaction between the two different constituents of the universe, we observed the evolution of the universe, from early deceleration to late

Bin Wang; Yungui Gong; Elcio Abdalla

2005-01-01

12

Is w{ne}-1 evidence for a dynamical dark energy equation of state?

Current constraints on the dark energy equation of state parameter, w, are expected to be improved by more than 1 order of magnitude in the next decade. If |w-1| > or approx. 0.01 around the present time, but the dark energy dynamics is sufficiently slow, it is possible that future constraints will rule out a cosmological constant while being consistent with a time-independent equation of state parameter. In this paper, we show that although models with such behavior can be constructed, they do require significant fine-tuning. Therefore, if the observed acceleration of the Universe is induced by a dark energy component, then finding w{ne}-1 would, on its own, constitute very strong evidence for a dynamical dark energy equation of state.

Avelino, P. P. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Trindade, A. M. M. [Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto (Portugal); Viana, P. T. P. [Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto (Portugal); Departamento de Matematica Aplicada da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal)

2009-09-15

13

The new form of the equation of state for dark energy fluid and accelerating universe

We suggest to generalize the dark energy equation of state (EoS) by introduction the relaxation equation for pressure which is equivalent to consideration of the inhomogeneous EoS cosmic fluid which often appears as the effective model from strings\\/brane-worlds. As another, more wide generalization we discuss the inhomogeneous EoS which contains derivatives of pressure. For several explicit examples motivated by the

Shin'ichi Nojiri; Sergei D. Odintsov

2006-01-01

14

Observational constraints on dark energy with a fast varying equation of state

NASA Astrophysics Data System (ADS)

We place observational constraints on models with the late-time cosmic acceleration based on a number of parametrizations allowing fast transitions for the equation of state of dark energy. In addition to the model of Linder and Huterer where the dark energy equation of state w monotonically grows or decreases in time, we propose two new parametrizations in which w has an extremum. We carry out the likelihood analysis with the three parametrizations by using the observational data of supernovae type Ia, cosmic microwave background, and baryon acoustic oscillations. Although the transient cosmic acceleration models with fast transitions can give rise to the total chi square smaller than that in the ?-Cold-Dark-Matter (?CDM) model, these models are not favored over ?CDM when one uses the Akaike information criterion which penalizes the extra degrees of freedom present in the parametrizations.

De Felice, Antonio; Nesseris, Savvas; Tsujikawa, Shinji

2012-05-01

15

Can the dark energy equation-of-state parameter w be less than -1?

Models of dark energy are conveniently characterized by the equation-of-state parameter w=p\\/rho, where rho is the energy density and p is the pressure. Imposing the dominant energy condition, which guarantees stability of the theory, implies that w=>-1. Nevertheless, it is conceivable that a well-defined model could (perhaps temporarily) have w<-1 , and indeed such models have been proposed. We study

Sean M. Carroll; Mark Hoffman; Mark Trodden

2003-01-01

16

Dynamical mutation of dark energy

We discuss the intriguing possibility that dark energy may change its equation of state in situations where large dark energy fluctuations are present. We show indications of this dynamical mutation in some generic models of dark energy.

Abramo, L. R.; Batista, R. C. [Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo (Brazil); Liberato, L.; Rosenfeld, R. [Instituto de Fisica Teorica, Universidade Estadual Paulista, R. Pamplona 145, 01405-900, Sao Paulo (Brazil)

2008-03-15

17

Fables of reconstruction: controlling bias in the dark energy equation of state

NASA Astrophysics Data System (ADS)

We develop an efficient, non-parametric Bayesian method for reconstructing the time evolution of the dark energy equation of state w(z) from observational data. Of particular importance is the choice of prior, which must be chosen carefully to minimise variance and bias in the reconstruction. Using a principal component analysis, we show how a correlated prior can be used to create a smooth reconstruction and also avoid bias in the mean behaviour of w(z). We test our method using Wiener reconstructions based on Fisher matrix projections, and also against more realistic MCMC analyses of simulated data sets for Planck and a future space-based dark energy mission. While the accuracy of our reconstruction depends on the smoothness of the assumed w(z), the relative error for typical dark energy models is lesssim10% out to redshift z = 1.5.

Crittenden, Robert G.; Zhao, Gong-Bo; Pogosian, Levon; Samushia, Lado; Zhang, Xinmin

2012-02-01

18

Future Type Ia Supernova Data as Tests of Dark Energy from Modified Friedmann Equations

NASA Astrophysics Data System (ADS)

In the Cardassian model, dark energy density arises from modifications to the Friedmann equation, which becomes H2=g(?M), where g(?M) is a new function of the energy density. The universe is flat, matter dominated, and accelerating. The distance-redshift relation predictions of generalized Cardassian models can be very different from generic quintessence models, and can be differentiated with data from upcoming pencil beam surveys of Type Ia supernovae such as Supernova/Acceleration Probe (SNAP). We have found the interesting result that, once ?m is known to 10% accuracy, SNAP will be able to determine the sign of the time dependence of the dark energy density. Knowledge of this sign (which is related to the weak energy condition) will provide a first discrimination between various cosmological models that fit the current observational data (cosmological constant, quintessence, Cardassian expansion). Further, we have performed Monte Carlo simulations to illustrate how well one can reproduce the form of the dark energy density with SNAP. To be concrete we study a class of two-parameter (n, q) generalized Cardassian models that includes the original Cardassian model (parameterized by n only) as a special case. Examples are given of modified polytropic (MP) Cardassian models that fit current supernova and cosmic microwave background data, and prospects for differentiating between MP Cardassian and other models in future data are discussed. We also note that some Cardassian models can satisfy the weak energy condition w>-1 even with a dark energy component that has an effective equation of state wX<-1.

Wang, Yun; Freese, Katherine; Gondolo, Paolo; Lewis, Matthew

2003-09-01

19

Properties and uncertainties of scalar field models of dark energy with barotropic equation of state

The dynamics of expansion and large scale structure formation in the multicomponent Universe with dark energy modeled by the minimally coupled scalar field with generalized linear barotropic equation of state are analyzed. It is shown that the past dynamics of expansion and future of the Universe - eternal accelerated expansion or turnaround and collapse - are completely defined by the current energy density of a scalar field and relation between its current and early equation of state parameters. The clustering properties of such models of dark energy and their imprints in the power spectrum of matter density perturbations depend on the same relation and, additionally, on the 'effective sound speed' of a scalar field, defined by its Lagrangian. It is concluded that such scalar fields with different values of these parameters are distinguishable in principle. This gives the possibility to constrain them by confronting the theoretical predictions with the corresponding observational data. For that we have used the 7-year Wilkinson Microwave Anisotropy Probe data on cosmic microwave background anisotropies, the Union2 data set on Supernovae Ia and the seventh data release of the Sloan Digital Sky Survey data on luminous red galaxies space distribution. Using the Markov Chain Monte Carlo technique the marginalized posterior and mean likelihood distributions are computed for the scalar fields with two different Lagrangians: Klein-Gordon and Dirac-Born-Infeld ones. The properties of such scalar field models of dark energy with best fitting parameters and uncertainties of their determination are also analyzed in the paper.

Novosyadlyj, Bohdan; Sergijenko, Olga; Apunevych, Stepan; Pelykh, Volodymyr [Astronomical Observatory of Ivan Franko National University of Lviv, Kyryla i Methodia str., 8, Lviv, 79005 (Ukraine); Ya. S. Pidstryhach Institute for Applied Problems of Mechanics and Mathematics, Naukova Street, 3-b, Lviv, 79060 (Ukraine)

2010-11-15

20

NASA Astrophysics Data System (ADS)

The discovery that the Universe is speeding up and not slowing down was greeted with open arms by theorists. First, because the dark energy powering the acceleration provided the "missing stuff" needed to make the Universe flat, in accord with a key prediction of inflation. Second, because theorists now have a new puzzle to solve, the nature of the mysterious dark energy. I have no doubt that the dark energy problem will be just as fundamental and just as interesting as the dark matter problem. Determining its nature will require the work of both astronomers and particle physics and will shed light on both fundamental physics and the fate of the Universe.

Turner, Michael S.

2001-04-01

21

Features in the dark energy equation of state and modulations in the Hubble diagram

NASA Astrophysics Data System (ADS)

We probe the time dependence of the dark energy equation of state (EOS) in light of three-year WMAP (WMAP3) and the combination with other tentative cosmological observations from galaxy clustering (SDSS) and Type Ia Supernova (SNIa). We mainly focus on cases where the EOS is oscillating or with local bumps. By performing a global analysis with the Markov Chain Monte Carlo (MCMC) method, we find the current observations, in particular, the WMAP3 + SDSS data combination, allow large oscillations of the EOS which can leave oscillating features on the (residual) Hubble diagram, and such oscillations are potentially detectable by future observations like SNAP, or even by the currently ongoing SNIa observations. Local bumps of dark energy EOS can also leave imprints on CMB, LSS and SNIa. In cases where the bumps take place at low redshifts and the effective EOS is close to -1, CMB and LSS observations cannot give stringent constraints on such possibilities. However, geometrical observations like (future) SNIa can possibly detect such features. On the other hand when the local bumps take place at higher redshifts beyond the detectability of SNIa, future precise observations like Gamma-ray bursts, CMB and LSS may possibly detect such features. In particular, we find that bumplike dark energy EOS on high redshifts might be responsible for the localized features of WMAP on ranges l˜20-40, which is interesting and deserves addressing further.

Xia, Jun-Qing; Zhao, Gong-Bo; Li, Hong; Feng, Bo; Zhang, Xinmin

2006-10-01

22

Features in the dark energy equation of state and modulations in the Hubble diagram

We probe the time dependence of the dark energy equation of state (EOS) in light of three-year WMAP (WMAP3) and the combination with other tentative cosmological observations from galaxy clustering (SDSS) and Type Ia Supernova (SNIa). We mainly focus on cases where the EOS is oscillating or with local bumps. By performing a global analysis with the Markov Chain Monte Carlo (MCMC) method, we find the current observations, in particular, the WMAP3 + SDSS data combination, allow large oscillations of the EOS which can leave oscillating features on the (residual) Hubble diagram, and such oscillations are potentially detectable by future observations like SNAP, or even by the currently ongoing SNIa observations. Local bumps of dark energy EOS can also leave imprints on CMB, LSS and SNIa. In cases where the bumps take place at low redshifts and the effective EOS is close to -1, CMB and LSS observations cannot give stringent constraints on such possibilities. However, geometrical observations like (future) SNIa can possibly detect such features. On the other hand when the local bumps take place at higher redshifts beyond the detectability of SNIa, future precise observations like Gamma-ray bursts, CMB and LSS may possibly detect such features. In particular, we find that bumplike dark energy EOS on high redshifts might be responsible for the localized features of WMAP on ranges l{approx}20-40, which is interesting and deserves addressing further.

Xia Junqing; Zhao Gongbo; Li Hong; Zhang Xinmin [Institute of High Energy Physics, Chinese Academy of Science, P.O. Box 918-4, Beijing 100049 (China); Feng Bo [Research Center for the Early Universe (RESCEU), Graduate School of Science, University of Tokyo, Tokyo 113-0033 (Japan)

2006-10-15

23

National Technical Information Service (NTIS)

Dark Energy is the dominant constituent of the universe and we have little understanding of it. We describe a new project aimed at measuring the dark energy equation of state parameter to a statistical precision of approx. 5 %, with four separate techniqu...

B. Flaugher

2004-01-01

24

Dark energy perturbations are normally either neglected or else included in a purely numerical way, obscuring their dependence on underlying parameters like the equation of state or the sound speed. However, while many different explanations for the dark energy can have the same equation of state, they usually differ in their perturbations so that these provide a fingerprint for distinguishing between different models with the same equation of state. In this paper we derive simple yet accurate approximations that are able to characterize a specific class of models (encompassing most scalar-field models) which is often generically called 'dark energy'. We then use the approximate solutions to look at the impact of the dark energy perturbations on the dark matter power spectrum and on the integrated Sachs-Wolfe effect in the cosmic microwave background radiation.

Sapone, Domenico; Kunz, Martin [Departement de Physique Theorique, Universite de Geneve, 24 quai Ernest Ansermet, CH-1211 Geneve 4 (Switzerland); Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom)

2009-10-15

25

Can the dark energy equation-of-state parameter w be less than -1?

NASA Astrophysics Data System (ADS)

Models of dark energy are conveniently characterized by the equation-of-state parameter w=p/?, where ? is the energy density and p is the pressure. Imposing the dominant energy condition, which guarantees stability of the theory, implies that w?-1. Nevertheless, it is conceivable that a well-defined model could (perhaps temporarily) have w<-1?, and indeed such models have been proposed. We study the stability of dynamical models exhibiting w<-1 by virtue of a negative kinetic term. Although naively unstable, we explore the possibility that these models might be phenomenologically viable if thought of as effective field theories valid only up to a certain momentum cutoff. Under our most optimistic assumptions, we argue that the instability time scale can be greater than the age of the universe, but only if the cutoff is at or below 10-3 eV. We conclude that it is difficult, although not necessarily impossible, to construct viable models of dark energy with w<-1; observers should keep an open mind, but the burden is on theorists to demonstrate that any proposed new models are not ruled out by rapid vacuum decay.

Carroll, Sean M.; Hoffman, Mark; Trodden, Mark

2003-07-01

26

Beyond the perfect fluid hypothesis for the dark energy equation of state

Abandoning the perfect fluid hypothesis, we investigate here the possibility that the dark energy equation of state (EoS) w is a nonlinear function of the energy density {rho}. To this aim, we consider four different EoS describing classical fluids near thermodynamical critical points and discuss the main features of cosmological models made out of dust matter and a dark energy term with the given EoS. Each model is tested against the data on the dimensionless coordinate distance to Type Ia Supernovae and radio galaxies, the shift and the acoustic peak parameters and the positions of the first three peaks in the anisotropy spectrum of the comic microwave background radiation. We propose a possible interpretation of each model in the framework of scalar field quintessence determining the shape of the self-interaction potential V({phi}) that gives rise to each one of the considered thermodynamical EoS. As a general result, we demonstrate that replacing the perfect fluid EoS with more general expressions gives both the possibility of successfully solving the problem of cosmic acceleration escaping the resort to phantom models.

Cardone, V.F.; Troisi, A. [Dipartimento di Fisica E.R. Caianiello, Universita di Salerno and INFN, Sez. di Napoli, Gruppo Coll. di Salerno, via S. Allende, 84081--Baronissi (Salerno) (Italy); Tortora, C.; Capozziello, S. [Dipartimento di Scienze Fisiche, Universita di Napoli Federico II and INFN, Sez. di Napoli, Compl. Univ. Monte S. Angelo, Edificio N, Via Cinthia, 80126, Naples (Italy)

2006-02-15

27

Variable equation of state for Bianchi type-VI0 dark energy models

NASA Astrophysics Data System (ADS)

We present dark energy models in an anisotropic Bianchi type-VI0 (B-VI0) space-time with a variable equation of state (EoS). The EoS for dark energy ? is found to be time dependent and its existing range for derived models is in good agreement with the recent observations of SNe Ia data (Knop et al. in Astrophys. J. 598:102 2003), SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al. in Astrophys. J. 606:702, 2004b) and latest a combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift type Ia supernovae and galaxy clustering (Hinshaw et al. in Astrophys. J. Suppl. 180:225, 2009; Komatsu et al. in Astrophys. J. Suppl. 180:330, 2009). The cosmological constant ? is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent supernovae Ia observations. The physical and geometric aspects of the models are also discussed in detail.

Amirhashchi, Hassan; Pradhan, Anirudh; Saha, Bijan

2011-05-01

28

Cosmological Evolution of Equation of State for Dark Energy in G-Essence Models

NASA Astrophysics Data System (ADS)

We explore the cosmological evolution of equation of state (EoS) for dark energy in g-essence models, the action of which is described by a function of both the canonical kinetic term of both the scalar and fermionic fields. We examine g-essence models with realizing the crossing of the phantom divide line of wDE = -1 as well as the models in which the universe always stays in the nonphantom (quintessence) phase (wDE > -1). In particular, we find an explicit g-essence model with the crossing from the nonphantom phase to the phantom one (wDE < -1). This transition behavior is consistent with the recent observational data analyses.

Bamba, Kazuharu; Razina, Olga; Yerzhanov, Koblandy; Myrzakulov, Ratbay

2013-05-01

29

Effects of voids on the reconstruction of the equation of state of dark energy

We quantify the effects of the voids known to exist in the Universe upon the reconstruction of the dark energy equation of state w. We show that the effect can start to be comparable with some of the other errors taken into account when analyzing supernova data, depending strongly upon the low redshift cutoff used in the sample. For the supernova data alone, the error induced in the reconstruction of w is much larger than the percent level. When the Baryonic Acoustic Oscillations and the Cosmic Microwave Background data are included in the fit, the effect of the voids upon the determination of w is much lessened but is not much smaller than some of the other errors taken into consideration when performing such fits. We also look at the effect of voids upon the estimation of the equation of state when we allow w to vary over time and show that even when supernova, Cosmic Microwave Background, and Baryonic Acoustic Oscillations data are used to constrain the equation of state, the best fit points in parameter space can change at the 10% level due to the presence of voids, and error-bars increase significantly.

Lavallaz, Arnaud de; Fairbairn, Malcolm [Department of Physics, King's College London, Strand, London, WC2R 2LS (United Kingdom)

2011-10-15

30

Cosmological model with variable equations of state for matter and dark energy

NASA Astrophysics Data System (ADS)

We construct a cosmological model which is physically reasonable, mathematically tractable, and extends the study of cold dark matter (CDM) models to the case where the equations of state (EoS) for matter and dark energy (DE) vary with time. It is based on the assumptions of (i) flatness, (ii) validity of general relativity, (iii) the presence of a DE component that varies between two asymptotic values, (iv) the matter of the universe smoothly evolves from an initial radiation stage—or a barotropic perfect fluid—to a phase where it behaves as cosmological dust at late times. The model approximates the CDM ones for small z but significantly differ from them for large z. We focus our attention on how the evolving EoS for matter and DE can modify the CDM paradigm. We discuss a number of physical scenarios. One of them includes, as a particular case, the so-called generalized Chaplygin gas models where DE evolves from non-relativistic dust. Another kind of models shows that the current accelerated expansion is compatible with a DE that behaves like pressureless dust at late times. We also find that a universe with variable DE can go from decelerated to accelerated expansion, and vice versa, several times.

Ponce de Leon, J.

2012-07-01

31

Gravitational wave (GW) signals from coalescing massive black hole (MBH) binaries could be used as standard sirens to measure cosmological parameters. The future space-based GW observatory Laser Interferometer Space Antenna (LISA) will detect up to a hundred of those events, providing very accurate measurements of their luminosity distances. To constrain the cosmological parameters, we also need to measure the redshift of the galaxy (or cluster of galaxies) hosting the merger. This requires the identification of a distinctive electromagnetic event associated with the binary coalescence. However, putative electromagnetic signatures may be too weak to be observed. Instead, we study here the possibility of constraining the cosmological parameters by enforcing statistical consistency between all the possible hosts detected within the measurement error box of a few dozen of low-redshift (z < 3) events. We construct MBH populations using merger tree realizations of the dark matter hierarchy in a {Lambda}CDM universe, and we use data from the Millennium simulation to model the galaxy distribution in the LISA error box. We show that, assuming that all the other cosmological parameters are known, the parameter w describing the dark energy equation of state can be constrained to a 4%-8% level (2{sigma} error), competitive with current uncertainties obtained by type Ia supernovae measurements, providing an independent test of our cosmological model.

Petiteau, Antoine; Babak, Stanislav; Sesana, Alberto [Max-Planck-Institut fuer Gravitationsphysik, Albert-Einstein-Institut, Am Muhlenberg 1, D-14476 Golm (Germany)

2011-05-10

32

Dark-energy thermodynamic models

We study cosmological consequences of dark-energy thermodynamic models. The assumption that dark energy is conformed of quanta, and an extensivity argument generalize its equation of state. This implies that dark energy and another key component exchange energy. The energy densities of dark energy and the other component then tend asymptotically to a constant, thus explaining the coincidence of dark matter and dark energy today. On the other hand, a model of non-relativistic particles in a Bose-Einstein condensate, with a short-range attractive interaction, produces acceleration. It is shown that the phantom-acceleration regime, at the beginning of the universe, solves the horizon problem.

Besprosvany, Jaime [Instituto de Fisica, Universidad Nacional Autonoma de Mexico, Apartado Postal 20-364, Mexico 01000, Distrito Federal (Mexico); Izquierdo, German [Institut de Mathematiques de Bourgogne, Universite de Bourgogne, 9 Av. Alain Savary, 21078 Dijon Cedex (France)

2010-12-07

33

Nonparametric reconstruction of the dark energy equation of state from diverse data sets

The cause of the accelerated expansion of the Universe poses one of the most fundamental questions in physics today. In the absence of a compelling theory to explain the observations, a first task is to develop a robust phenomenological approach: If the acceleration is driven by some form of dark energy, then the phenomenology is determined by the form of the dark energy equation of state w(z) as a function of redshift. A major aim of ongoing and upcoming cosmological surveys is to measure w and its evolution at high accuracy. Since w(z) is not directly accessible to measurement, powerful reconstruction methods are needed to extract it reliably from observations. We have recently introduced a new reconstruction method for w(z) based on Gaussian process modeling. This method can capture nontrivial w(z) dependences and, most importantly, it yields controlled and unbiased error estimates. In this paper we extend the method to include a diverse set of measurements: baryon acoustic oscillations, cosmic microwave background measurements, and supernova data. We analyze currently available data sets and present the resulting constraints on w(z), finding that current observations are in very good agreement with a cosmological constant. In addition, we explore how well our method captures nontrivial behavior of w(z) by analyzing simulated data assuming high-quality observations from future surveys. We find that the baryon acoustic oscillation measurements by themselves already lead to remarkably good reconstruction results and that the combination of different high-quality probes allows us to reconstruct w(z) very reliably with small error bounds.

Holsclaw, Tracy; Sanso, Bruno; Lee, Herbie [Department of Applied Mathematics and Statistics, University of California, Santa Cruz, California 95064 (United States); Alam, Ujjaini [ISR-1, MS D466, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Heitmann, Katrin [ISR-1, MS D466, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); High Energy Physics Division and Mathematics and Computer Science Division, Argonne National Laboratory, Argonne, Illinois, 60439 (United States); Habib, Salman [T-2, MS B285, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); High Energy Physics Division and Mathematics and Computer Science Division, Argonne National Laboratory, Argonne, Illinois, 60439 (United States); Higdon, David [CCS-6, MS F600, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2011-10-15

34

We have reanalyzed constraints on the equation of state parameter, wQ?P\\/rho, of the dark energy, using several cosmological data sets and relaxing the usual constraint wQ>=-1. We find that combining cosmic microwave background, large scale structure, and type Ia supernova data yields a nontrivial lower bound on wQ. At 95.4% confidence we find, assuming a flat geometry of the universe,

Steen Hannestad; Edvard Mörtsell

2002-01-01

35

Discretuum versus continuum dark energy

The dark energy equation of state for theories with either a discretuum or continuum distribution of vacua is investigated. In the discretuum case the equation of state is constant w=p\\/?=?1. The continuum case may be realized by an action with large wave function factor Z for the dark energy modulus and generic potential. This form of the action is quantum

Savas Dimopoulos; Scott Thomas

2003-01-01

36

The dynamical age of the universe depends upon the rate of the expansion of the universe, which explicitly involves the dark energy equation of state parameter w(z). Consequently, the evolution of w(z) has a direct imprint on the age of the universe. We have shown that the dynamical age of the universe as derived from CMB data can be used as an authentic criterion, being independent of the prior assumptions likethe present value of the Hubble constant H{sub 0} and the cosmological density parameter {omega}{sub M}{sup 0}, to constrain the range of admissible values of w for quiessence models and to test the physically viable parametrizations of the equation of state w(z) in kinessence models. An upper bound on variation of dark energy density is derived and a relation between cosmological density parameters and the transition redshift is established.

Johri, V. B.; Rath, P. K. [Department of Mathematics and Astronomy, University of Lucknow, Lucknow-226007 (India); Department of Physics, University of Lucknow, Lucknow-226007 (India)

2006-12-15

37

The dynamical age of the universe depends upon the rate of the expansion of\\u000athe universe, which explicitly involves the dark energy equation of state\\u000aparameter $w(z)$. Consequently, the evolution of $w(z)$ has a direct imprint on\\u000athe age of the universe. We have shown that the dynamical age of the universe\\u000aas derived from CMB data can be used

Vinod B. Johri; P. K. Rath

2006-01-01

38

Dark Energy is the dominant constituent of the universe and they have little understanding of it. They describe a new project aimed at measuring the dark energy equation of state parameter, w, to a statistical precision of {approx} 5%, with four separate techniques. The survey will image 5000 deg{sup 2} in the southern sky and collect 300 million galaxies, 30,000 galaxy clusters, and 2000 Type Ia supernovae. The survey will be carried out using a new 3 deg{sup 2} mosaic camera mounted at the prime focus of the 4m Blanco telescope at CTIO.

Flaugher, Brenna; /Fermilab

2004-11-01

39

NASA Astrophysics Data System (ADS)

The gravastar picture is an alternative model to the concept of a black hole, where there is an effective phase transition at or near where the event horizon is expected to form, and the interior is replaced by a de Sitter condensate. In this work a generalization of the gravastar picture is explored by considering matching of an interior solution governed by the dark energy equation of state, ? ? p/? < -1/3, to an exterior Schwarzschild vacuum solution at a junction interface. The motivation for implementing this generalization arises from the fact that recent observations have confirmed an accelerated cosmic expansion, for which dark energy is a possible candidate. Several relativistic dark energy stellar configurations are analysed by imposing specific choices for the mass function. The first case considered is that of a constant energy density, and the second choice that of a monotonic decreasing energy density in the star's interior. The dynamical stability of the transition layer of these dark energy stars to linearized spherically symmetric radial perturbations about static equilibrium solutions is also explored. It is found that large stability regions exist that are sufficiently close to where the event horizon is expected to form, so that it would be difficult to distinguish the exterior geometry of the dark energy stars, analysed in this work, from an astrophysical black hole.

Lobo, Francisco S. N.

2006-03-01

40

Dark Energy Survey Instrument Design.

National Technical Information Service (NTIS)

We describe a new project, the Dark Energy Survey (DES), aimed at measuring the dark energy equation of state parameter, w, to a statistical precision of approximately 5%, with four complementary techniques. The survey will use a new 3 sq. deg. mosaic cam...

B. Flaugher

2006-01-01

41

Thermodynamical properties of dark energy

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature T{approx}a{sup -n} and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state w>-1 and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation, and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously.

Gong Yungui; Wang Bin; Wang Anzhong [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China) and CASPER, Department of Physics, Baylor University, Waco, Texas 76798 (United States); Department of Physics, Fudan University, Shanghai 200433 (China); CASPER, Department of Physics, Baylor University, Waco, Texas 76798 (United States)

2007-06-15

42

Beyond two dark energy parameters.

Our ignorance of dark energy is generally described by a two-parameter equation of state. In these approaches, a particular ad hoc functional form is assumed, and only two independent parameters are incorporated. We propose a model-independent, multiparameter approach to fitting dark energy and show that next-generation surveys will constrain the equation of state in three or more independent redshift bins to better than 10%. Future knowledge of dark energy will surpass two numbers (e.g., [w{0},w{1}] or [w{0},w{a}]), and we propose a more flexible approach to the analysis of present and future data. PMID:18643568

Sarkar, Devdeep; Sullivan, Scott; Joudaki, Shahab; Amblard, Alexandre; Holz, Daniel E; Cooray, Asantha

2008-06-19

43

Cosmological Consequences of a Dark Energy Component with Super-Negative Equation of State

It is extraordinary that a number of observations indicate that we live in a spatially flat, low matter density Universe, which is currently undergoing a period of accelerating expansion. The effort to explain this current state has focused attention on cosmological models in which the dominant component of the cosmic energy density has negative pressure, with an equation of state

R. R. Caldwell

44

Inflation, Dark Matter, Dark Energy

NASA Astrophysics Data System (ADS)

Remarkable 20th-century cosmological discoveries and theoretical ideas led to the development of the present cosmological "standard model." In this lecture I will focus on one of the more recent ideas that may now be regarded as part of the framework of the standard big-bang model; namely, that structure in the universe results from the growth of small seed density fluctuations produced during the inflationary universe. In order to complete this picture, I will also discuss the idea that the present mass density is dominated by dark matter and that there is now a preponderance of dark energy.

Kolb, Edward W.

2005-06-01

45

Perturbation of a planetary orbit by the Lambda-term (dark energy) in Einstein equations

NASA Astrophysics Data System (ADS)

The problem of cosmological influences at small (e.g. interplanetary) scales is discussed for a few decades, starting from the early 1930's, but still remains unsolved definitively by now [1]. This subject became especially topical in the context of the dark-energy-dominated cosmology, because the commonly-used arguments against the local Hubble expansion (such as Einstein-Straus theorem [2]) are inapplicable when the most contribution to the energy density of the Universe comes from the perfectly-uniform dark energy (Lambda-term). Moreover, there are some empirical evidences in favor of the local cosmological influences. For example, inclusion of the local Hubble expansion into dynamics of the Earth-Moon system enables us to resolve a long-standing discrepancy in the rates of secular increase of the lunar semi-major axis (a) mea-sured by the lunar laser ranging and (b) derived from the astrometric observations of the Earth's rotation deceleration [3, 4]. The aim of the present report is to provide a detailed mathematical treatment of the respective two-body problem in the framework of General Relativity, which is based on the Kottler metric reduced to the Robertson-Walker cosmological asymptotics, as outlined in our earlier work [5]. References: 1. W.B. Bonnor. Gen. Rel. Grav., v.32, p.1005 (2000). 2. A. Einstein and E.G. Straus. Rev. Mod. Phys., v.17, p.120 (1945). 3. Yu.V. Dumin. Adv. Space Res., v.31, p.2461 (2003). 4. Yu.V. Dumin. In Proc. 11th Marcel Grossmann Meeting on General Relativity, World Sci., Singapore, p.1752 (2008). 5. Yu.V. Dumin. Phys. Rev. Lett., v.98, p.059001 (2007).

Dumin, Yurii

46

Dark energy perturbations revisited

NASA Astrophysics Data System (ADS)

In this Letter we study the evolution of cosmological perturbations in the presence of dynamical dark energy, and revisit the issue of dark energy perturbations. For a generally parameterized equation of state (EoS) such as w(z)=w+wz1+z (for a single fluid or a single scalar field), the dark energy perturbation diverges when its EoS crosses the cosmological constant boundary w=-1. In this Letter we present a method of treating the dark energy perturbations during the crossing of the w=-1 surface by imposing matching conditions which require the induced 3-metric on the hypersurface of w=-1 and its extrinsic curvature to be continuous. These matching conditions have been used widely in the literature to study perturbations in various models of early universe physics, such as Inflation, the Pre-Big-Bang and Ekpyrotic scenarios, and bouncing cosmologies. In all of these cases the EoS undergoes a sudden change. Through a detailed analysis of the matching conditions, we show that ? and ? are continuous on the matching hypersurface. This justifies the method used (Zhao et al., 2005, 2007; Xia et al., 2006, 2008) [1-4] in the numerical calculation and data fitting for the determination of cosmological parameters. We discuss the conditions under which our analysis is applicable.

Li, Mingzhe; Cai, Yifu; Li, Hong; Brandenberger, Robert; Zhang, Xinmin

2011-08-01

47

The physical process leading to the acceleration of the expansion of the universe is unknown. It may involve new high energy physics or extensions to gravitation. Calling this generically dark energy, we examine the consistencies and relations between these two approaches, showing that an effective equation of state function w(z) is broadly useful in describing the properties of the dark energy. A variety of cosmological observations can provide important information on the dynamics of dark energy and the future looks bright for constraining dark energy, though both the measurements and the interpretation will be challenging. We also discuss a more direct relation between the spacetime geometry and acceleration, via ''geometric dark energy'' from the Ricci scalar, and superacceleration or phantom energy where the fate of the universe may be more gentle than the Big Rip.

Linder, Eric V.

2004-04-01

48

Measuring the speed of dark: Detecting dark energy perturbations

The nature of dark energy can be probed not only through its equation of state but also through its microphysics, characterized by the sound speed of perturbations to the dark energy density and pressure. As the sound speed drops below the speed of light, dark energy inhomogeneities increase, affecting both cosmic microwave background and matter power spectra. We show that current data can put no significant constraints on the value of the sound speed when dark energy is purely a recent phenomenon, but can begin to show more interesting results for early dark energy models. For example, the best fit model for current data has a slight preference for dynamics [w(a){ne}-1], degrees of freedom distinct from quintessence (c{sub s{ne}}1), and early presence of dark energy [{Omega}{sub de}(a<<1){ne}0]. Future data may open a new window on dark energy by measuring its spatial as well as time variation.

Putter, Roland de [Berkeley Lab and University of California, Berkeley, California 94720 (United States); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan, 48109 (United States); Linder, Eric V. [Berkeley Lab and University of California, Berkeley, California 94720 (United States); Institute for the Early Universe, Ewha Womans University, Seoul (Korea, Republic of)

2010-05-15

49

Evaluating dark energy probes using multidimensional dark energy parameters

We investigate the value of future dark-energy experiments by modeling their ability to constrain the dark-energy equation of state. Similar work was recently reported by the Dark Energy Task Force (DETF) using a two dimensional parameterization of the equation-of-state evolution. We examine constraints in a nine-dimensional dark-energy parameterization, and find that the best experiments constrain significantly more than two dimensions in our 9D space. Consequently the impact of these experiments is substantially beyond that revealed in the DETF analysis, and the estimated cost per 'impact' drops by about a factor of 10 as one moves to the very best experiments. The DETF conclusions about the relative value of different techniques and of the importance of combining techniques are unchanged by our analysis.

Albrecht, Andreas; Bernstein, Gary [Department of Physics, University of California at Davis, One Shields Avenue, Davis, California 95616 (United States); Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104 (United States)

2007-05-15

50

Weak lensing and dark energy: The impact of dark energy on nonlinear dark matter clustering

We examine the influence of percent-level dark energy corrections to the nonlinear matter power spectrum on constraints of the dark energy equation of state from future weak lensing probes. We explicitly show that a poor approximation (off by > or approx.10%) to the nonlinear corrections causes a > or approx. 1{sigma} bias on the determination of the dark energy equation of state. Future weak lensing surveys must therefore incorporate dark energy modifications to the nonlinear matter power spectrum accurate to the percent-level, to avoid introducing significant bias in their measurements. For the WMAP5 cosmology, the more accurate power spectrum is more sensitive to dark energy properties, resulting in a factor of 2 improvement in dark energy equation of state constraints. We explore the complementary constraints on dark energy from future weak lensing and supernova surveys. A space-based, Joint Dark Energy Mission-like survey measures the equation of state in five independent redshift bins to {approx}10%, while this improves to {approx}5% for a wide-field ground-based survey like the Large Synoptic Survey Telescope. These constraints are contingent upon our ability to control weak lensing systematic uncertainties to the sub-percent level.

Joudaki, Shahab; Cooray, Asantha; Holz, Daniel E. [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2009-07-15

51

Entropy bounds and dark energy

NASA Astrophysics Data System (ADS)

Entropy bounds render quantum corrections to the cosmological constant ? finite. Under certain assumptions, the natural value of ? is of order the observed dark energy density ~10-10 eV4, thereby resolving the cosmological constant problem. We note that the dark energy equation of state in these scenarios is w?p/?=0 over cosmological distances, and is strongly disfavored by observational data. Alternatively, ? in these scenarios might account for the diffuse dark matter component of the cosmological energy density. Permanent address: Institute of Theoretical Science and Department of Physics, University of Oregon, Eugene, OR 97403.

Hsu, Stephen D. H.

2004-07-01

52

Entropy bounds and dark energy

Entropy bounds render quantum corrections to the cosmological constant ? finite. Under certain assumptions, the natural value of ? is of order the observed dark energy density ?10?10 eV4, thereby resolving the cosmological constant problem. We note that the dark energy equation of state in these scenarios is w?p\\/?=0 over cosmological distances, and is strongly disfavored by observational data. Alternatively, ?

Stephen D. H. Hsu

2004-01-01

53

How many dark energy parameters?

For exploring the physics behind the accelerating universe a crucial question is how much we can learn about the dynamics through next generation cosmological experiments. For example, in defining the dark energy behavior through an effective equation of state, how many parameters can we realistically expect to tightly constrain? Through both general and specific examples (including new parametrizations and principal component analysis) we argue that the answer is 42 - no, wait, two. Cosmological parameter analyses involving a measure of the equation of state value at some epoch (e.g., w_0) and a measure of the change in equation of state (e.g., w') are therefore realistic in projecting dark energy parameter constraints. More elaborate parametrizations could have some uses (e.g., testing for bias or comparison with model features), but do not lead to accurately measured dark energy parameters.

Linder, Eric V.; Huterer, Dragan

2005-05-16

54

NASA Astrophysics Data System (ADS)

In cosmology we are facing the dark energy crisis: How can we survive huge vacuum energy, meanwhile living with tiny dark energy? For the solution to this crisis, we raise several clues and hints, in particular, supersymmetry and the double hierarchy, Mp-MSM-MDE (Planck-Standard Model-dark energy scales). These two clues naturally lead to a solution with a supersymmetry-breaking brane-world. The train of thought from the clues to the solution is elucidated.

Gu, Je-An

2010-11-01

55

Cosmology with Ricci dark energy

NASA Astrophysics Data System (ADS)

We assume the cosmological dark sector to consist of pressureless matter and holographic dark energy with a cutoff length proportional to the Ricci scale. The requirement of separate energy-momentum conservation of the components is shown to establish a relation between the matter fraction and the (necessarily time-dependent) equation-of-state parameter of the dark energy. Focusing on intrinsically adiabatic pressure perturbations of the dark-energy component, the matter perturbations are found as linear combinations of the total energy-density perturbations of the cosmic medium and the relative (nonadiabatic) perturbations of the components. The resulting background dynamics is consistent with observations from supernovae of type Ia, baryonic acoustic oscillations and the differential age of old objects. The perturbation dynamics, on the other hand, is plagued by instabilities which excludes any phantom-type equation of state. The only stable configuration is singled out by a fixed relation between the present matter fraction ?m0 and the present value ?0 of the equation-of-state parameter of the dark energy. However, this instability-avoiding configuration is only marginally consistent with the observationally preferred background values of the mentioned parameters.

del Campo, Sergio; Fabris, Júlio C.; Herrera, Ramón; Zimdahl, Winfried

2013-06-01

56

Direct reconstruction of dark energy.

An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With so few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space. We present a new nonparametric method which can accurately reconstruct a wide variety of dark energy behavior with no prior assumptions about it. It is simple, quick and relatively accurate, and involves no expensive explorations of parameter space. The technique uses principal component analysis and a combination of information criteria to identify real features in the data, and tailors the fitting functions to pick up trends and smooth over noise. We find that we can constrain a large variety of w(z) models to within 10%-20% at redshifts z?1 using just SNAP-quality data. PMID:20867085

Clarkson, Chris; Zunckel, Caroline

2010-05-27

57

I briefly review our current understanding of dark matter and dark energy. The first part of this review focusses on issues pertaining to dark matter including observational evidence for its existence, current constraints and the a...\\/a H3 = 1\\\\stackrel{...}{a}\\/a H^3 = 1 for LCDM, which is helpful for differentiating between LCDM and rival dark energy models. The review ends with

Varun Sahni

2005-01-01

58

Holographic dark energy reexamined

We have reexamined the holographic dark energy model by considering the spatial curvature. We have refined the model parameter and observed that the holographic dark energy model does not behave as phantom model. Comparing the holographic dark energy model to the supernova observation alone, we found that the closed Universe is favored. Combining with the Wilkinson microwave anisotropy probe (WMAP) data, we obtained the reasonable value of the spatial curvature of our Universe.

Gong Yungui; Wang Bin; Zhang Yuanzhong [College of Electronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China); Department of Physics, Fudan University, Shanghai 200433 (China); CCAST (World Laboratory), P.O. Box 8730, Beijing 100080, China and Institute of Theoretical Physics, Chinese Academy of Sciences, P.O. Box 2735, Beijing 100080 (China)

2005-08-15

59

Scattering of dark matter and dark energy

We demonstrate how the two dominant constituents of the Universe, dark energy and dark matter, could possess a large scattering cross section without considerably impacting observations. Unlike interacting models which invoke energy exchange between the two fluids, the background cosmology remains unaltered, leaving fewer observational signatures. Following a brief review of the scattering cross sections between cosmologically significant particles, we explore the implications of an elastic interaction between dark matter and dark energy. The growth of large scale structure is suppressed, yet this effect is found to be weak due to the persistently low dark energy density. Thus we conclude that the dark matter-dark energy cross section may exceed the Thomson cross section by several orders of magnitude.

Simpson, Fergus [SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)

2010-10-15

60

Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields, the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multifield system in details with a general Lagrangian density p={sub i=1}{sup n}X{sub i}g(X{sub i}e{sup {lambda}{sub i}}{sup {phi}{sub i}}), where X{sub i}=-({nabla}{phi}{sub i}){sup 2}/2 is the kinetic energy of the ith field {phi}{sub i}, {lambda}{sub i} is a constant, and g is an arbitrary function in terms of Y{sub i}=X{sub i}e{sup {lambda}{sub i}}{sup {phi}{sub i}}. This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from big-bang nucleosynthesis and the condition under which each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a thawing property of assisting scalar fields is crucial to determine the evolution of the field equation of state w{sub {phi}}, and (iii) the field equation of state today can be consistent with the observational bound w{sub {phi}}<-0.8 in the presence of multiple scalar fields.

Ohashi, Junko; Tsujikawa, Shinji [Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)

2009-11-15

61

Non-negligible dark energy density at high redshifts would indicate dark energy physics distinct from a cosmological constant or"reasonable'" canonical scalar fields. Such dark energy can be constrained tightly through investigation of the growth of structure, with limits of<~;;2percent of total energy density at z>> 1 for many models. Intermediate dark energy can have effects distinct from its energy density; the dark ages acceleration can be constrained to last less than 5percent of a Hubble e-fold time, exacerbating the coincidence problem. Both the total linear growth, or equivalently sigma 8, and the shape and evolution of the nonlinear mass power spectrum for z<2 (using the Linder-White nonlinear mapping prescription) provide important windows. Probes of growth, such as weak gravitational lensing, can interact with supernovae and CMB distance measurements to scan dark energy behavior over the entire range z=0-1100.

Linder, Eric V.

2006-04-11

62

NASA Astrophysics Data System (ADS)

Three decades ago Heath found the integral form of the exact analytic growing mode solution of the linear density perturbation ? on sub-horizon scales including the cosmological constant or the curvature term. Recently, we obtained the exact analytic form of this solution in our previous work [1]. Interestingly, we are able to extend this solution for general dark energy models with the constant equation of state ? in a flat universe. This analytic solution provides the accurate and efficient tools for probing the properties of dark energy models such as the behavior of the growth factor and the growth index. We investigate the growth index and its parameter at any epoch with this exact solution for different dark energy models and find that the growth index is quite model dependent in the redshift space, 0.25?z?1.5, so observations of the structure growth around this epoch would be very interesting. Also one may be able to rule out some dark energy models by using the analysis from this exact solution. Thus, the analytic solution for the growth factor provides the very useful tools for future observations to constrain the exact values of observational quantities at any epoch related to the growth factor in the dark energy models.

Lee, Seokcheon; Ng, Kin-Wang

2010-04-01

63

Dark radiation as a signature of dark energy

We propose a simple dark energy model with the following properties: the model predicts a late-time dark radiation component that is not ruled out by current observational data, but which produces a distinctive time-dependent equation of state w(z) for z<3. The dark energy field can be coupled strongly enough to standard model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

Dutta, Sourish; Scherrer, Robert J. [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States); Hsu, Stephen D. H.; Reeb, David [Institute of Theoretical Science, University of Oregon, Eugene, Oregon 97403-5203 (United States)

2009-05-15

64

A model of holographic dark energy

A model for holographic dark energy is proposed, following the idea that the short distance cut-off is related to the infrared cut-off. We assume that the infrared cut-off relevant to the dark energy is the size of the event horizon. With the input ??=0.73, we predict the equation of state of the dark energy at the present time be characterized

Miao Li

2004-01-01

65

Possible equilibria of interacting dark energy models

Interacting dark energy and the holographic principle offer a possible way of addressing the cosmic coincidence problem as well as accounting for the size of the dark energy component. The equilibrium points of the Friedmann equations which govern the evolution behavior of dark energy, matter, and curvature components can determine the qualitative behavior of the cosmological models. These possible equilibria and their behavior are examined in a general framework, and some illustrative examples are presented.

Berger, Micheal S.; Shojaei, Hamed [Physics Department, Indiana University, Bloomington, Indiana 47405 (United States)

2008-06-15

66

We review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark

Edmund J. Copeland; M. Sami; Shinji Tsujikawa

2006-01-01

67

Supernova Constraints on Braneworld Dark Energy

Braneworld models of dark energy are examined in the light of observations of high redshift type Ia supernovae. Braneworld models admit several novel and even exotic possibilities which include: (i) The effective equation of state of dark energy can be more negative than in LCDM: $w \\\\leq -1$; (ii) A class of braneworld models can encounter a `quiescent' future singularity

Ujjaini Alam; Varun Sahni

2002-01-01

68

Interacting Dark Matter and Dark Energy

We discuss models for the cosmological dark sector in which the energy\\u000adensity of a scalar field approximates Einstein's cosmological constant and the\\u000ascalar field value determines the dark matter particle mass by a Yukawa\\u000acoupling. A model with one dark matter family can be adjusted so the\\u000aobservational constraints on the cosmological parameters are close to but\\u000adifferent from

Glennys R. Farrar; P. J. E. Peebles

2003-01-01

69

Field theory model for dark matter and dark energy in interaction

We propose a field theory model for dark energy and dark matter in interaction. Comparing the classical solutions of the field equations with the observations of the CMB shift parameter, baryonic acoustic oscillations, lookback time, and the Gold supernovae sample, we observe a possible interaction between dark sectors with energy decay from dark energy into dark matter. The observed interaction provides an alleviation to the coincidence problem.

Micheletti, Sandro; Abdalla, Elcio [Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo (Brazil); Wang Bin [Department of Physics, Fudan University, 200433 Shanghai (China)

2009-06-15

70

Dark energy dynamics in the recent universe is influenced by its evolution through the long, matter-dominated expansion history. A particular dynamical property, the flow variable, remains constant in several classes of scalar field models as long as matter dominates; the dark energy is only free to change this behavior at recent times. This gives natural initial conditions for Monte Carlo studies of dark energy dynamics. We propose a parametrization for the later evolution that covers a wide range of possible behaviors, is tractable in making predictions, and can be constrained by observations. We compare the approach to directly parametrizing the potential, which does not take into account the history of the dark energy dynamics.

Cortes, Marina [Berkeley Lab, Berkeley, California 94720 (United States); Linder, Eric V. [Berkeley Lab, Berkeley, California 94720 (United States); University of California, Berkeley, California 94720 (United States); Institute for the Early Universe, Ewha Womans University, Seoul (Korea, Republic of)

2010-03-15

71

NASA Astrophysics Data System (ADS)

We look for evidence for the evolution in dark energy density by employing Principal Component Analysis (PCA). Distance redshift data from supernovae and baryon acoustic oscillations (BAO) along with WMAP7 distance priors are used to put constraints on curvature parameter ?k and dark energy parameters. The data sets are consistent with a flat Universe. The constraints on the dark energy evolution parameters obtained from supernovae (including CMB distance priors) are consistent with a flat ?CDM Universe. On the other hand, in the parameter estimates obtained from the addition of BAO data the second principal component, which characterize a non-constant contribution from dark energy, is non-zero at 1?. This could be a systematic effect and future BAO data holds key to making more robust claims.

Nair, Remya; Jhingan, Sanjay

2013-02-01

72

I review the problem of dark energy focussing on cosmological constant as the candidate and discuss what it tells us regarding\\u000a the nature of gravity. Section 1 briefly overviews the currently popular “concordance cosmology” and summarizes the evidence\\u000a for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as a\\u000a candidate and emphasizes

T. Padmanabhan

2008-01-01

73

Thermodynamics of dark energy interacting with dark matter and radiation

We investigate the validity of the generalized second law of thermodynamics, in the cosmological scenario where dark energy interacts with both dark matter and radiation. Calculating separately the entropy variation for each fluid component and for the apparent horizon itself, we show that the generalized second law is always and generally valid, independently of the specific interaction form, of the fluids equation-of-state parameters and of the background geometry.

Jamil, Mubasher [Center for Advanced Mathematics and Physics, National University of Sciences and Technology, Rawalpindi, 46000 (Pakistan); Saridakis, Emmanuel N. [Department of Physics, University of Athens, GR-15771 Athens (Greece); Setare, M. R. [Department of Science, Payame Noor University, Bijar (Iran, Islamic Republic of)

2010-01-15

74

From Dark Energy & Dark Matter to Dark Metric

NASA Astrophysics Data System (ADS)

We present a new approach to the mathematical objects of General Relativity in terms of which a generic f( R)-gravity theory gravitation is written in a first-order ( à la Palatini) formalism, and introduce the concept of Dark Metric which could bypass the emergence of disturbing concepts as Dark Energy and Dark Matter. These issues are related to the fact that General Relativity could not be the definitive theory of Gravitation due to several shortcomings that come out both from theoretical and experimental viewpoints. At large scales, the attempts to match it with the recent observational data lead to invoke Dark Energy and Dark Matter as the bulk components of the cosmic fluid. Since no final evidence, at fundamental level, exists for such ingredients, it could be useful to reconsider the gravitational sector in order to see if suitable extensions of General Relativity could solve the shortcomings present at infrared scales.

Capozziello, S.; de Laurentis, M.; Francaviglia, M.; Mercadante, S.

2009-10-01

75

Dark energy and dark matter haloes

NASA Astrophysics Data System (ADS)

We investigate the effect of dark energy on the density profiles of dark matter haloes with a suite of cosmological N-body simulations and use our results to test analytic models. We consider constant equation of state models, and allow both w>=-1 and w < -1. Using five simulations with w ranging from -1.5 to -0.5, and with more than ~1600 well-resolved haloes each, we show that the halo concentration model of Bullock et al. accurately predicts the median concentrations of haloes over the range of w, halo masses and redshifts that we are capable of probing. We find that the Bullock et al. model works best when halo masses and concentrations are defined relative to an outer radius set by a cosmology-dependent virial overdensity. For a fixed power spectrum normalization and fixed-mass haloes, larger values of w lead to higher concentrations and higher halo central densities, both because collapse occurs earlier and because haloes have higher virial densities. While precise predictions of halo densities are quite sensitive to various uncertainties, we make broad comparisons to galaxy rotation curve data. At fixed power spectrum normalization (fixed ?8), w > -1 quintessence models seem to exacerbate the central density problem relative to the standard w=-1 model. For example, models with w~=- 0.5 seem disfavoured by the data, which can be matched only by allowing extremely low normalizations, ?8<~ 0.6. Meanwhile w < -1 models help to reduce the apparent discrepancy. We confirm that the halo mass function of Jenkins et al. provides an excellent approximation to the abundance of haloes in our simulations and extend its region of validity to include models with w < -1.

Kuhlen, Michael; Strigari, Louis E.; Zentner, Andrew R.; Bullock, James S.; Primack, Joel R.

2005-02-01

76

Dipolar dark matter and dark energy

In previous work [L. Blanchet and A. Le Tiec, Phys. Rev. D 78, 024031 (2008)], a model of dark matter and dark energy based on the concept of gravitational polarization was investigated. This model was shown to recover the concordance cosmological scenario ({lambda}-cold dark matter) at cosmological scales, and the phenomenology of the modified Newtonian dynamics at galactic scales. In this article we prove that the model can be formulated with a simple and physically meaningful matter action in general relativity. We also provide alternative derivations of the main results of the model, and some details on the variation of the action.

Blanchet, Luc; Le Tiec, Alexandre [GReCO Institut d'Astrophysique de Paris-UMR 7095 du CNRS, Universite Pierre and Marie Curie, 98bis boulevard Arago, 75014 Paris (France)

2009-07-15

77

NASA Astrophysics Data System (ADS)

A generic feature of viable F(R) gravity is investigated: it is demonstrated that during the matter dominated era the large frequency oscillations of the effective dark energy may influence the behavior of higher derivatives of the Hubble parameter with the risk to produce some singular unphysical solutions at high redshift. This behavior is explicitly analyzed for realistic F(R) models, in particular, exponential gravity and a power form model. To stabilize such oscillations, we consider the additional modification of the models via a correction term which does not destroy the viability properties. A detailed analysis on the future evolution of the universe and the evolution history of the growth index of the matter density perturbations are performed. Furthermore, we explore two applications of exponential gravity to the inflationary scenario. We show how it is possible to obtain different numbers of e-folds during the early-time acceleration by making different choices of the model parameters in the presence of ultrarelativistic matter, which destabilizes inflation and eventually leads to the exit from the inflationary stage. We execute the numerical analysis of inflation in two viable exponential gravity models. It is proved that at the end of the inflation, the effective energy density and curvature of the universe decrease and thus a unified description between inflation and the ?CDM-like dark energy dominated era can be realized.

Bamba, Kazuharu; Lopez-Revelles, Antonio; Myrzakulov, R.; Odintsov, S. D.; Sebastiani, L.

2013-01-01

78

We review two mechanisms rooted in the infrared sector of QCD which, by exploiting the properties of the QCD ghost, as introduced by Veneziano, provide new insight on the cosmological dark energy problem, first, in the form of a Casimir-like energy from quantising QCD in a box, and second, in the form of additional, time-dependent, vacuum energy density in an expanding universe. Based on [1, 2].

Urban, Federico R.; Zhitnitsky, Ariel R. [Department of Physics and Astronomy, University of British Columbia, Vancouver, B.C. V6T 1Z1 (Canada)

2010-08-30

79

A Modified Dark Energy Model and Quintessence

NASA Astrophysics Data System (ADS)

The observational data indicate that about 70 % of the total energy density of the current state universe has been occupied by Dark Energy. This is said to be the cause of the accelerated expansion of universe. In this letter we shall use a curvature constant as a scalar field in the quintessence Dark Energy model, for an isotropic universe. Connected to the so-called model, we will specify a definite dynamical field equation from the initial action of the theory.

Zare Dehnavi, Naser; Fathi, Mohsen; Tavakoli, Farhad

2013-11-01

80

Cosmological model with viscosity media (dark fluid) described by an effective equation of state

A generally parameterized equation of state (EOS) is investigated in the cosmological evolution with bulk viscosity media modelled as dark fluid, which can be regarded as a unification of dark energy and dark matter. Compared with the case of the perfect fluid, this EOS has possessed four additional parameters, which can be interpreted as the case of the non-perfect fluid

Jie Ren; Xin-He Meng

2006-01-01

81

This review summarizes recent attempts to reconstruct the expansion history of the universe and to probe the nature of dark energy. Reconstruction methods can be broadly classified into parametric and non-parametric approaches. It is encouraging that, even with the limited observational data currently available, different approaches give consistent results for the reconstruction of the Hubble parameter H(z) and the effective

Varun Sahni; Alexei Starobinsky

2006-01-01

82

Extended holographic dark energy

The idea of relating the infrared and ultraviolet cutoffs is applied to the Brans-Dicke theory of gravitation. We find that the Hubble scale or the particle horizon as the infrared cutoff will not give accelerating expansion. The dynamical cosmological constant with the event horizon as the infrared cutoff is a viable dark energy model.

Gong Yungui [Institute of Applied Physics and College of Electronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China)

2004-09-15

83

Dark Energy from Entanglement Entropy

NASA Astrophysics Data System (ADS)

We show that quantum decoherence, in the context of observational cosmology, can be connected to the cosmic dark energy. The decoherence signature could be characterized by the existence of quantum entanglement between cosmological eras. As a consequence, the Von Neumann entropy related to the entanglement process, can be compared to the thermodynamical entropy in a homogeneous and isotropic universe. The corresponding cosmological models are compatible with the current observational bounds being able to reproduce viable equations of state without introducing a priori any cosmological constant. In doing so, we investigate two cases, corresponding to two suitable cosmic volumes, V? a 3 and V? H -3, and find two models which fairly well approximate the current cosmic speed up. The existence of dark energy can be therefore reinterpreted as a quantum signature of entanglement, showing that the cosmological constant represents a limiting case of a more complicated model derived from the quantum decoherence.

Capozziello, Salvatore; Luongo, Orlando

2013-08-01

84

Traditionally, inflationary models are analyzed in terms of parameters such as the scalar spectral index n{sub s} and the tensor to scalar ratio r, while dark energy models are studied in terms of the equation of state parameter w. Motivated by the fact that both deal with periods of accelerated expansion, we study the evolution of w during inflation, in order to derive observational constraints on its value during an earlier epoch likely dominated by a dynamic form of dark energy. We find that the cosmic microwave background and large-scale structure data is consistent with w{sub inflation}=-1 and provides an upper limit of 1+w < or approx. 0.02. Nonetheless, an exact de Sitter expansion with a constant w=-1 is disfavored since this would result in n{sub s}=1.

Ilic, Stephane [Magistere de Physique Fondamentale, Universite Paris-Sud XI, Orsay 91405 (France); Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom); Kunz, Martin [Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom); Departement de Physique Theorique, Universite de Geneve, 1211 Geneva 4 (Switzerland); Liddle, Andrew R. [Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom); Frieman, Joshua A. [Fermilab Center for Particle Astrophysics, Batavia, Illinois 60510 (United States); Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, Illinois 60637 (United States)

2010-05-15

85

Cosmic steps in modeling dark energy

Past and recent data analyses gave some hints of steps in dark energy. Considering dark energy as a dynamical scalar field, we investigate several models with various steps: a step in the scalar potential, a step in the kinetic term, a step in the energy density, and a step in the equation-of-state parameter w. These toy models provide a workable mechanism to generate steps and features of dark energy. Remarkably, a single real scalar can cross w=-1 dynamically with a step in the kinetic term.

Wang Tower [Center for High-Energy Physics, Peking University, Beijing 100871 (China)

2009-11-15

86

It is extraordinary that a number of observations indicate that we live in a spatially flat, low matter density Universe, which is currently undergoing a period of accelerating expansion. The effort to explain this current state has focused attention on cosmological models in which the dominant component of the cosmic energy density has negative pressure, with an equation of state

R. R. Caldwell

2002-01-01

87

What is the Universe made of? This question has been asked as long as humans have been questioning, and astronomers and physicists are finally converging on an answer. The picture which has emerged from numerous complementary observations over the past decade is a surprising one: most of the matter in the Universe isn't visible, and most of the Universe isn't even made of matter. In this talk, I will explain what the rest of this stuff, known as 'Dark Energy' is, how it is related to the so-called 'Dark Matter', how it impacts the evolution of the Universe, and how we can study the dark universe using observations of light from current and future telescopes.

88

Roles of dark energy perturbations in dynamical dark energy models: can we ignore them?

We show the importance of properly including the perturbations of the dark energy component in the dynamical dark energy models based on a scalar field and modified gravity theories in order to meet with present and future observational precisions. Based on a simple scaling scalar field dark energy model, we show that observationally distinguishable substantial differences appear by ignoring the dark energy perturbation. By ignoring it the perturbed system of equations becomes inconsistent and deviations in (gauge-invariant) power spectra depend on the gauge choice. PMID:19905618

Park, Chan-Gyung; Hwang, Jai-chan; Lee, Jae-heon; Noh, Hyerim

2009-10-09

89

We study the power of upcoming weak lensing surveys to probe dark energy. Dark energy modifies the distance-redshift relation as well as the matter power spectrum, both of which affect the weak lensing convergence power spectrum. Some dark-energy models predict additional clustering on very large scales, but this probably cannot be detected by weak lensing alone due to cosmic variance.

Dragan Huterer

2002-01-01

90

Signature of the interaction between dark energy and dark matter in observations

We investigate the effect of an interaction between dark energy and dark matter upon the dynamics of galaxy clusters. This effect is computed through the Layser-Irvine equation, which describes how an astrophysical system reaches virial equilibrium and was modified to include the dark interactions. Using observational data from almost 100 purportedly relaxed galaxy clusters we put constraints on the strength of the couplings in the dark sector. We compare our results with those from other observations and find that a positive (in the sense of energy flow from dark energy to dark matter) nonvanishing interaction is consistent with the data within several standard deviations.

Abdalla, Elcio; Abramo, L. Raul; Souza, Jose C. C. de [Instituto de Fisica, Universidade de Sao Paulo, C. P. 66318, 05315-970, Sao Paulo, SP (Brazil)

2010-07-15

91

We examine a dark energy model where a scalar unparticle degree of freedom plays the role of quintessence. In particular, we study a model where the unparticle degree of freedom has a standard kinetic term and a simple mass potential, the evolution is slowly rolling and the field value is of the order of the unparticle energy scale ({lambda}{sub u}). We study how the evolution of w depends on the parameters B (a function of unparticle scaling dimension d{sub u}), the initial value of the field {phi}{sub i} (or equivalently, {lambda}{sub u}) and the present matter density {omega}{sub m0}. We use observational data from type Ia supernovae, baryon acoustic oscillations and the cosmic microwave background to constrain the model parameters and find that these models are not ruled out by the observational data. From a theoretical point of view, unparticle dark energy model is very attractive, since unparticles (being bound states of fundamental fermions) are protected from radiative corrections. Further, coupling of unparticles to the standard model fields can be arbitrarily suppressed by raising the fundamental energy scale M{sub F}, making the unparticle dark energy model free of most of the problems that plague conventional scalar field quintessence models.

Dai, D.-C.; Stojkovic, Dejan [HEPCOS, Department of Physics, SUNY at Buffalo, Buffalo, New York 14260-1500 (United States); Dutta, Sourish [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States)

2009-09-15

92

Cluster Physics with Dark Energy

NASA Astrophysics Data System (ADS)

Dark energy affects the abundance and evolution of clusters owing to their dependence on the geometry of the Universe and the power spectrum. Usually, there exits the degeneracy between ?8 and the matter energy density contrast {? { m}0}. We avoid this by using the explicit dark energy dependent rms linear mass fluctuation ?8 which is consistent with the CMB normalization for general constant dark energy equation of state, ?Q. When we use the correct value of the critical density threshold ?c = 1.58 into the cluster number density n calculation in the Press-Schechter (PS) formalism, PS formalism predicts the cluster number consistent with both simulation and observed data at the high mass region. The improved coefficients of Sheth-Tormen (ST) formalism by using the correct ?c is also obtained. We found that changing ?Q by ??Q = -0.1 from ?Q = -1.0 causes the changing of the comoving numbers of high mass clusters of M = 1016h-1M? by about 20 and 40% at z = 0 and 1, respectively.

Lee, Seokcheon

93

Can dark energy evolve to the phantom?

Dark energy with the equation of state w(z) rapidly evolving from the dustlike (w~=0 at z˜1) to the phantomlike (-1.2≲w≲-1 at z~=0) has been recently proposed as the best fit for the supernovae Ia data. Assuming that a dark energy component with an arbitrary scalar-field Lagrangian p(phi,?muphi) dominates in the flat Friedmann universe, we analyze the possibility of a dynamical

Alexander Vikman

2005-01-01

94

Decaying dark matter mimicking time-varying dark energy

A {Lambda}CDM model with dark matter that decays into inert relativistic energy on a time scale longer than the Hubble time will produce an expansion history that can be misinterpreted as stable dark matter with time-varying dark energy. We calculate the corresponding spurious equation of state parameter, w-tilde{sub {phi}}, as a function of redshift, and show that the evolution of w-tilde{sub {phi}} depends strongly on the assumed value of the dark-matter density, erroneously taken to scale as a{sup -3}. Depending on the latter, one can obtain models that mimic quintessence (w-tilde{sub {phi}>}-1), phantom models (w-tilde{sub {phi}<}-1), or models in which the equation of state parameter crosses the phantom divide, evolving from w-tilde{sub {phi}>}-1 at high redshift to w-tilde{sub {phi}<}-1 at low redshift. All of these models generically converge toward w-tilde{sub {phi}{approx_equal}-}1 at the present. The degeneracy between the {Lambda}CDM model with decaying dark matter and the corresponding spurious quintessence model is broken by the growth of density perturbations.

Dutta, Sourish; Scherrer, Robert J. [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States)

2010-08-15

95

Dark energy and extended dark matter halos

NASA Astrophysics Data System (ADS)

The cosmological mean matter (dark and baryonic) density measured in the units of the critical density is ?m = 0.27. Independently, the local mean density is estimated to be ?loc = 0.08-0.23 from recent data on galaxy groups at redshifts up to z = 0.01-0.03 (as published by Crook et al. 2007, ApJ, 655, 790 and Makarov & Karachentsev 2011, MNRAS, 412, 2498). If the lower values of ?loc are reliable, as Makarov & Karachentsev and some other observers prefer, does this mean that the Local Universe of 100-300 Mpc across is an underdensity in the cosmic matter distribution? Or could it nevertheless be representative of the mean cosmic density or even be an overdensity due to the Local Supercluster therein. We focus on dark matter halos of groups of galaxies and check how much dark mass the invisible outer layers of the halos are able to host. The outer layers are usually devoid of bright galaxies and cannot be seen at large distances. The key factor which bounds the size of an isolated halo is the local antigravity produced by the omnipresent background of dark energy. A gravitationally bound halo does not extend beyond the zero-gravity surface where the gravity of matter and the antigravity of dark energy balance, thus defining a natural upper size of a system. We use our theory of local dynamical effects of dark energy to estimate the maximal sizes and masses of the extended dark halos. Using data from three recent catalogs of galaxy groups, we show that the calculated mass bounds conform with the assumption that a significant amount of dark matter is located in the invisible outer parts of the extended halos, sufficient to fill the gap between the observed and expected local matter density. Nearby groups of galaxies and the Virgo cluster have dark halos which seem to extend up to their zero-gravity surfaces. If the extended halo is a common feature of gravitationally bound systems on scales of galaxy groups and clusters, the Local Universe could be typical or even an overdense region, with a low density contrast ~1.

Chernin, A. D.; Teerikorpi, P.; Valtonen, M. J.; Dolgachev, V. P.; Domozhilova, L. M.; Byrd, G. G.

2012-03-01

96

Weak Lensing: Dark Matter, Dark Energy

The light rays from distant galaxies are deflected by massive structures along the line of sight, causing the galaxy images to be distorted. Measurements of these distortions, known as weak lensing, provide a way of measuring the distribution of dark matter as well as the spatial geometry of the universe. I will describe the ideas underlying this approach to cosmology. With planned large imaging surveys, weak lensing is a powerful probe of dark energy. I will discuss the observational challenges ahead and recent progress in developing multiple, complementary approaches to lensing measurements.

Jain, Bhuvnesh (University of Pennsylvania)

2006-02-27

97

Weak Lensing: Dark Matter, Dark Energy

The light rays from distant galaxies are deflected by massive structures along the line of sight, causing the galaxy images to be distorted. Measurements of these distortions, known as weak lensing, provide a way of measuring the distribution of dark matter as well as the spatial geometry of the universe. I will describe the ideas underlying this approach to cosmology. With planned large imaging surveys, weak lensing is a powerful probe of dark energy. I will discuss the observational challenges ahead and recent progress in developing multiple, complementary approaches to lensing measurements.

98

Weak Lensing: Dark Matter, Dark Energy

The light rays from distant galaxies are deflected by massive structures along the line of sight, causing the galaxy images to be distorted.Measurements of these distortions, known as weak lensing, provide a way of measuring the distribution of dark matter as well as the spatial geometry of the universe. I will describe the ideas underlying this approach to cosmology. With planned large imaging surveys, weak lensing is a powerful probe of dark energy. I will discuss the observational challenges ahead and recent progress in developing multiple, complementary approaches to lensing measurements.

99

Dark Energy: fiction or reality?

Is Dark Energy justified as an alternative to the cosmological constant LAMBDA in order to explain the acceleration of the cosmic expansion? It turns out that a straightforward dimensional analysis of Einstein equation provides us with clear evidences that the geometrical nature of LAMBDA is the only viable source to this phenomenon, in addition of the application of Ockham's razor principle. This contribution is primarily a review of the main stream in the interpretation of LAMBDA because it is at the origin of such a research program.

Triay, Roland [Centre de Physique Theorique, CNRS Luminy Case 907, 13288 Marseille Cedex 9 (France)

2010-06-15

100

1 construct a general description for neutrino dark energy models, that do not require exotic particles or strange couplings. With the help of the above, this class of models is reduced to a single function with several constraints. It is shown that these models lead to some concrete predictions that can be verified (or disproved) within the next decade, using results from PLANK, EUCLID and JDEM.

Gurwich, Ilya [Physics Department, Ben-Gurion University, Beer-Sheva 84105 (Israel)

2010-06-23

101

Dark energy, inflation, and extra dimensions

We consider how accelerated expansion, whether due to inflation or dark energy, imposes strong constraints on fundamental theories obtained by compactification from higher dimensions. For theories that obey the null energy condition (NEC), we find that inflationary cosmology is impossible for a wide range of compactifications; and a dark energy phase consistent with observations is only possible if both Newton's gravitational constant and the dark energy equation of state vary with time. If the theory violates the NEC, inflation and dark energy are only possible if the NEC-violating elements are inhomogeneously distributed in the compact dimensions and vary with time in precise synchrony with the matter and energy density in the noncompact dimensions. Although our proofs are derived assuming general relativity applies in both four and higher dimensions and certain forms of metrics, we argue that similar constraints must apply for more general compactifications.

Steinhardt, Paul J. [Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544 (United States); Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544 (United States); Wesley, Daniel [Centre for Theoretical Cosmology, DAMTP, Cambridge University, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)

2009-05-15

102

NASA Astrophysics Data System (ADS)

This dissertation describes three research projects on the topic of dark energy. The first project is an analysis of a scalar field model of dark energy with an exponential potential using the Dark Energy Task Force (DETF) simulated data models. Using Markov Chain Monte Carlo sampling techniques we examine the ability of each simulated data set to constrain the parameter space of the exponential potential for data sets based on a cosmological constant and a specific exponential scalar field model. We compare our results with the constraining power calculated by the DETF using their "w 0--wa" parameterization of the dark energy. We find that respective increases in constraining power from one stage to the next produced by our analysis give results consistent with DETF results. To further investigate the potential impact of future experiments, we also generate simulated data for an exponential model background cosmology which can not be distinguished from a cosmological constant at DETF Stage 2, and show that for this cosmology good DETF Stage 4 data would exclude a cosmological constant by better than 3sigma. The second project details this analysis on a Inverse Power Law (IPL) or "Ratra-Peebles" (RP) model. This model is a member of a popular subset of scalar field quintessence models that exhibit "tracking" behavior that make this model particularly theoretically interesting. We find that the relative increase in constraining power on the parameter space of this model is consistent to what was found in the first project and the DETF report. We also show, using a background cosmology based on an IPL scalar field model that is consistent with a cosmological constant with Stage 2 data, that good DETF Stage 4 data would exclude a cosmological constant by better than 3sigma. The third project extends the Causal Entropic Principle to predict the preferred curvature within the "multiverse". The Causal Entropic Principle (Bousso, et al.) provides an alternative approach to anthropic attempts to predict our observed value of the cosmological constant by calculating the entropy created within a causal diamond. We have found that values larger than rhok = 40rho m are disfavored by more than 99.99% and a peak value at rho ? = 7.9 x 10-123 and rho k = 4.3rhom for open universes. For universes that allow only positive curvature or both positive and negative curvature, we find a correlation between curvature and dark energy that leads to an extended region of preferred values. Our universe is found to be disfavored to an extent depending the priors on curvature. We also provide a comparison to previous anthropic constraints on open universes and discuss future directions for this work.

Bozek, Brandon

103

Dark Matter, Dark Energy and General Relativity

NASA Astrophysics Data System (ADS)

From an earlier model of DM, we derived: An equation of state for DM. An equation governing the coupled distributions of baryonic and dark matters; galactic flat rotation curves; the Tully-Fisher relation; Milgrom's MOND relation; and a mechanism for the accelerated expansion of the Universe (J of Modern Physics, Dec. 2011). In the present paper, we add the derivation of: a mechanism for the formation of a super massive black hole at the center of every large galaxy; a rational for the finding (by Oort) that the Sun mass on the Solar Neighborhood scale is twice that found on the Solar System scale. Results are discussed; we conclude that general relativity may neither be applied to DM nor to the description of the Universe and its evolution. The correct description must be based not on General Relativity but on Newton's Gravitation, on the proposed model of DM, and on Einstein's relativity of 1905.

Leibovitz, Jacques

2013-04-01

104

Dark energy and dark gravity: theory overview

Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological\\u000a observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations\\u000a of cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models\\u000a are nearly all based on a dark energy field with fine-tuned, unnatural

Ruth Durrer; Roy Maartens

2008-01-01

105

NASA Astrophysics Data System (ADS)

Future advances in cosmology will depend on the next generation of cosmological observations and how they shape our theoretical understanding of the universe. Current theoretical ideas, however, have an important role to play in guiding the design of such observational programs. The work presented in this thesis concerns the intersection of observation and theory, particularly as it relates to advancing our understanding of the accelerated expansion of the universe (or the dark energy). Chapters 2 - 4 make use of the simulated data sets developed by the Dark Energy Task Force (DETF) for a number of cosmological observations currently in the experimental pipeline. We use these forecast data in the analysis of four quintessence models of dark energy: the PNGB, Exponential, Albrecht-Skordis and Inverse Power Law (IPL) models. Using Markov Chain Monte Carlo sampling techniques we examine the ability of each simulated data set to constrain the parameter space of these models. We examine the potential of the data for differentiating time-varying models from a pure cosmological constant. Additionally, we introduce an abstract parameter space to facilitate comparison between models and investigate the ability of future data to distinguish between these quintessence models. In Chapter 5 we present work towards understanding the effects of systematic errors associated with photometric redshift estimates. Due to the need to sample a vast number of deep and faint galaxies, photometric redshifts will be used in a wide range of future cosmological observations including gravitational weak lensing, baryon accoustic oscillations and type 1A supernovae observations. The uncertainty in the redshift distributions of galaxies has a significant potential impact on the cosmological parameter values inferred from such observations. We introduce a method for parameterizing uncertainties in modeling assumptions affecting photometric redshift calculations and for propagating these uncertainties through to uncertainties in the cosmological parameters.

Abrahamse, Augusta

2010-12-01

106

Constraints on generalized dark energy from recent observations

Effects of a generalized dark energy fluid are investigated on cosmic density fluctuations such as a cosmic microwave background. As a general dark energy fluid, we take into consideration the possibility of the anisotropic stress for dark energy, which has not been discussed much in the literature. We comprehensively study its effects on the evolution of density fluctuations along with that of the nonadiabatic pressure fluctuation of dark energy, then give constraints on such a generalized dark energy from current observations. We show that, though we cannot find any stringent limits on the anisotropic stress or the nonadiabatic pressure fluctuation themselves, the constraints on the equation of state of dark energy can be affected in some cases by the nature of dark energy fluctuation characterized by these properties. This may have important implications in the strategy to study the nature of dark energy.

Ichiki, Kiyotomo [Research Center for the Early Universe, University of Tokyo, Tokyo 113-0033 (Japan); Takahashi, Tomo [Department of Physics, Saga University, Saga 840-8502 (Japan)

2007-06-15

107

We present a model in which the parameter w approaches -1 near a particular value of z, and has significant negative values in a restricted range of z. For example, one can have w {approx} -1 near z = 1, and w > -0.2 from z = 0 to z = 0.3, and for z > 9. The ingredients of the model are neutral fermions (which may be neutrinos, neutralinos, etc) which are very weakly coupled to a light scalar field. This model emphasizes the importance of the proposed studies of the properties of dark energy into the region z > 1.

Goldman, Terry J [Los Alamos National Laboratory; Mckellar, B H J [UNIV 0F MELBOURNE; Stephenson, G J [UNM; Alsing, P M [UNM

2009-01-01

108

Dark matter and dark energy: The critical questions

Stars account for only about 0.5% of the content of the Universe; the bulk of the Universe is optically dark. The dark side of the Universe is comprised of: at least 0.1% light neutrinos; 3.5% {+-} 1% baryons; 29% {+-} 4% cold dark matter; and 66% {+-} 6% dark energy. Now that we have characterized the dark side of the Universe, the challenge is to understand it. The critical questions are: (1) What form do the dark baryons take? (2) What is (are) the constituent(s) of the cold dark matter? (3) What is the nature of the mysterious dark energy that is causing the Universe to speed up.

Michael S. Turner

2002-11-19

109

Analysis of pilgrim dark energy models

NASA Astrophysics Data System (ADS)

The proposal of pilgrim dark energy is based on the idea that phantom dark energy possesses enough resistive force to preclude black hole formation. We work on this proposal by choosing an interacting framework with cold dark matter and three cutoffs such as Hubble as well as event horizon and conformal age of the universe. We present a graphical analysis and focus our study on the pilgrim dark energy as well as interacting parameters. It is found that these parameters play an effective role on the equation of state parameter for exploring the phantom region of the universe. We also make the analysis of ?- ?' and point out freezing region in the ?- ?' plane. Finally, it turns out that the ?CDM is achieved in the statefinders plane for all models.

Sharif, M.; Jawad, Abdul

2013-04-01

110

Constraining Dark Energy with Neutrino Physics

We show that the particle properties of the Standard Model fields can determine the nature of Dark Energy. In particular, the mass of the neutrinos plays an important role in the determination of the equation of state w of Dark Energy. Using the Heidelberg-Moscow double beta decay experiment, which detects a large neutrino mass, we show that the impact of this measurement with the other cosmological data sets constrains the equation of state to -1.67

Macorra, A. de la [Instituto de Fisica, Universidad Nacional Autonoma de Mexico Apdo. Postal 20-364, 01000 Mxico D.F. (Mexico)

2009-04-20

111

Dark energy in hybrid inflation

The situation that a scalar field provides the source of the accelerated expansion of the Universe while rolling down its potential is common in both the simple models of the primordial inflation and the quintessence-based dark energy models. Motivated by this point, we address the possibility of causing the current acceleration via the primordial inflation using a simple model based on hybrid inflation. We trigger the onset of the motion of the quintessence field via the waterfall field, and find that the fate of the Universe depends on the true vacuum energy determined by choosing the parameters. We also briefly discuss the variation of the equation of state and the possible implementation of our scenario in supersymmetric theories.

Gong, Jinn-Ouk [Department of Physics, KAIST, Daejeon, Republic of Korea (Korea, Republic of); International Center for Astrophysics, KASI, Daejeon (Korea, Republic of); Kim, Seongcheol [Department of Physics, KAIST, Daejeon (Korea, Republic of)

2007-03-15

112

Testing the interaction between dark energy and dark matter via the latest observations

Cosmological analyses based on currently available observations are unable to rule out a sizeable coupling between dark energy and dark matter. However, the signature of the coupling is not easy to grasp, since the coupling is degenerate with other cosmological parameters, such as the dark energy equation of state and the dark matter abundance. We discuss possible ways to break such degeneracy. Based on the perturbation formalism, we carry out the global fitting by using the latest observational data and get a tight constraint on the interaction between dark sectors. We find that the appropriate interaction can alleviate the coincidence problem.

He Jianhua; Wang Bin; Abdalla, Elcio [Institute of Nuclear and Particle Physics, Astronomy, and Cosmology (INPAC) and the Department of Physics, Shanghai Jiao Tong University, 200240 Shanghai (China); Instituto de Fisica, Universidade de Sao Paulo, Caminho Particular 66318, 05315-970, Sao Paulo (Brazil)

2011-03-15

113

Revival of the unified dark energy-dark matter model?

We consider the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and show that it admits an unique decomposition into dark energy and dark matter components once phantomlike dark energy is excluded. Within this framework, we study structure formation and show that difficulties associated to unphysical oscillations or blowup in the matter power spectrum can be circumvented. Furthermore, we show that the dominance of dark energy is related to the time when energy density fluctuations start deviating from the linear {delta}{approx}a behavior.

Bento, M.C.; Bertolami, O.; Sen, A.A. [Departamento de Fisica, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal); Departamento de Fisica and Centro de Fisica das Interaccoes Fundamentais, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal)

2004-10-15

114

Cosmological bounds on the equation of state of dark matter

In this exploratory study, we investigate the bounds on the equation of state of dark matter. Modeling dark matter as a fluid component, we take into account both positive and negative fixed equations of state. Using CMB, supernovae Ia and large scale structure data we find constraints on the equation of state in a modified Î»CDM cosmology. We obtain -1.50x10â»â¶

Christian M

2005-01-01

115

Cosmological bounds on the equation of state of dark matter

In this exploratory study, we investigate the bounds on the equation of state of dark matter. Modeling dark matter as a fluid component, we take into account both positive and negative fixed equations of state. Using CMB, supernovae Ia and large scale structure data we find constraints on the equation of state in a modified LambdaCDM cosmology. We obtain -1.50×10-6

Christian M. Müller

2005-01-01

116

On cosmic acceleration without dark energy

We elaborate on the proposal that the observed acceleration of the Universe is the result of the backreaction of cosmological perturbations, rather than the effect of a negative-pressure dark-energy fluid or a modification of general relativity. Through the effective Friedmann equations describing an inhomogeneous Universe after smoothing, we demonstrate that acceleration in our local Hubble patch is possible even if

Edward W. Kolb; Sabino Matarrese; Antonio Riotto

2006-01-01

117

The dark energy dominated Universe

In this paper we investigate the epochs in which the Universe started accelerating and when it began to become dark energy dominated (i.e., the dynamics of the expansion of the Universe dominated by the dark energy). We provide analytic expressions to calculate the redshifts of these epochs as a function of density parameters. Moreover, we review and discuss cosmological models

José Carlos N. de Araujo

2005-01-01

118

Exact dark energy star solutions

NASA Astrophysics Data System (ADS)

Adopting the phantom (ghost) scalar field description of dark energy, we construct a general class of exact interior solutions describing mixed relativistic stars containing both ordinary matter and dark energy in different proportions. The exterior solution that continuously matches the interior solutions is also found. Exact solutions describing extremal configurations with zero ordinary matter pressure are also constructed.

Yazadjiev, Stoytcho S.

2011-06-01

119

Dark Energy Rules the Universe

The revolutionary discovery that the expansion of the universe is speeding up, not slowing down from gravity, means that 75 percent of our universe consists of mysterious dark energy. Berkeley Lab theoretical physicist Eric Linder delves into the mystery of dark energy as part of the Science in the Theatre lecture series on Nov. 24, 2008.

120

Non-linear dark energy clustering

NASA Astrophysics Data System (ADS)

We consider a dark energy fluid with arbitrary sound speed and equation of state and discuss the effect of its clustering on the cold dark matter distribution at the non-linear level. We write the continuity, Euler and Poisson equations for the system in the Newtonian approximation. Then, using the time renormalization group method to resum perturbative corrections at all orders, we compute the total clustering power spectrum and matter power spectrum. At the linear level, a sound speed of dark energy different from that of light modifies the power spectrum on observationally interesting scales, such as those relevant for baryonic acoustic oscillations. We show that the effect of varying the sound speed of dark energy on the non-linear corrections to the matter power spectrum is below the per cent level, and therefore these corrections can be well modelled by their counterpart in cosmological scenarios with smooth dark energy. We also show that the non-linear effects on the matter growth index can be as large as 10-15 per cent for small scales.

Anselmi, Stefano; Ballesteros, Guillermo; Pietroni, Massimo

2011-11-01

121

Theoretical Models of Dark Energy

NASA Astrophysics Data System (ADS)

Mounting observational data confirm that about 73% of the energy density consists of dark energy which is responsible for the current accelerated expansion of the Universe. We present observational evidences and dark energy projects. We then review various theoretical ideas that have been proposed to explain the origin of dark energy; they contain the cosmological constant, modified matter models, modified gravity models and the inhomogeneous model. The cosmological constant suffers from two major problems: one regarding fine-tuning and the other regarding coincidence. To solve them there arose modified matter models such as quintessence, k-essence, coupled dark energy and unified dark energy. We compare those models by presenting attractive aspects, new rising problems and possible solutions. Furthermore, we review modified gravity models that lead to late-time accelerated expansion without invoking a new form of dark energy; they contain f(R) gravity and the Dvali-Gabadadze-Porrati (DGP) model. We also discuss observational constraints on those models and on future modified gravity theories. Finally we review the inhomogeneous Lemaître-Tolman-Bondi (LTB) model that drops an assumption of the spatial homogeneity of the Universe. We also present basics of cosmology and scalar field theory, which are useful especially for students and novices to understand dark energy models.

Yoo, Jaewon; Watanabe, Yuki

2012-11-01

122

Fingerprinting dark energy. II. Weak lensing and galaxy clustering tests

The characterization of dark energy is a central task of cosmology. To go beyond a cosmological constant, we need to introduce at least an equation of state and a sound speed and consider observational tests that involve perturbations. If dark energy is not completely homogeneous on observable scales, then the Poisson equation is modified and dark matter clustering is directly affected. One can then search for observational effects of dark energy clustering using dark matter as a probe. In this paper we exploit an analytical approximate solution of the perturbation equations in a general dark energy cosmology to analyze the performance of next-decade large-scale surveys in constraining equation of state and sound speed. We find that tomographic weak lensing and galaxy redshift surveys can constrain the sound speed of the dark energy only if the latter is small, of the order of c{sub s} < or approx. 0.01 (in units of c). For larger sound speeds the error grows to 100% and more. We conclude that large-scale structure observations contain very little information about the perturbations in canonical scalar field models with a sound speed of unity. Nevertheless, they are able to detect the presence of cold dark energy, i.e. a dark energy with nonrelativistic speed of sound.

Sapone, Domenico [Departamento de Fisica Teorica and Instituto de Fisica Teorica, Universidad Autonoma de Madrid IFT-UAM/CSIC, Cantoblanco, Madrid (Spain); Kunz, Martin [Departement de Physique Theorique, Universite de Geneve, 24 quai Ernest Ansermet, CH-1211 Geneve 4 (Switzerland); Institut d'Astrophysique Spatiale, Universite Paris-Sud XI, Orsay 91405 (France); Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom); Amendola, Luca [University of Heidelberg, Philosophenweg 16, 69120 Heidelberg (Germany); INAF/Rome (Italy)

2010-11-15

123

Dark energy, dark matter and the Chaplygin gas

The possibility that the dark energy may be described by the Chaplygin gas is discussed. Some observational constraints are established. These observational constraints indicate that a unified model for dark energy and dark matter through the employement of the Chaplygin gas is favored.

R. Colistete J; J. C. Fabris; S. V. B. Goncalves; P. E. de Souza

2002-01-01

124

Dark energy, dark matter and the Chaplygin gas

The possibility that the dark energy may be described by the Chaplygin gas is\\u000adiscussed. Some observational constraints are established. These observational\\u000aconstraints indicate that a unified model for dark energy and dark matter\\u000athrough the employement of the Chaplygin gas is favored.

R. Colistete Jr.; J. C. Fabris; S. V. B. Goncalves; P. E. de Souza

2002-01-01

125

Inflation, dark matter, and dark energy in the string landscape.

We consider the conditions needed to unify the description of dark matter, dark energy, and inflation in the context of the string landscape. We find that incomplete decay of the inflaton field gives the possibility that a single field is responsible for all three phenomena. By contrast, unifying dark matter and dark energy into a single field, separate from the inflaton, appears rather difficult. PMID:17155379

Liddle, Andrew R; Ureña-López, L Arturo

2006-10-17

126

Properties of singularities in the (phantom) dark energy universe

The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an

Shinichi Nojiri; Sergei D. Odintsov; Shinji Tsujikawa

2005-01-01

127

Extraterrestrial life contradicts dark energy

NASA Astrophysics Data System (ADS)

Extraterrestrial life contradicts the Cold Dark Matter (CDM) Hierarchical Clustering (HC) model for cosmology, as well as its dark energy extension (by the 2011 Nobel Prize in Physics) to include an accelerating expansion of the universe (?CDMHC). The expansion is driven by the antigravitational property of dark energy that justified Einstein's cosmological constant (?). CDM stars appear only after a dark-age period lasting 300 Myr, rendering cosmic scale extraterrestrial life problematic. Turbulence stresses of Hydro-Gravitational-Dynamics (HGD) cosmology during the big bang are powerful but temporary, so CDM and dark energy ??are unnecessary. Superclusters fragment at 0.03 Myr. Hydrogen planets in proto-globular-star-cluster (PGC) clumps fragment protogalaxies at the transition to gas (0.3 Myr). The density at 0.03 Myr is preserved by old globular clusters (OGC) as a fossil of first fragmentation. Infrared observations support the HGD prediction (Gibson 1996) and quasar microlensing observation (Schild 1996) that the dark matter of galaxies is Earth-mass gas planets in dense PGC clumps. Water oceans seeded by dust of the first exploding stars at 2 Myr hosted extraterrestrial life spread on cosmic scales. Life anywhere falsifies dark energy.

Gibson, Carl H.

2012-10-01

128

Numerical study of halo concentrations in dark-energy cosmologies

We study the concentration parameters, their mass dependence and redshift evolution, of dark-matter halos in different dark-energy cosmologies with constant and time-variable equation of state, and compare them with ``standard'' LambdaCDM and OCDM models. We find that previously proposed algorithms for predicting halo concentrations can be well adapted to dark-energy models. When centred on the analytically expected values, halo concentrations

Klaus Dolag; Matthias Bartelmann; Francesca Perrotta; Carlo Baccigalupi; Lauro Moscardini; Massimo Meneghetti; Giuseppe Tormen

2004-01-01

129

Interacting Tachyon Dark Energy Model in Gauss-Bonnet Cosmology

NASA Astrophysics Data System (ADS)

We investigate the tachyon scalar filed model of dark energy in the framework of Gauss-Bonnet cosmology. We consider a spatially non-flat universe containing interacting tachyon dark energy with pressureless dark matter. We obtain the equation of state and deceleration parameters. We also reconstruct the potential and the dynamics for the tachyon scalar field model, which describe accelerated expansion of the universe.

Fayaz, V.; Felegary, F.; Hossienkhani, H.

2013-01-01

130

Constraining dark energy from the abundance of weak gravitational lenses

We examine the prospect of using the observed abundance of weak gravitational lenses to constrain the equation-of-state parameter w=p\\/rho of dark energy. Dark energy modifies the distance-redshift relation, the amplitude of the matter power spectrum, and the rate of structure growth. As a result, it affects the efficiency with which dark-matter concentrations produce detectable weak-lensing signals. Here we solve the

Nevin N. Weinberg; Marc Kamionkowski

2002-01-01

131

The Localized Energy Distribution of Dark Energy Star Solutions

NASA Astrophysics Data System (ADS)

We examine the question of energy localization for an exact solution of Einstein's equations with a scalar field corresponding to the phantom energy interpretation of dark energy. We apply three different energy-momentum complexes, the Einstein, Papapetrou and M{\\o}ller prescriptions, to the exterior metric and determine the energy distribution for each. Comparing the results, we find that the three prescriptions yield identical energy distributions.

Halpern, Paul; Pecorino; Michael

132

Dark Energy and dark matter of the universe from one-loop renormalization of riccion

Here, creation of the universe is obtained only from gravity sector. The dynamical universe begins with two basic ingredients (i) vacuum energy, also called dark energy (as vacuum energy is not observed) and (ii) background radiation. These two are obtained through one-loop renormalization of riccion. Solutions of renormalization group equations yield initial value of vaccum energy density $\\\\rho_{\\\\Lambda_{\\\\rm ew}} =

S. K. Srivastava

2004-01-01

133

Scalar field oscillations contributing to dark energy

We use action-angle variables to describe the basic physics of coherent scalar field oscillations in the expanding universe. These analytical mechanics methods have some advantages, like the identification of adiabatic invariants. As an application, we show some instances of potentials leading to equations of state with p<-{rho}/3, thus contributing to the dark energy that causes the observed acceleration of the universe.

Masso, Eduard; Rota, Francesc; Zsembinszki, Gabriel [Grup de Fisica Teorica and Institut de Fisica d'Altes Energies, Universitat Autonoma de Barcelona 08193 Bellaterra, Barcelona (Spain)

2005-10-15

134

Simulation studies of dark energy clustering induced by the formation of dark matter halos

NASA Astrophysics Data System (ADS)

In this paper, we present a simulation method within the two-component spherical collapse model to investigate dark energy perturbations associated with the formation of dark matter halos. The realistic mass accretion history of a dark matter halo taking into account its fast and slow growth is considered by imposing suitable initial conditions and isotropized virializations for the spherical collapse process. The dark energy component is treated as a perfect fluid described by two important parameters, the equation of state parameter w and the sound speed cs. Quintessence models with w>-1 are analyzed. We adopt the Newtonian gauge to describe the spacetime which is perturbed mainly by the formation of a dark matter halo. It is found that the dark energy (DE) density perturbation ?DE depends on w and cs, and its behavior follows closely the gravitational potential ? of the dark matter halo with ?DE?-(1+w)?/cs2. For w>-1, the dark energy perturbation presents a clustering behavior with ?DE>0 during the entire formation of the dark matter halo, from linear to nonlinear and virialized stages. The value of ?DE increases with the increase of the halo mass. For a cluster of mass M˜1015M?, ?DE˜10-5 within the virialized region for cs2?[0.5,1], and it can reach ?DE=O(1) with cs2=0.00001. For a scalar-field dark energy model, we find that with suitably modeled w and cs, its perturbation behavior associated with the nonlinear formation of dark matter halos can well be analyzed using the fluid approach, demonstrating the validity of the fluid description for dark energy even considering its perturbation in the stage of nonlinear dark matter structure formation.

Wang, Qiao; Fan, Zuhui

2012-01-01

135

Oscillatory Universe, dark energy and general relativity

NASA Astrophysics Data System (ADS)

The concept of oscillatory Universe appears to be realistic and buried in the dynamic dark energy equation of state. We explore its evolutionary history under the framework of general relativity. We observe that oscillations do not go unnoticed with such an equation of state and that their effects persist later on in cosmic evolution. The `classical' general relativity seems to retain the past history of oscillatory Universe in the form of increasing scale factor as the classical thermodynamics retains this history in the form of increasing cosmological entropy.

Ghosh, Partha Pratim; Ray, Saibal; Usmani, A. A.; Mukhopadhyay, Utpal

2013-06-01

136

Dark matter and energy of universe

We describe the different components of the present energy of the universe starting from the well established radiation and luminous matter, following with the dark baryonic matter determined by primordial nucleosynthesis, the likely cold dark matter with its theoretical candidates, the sure but not yet defined hot dark matter represented by neutrinos and the evidence of dark energy given by

Luis Masperi

2001-01-01

137

Dark energy and Josephson junctions

NASA Astrophysics Data System (ADS)

It has been recently claimed that dark energy can be (and has been) observed in laboratory experiments by measuring the power spectrum SI(?) of the noise current in a resistively shunted Josephson junction and that in new dedicated experiments, which will soon test a higher frequency range, SI(?) should show a deviation from the linear rising observed in the lower frequency region because higher frequencies should not contribute to dark energy. Based on previous work on theoretical aspects of the fluctuation-dissipation theorem, we carefully investigate these issues and show that these claims are based on a misunderstanding of the physical origin of the spectral function SI(?). According to our analysis, dark energy has never been (and will never be) observed in Josephson junctions experiments. We also predict that no deviation from the linear rising behavior of SI(?) will be observed in forthcoming experiments. Our findings provide new (we believe definite) arguments which strongly support previous criticisms.

Branchina, Vincenzo; Di Liberto, Marco; Lodato, Ivano

2009-08-01

138

On the determination of dark energy

I consider some of the issues we face in trying to understand dark energy. Huge fluctuations in the unknown dark energy equation of state can be hidden in distance data, so I argue that model-independent tests which signal if the cosmological constant is wrong are valuable. These can be constructed to remove degeneracies with the cosmological parameters. Gravitational effects can play an important role. Even small inhomogeneity clouds our ability to say something definite about dark energy. I discuss how the averaging problem confuses our potential understanding of dark energy by considering the backreaction from density perturbations to second-order in the concordance model: this effect leads to at least a 10% increase in the dynamical value of the deceleration parameter, and could be significantly higher. Large Hubble-scale inhomogeneity has not been investigated in detail, and could conceivably be the cause of apparent cosmic acceleration. I discuss void models which defy the Copernican principle in our Hubble patch, and describe how we can potentially rule out these models.This article is a summary of two talks given at the Invisible Universe Conference, Paris, 2009.

Clarkson, Chris [Cosmology and Gravity Group, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch 7701 (South Africa)

2010-06-23

139

NASA Astrophysics Data System (ADS)

We have considered a cosmological model of holographic dark energy interacting with dark matter and another unknown component of dark energy of the universe. We have assumed two interaction terms Q and Q' in order to include the scenario in which the mutual interaction between the two principal components (i.e., holographic dark energy and dark matter) of the universe leads to some loss in other forms of cosmic constituents. Our model is valid for any sign of Q and Q'. If Q< Q', then part of the dark energy density decays into dark matter and the rest in the other unknown energy density component. But if Q> Q', then dark matter energy receives from dark energy and from the unknown component of dark energy. Observation suggests that dark energy decays into dark matter. Here we have presented a general prescription of a cosmological model of dark energy which imposes mutual interaction between holographic dark energy, dark matter and another fluid. We have obtained the equation of state for the holographic dark energy density which is interacting with dark matter and other unknown component of dark energy. Using first law of thermodynamics, we have obtained the entropies for holographic dark energy, dark matter and other component of dark energy, when holographic dark energy interacting with two fluids (i.e., dark matter and other component of dark energy). Also we have found the entropy at the horizon when the radius ( L) of the event horizon measured on the sphere of the horizon. We have investigated the GSL of thermodynamics at the present time for the universe enveloped by this horizon. Finally, it has been obtained validity of GSL which implies some bounds on deceleration parameter q.

Debnath, Ujjal

2012-01-01

140

NASA Astrophysics Data System (ADS)

Event horizons and closed time-like curves cannot exist in the real world for the simple reason that they are inconsistent with quantum mechanics. Following ideas originated by Robert Laughlin, Pawel Mazur, Emil Mottola, David Santiago, and the speaker it is now possible to describe in some detail what happens physically when one approaches and crosses a region of space-time where classical general relativity predicts there should be an infinite red shift surface. This quantum critical physics provides a new perspective on a variety of enigmatic astrophysical phenomena including supernovae explosions, gamma ray bursts, positron emission, and dark matter.

Chapline, G.

141

Event horizons and closed time-like curves cannot exist in the real world for the simple reason that they are inconsistent with quantum mechanics. Following ideas originated by Robert Laughlin, Pawel Mazur, Emil Mottola, David Santiago, and the speaker it is now possible to describe in some detail what happens physically when one approaches and crosses a region of space-time where classical general relativity predicts there should be an infinite red shift surface. This quantum critical physics provides a new perspective on a variety of enigmatic astrophysical phenomena including supernovae explosions, gamma ray bursts, positron emission, and dark matter.

Chapline, G

2005-03-08

142

Scalar field dark energy evolving from a long radiation- or matter-dominated epoch has characteristic dynamics. While slow-roll approximations are invalid, a well defined field expansion captures the key aspects of the dark energy evolution during much of the matter-dominated epoch. Since this behavior is determined, it is not faithfully represented if priors for dynamical quantities are chosen at random. We demonstrate these features for both thawing and freezing fields, and for some modified gravity models, and unify several special cases in the literature.

Cahn, Robert N.; de Putter, Roland; Linder, Eric V.

2008-07-08

143

Observing dark energy with SNAP

The nature of dark energy is of such fundamental importance -- yet such a mystery -- that a dedicated dark energy experiment should be as comprehensive and powerfully incisive as possible. The Supernova/Acceleration Probe robustly controls for a wide variety of systematic uncertainties, employing the Type Ia supernova distance method, with high signal to noise light curves and spectra over the full redshift range from z=0.1-1.7, and the weak gravitational lensing method with an accurate and stable point spread function.

Linder, Eric V.; SNAP Collaboration

2004-06-07

144

QCD Modified Ghost Scalar Field Dark Energy Models

NASA Astrophysics Data System (ADS)

Within the framework of FRW cosmology, we study the QCD modified ghost scalar field models of dark energy (DE) in the presence of both interaction and viscosity. For a spatially nonflat FRW universe containing modified ghost dark energy (MGDE) and dark matter (DM), we obtain the equation of state of MGDE, the deceleration parameter as well as a differential equation governing the MGDE density parameter. We also investigate the growth of structure formation for our model in a linear perturbation regime. Furthermore, we reconstruct both the dynamics and potentials of the quintessence, tachyon, K-essence and dilaton scalar field DE models according to the evolution of the MGDE density.

Karami, K.; Asadzadeh, S.; Mousivand, M.; Safari, Z.

2013-04-01

145

Models where the dark matter component of the Universe interacts with the dark energy field have been proposed as a solution to the cosmic coincidence problem, since in the attractor regime both dark energy and dark matter scale in the same way. In these models the mass of the cold dark matter particles is a function of the dark energy field responsible for the present acceleration of the Universe, and different scenarios can be parametrized by how the mass of the cold dark matter particles evolves with time. In this article we study the impact of a constant coupling {delta} between dark energy and dark matter on the determination of a redshift dependent dark energy equation of state w{sub DE}(z) and on the dark matter density today from SNIa data. We derive an analytical expression for the luminosity distance in this case. In particular, we show that the presence of such a coupling increases the tension between the cosmic microwave background data from the analysis of the shift parameter in models with constant w{sub DE} and SNIa data for realistic values of the present dark matter density fraction. Thus, an independent measurement of the present dark matter density can place constraints on models with interacting dark energy.

Amendola, Luca; Campos, Gabriela Camargo; Rosenfeld, Rogerio [INAF/Osservatorio Astronomico di Roma, V. Frascati 33, 00040, Monteporzio Catone (Rome) (Italy); Instituto de Fisica Teorica - UNESP, Rua Pamplona 145, 01405-900, Sao Paulo, SP (Brazil)

2007-04-15

146

QCD Equation of State and Dark Matter

The relic density of weakly interacting massive particles (WIMPs) depends on the entropy and the expansion rate of the Universe when they freeze out, at a temperature in the range 100 MeV - 10GeV. We use recent calculations of the QCD pressure to produce a new equation of state suitable for use in relic density calculations. We show that relic

Mark Hindmarsh; Owe Philipsen

2005-01-01

147

The Hubble constant and dark energy from cosmological distance measures

We study how the determination of the Hubble constant from cosmological distance measures is affected by models of dark energy and vice versa. For this purpose, constraints on the Hubble constant and dark energy are investigated using the cosmological observations of cosmic microwave background, baryon acoustic oscillations and type Ia supernovae. When one investigates dark energy, the Hubble constant is often a nuisance parameter; thus it is usually marginalized over. On the other hand, when one focuses on the Hubble constant, simple dark energy models such as a cosmological constant and a constant equation of state are usually assumed. Since we do not know the nature of dark energy yet, it is interesting to investigate the Hubble constant assuming some types of dark energy and see to what extent the constraint on the Hubble constant is affected by the assumption concerning dark energy. We show that the constraint on the Hubble constant is not affected much by the assumption for dark energy. We furthermore show that this holds true even if we remove the assumption that the universe is flat. We also discuss how the prior on the Hubble constant affects the constraints on dark energy and/or the curvature of the universe.

Ichikawa, Kazuhide [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan); Takahashi, Tomo, E-mail: kazuhide@icrr.u-tokyo.ac.jp, E-mail: tomot@cc.saga-u.ac.jp [Department of Physics, Saga University, Saga 840-8502 (Japan)

2008-04-15

148

Adiabatic instability in coupled dark energy/dark matter models

We consider theories in which there exists a nontrivial coupling between the dark matter sector and the sector responsible for the acceleration of the Universe. Such theories can possess an adiabatic regime in which the quintessence field always sits at the minimum of its effective potential, which is set by the local dark matter density. We show that if the coupling strength is much larger than gravitational, then the adiabatic regime is always subject to an instability. The instability, which can also be thought of as a type of Jeans instability, is characterized by a negative sound speed squared of an effective coupled dark matter/dark energy fluid, and results in the exponential growth of small scale modes. We discuss the role of the instability in specific coupled cold dark matter and mass varying neutrino models of dark energy and clarify for these theories the regimes in which the instability can be evaded due to nonadiabaticity or weak coupling.

Bean, Rachel [Department of Astronomy, Cornell University, Ithaca, New York 14853 (United States); Flanagan, Eanna E. [Department of Astronomy, Cornell University, Ithaca, New York 14853 (United States); Laboratory for Elementary Particle Physics, Cornell University, Ithaca, New York 14853 (United States); Trodden, Mark [Department of Physics, Syracuse University, Syracuse, New York 13244 (United States)

2008-07-15

149

The cosmological constant and dark energy

Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, Lambda; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is

P. J. Peebles; Bharat Ratra

2003-01-01

150

Cosmological dark energy and entanglement

Astrophysical observations of recent years indicate that the universe is composed of a large amount of dark energy (DE) responsible for an accelerated expansion of the universe. Here, we argue that DE can arise entirely from the standard principles of strong interaction physics and quantum entanglement. Good quantitative agreement of our estimate with the observation, without the need for any adjustable parameters, is encouraging.

Ghosh, Sanjay K.; Raha, Sibaji [Department of Physics, Bose Institute, 93/1 A.P.C. Road, Kolkata-700009, India Centre for Astroparticle Physics and Space Science, Bose Institute, Block EN, Sector V, Kolkata 700091 (India)

2011-09-23

151

Dark Energy from a Positive Jerk Parameter

NASA Astrophysics Data System (ADS)

In this paper, we demonstrate that the variation of acceleration, namely the jerk parameter j, could give hints for determining the dark energy equation of state (EoS). In particular, it is possible to show that a viable cosmological model is compatible with a constant jerk, here conventionally rewritten as j = 1+?, with ? > 0 representing a departure from the ?CDM model. This suggests that the cosmological constant could be seen as a limiting case of a more general dark energy model. We use the most recent union 2.1 compilation of supernovae Ia, showing at 1? confidence level, that j is compatible with the condition j ? 1. In doing so, we infer a corresponding cosmological model, viable with a negative acceleration parameter, in the observed range -1 < q0 < 0.

Luongo, Orlando

2013-06-01

152

Two-component model of dark energy

We consider the possibility that the dark energy is made up of two or more independent components, each having a different equation of state. We fit the model with supernova and gamma-ray burst data from recent observations, and use the Markov Chain Monte Carlo technique to estimate the allowed parameter regions. We also use various model selection criteria to compare the two-component model with the {lambda}CDM, one-component dark energy model with static or variable w (XCDM) and other multicomponent models. We find that the two-component models can give a reasonably good fit to the current data. For some data sets, and depending somewhat on the model selection criteria, the two-component model can give a better fit to the data than XCDM with static w and XCDM with variable w parametrized by w=w{sub 0}+w{sub a}z/(1+z)

Gong Yan; Chen Xuelei [National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012 (China)

2007-12-15

153

Cluster probes of dark energy clustering

NASA Astrophysics Data System (ADS)

Cluster abundances are oddly insensitive to canonical early dark energy. Early dark energy with sound speed equal to the speed of light cannot be distinguished from a quintessence model with the equivalent expansion history for z<2 but negligible early dark energy density, despite the different early growth rate. However, cold early dark energy, with a sound speed much smaller than the speed of light, can give a detectable signature. Combining cluster abundances with cosmic microwave background (CMB) power spectra can determine the early dark energy fraction to 0.3% and distinguish a true sound speed of 0.1 from 1 at 99% confidence. We project constraints on early dark energy from the Euclid cluster survey, as well as the Dark Energy Survey, using both current and projected Planck CMB data, and assess the impact of cluster mass systematics. We also quantify the importance of dark energy perturbations, and the role of sound speed during a crossing of w=-1.

Appleby, Stephen A.; Linder, Eric V.; Weller, Jochen

2013-08-01

154

Cluster number counts dependence on dark energy inhomogeneities and coupling to dark matter

Cluster number counts can be used to test dark energy models. We investigate dark energy candidates which are coupled to dark matter. We analyse the cluster number counts dependence on the amount of dark matter coupled to dark energy. Furthermore, we study how dark energy inhomogeneities affect cluster abundances. It is shown that increasing the coupling reduces significantly the cluster

M. Manera; D. F. Mota

2006-01-01

155

Late Forming Dark Matter in Theories of Neutrino Dark Energy

We study the possibility of Late Forming Dark Matter (LFDM), where a scalar field, previously trapped in a metastable state by thermal or finite density effects, begins to oscillate near the era matter-radiation equality about its true minimum. Such a theory is motivated generally if the dark energy is of a similar form, but has not yet made the transition

Subinoy Das; Neal Weiner

2006-01-01

156

Cosmic structure growth and dark energy

Dark energy has a dramatic effect on the dynamics of the Universe, causing the recently discovered acceleration of the expansion. The dynamics are also central to the behaviour of the growth of large-scale structure, offering the possibility that observations of structure formation provide a sensitive probe of the cosmology and dark energy characteristics. In particular, dark energy with a time-varying

Eric V. Linder; Adrian Jenkins

2003-01-01

157

OPERA and a Neutrino Dark Energy Model

We consider a neutrino dark energy model and study its implications for the neutrino superluminality reported recently by the OPERA collaboration. In our model the derivative couplings of the neutrino to the dark energy scalar result in a Lorentz violation in the neutrino sector. Furthermore, the coupling of the dark energy scalar field to the stress tensor of the Earth

Emilio Ciuffoli; Jarah Evslin; Jie Liu; Xinmin Zhang

2011-01-01

158

Observational approaches to understanding dark energy

Illuminating the nature of dark energy is one of the most important challenges in cosmology today. In this review I discuss several promising observational approaches to understanding dark energy, in the context of the recommendations by the U.S. Dark Energy Task Force and the ESA-ESO Working Group on Fundamental Cosmology.

Yun Wang; Homer L. Dodge; W. Brooks

2007-01-01

159

A universe unexpected: Dark matter, dark energy, and the accelerating cosmos

NASA Astrophysics Data System (ADS)

The problems of dark matter and dark energy in the Universe are somewhat astonishing. After centuries of scientific endeavor, after breathtaking advances in technology, and after the innumerable predictive successes of physical science, we find ourselves in an odd position: we do not know what constitutes 96% of the Universe. The recent measurements that indicate an accelerating Universe imply that 73% of the mass of the Universe consists of an unknown, unseen and uniformly distributed dark energy. Only 4% of the mass resides in familiar baryonic objects such as stars, gas, and planets. The remaining 23% is composed of an unknown dark matter, possibly consisting of thermal relics known as WIMPS (weakly interacting massive particles). The resolution to these problems of dark matter and dark energy may involve an overlooked aspect of contemporary physics, or may entail the development of entirely new physical theories. This dissertation grapples with aspects of both of these issues, and may be broadly divided into two parts. In the first part, we introduce a series of modifications to the Friedmann equation that can explain the observed acceleration of the Universe without recourse to conventional dark energy and that can be motivated in the context of: (1)brane-world scenarios, in which the Universe is assumed to be a four-dimensional hyperplane embedded in a higher dimensional space; or, (2)generic fluid models, in which the effective modifications to the Friedmann equations arise from the presence of an exotic fluid in the Universe, possibly due to the long range interaction of dark matter particles. We investigate the phenomenology of each of these models, focusing on the observational consequences of each. In the second part, we confront the problem of understanding the dark galactic halo. In particular, we would like to use WIMP direct detection experiments to search for dark halo substructure. Such experiments seek to measure the energy deposited when a WIMP interacts with a nuclei in a detector. We describe a Bayesian multiscale method for investigating the structure of the dark halo using non-directional WIMP detection experiments. The WIMP interaction rate, the energy deposited per collision, and the time dependence of these effects all depend sensitively on the velocity distribution of the dark halo, so that direct detection experiments can not only detect the presence of WIMPs, but also probe the structure of the dark halo itself. The methods developed in the dissertation estimate the dark halo properties from the raw Poisson distributed detector counts without requiring assumptions about the underlying halo structure.

Lewis, Matthew James

2003-12-01

160

Crossing the phantom divide: Dark energy internal degrees of freedom

Dark energy constraints have forced viable alternatives that differ substantially from a cosmological constant {lambda} to have an equation of state w that evolves across the phantom divide set by {lambda}. Naively, crossing this divide makes the dark energy gravitationally unstable, a problem that is typically finessed by unphysically ignoring the perturbations. While this procedure does not affect constraints near the favored cosmological constant model it can artificially enhance the confidence with which alternative models are rejected. Similar to the general problem of stability for w<0, the solution lies in the internal degrees of freedom in the dark energy sector. We explicitly show how to construct a two scalar field model that crosses the phantom divide and mimics the single field behavior on either side to substantially better than 1% in all observables. It is representative of models where the internal degrees of freedom keep the dark energy smooth out to the horizon scale independently of the equation of state.

Hu, Wayne [Kavli Institute for Cosmological Physics, Department of Astronomy and Astrophysics, Enrico Fermi Institute, University of Chicago, Chicago IL 60637 (United States)

2005-02-15

161

Dark Energy and Dark Matter from the same Vacuum Condensate

NASA Astrophysics Data System (ADS)

The micro-quantum Dirac negative energy electron Fermi sphere with Planck scale cutoff is unstable to the formation of off-mass-shell Cooper pairs of virtual electrons and positrons from their static Coulomb attraction. The resulting virtual BEC complex macro-quantum coherent local order parameter (0|e+e-|0) gives rise to both spin 2 gravity guv and spin 0 quintessence / from the Goldstone and Higgs oscillations respectively, Susskind's "world hologram" conjecture replaces the Planck scale Lp with Lp^2/3L^1/3 at scale L. Hagen Kleinert's strain tensor for the "world crystal" is Einstein's geometrodynamic field: guv = nuv + Lp^4/3L^2/3Du,Dvarg(0|e+e-|0)/2 nuv = Minkowski metric, = anti-commutator Du = ,u + TaAu^a is the spin 1 gauge covariant derivative for Lie group P with Lie algebra [Ta,Tb] = Cab^cTc / = Lp-4/3L-2/3[1 - Lp^2L|(0|e+e-|0)|^2] When L = size of visible universe 10^28 cm, Lp^2/3L^1/3 1 fermi / > 0 is anti-gravitating zero point vacuum dark energy, i.e. Kip Thorne's "exotic matter" for traversable wormhole time machines. / < 0 is gravitating zero point vacuum dark matter The non-perturbative BCS energy gap equation for a basic vacuum polarization closed loop with one virtual photon Feynman diagram is: z^2 = ge^-(1/gz) z = (Lp/L)^1/3 and the dimensionless coupling vertex is g^1/2 http://stardrive.org/Jack/nambu.pdf http://stardrive.org/Jack/Lambda1.pdf

Sarfatti, Jack

2003-04-01

162

Figures of merit for present and future dark energy probes

We compare current and forecasted constraints on dynamical dark energy models from Type Ia supernovae and the cosmic microwave background using figures of merit based on the volume of the allowed dark energy parameter space. For a two-parameter dark energy equation of state that varies linearly with the scale factor, and assuming a flat universe, the area of the error ellipse can be reduced by a factor of {approx}10 relative to current constraints by future space-based supernova data and CMB measurements from the Planck satellite. If the dark energy equation of state is described by a more general basis of principal components, the expected improvement in volume-based figures of merit is much greater. While the forecasted precision for any single parameter is only a factor of 2-5 smaller than current uncertainties, the constraints on dark energy models bounded by -1{<=}w{<=}1 improve for approximately 6 independent dark energy parameters resulting in a reduction of the total allowed volume of principal component parameter space by a factor of {approx}100. Typical quintessence models can be adequately described by just 2-3 of these parameters even given the precision of future data, leading to a more modest but still significant improvement. In addition to advances in supernova and CMB data, percent-level measurement of absolute distance and/or the expansion rate is required to ensure that dark energy constraints remain robust to variations in spatial curvature.

Mortonson, Michael J. [Center for Cosmology and AstroParticle Physics, Ohio State University, Columbus, Ohio 43210 (United States); Huterer, Dragan [Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040 (United States); Hu, Wayne [Kavli Institute for Cosmological Physics, Department of Astronomy and Astrophysics, and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States)

2010-09-15

163

Studies of dark energy with X-ray observatories.

I review the contribution of Chandra X-ray Observatory to studies of dark energy. There are two broad classes of observable effects of dark energy: evolution of the expansion rate of the Universe, and slow down in the rate of growth of cosmic structures. Chandra has detected and measured both of these effects through observations of galaxy clusters. A combination of the Chandra results with other cosmological datasets leads to 5% constraints on the dark energy equation-of-state parameter, and limits possible deviations of gravity on large scales from general relativity. PMID:20404207

Vikhlinin, Alexey

2010-04-19

164

Stable Dark Energy Stars:. AN Alternative to Black Holes?

NASA Astrophysics Data System (ADS)

In this work, a generalization of the Mazur-Mottola gravastar model is explored, by considering a matching of an interior solution governed by the dark energy equation of state, w ? p/? < -1/3, to an exterior Schwarzschild vacuum solution at a junction interface, situated near to where the event horizon is expected to form. The motivation for implementing this generalization arises from the fact that recent observations have confirmed an accelerated cosmic expansion, for which dark energy is a possible candidate.

Lobo, Francisco S. N.

2008-09-01

165

A Data Management System for the Dark Energy Survey

The Dark Energy Survey (DES) is a 5-year (2009-2014), multi-institutional endeavor to image 5000 deg2 in g, r, i, and z filters using a new 3 deg2 imager for the CTIO Blanco 4-m telescope. The primary goal of the DES is to precisely constrain the dark energy equation of state parameter and its variation with redshift using four complementary techniques:

Wayne Barkhouse; T. Alam; C. Beldica; D. Cai; G. Daues; J. J. Mohr; C. Ngeow; R. Plante; J. Annis; H. Lin; D. Tucker; R. C. Smith

2006-01-01

166

Constraints on holographic dark energy from type Ia supernova observations

In this paper, we use the type Ia supernovae data to constrain the holographic dark energy model proposed by Li. We also apply a cosmic age test to this analysis. We consider in this paper a spatially flat Friedmann-Robertson-Walker universe with a matter component and a holographic dark energy component. The fit result shows that the case c<1 (c=0.21) is favored, which implies that the holographic dark energy behaves as a quintom-type dark energy. Furthermore, we also perform a joint analysis of SNe+CMB+LSS to this model; the result is well improved and still upholds the quintom dark energy conclusion. The best fit results in our analysis are c=0.81, {omega}{sub m}{sup 0}=0.28, and h=0.65, which lead to the present equation of state of dark energy w{sub 0}=-1.03 and the deceleration/acceleration transition redshift z{sub T}=0.63. Finally, an expected supernova/acceleration probe simulation using {lambda}CDM as a fiducial model is performed on this model, and the result shows that the holographic dark energy model takes on c<1 (c=0.92) even though the dark energy is indeed a cosmological constant.

Zhang Xin; Wu Fengquan [CCAST (World Laboratory), P.O. Box 8730, Beijing 100080 (China) and Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918(4), Beijing 100049 (China)

2005-08-15

167

Bistable dark solitons of a cubic-quintic Helmholtz equation

We provide a report on exact analytical bistable dark spatial solitons of a nonlinear Helmholtz equation with a cubic-quintic refractive-index model. Our analysis begins with an investigation of the modulational instability characteristics of Helmholtz plane waves. We then derive a dark soliton by mapping the desired asymptotic form onto a uniform background field and obtain a more general solution by deploying rotational invariance laws in the laboratory frame. The geometry of the new soliton is explored in detail, and a range of new physical predictions is uncovered. Particular attention is paid to the unified phenomena of arbitrary-angle off-axis propagation and nondegenerate bistability. Crucially, the corresponding solution of paraxial theory emerges in a simultaneous multiple limit. We conclude with a set of computer simulations that examine the role of Helmholtz dark solitons as robust attractors.

Christian, J. M.; McDonald, G. S.; Chamorro-Posada, P. [Joule Physics Laboratory, School of Computing, Science and Engineering, Materials and Physics Research Centre, University of Salford, Salford M5 4WT (United Kingdom); Departamento de Teoria de la Senal y Comunicaciones e Ingenieria Telematica, Universidad de Valladolid, ETSI Telecomunicacion, Campus Miguel Delibes s/n, 47011 Valladolid (Spain)

2010-05-15

168

Cosmological evolution of a quintom model of dark energy

We investigate in this Letter the cosmological evolution of a dark energy model with two scalar fields where one of the scalar has canonical kinetic energy and another scalar has negative kinetic energy term. For such a system with exponential potentials we find that during the evolution of the universe the equation of state w changes from w??1 to w?1,

Zong-Kuan Guo; Yun-Song Piao; Xinmin Zhang; Yuan-Zhong Zhang

2005-01-01

169

Dark Matter and Dark Energy in the Universe

While images of the distant Universe reveal millions of colorful galaxies in every square degree of the sky, the Universe is actually held together by dark matter as it is being pushed apart by the gravitational effects of dark energy. According to our first, complete accounting of matter and energy, the recipe for our Universe is: 0.5% bright stars; 5% ordinary matter; 30% slowly moving elementary particles left over from the earliest moments; and 65% dark energy, totaling to the critical density. The elementary particles are thought to be either neutralinos or axions, and their gravity holds together all structures in the Universe -- from galaxies to the great walls of galaxies. (Neutrinos left over from the big bang account for as much mass as do bright stars.) The dark energy, whose fundamental character is still a mystery, is causing the expansion of the Universe to accelerate rather than slow down. Now, the big question is, 'Who ordered that combination?'

Turner, Mike (Fermilab/University of Chicago)

2001-01-17

170

Dark energy anisotropic stress and large scale structure formation

We investigate the consequences of an imperfect dark energy fluid on the large scale structure. A phenomenological three parameter fluid description is used to study the effect of dark energy on the cosmic microwave background radiation (CMBR) and matter power spectrum. In addition to the equation of state and the sound speed, we allow a nonzero viscosity parameter for the fluid. Then anisotropic stress perturbations are generated in dark energy. In general, we find that this possibility is not excluded by the present day cosmological observations. In the simplest case when all of the three parameters are constant, we find that the observable effects of the anisotropic stress can be closely mimicked by varying the sound speed of perfect dark energy. However, now also negative values for the sound speed, as expected for adiabatic fluid model, are tolerable and in fact could explain the observed low quadrupole in the CMBR spectrum. We investigate also structure formation of imperfect fluid dark energy characterized by an evolving equation of state. In particular, we study models unifying dark energy with dark matter, such as the Chaplygin gas or the Cardassian expansion, with a shear perturbation included. This can stabilize the growth of inhomogeneities in these models, thus somewhat improving their compatibility with large scale structure observations.

Koivisto, Tomi [Helsinki Institute of Physics, FIN-00014 Helsinki (Finland); Mota, David F. [Institute of Theoretical Astrophysics, University of Oslo, Box 1029, 0315 Oslo (Norway)

2006-04-15

171

Dark energy versus modified gravity.

There is now strong observational evidence that the expansion of the Universe is accelerating. The standard explanation invokes an unknown "dark energy" component. But such scenarios are faced with serious theoretical problems, which has led to increased interest in models where instead general relativity is modified in a way that leads to the observed accelerated expansion. The question then arises whether the two scenarios can be distinguished. Here we show that this may not be so easy, demonstrating explicitly that a generalized dark energy model can match the growth rate of the Dvali-Gabadadze-Porrati model and reproduce the 3+1 dimensional metric perturbations. Cosmological observations are then unable to distinguish the two cases. PMID:17501110

Kunz, Martin; Sapone, Domenico

2007-03-21

172

Parametrizing dark sector perturbations via equations of state

NASA Astrophysics Data System (ADS)

The evolution of perturbations is a crucial part of the phenomenology of the dark sector cosmology. We advocate parametrizing these perturbations using equations of state for the entropy perturbation and the anisotropic stress. For small perturbations, these equations of state will be linear in the density, velocity and metric perturbations, and in principle these can be related back to the field content of the underlying model allowing for confrontation with observations. We illustrate our point by constructing gauge-invariant entropy perturbations for theories where the dark sector Lagrangian is a general function of a scalar field, its first and second derivatives, and the metric and its first derivative, L=L(?,???,?????,g??,??g??). As an example, we show how to apply this approach to the case of models of kinetic gravity braiding.

Battye, Richard A.; Pearson, Jonathan A.

2013-09-01

173

Clustering properties of dynamical dark energy models

We provide a generic but physically clear discussion of the clustering properties of dark energy models. We explicitly show that in quintessence-type models the dark energy fluctuations, on scales smaller than the Hubble radius, are of the order of the perturbations to the Newtonian gravitational potential, hence necessarily small on cosmological scales. Moreover, comparable fluctuations are associated with different gauge choices. We also demonstrate that the often used homogeneous approximation is unrealistic, and that the so-called dark energy mutation is a trivial artifact of an effective, single fluid description. Finally, we discuss the particular case where the dark energy fluid is nonminimally coupled to dark matter.

Avelino, P. P.; Beca, L. M. G. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Martins, C. J. A. P. [Centro de Astrofisica, Universidade do Porto, Rua das Estrelas s/n, 4150-762 Porto (Portugal); DAMTP, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)

2008-05-15

174

Dark energy, matter creation and curvature

NASA Astrophysics Data System (ADS)

The most studied way to explain the current accelerated expansion of the universe is to assume the existence of dark energy; a new component that fills the universe, does not form clumps, currently dominates the evolution, and has a negative pressure. In this work I study an alternative model proposed by Lima et al. (Abramo and Lima in Class. Quantum Gravity 13:2953, 1996; Zimdahl in Phys. Rev. D 53:5483, 1996; Zimdahl and Pavón in Mon. Not. R. Astron. Soc. 266:872, 1994), which does not need an exotic equation of state, but assumes instead the existence of gravitational particle creation. Because this model fits the supernova observations as well as the ?CDM model, I perform in this work a thorough study of this model, considering an explicit spatial curvature. I found that in this scenario we can alleviate the cosmic coincidence problem, basically showing that these two components, dark matter and dark energy, are of the same nature, but they act at different scales. I also shown the inadequacy of some particle creation models, and I study a previously proposed new model that overcomes these difficulties.

Cárdenas, Víctor H.

2012-09-01

175

Formation of Dark Matter Haloes in a Homogeneous Dark Energy Universe

NASA Astrophysics Data System (ADS)

Several independent cosmological tests have shown evidences that the energy density of the universe is dominated by a dark energy component, which causes the present accelerated expansion. The large scale structure formation can be used to probe dark energy models, and the mass function of dark matter haloes is one of the best statistical tools to perform this study. We present here a statistical analysis of mass functions of galaxies under a homogeneous dark energy model, proposed in the work of Percival (2005), using an observational flux-limited X-ray cluster survey, and CMB data from WMAP. We compare, in our analysis, the standard Press-Schechter (PS) approach (where a Gaussian distribution is used to describe the primordial density fluctuation field of the mass function), and the PL (power-law) mass function (where we apply a non-extensive q-statistical distribution to the primordial density field). We conclude that the PS mass function cannot explain at the same time the X-ray and the CMB data (even at 99% confidence level), and the PS best fit dark energy equation of state parameter is ? = -0.58, which is distant from the cosmological constant case. The PL mass function provides better fits to the HIFLUGCS X-ray galaxy data and the CMB data; we also note that the ? parameter is very sensible to modifications in the PL free parameter, q, suggesting that the PL mass function could be a powerful tool to constrain dark energy models.

Marassi, L.

176

Gauge conditions in combined dark energy and dark matter systems

When analyzing a system consisting of both dark matter and dark energy, an often used practice in the literature is to neglect the perturbations in the dark energy component. However, it has recently been argued, through the use of numerical simulations, that one cannot do so. In this work we show that by neglecting such perturbations one is implicitly making a choice of gauge. As such, one no longer has the freedom to choose, for example, a gauge comoving with the dark matter - in fact doing so will give erroneous, gauge dependent results. We obtain results consistent with the numerical simulations by using the formalism of cosmological perturbation theory, and thus without resorting to involved numerical calculations.

Christopherson, Adam J. [Astronomy Unit, School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London, E1 4NS (United Kingdom)

2010-10-15

177

A Kinematical Approach to Dark Energy Studies

We present and employ a new kinematical approach to cosmological ''dark energy'' studies. We construct models in terms of the dimensionless second and third derivatives of the scale factor a(t) with respect to cosmic time t, namely the present-day value of the deceleration parameter q{sub 0} and the cosmic jerk parameter, j(t). An elegant feature of this parameterization is that all {Lambda}CDM models have j(t) = 1 (constant), which facilitates simple tests for departures from the {Lambda}CDM paradigm. Applying our model to the three best available sets of redshift-independent distance measurements, from type Ia supernovae and X-ray cluster gas mass fraction measurements, we obtain clear statistical evidence for a late time transition from a decelerating to an accelerating phase. For a flat model with constant jerk, j(t) = j, we measure q{sub 0} = -0.81 {+-} 0.14 and j = 2.16{sub -0.75}{sup +0.81}, results that are consistent with {Lambda}CDM at about the 1{sigma} confidence level. A standard ''dynamical'' analysis of the same data, employing the Friedmann equations and modeling the dark energy as a fluid with an equation of state parameter, w (constant), gives {Omega}{sub m} = 0.306{sub -0.040}{sup +0.042} and w = -1.15{sub -0.18}{sup +0.14}, also consistent with {Lambda}CDM at about the 1{sigma} level. In comparison to dynamical analyses, the kinematical approach uses a different model set and employs a minimum of prior information, being independent of any particular gravity theory. The results obtained with this new approach therefore provide important additional information and we argue that both kinematical and dynamical techniques should be employed in future dark energy studies, where possible. Our results provide further interesting support for the concordance {Lambda}CDM paradigm.

Rapetti, David; Allen, Steven W.; Amin, Mustafa A.; Blandford, Roger D.; /KIPAC, Menlo Park

2006-06-06

178

Plane symmetric cosmological models with perfect fluid and dark energy

NASA Astrophysics Data System (ADS)

We consider a self-consistent system of Plane symmetric cosmology and binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be one obeying the usual equation of state p= ?? with ??[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding Einstein's field equations are obtained as a quadrature. The cases of Zeldovich Universe, Dust Universe and Radiation Universe and models with power-law and exponential expansion have discussed in detail. For large t, the models tend to be isotropic.

Katore, S. D.; Adhav, K. S.; Shaikh, A. Y.; Sancheti, M. M.

2011-05-01

179

Constraining dark energy with clusters: Complementarity with other probes

The Figure of Merit Science Working Group recently forecast the constraints on dark energy that will be achieved prior to the Joint Dark Energy Mission by ground-based experiments that exploit baryon acoustic oscillations, type Ia supernovae, and weak gravitational lensing. We show that cluster counts from ongoing and near-future surveys should provide robust, complementary dark energy constraints. In particular, we find that optimally combined optical and Sunyaev-Zel'dovich effect cluster surveys should improve the Dark Energy Task Force figure of merit for pre-Joint Dark Energy Mission projects by a factor of 2 even without prior knowledge of the nuisance parameters in the cluster mass-observable relation. Comparable improvements are achieved in the forecast precision of parameters specifying the principal component description of the dark energy equation of state parameter, as well as in the growth index {gamma}. These results indicate that cluster counts can play an important complementary role in constraining dark energy and modified gravity even if the associated systematic errors are not strongly controlled.

Cunha, Carlos; Huterer, Dragan [Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan 48109-1040 (United States); Frieman, Joshua A. [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, Illinois 60510 (United States); Kavli Institute for Cosmological Physics, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637 (United States)

2009-09-15

180

Dark energy constraints from galaxy cluster peculiar velocities

Future multifrequency microwave background experiments with arcminute resolution and micro-Kelvin temperature sensitivity will be able to detect the kinetic Sunyaev-Zeldovich effect, providing a way to measure radial peculiar velocities of massive galaxy clusters. We show that cluster peculiar velocities have the potential to constrain several dark energy parameters. We compare three velocity statistics (the distribution of radial velocities, the mean pairwise streaming velocity, and the velocity correlation function) and analyze the relative merits of these statistics in constraining dark energy parameters. Of the three statistics, mean pairwise streaming velocity provides constraints that are least sensitive to velocity errors: the constraints on parameters degrade only by a factor of 2 when the random error is increased from 100 to 500 km/s. We also compare cluster velocities with other dark energy probes proposed in the Dark Energy Task Force report. For cluster velocity measurements with realistic priors, the eventual constraints on the dark energy density, the dark energy equation of state and its evolution are comparable to constraints from supernovae measurements, and better than cluster counts and baryon acoustic oscillations; adding velocity to other dark energy probes improves constraints on the figure of merit by more than a factor of 2. For upcoming Sunyaev-Zeldovich galaxy cluster surveys, even velocity measurements with errors as large as 1000 km/s will substantially improve the cosmological constraints compared to using the cluster number density alone.

Bhattacharya, Suman; Kosowsky, Arthur [Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, Pennsylvania 15260 (United States)

2008-04-15

181

Cosmological perturbations in models of coupled dark energy

Models in which dark energy interacts with dark matter have been proposed in the literature to help explain why dark energy should only come to dominate in recent times. In this paper, we present a dynamical framework to calculate cosmological perturbations for a general quintessence potential and interaction term. Our formalism is built upon the powerful phase-space approach often used to analyze the dynamical attractors in the background. We obtain a set of coupled differential equations purely in terms of dimensionless, bounded variables and apply these equations to calculate perturbations in a number of scenarios. Interestingly, in the presence of dark-sector interactions, we find that dark energy perturbations do not redshift away at late times, but can cluster even on small scales. We also clarify the initial conditions for the perturbations in the dark sector, showing that adiabaticity is no longer conserved in the presence of dark-sector interactions, even on large scales. Some issues of instability in the perturbations are also discussed.

Chongchitnan, Sirichai [Oxford Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH (United Kingdom)

2009-02-15

182

Baryonic Matter and Dark Energy

NASA Astrophysics Data System (ADS)

Estimates of the baryonic density of the universe can be obtained applicable to different epochs of the cosmic history. Ancient values are in satisfactory agreement with each others. However, there seems to be a deficit in the contemporary universe. One aim of this meeting was an exploration of the various sites where these baryons could possibly hide. In the second part of this talk, the status of the recently discovered `dark energy' and its possible influence in the far future of the universe are briefly discussed.

Reeves, Hubert

2002-01-01

183

I present galaxy clustering results from the Sloan Digital Sky Survey that reveal the signature of acoustic oscillations of the photon-baryon fluid in the first million years of the Universe. The scale of this feature can be computed and hence the detection in the galaxy clustering serves as a standard ruler, giving a geometric distance to a redshift of 0.35. I will discuss the implications of this measurement for the composition of the universe, including dark energy and spatial curvature. I will close with a discussion of the prospects for future redshift surveys to use the acoustic peak to map the expansion history of the universe.

184

The idea that we live near the center of a large, nonlinear void has attracted attention recently as an alternative to dark energy or modified gravity. We show that an appropriate void profile can fit both the latest cosmic microwave background and supernova data. However, this requires either a fine-tuned primordial spectrum or a Hubble rate so low as to rule these models out. We also show that measurements of the radial baryon acoustic scale can provide very strong constraints. Our results present a serious challenge to void models of acceleration. PMID:19113691

Zibin, James P; Moss, Adam; Scott, Douglas

2008-12-18

185

NASA Astrophysics Data System (ADS)

Dark energy is one of the biggest mysteries in science. In this article the origin of the concept is traced as far back as Newton and Hooke in the 17th century. Newton considered, along with the inverse-square law, a force of attraction that varies linearly with distance. A direct link can be made between this term and Einstein's cosmological constant, ?, and this leads to a possible relation between ? and the total mass of the universe. Mach's influence on Einstein is discussed and the convoluted history of ? throughout the last 90 years is coherently presented.

Calder, Lucy; Lahav, Ofer

2008-02-01

186

Holographic Dark Energy in Higher Derivative Gravity with Varying Gravitational Constant

NASA Astrophysics Data System (ADS)

In this paper we investigate the holographic dark energy scenario in higher derivative gravity with a varying gravitational constant. We introduce a kind of energy density from higher derivative gravity which has role of the same as holographic dark energy. We obtain the exact differential equation , which determine the evolution of the dark energy density based on varying gravitational constant G. We also find out a cosmological application of our work by evaluating a relation for the equation of state of dark energy for low redshifts containing varying G correction.

Borah, Bharat; Ansari, M.

2013-04-01

187

Energy Conservation (Bernoulli's Equation)

NSDL National Science Digital Library

Professor Chiang Shih's Thermal-Fluids I course at the FAMU-FSU College of Engineering combines "the traditional thermal disciplines in Thermodynamics, heat transfer and fluid mechanics." This PowerPoint presentation, from Shih's lecture notes, illustrates Benoulli's Equation and energy conservation. Along with mathematical equations, there are also a number of illustrations and examples which show the concepts discussed.

Shih, Chiang

2008-09-24

188

[Dark matter and dark energy of the universe].

At the turn of the 20th Century, the Universe was thought to consist of our solar system, the Sun, planets, satellites and comets, floating under the Milky Way. The astronomers were ignorant of the existence of galaxies, clusters, quasars and black holes. Over the last ten years the Cosmology has made remarkable progress in our understanding of the composition of the Universe: 23 per cent is in an unknown form called dark matter; 73 per cent in another form called dark energy; 3 per cent is made of free hydrogen and helium atoms; 0.5 per cent makes up all the light we see in the night including the stars, clusters and superclusters; 0.3 per cent is in free neutrino particles; and finally, 0.03 per cent is in the heavier nuclei of which the Sun, the Earth and ourselves are made. In this work we study specially the dark matter and the dark energy. The first one appears to be attached to galaxies, and astronomers agree that it is cold, meaning that the particles that make up that matter are not moving fast. Very recently astronomers discovered that a tremendous amount of the so-cahled dark energy exists and that it is pushing and accelerating the expansion of the Universe. Should this expansion continue for another 14,000 million years, the sky will darken with only a handful of galaxies remaining visible. PMID:16463572

Aguilar Peris, José

2005-01-01

189

Describing Dark Matter and Dark Energy through a non-minimal gravitational coupling

NASA Astrophysics Data System (ADS)

A model exhibiting a non-minimal coupling between matter and the scalar curvature is pre-sented, generalizing the framework of the so-called f(R) theories; its main fundamental impli-cations are discussed, namely the non-conservation of the energy-momentum tensor and the deviation from geodesic motion. The flexibility of this model is first presented through a suitable application to the description of known dark matter profiles, thus accounting for the reported flattening of the galaxy rotation curves and encompassing the phenomenological Tully-Fisher law. In a cosmological context, the non-minimal gravitational coupling is further explored, and a description of a dynamic negative pressure perfect fluid (i.e. Dark Energy) is obtained; its main features are discussed, with a focus on the evolution of the deceleration parameter and the dark energy equation of state parameter.

Páramos, Jorge; Bertolami, Orfeu

190

Can cosmological observations uniquely determine the nature of dark energy?

The observational effect of all minimally coupled scalar field models of dark energy can be determined by the behavior of the following two parameters: (1) equation of state parameter w, which relates dark energy pressure to its energy density, and (2) effective speed of sound c{sub e}{sup 2}, which relates dark energy pressure fluctuation to its density fluctuation. In this paper we show that these two parameters do not uniquely determine the form of a scalar field dark energy Lagrangian even after taking into account the perturbation in the scalar field. We present this result by showing that two different forms of scalar field Lagrangian can lead to the same values for these paired parameters. It is well known that from the background evolution the Lagrangian of the scalar field dark energy cannot be uniquely determined. The two models of dark energy presented in this paper are indistinguishable from the evolution of background as well as from the evolution of perturbations from a Friedmann-Robertson-Walker metric.

Unnikrishnan, Sanil [Department of Physics and Astrophysics, University of Delhi, Delhi 110007 (India)

2008-09-15

191

Constraining the Runaway Dilaton and Quintessential Dark Energy

NASA Astrophysics Data System (ADS)

Dark energy is some of the weirdest and most mysterious stuff in the universe that tends to increase the rate of expansion of the universe. Two commonly known forms of dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli whose energy density can vary with time. We explore one particular model for dynamic dark energy: quintessence driven by a scalar dilaton field. We propose an ansatz for the form of the dilaton field, |?(a)|mP ? ?1 ln t + ?2tn = ? ln a + ?a2?, where a is the scale factor and ? and ? are parameters of the model. This phenomenological ansatz for ? can be motivated by generic solutions of a scalar dilaton field in many effective string theory and string-inspired gravity models in four dimensions. Most of the earlier discussions in the literature correspond to the choice that ? = 0 so that ?(t) ? ln t or ?(t) ? ln a(t). Using a compilation of current data including type Ia supernovae, we impose observational constraints on the slope parameters like ? and ? and then discuss the relation of our results to analytical constraints on various cosmological parameters, including the dark energy equation of state. Some useful constraints are imposed on model parameters like ? and ? as well as on the dark energy/dark matter couplings using results from structure formation. The constraints of this model are shown to encompass the cosmological constant limit within 1? error bars.

Neupane, Ishwaree P.; Trowland, Holly

192

A numerical approach is considered for spherically symmetric spacetimes that generalize Lemaitre-Tolman-Bondi dust solutions to nonzero pressure ('LTB spacetimes'). We introduce quasilocal (QL) variables that are covariant LTB objects satisfying evolution equations of Friedman-Lemaitre-Robertson-Walker (FLRW) cosmologies. We prove rigorously that relative deviations of the local covariant scalars from the QL scalars are nonlinear, gauge invariant and covariant perturbations on a FLRW formal background given by the QL scalars. The dynamics of LTB spacetimes is completely determined by the QL scalars and these exact perturbations. Since LTB spacetimes are compatible with a wide variety of ''equations of state,'' either single fluids or mixtures, a large number of known solutions with dark matter and dark energy sources in a FLRW framework (or with linear perturbations) can be readily examined under idealized but nontrivial inhomogeneous conditions. Coordinate choices and initial conditions are derived for a numerical treatment of the perturbation equations, allowing us to study nonlinear effects in a variety of phenomena, such as gravitational collapse, nonlocal effects, void formation, dark matter and dark energy couplings, and particle creation. In particular, the embedding of inhomogeneous regions can be performed by a smooth matching with a suitable FLRW solution, thus generalizing the Newtonian 'top hat' models that are widely used in astrophysical literature. As examples of the application of the formalism, we examine numerically the formation of a black hole in an expanding Chaplygin gas FLRW universe, as well as the evolution of density clumps and voids in an interactive mixture of cold dark matter and dark energy.

Sussman, Roberto A. [Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico (ICN-UNAM), A. P. 70--543, 04510 Mexico D. F. (Mexico)

2009-01-15

193

Dark Energy and the Cosmological Constant: A Brief Introduction

ERIC Educational Resources Information Center

|The recently observed acceleration of the expansion of the universe is a topic of intense interest. The favoured causes are the "cosmological constant" or "dark energy". The former, which appears in the Einstein equations as the term [lambda]g[subscript [mu]v], provides an extremely simple, well-defined mechanism for the acceleration. However,…

Harvey, Alex

2009-01-01

194

Hessence: a new view of quintom dark energy

Recently a lot of attention has been given to building a dark energy model in which the equation-of-state parameter w can cross the phantom divide w = -1. One of the models to realize crossing the phantom divide is called the quintom model, in which two real scalar fields appear, one is a normal scalar field and the other is

Hao Wei; Rong-Gen Cai; Ding-Fang Zeng

2005-01-01

195

Cosmological Surprises from Braneworld models of Dark Energy

Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w -1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future `Big Rip' singularity which is usually encountered by Phantom models.

Varun Sahni

2005-01-01

196

Nonlocal String Tachyon as a Model for Cosmological Dark Energy

There are many different phenomenological models describing the cosmological dark energy and accelerating Universe by choosing adjustable functions. In this paper we consider a specific model of scalar tachyon field which is derived from the NSR string field theory and study its cosmological applications. We find that in the effective field theory approximation the equation of state parameter w <

I. Ya

197

Dark energy: Vacuum fluctuations, the effective phantom phase, and holography

We aim at the construction of dark energy models without exotic matter but with a phantomlike equation of state (an effective phantom phase). The first model we consider is decaying vacuum cosmology where the fluctuations of the vacuum are taken into account. In this case, the phantom cosmology (with an effective, observational omega being less than -1 ) emerges even

Emilio Elizalde; Shin'ichi Nojiri; S. D. Odintsov; Peng Wang

2005-01-01

198

Models of dark matter coupled to dark energy

NASA Astrophysics Data System (ADS)

We present three distinct types of models of dark energy in the form of a scalar field which is explicitly coupled to dark matter. Our construction draws from the pull-back formalism for fluids and generalizes the fluid action to involve couplings to the scalar field. We investigate the cosmology of each class of model both at the background and linearly perturbed level. We choose a potential for the scalar field and a specific coupling function for each class of models and we compute the cosmic microwave background and matter power spectra.

Pourtsidou, A.; Skordis, C.; Copeland, E. J.

2013-10-01

199

The Dark Energy Survey instrument design

We describe a new project, the Dark Energy Survey (DES), aimed at measuring the dark energy equation of state parameter, w, to a statistical precision of {approx}5%, with four complementary techniques. The survey will use a new 3 sq. deg. mosaic camera (DECam) mounted at the prime focus of the Blanco 4m telescope at the Cerro-Tololo International Observatory (CTIO). DECam includes a large mosaic camera, a five element optical corrector, four filters (g,r,i,z), and the associated infrastructure for operation in the prime focus cage. The focal plane consists of 62 2K x 4K CCD modules (0.27''/pixel) arranged in a hexagon inscribed within the 2.2 deg. diameter field of view. We plan to use the 250 micron thick fully-depleted CCDs that have been developed at the Lawrence Berkeley National Laboratory (LBNL). At Fermilab, we will establish a packaging factory to produce four-side buttable modules for the LBNL devices, as well as to test and grade the CCDs. R&D is underway and delivery of DECam to CTIO is scheduled for 2009.

Flaugher, B.; /Fermilab

2006-05-01

200

NASA Astrophysics Data System (ADS)

Recently, it has been shown that the inclusion of higher signal harmonics in the inspiral signals of binary supermassive black holes (SMBH) leads to dramatic improvements in the parameter estimation with Laser Interferometer Space Antenna (LISA). In particular, the angular resolution becomes good enough to identify the host galaxy or galaxy cluster, in which case the redshift can be determined by electromagnetic means. The gravitational wave signal also provides the luminosity distance with high accuracy, and the relationship between this and the redshift depends sensitively on the cosmological parameters, such as the equation-of-state parameter w = pDE/?DE of dark energy. Using binary SMBH events at z < 1 with appropriate masses and orientations, one would be able to constrain w to within a few per cent. We show that, if the measured sky location is folded into the error analysis, the uncertainty on w goes down by an additional factor of 2-3, leaving weak lensing as the only limiting factor in using LISA as a dark energy probe.

Arun, K. G.; Mishra, Chandra Kant; Van Den Broeck, Chris; Iyer, B. R.; Sathyaprakash, B. S.; Sinha, Siddhartha

2009-05-01

201

The dark energy–dominated Universe

In this paper we investigate the epochs in which the Universe started accelerating and when it began to become dark energy–dominated (i.e., the dynamics of the expansion of the Universe dominated by the dark energy). We provide analytic expressions to calculate the redshifts of these epochs as a function of density parameters. Moreover, we review and discuss cosmological models with

José Carlos N. de Araujo; N. de Araujo

2005-01-01

202

Singularity-free dark energy star

NASA Astrophysics Data System (ADS)

We propose a model for an anisotropic dark energy star where we assume that the radial pressure exerted on the system due to the presence of dark energy is proportional to the isotropic perfect fluid matter density. We discuss various physical features of our model and show that the model satisfies all the regularity conditions and is stable as well as singularity-free.

Rahaman, Farook; Maulick, Raju; Yadav, Anil Kumar; Ray, Saibal; Sharma, Ranjan

2012-01-01

203

Probing Dark Energy with Constellation-X.

National Technical Information Service (NTIS)

Constellation-X (Con-X) will carry out two powerful and independent sets of tests of dark energy based on X-ray observations of galaxy clusters, providing comparable accuracy to other leading dark energy probes. The first group of tests will measure the a...

D. A. Rapetti S. W. Allen

2006-01-01

204

Probing dark energy: Methods and strategies

The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and, indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about two-thirds of the critical density, is smoothly distributed, has large negative pressure, and is very mysterious. For now, all of its discernible cosmological consequences follow from its effect

Dragan Huterer; Michael S. Turner

2001-01-01

205

The case for dynamical dark energy revisited

We investigate the behaviour of dark energy using the recently released supernova data of Riess et al 2004 and a model independent parametrization for dark energy (DE). We find that, if no priors are imposed on Omega0m and h, DE which evolves with time provides a better fit to the SNe data than LambdaCDM. This is also true if we

Ujjaini Alam; Varun Sahni; A. A. Starobinsky

2004-01-01

206

Universe Filled with Dark Energy (DE) from a Wet Dark Fluid (WDF) in f( R, T) Gravity

NASA Astrophysics Data System (ADS)

We studied the Bianchi type-V universe filled with dark energy (DE) from a wet dark fluid (WDF) in the framework of f( R, T) gravity (Harko in Phys. Rev. D 84:024020, 2011). A new equation of state for the dark energy (DE) component of the universe has been used. It is modeled on the equation of state p= w( ?- ? ?) which can be describing a liquid, for example water. The exact solutions to the corresponding field equations are obtained for exponential and power-law volumetric expansion. It is observed that the universe can approach to isotropy monotonically even in the presence of wet dark fluid. Also we have discussed the well-known astrophysical phenomena, namely the look-back time, proper distance, the luminosity distance and angular diameter distance with redshift.

Samanta, G. C.

2013-07-01

207

GALAXY CLUSTERS AS A PROBE OF EARLY DARK ENERGY

We study a class of early dark energy (EDE) models, in which, unlike in standard dark energy models, a substantial amount of dark energy exists in the matter-dominated era. We self-consistently include dark energy perturbations, and show that these models may be successfully constrained using future observations of galaxy clusters, in particular the redshift abundance, and the Sunyaev-Zel'dovich (SZ) power spectrum. We make predictions for EDE models, as well as {Lambda}CDM for incoming X-ray (eROSITA) and microwave (South Pole Telescope) observations. We show that galaxy clusters' mass function and the SZ power spectrum will put strong constraints both on the equation of state of dark energy today and the redshift at which EDE transits to present-day {Lambda}CDM-like behavior for these models, thus providing complementary information to the geometric probes of dark energy. Not including perturbations in EDE models leads to those models being practically indistinguishable from {Lambda}CDM. An MCMC analysis of future galaxy cluster surveys provides constraints for EDE parameters that are competitive with and complementary to background expansion observations such as supernovae.

Alam, Ujjaini [ISR-1, ISR Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Lukic, Zarija; Bhattacharya, Suman, E-mail: ujjaini@lanl.gov, E-mail: zarija@lanl.gov, E-mail: sumanb@lanl.gov [T-2, T Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

2011-02-01

208

Is there supernova evidence for dark energy metamorphosis?

We reconstruct the equation of state w(z) of dark energy (DE) using a recently released data set containing 172 Type Ia supernovae (SNe) without assuming the prior w(z) >=-1 (in contrast to previous studies). We find that DE evolves rapidly and metamorphoses from dust-like behaviour at high z (w~= 0 at z~ 1) to a strongly negative equation of state

Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky

2004-01-01

209

Physical evidence for dark energy

The authors present measurements of the angular cross-correlation between luminous red galaxies from the Sloan Digital Sky Survey and the cosmic microwave background temperature maps from the Wilkinson Microwave Anisotropy Probe. They find a statistically significant achromatic positive correlation between these two data sets, which is consistent with the expected signal from the late Integrated Sachs-Wolfe (ISW) effect. they do not detect any anti-correlation on small angular scales as would be produced from a large Sunyaev-Zel'dovich (SZ) effect, although they do see evidence for some SZ effect for their highest redshift samples. Assuming a flat universe, their preliminary detection of the ISW effect provides independent physical evidence for the existence of dark energy.

Scranton, Ryan; Connolly, Andrew J.; Nichol, Robert C.; Stebbins, Albert; Szapudi, Istvan; Eisenstein, Daniel J.; Afshordi, Niayesh; Budavari, Tamas; Csabai, Istvan; Frieman, Joshua A.; Gunn, James E.; Johnston, David; Loh, Yeong-Shang; Lupton, Robert H.; Miller, Christopher J.; Sheldon, Erin Scott; Sheth, Ravi K.; Szalay, Alexander S.; Tegmark, Max; Xu, Yongzhong; Anderson, Scott F.; /Pittsburgh U. /Carnegie Mellon U. /Fermilab /Inst. Astron., Honolulu /Arizona U., Astron. Dept. - Steward Observ. /Princeton U. Observ. /Johns Hopkins U. /Eotvos U. /Chicago U., Astron. Astrophys. Ctr. /KICP, Chicago /Pennsylvania U. /Washington U., Seattle, Astron. Dept. /Apache Point Observ. /Illinois U., Urbana, Astron. Dept. /Tokyo U., ICRR /LLNL, Livermore /Sussex U., Astron. Ctr. /Baltimore, Space Telescope Sci. /Michigan U. /Naval Observ., Flagstaff /Penn State U., Astron. Astrophys.

2003-07-01

210

CMB lensing constraints on neutrinos and dark energy

Signatures of lensing of the cosmic microwave background radiation by gravitational potentials along the line of sight carry with them information on the matter distribution, neutrino masses, and dark energy properties. We examine the constraints that Planck, PolarBear, and CMBpol future data, including from the B-mode polarization or the lensing potential, will be able to place on these quantities. We simultaneously fit for neutrino mass and dark energy equation of state including time variation and early dark energy density, and compare the use of polarization power spectra with an optimal quadratic estimator of the lensing. Results are given as a function of systematics level from residual foreground contamination. A realistic CMBpol experiment can effectively constrain the sum of neutrino masses to within 0.05 eV and the fraction of early dark energy to 0.002. We also present a surprisingly simple prescription for calculating dark energy equation of state constraints in combination with supernova distances from JDEM.

Putter, Roland de; Zahn, Oliver; Linder, Eric V. [Berkeley Lab and Berkeley Center for Cosmological Physics, University of California, Berkeley, California 94720 (United States)

2009-03-15

211

Possible dark energy imprints in the gravitational wave spectrum of mixed neutron-dark-energy stars

NASA Astrophysics Data System (ADS)

In the present paper we study the oscillation spectrum of neutron stars containing both ordinary matter and dark energy in different proportions. Within the model we consider, the equilibrium configurations are numerically constructed and the results show that the properties of the mixed neuron-dark-energy star can differ significantly when the amount of dark energy in the stars is varied. The oscillations of the mixed neuron-dark-energy stars are studied in the Cowling approximation. As a result we find that the frequencies of the fundamental mode and the higher overtones are strongly affected by the dark energy content. This can be used in the future to detect the presence of dark energy in the neutron stars and to constrain the dark-energy models.

Yazadjiev, Stoytcho S.; Doneva, Daniela D.

2012-03-01

212

Dark Matter and Dark Energy in the Universe

For the first time, we have a plausible and complete accounting of matter and energy in the Universe. Expressed a fraction of the critical density it goes like this: neutrinos, between 0.3% and 15%; stars, between 0.3% and 0.6%; baryons (total), 5% ± 0.5%; matter (total), 40% ± 10%; smooth, dark energy, 80% ± 20%; totaling to the critical density

Michael S. Turner

2000-01-01

213

Constraints on the anisotropy of dark energy

If the equation of state of dark energy is anisotropic there will be additional quadrupole anisotropy in the cosmic microwave background induced by the time-dependent anisotropic stress quantified in terms of {Delta}w. Assuming that the entire amplitude of the observed quadrupole is due to this anisotropy, we conservatively impose a limit of |{Delta}w|<2.1x10{sup -4} for any value of w{>=}-1 assuming that {Omega}{sub m}<0.5. This is considerably tighter than that which comes from supernovae. Stronger limits, up to a factor of 10, are possible for specific values of {Omega}{sub m} and w. Since we assume this component is uncorrelated with the stochastic component from inflation, we find that both the expectation value and the sample variance are increased. There is no improvement in the likelihood of an anomalously low quadrupole as suggested by previous work on an elliptical universe.

Appleby, Stephen; Battye, Richard [Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL (United Kingdom); Moss, Adam [Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 (Canada)

2010-04-15

214

Precision Studies of Dark Energy with LSST

Starting around 2013, data from the Large Synoptic Survey Telescope (LSST) will be analyzed for a wide range of phenomena. By separately tracing the development of mass structure and rate of expansion of the universe, these data will address the physics of dark matter and dark energy, the possible existence of modified gravity on large scales, large extra dimensions, the neutrino mass, and possible self interaction of dark matter particles.

Tyson, J. Anthony [Department of Physics, University of California, Davis, CA 95616 (United States)

2006-11-17

215

Dark Energy from Fifth-Dimensional Brans-Dicke Theory

NASA Astrophysics Data System (ADS)

Following the approach of the induced-matter theory, we investigate the cosmological implications of a five-dimensional Brans-Dicke (BD) theory, and propose to explain the acceleration of the universe. After inducing in a four-dimensional hypersurface, we classify the energy-momentum tensor into two parts in a way that, one part represents all kind of the matter (the baryonic and dark) and the other one contains every extra terms emerging from the scale factor of the fifth dimension and the scalar field, which we consider as the energy-momentum tensor of dark energy. We also separate the energy-momentum conservation equation into two conservation equations, one for matter and the other for dark energy. We perform this procedure for different cases, without interacting term and with two particular (suitable) interacting terms between the two parts. By assuming the parameter of the state equation for dark energy to be constant, the equations of the model admit the power-law solutions. Though, the noninteracting case does not give any accelerated universe, but the interacting cases give both decelerated and accelerated universes. For the interacting cases, we figure out analytically the acceptable ranges of some parameters of the model, and also investigate the data analysis to test the model parameter values consistency with the observational data of the distance modulus of 580 SNe Ia compiled in Union2.1. For one of these interacting cases, the best fitted values suggest that BD coupling constant (?) is ? -7.75, however, it also gives the state parameter of dark energy (wX) equal to ? -0.67. In addition, the model gives the Hubble and deceleration parameters at the present time to be H? ? 69.4 (km/s)/Mpc and q? ? -0.38 (within their confidence intervals), where the scale factor of the fifth dimension shrinks with the time.

Bahrehbakhsh, Amir F.; Farhoudi, Mehrdad; Vakili, Hajar

2013-08-01

216

Unknowns and unknown unknowns: from dark sky to dark matter and dark energy

NASA Astrophysics Data System (ADS)

Answering well-known fundamental questions is usually regarded as the major goal of science. Discovery of other unknown and fundamental questions is, however, even more important. Recognition that "we didn't know anything" is the basic scientific driver for the next generation. Cosmology indeed enjoys such an exciting epoch. What is the composition of our universe ? This is one of the well-known fundamental questions that philosophers, astronomers and physicists have tried to answer for centuries. Around the end of the last century, cosmologists finally recognized that "We didn't know anything". Except for atoms that comprise slightly less than 5% of the universe, our universe is apparently dominated by unknown components; 23% is the known unknown (dark matter), and 72% is the unknown unknown (dark energy). In the course of answering a known fundamental question, we have discovered an unknown, even more fundamental, question: "What is dark matter? What is dark energy?" There are a variety of realistic particle physics models for dark matter, and its experimental detection may be within reach. On the other hand, it is fair to say that there is no widely accepted theoretical framework to describe the nature of dark energy. This is exactly why astronomical observations will play a key role in unveiling its nature. I will review our current understanding of the "dark sky", and then present on-going Japanese project, SuMIRe, to discover even more unexpected questions.

Suto, Yasushi

2010-07-01

217

Quintom dark energy models with nearly flat potentials

We examine quintom dark energy models, produced by the combined consideration of a canonical and a phantom field, with nearly flat potentials and dark energy equation-of-state parameter w{sub DE} close to -1. We find that all such models converge to a single expression for w{sub DE}(z), depending only on the initial field values and their derivatives. We show that this quintom paradigm allows for a description of the transition through -1 in the near cosmological past. In addition, we provide the necessary conditions for the determination of the direction of the -1 crossing.

Setare, M. R.; Saridakis, E. N. [Department of Science, Payame Noor University, Bijar (Iran, Islamic Republic of); Department of Physics, University of Athens, GR-15771 Athens (Greece)

2009-02-15

218

Cosmological neutrino mass limit and the dynamics of dark energy

NASA Astrophysics Data System (ADS)

We investigate the correlation between the neutrino mass limit and dark energy with the time evolving equation of state. Parametrizing dark energy as w=w0+w1*z/(1+z), we make a global fit using Markov Chain Monte Carlo technique to determine w0, w1, neutrino mass as well as other cosmological parameters simultaneously. We pay particular attention to the correlation between neutrino mass ?m? and w1 using current cosmological observations as well as the future simulated data sets such as PLANCK, SNAP, and LAMOST.

Xia, Jun-Qing; Zhao, Gong-Bo; Zhang, Xinmin

2007-05-01

219

Measuring the dark matter equation of state and its cosmological consequences

NASA Astrophysics Data System (ADS)

We explore the consequences of the measurements of the equation of state of dark matter7, on the homogenous FRW universe dynamics and build an alternative cosmological scenario to the concordance ?CDM universe. The new paradigm is based on the introduction of an effective scalar field replacing the undetected components of the dark sector: dark matter and dark energy in the form of a cosmological constant. The scalar field obeys a barotropic equation of state p = ?? with ? = -1/3 and dominates the cosmological dynamics in the last 14.27 Gyr, in a universe with an age of 14.83 Gyr . Before that epoch, baryons and photons drove the general behaviour of the universe as in the standard ?CDM scenario. We compute a minimal set of cosmological parameters which allow us to reproduce several observational results such us baryon abundance, constrains on the age of the universe, the astronomical scale of distance and the high redshift supernova data with a high degree of precision. However, it should be emphasized that the new model is not accelerating, instead expands asymptotically towards an Einstein Static Universe. We briefly mention the possible mechanisms behind the origin of such dominant component and analyze the prospective of reproducing the success of the standard cosmological model explaining the process of structure formation.

Domínguez Romero, Mariano Javier de León; Ruiz, Andrés Nicolás

2012-10-01

220

Holographic Dark Energy Model with Quintessence in Bianchi Type-I Space-Time

NASA Astrophysics Data System (ADS)

In this paper, we have studied a homogeneous and anisotropic universe filled with matter and holographic dark energy components. Assuming deceleration parameter to be a constant, an exact solution to Einstein's field equations in axially symmetric Bianchi type-I line element is obtained. A correspondence between the holographic dark energy models with the quintessence dark energy models is also established. Quintessence potential and the dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe.

Sarkar, Sanjay; Mahanta, Chandra Rekha

2013-05-01

221

Interactions of Dark Energy with Other Components

NASA Astrophysics Data System (ADS)

In this paper, we studied the interactions of dark energy with dark matter, black hole, and wormhole. It was shown that, in phantom case, the interaction terms make arise the new behaviors, such as avoidance from big rip, the decrease of black hole mass, and the increase of the wormhole throat size.

Kim, Sung-Won; Keum, Yong-Yeon

2010-04-01

222

Interactions of Dark Energy with Other Components

In this paper, we studied the interactions of dark energy with dark matter, black hole, and wormhole. It was shown that, in phantom case, the interaction terms make arise the new behaviors, such as avoidance from big rip, the decrease of black hole mass, and the increase of the wormhole throat size.

Sung-Won Kim; Yong-Yeon Keum

2010-01-01

223

Report of the Dark Energy Task Force

Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short

Andreas Albrecht; Gary Bernstein; Robert Cahn; Wendy L. Freedman; Jacqueline Hewitt; Wayne Hu; John Huth; Marc Kamionkowski; Edward W. Kolb; Lloyd Knox; John C. Mather; Suzanne Staggs; Nicholas B. Suntzeff

2006-01-01

224

Holographic dark matter and dark energy with second order invariants

NASA Astrophysics Data System (ADS)

One of the main goals of modern cosmology remains to summon up a self-consistent policy, able to explain, in the framework of Einstein’s theory, the cosmic speedup and the presence of dark matter in the Universe. According to the holographic principle, which postulates the existence of a minimal size of a physical region, we argue, in this paper, that if this size exists for the Universe and it is accrued from the independent geometrical second order invariants, it would be possible to ensure a surprising source for dark matter and a viable candidate for explaining the late acceleration of the Universe. We develop low redshift tests, such as supernovae Ia and kinematical analysis, compiled by the use of cosmography, and we compare the outcomes with higher redshift tests, such as the CMB peak and anisotropy of the cosmic power spectrum. All the results indicate that the models presented here can be interpreted as unified models that are capable of describing both dark matter and dark energy.

Aviles, Alejandro; Bonanno, Luca; Luongo, Orlando; Quevedo, Hernando

2011-11-01

225

Dark matter from dark energy-baryonic matter couplings

We present a scenario in which a scalar field dark energy is coupled to the trace of the energy momentum tensor of the baryonic matter fields. In the slow-roll regime, this interaction could give rise to the cosmological features of dark matter. We work out the cosmological background solutions and fit the parameters of the model using the Union 2 supernovae data set. Then, we develop cosmological perturbations up to linear order, and we find that the perturbed variables have an acceptable behavior, in particular, the density contrast of baryonic matter grows similar to that in the {Lambda}CDM model for a suitable choice of the strength parameter of the coupling.

Aviles, Alejandro [Instituto de Ciencias Nucleares, UNAM (Mexico); Departamento de Fisica, Instituto Nacional de Investigaciones Nucleares (Mexico); Cervantes-Cota, Jorge L. [Departamento de Fisica, Instituto Nacional de Investigaciones Nucleares (Mexico); Berkeley Center for Cosmological Physics, University of California, Berkeley, California 94720 (United States)

2011-01-15

226

Dark energy and the cosmological constant: a brief introduction

NASA Astrophysics Data System (ADS)

The recently observed acceleration of the expansion of the universe is a topic of intense interest. The favoured causes are the 'cosmological constant' or 'dark energy'. The former, which appears in the Einstein equations as the term ?g??, provides an extremely simple, well-defined mechanism for the acceleration. However, there is a school of thought that suggests that the cosmological constant is inadequate to explain fully the evolution of the universe and should be replaced by a slowly varying scalar field—the aforementioned dark energy. The situation is complicated by the circumstance that the expression for the cosmological constant is precisely that of the zero-point energy of a quantum field, epsilonog??. This strongly suggests the two are related. There are several problems here. The calculated value of the various contributions to epsilono may be as much as 120 orders of magnitude greater than the observed value of ?. How these contributions add up to so minute a quantity is a major part of what is commonly termed 'the cosmological constant problem'. Traces of dark energy have yet to be found. We review some of the history and problems associated with the concepts of the cosmological constant and dark energy, provide a brief review of the relevant cosmology and review possible candidates for dark energy.

Harvey, Alex

2009-07-01

227

Dark energy models in the w-w{sup '} plane

We examine the behavior of dark energy models in the plane defined by w (the equation of state parameter for the dark energy) and w{sup '} (the derivative of w with respect to the logarithm of the scale factor). For nonphantom barotropic fluids with positive squared sound speed, we find that w{sup '}<3w(w+1), the opposite of the bound on quintessence models previously derived by Caldwell and Linder. Thus, these barotropic models and quintessence models for the dark energy occupy disjoint regions in the w-w{sup '} plane. We also derive two new bounds for quintessence models in the w-w{sup '} plane: the first is a general bound for any scalar field with a monotonic potential, while the second improves on the Caldwell-Linder bound for tracker quintessence models. Observationally distinguishing barotropic models from quintessence models requires {sigma}(w{sup '}) < or approx. 1+w.

Scherrer, Robert J. [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States)

2006-02-15

228

Dark energy models in the w-w' plane

NASA Astrophysics Data System (ADS)

We examine the behavior of dark energy models in the plane defined by w (the equation of state parameter for the dark energy) and w' (the derivative of w with respect to the logarithm of the scale factor). For nonphantom barotropic fluids with positive squared sound speed, we find that w'<3w(w+1), the opposite of the bound on quintessence models previously derived by Caldwell and Linder. Thus, these barotropic models and quintessence models for the dark energy occupy disjoint regions in the w-w' plane. We also derive two new bounds for quintessence models in the w-w' plane: the first is a general bound for any scalar field with a monotonic potential, while the second improves on the Caldwell-Linder bound for tracker quintessence models. Observationally distinguishing barotropic models from quintessence models requires ?(w')?1+w.

Scherrer, Robert J.

2006-02-01

229

What We Know About Dark Energy From Supernovae

The measured distances of type Ia (white dwarf) supernovae as a function of redshift (z) have shown that the expansion of the Universe is currently accelerating, probably due to the presence of dark energy (X) having a negative pressure. Combining all of the data with existing results from large-scale structure surveys, we find a best fit for Omega M and Omega X of 0.28 and 0.72 (respectively), in excellent agreement with the values derived independently from WMAP measurements of the cosmic microwave background radiation. Thus far, the best-fit value for the dark energy equation-of-state parameter is -1, and its first derivative is consistent with zero, suggesting that the dark energy may indeed be Einstein's cosmological constant.

230

What We Know About Dark Energy from Supernovae

The measured distances of type Ia (white dwarf) supernovae as a function of redshift (z) have shown that the expansion of the Universe is currently accelerating, probably due to the presence of dark energy (X) having a negative pressure. Combining all of the data with existing results from large-scale structure surveys, we find a best fit for Omega M and Omega X of 0.28 and 0.72 (respectively), in excellent agreement with the values derived independently from WMAP measurements of the cosmic microwave background radiation. Thus far, the best-fit value for the dark energy equation-of-state parameter is -1, and its first derivative is consistent with zero, suggesting that the dark energy may indeed be Einstein's cosmological constant.

Filippenko, Alex (University of California, Berkeley)

2008-05-21

231

Voids as a precision probe of dark energy

The shapes of cosmic voids, as measured in spectroscopic galaxy redshift surveys, constitute a promising new probe of dark energy (DE). We forecast constraints on the DE equation of state and its variation from current and future surveys and find that the promise of void shape measurements compares favorably to that of standard methods such as supernovae and cluster counts even for currently available data. Owing to the complementary nature of the constraints, void shape measurements improve the Dark Energy Task Force figure of merit by 2 orders of magnitude for a future large scale experiment such as EUCLID when combined with other probes of dark energy available on a similar time scale. Modeling several observational and theoretical systematics has only moderate effects on these forecasts. We discuss additional systematics which will require further study using simulations.

Biswas, Rahul; Alizadeh, Esfandiar [Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, Illinois 61801 (United States); Wandelt, Benjamin D. [UPMC Universite Paris-06, Institut d'Astrophysique de Paris, 98 bis, boulevard Arago, 75014 Paris (France); Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, Illinois 61801 (United States); Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, Illinois 61801 (United States)

2010-07-15

232

What We Know About Dark Energy from Supernovae

233

On the Triple Interacting Dark Energy Model

NASA Astrophysics Data System (ADS)

Three aspects of the triple interacting dark energy model are studied. The relation between two types of the triple interacting dark energy models is investigated first. Then, the concrete forms of the interacting terms are given by supposing ratios between different energy components is stationary. Furthermore, the stability of the triple interacting dark energy model with different transfer terms is studied in detail, and the complete table of relations between the stability and the transfer terms is given, we find that only models with transformation between matter and dark energy proportional to ?c or ?DE, while the transformation between radiation and matter is not proportional to ?R, are stable against perturbation, which give strong restriction on the model building of the triple interacting.

Huang, Peng; Huang, Yong-Chang

2013-07-01

234

Constraining the evolution of dark energy with type Ia supernovae and gamma-ray bursts

Aims: The behavior of the dark energy equation of state (EOS) is crucial in distinguishing different cosmological models. With a model independent approach, we constrain the possible evolution of the dark energy EOS. Methods: Gamma-ray bursts (GRBs) of redshifts up to z>6 are used, in addition to type Ia supernovae (SNe Ia). We separate the redshifts into 4 bins and

Shi Qi; Fa-Yin Wang; Tan Lu

2008-01-01

235

Self-consistency and calibration of cluster number count surveys for dark energy

Cluster number counts offer sensitive probes of the dark energy if and only if the evolution of the cluster mass versus observable relation(s) is well calibrated. We investigate the potential for internal calibration by demanding consistency in the counts as a function of the observable. In the context of a constant dark energy equation of state, the known initial fluctuation

Wayne Hu

2003-01-01

236

Dark matter and dark energy: approaches and constraints

NASA Astrophysics Data System (ADS)

We will introduce problems of Dark Matter (DM) and Dark Energy (DE), namely we will describe a development of these concepts and their present status. We will demonstrate ap-proaches to these problems. As specific issues we will discuss limits on DM concentration near the black hole at the Galactic Center and ways to solve DE problem introducing alternative theories of gravity such as f (R)-theories. The existence of dark matter (DM) at scales of few pc down to 10-5 pc around the centers of galaxies and in particular in the Galactic Center region has been considered in the literature. Under the assumption that such a DM clump, principally constituted by non-baryonic matter (like WIMPs) does exist at the center of our galaxy, the study of the ?-ray emission from the Galactic Center region allows us to constrain both the mass and the size of this DM sphere. Moreover, if a DM cusp does exist around the Galactic Center it could modify the trajectories of stars moving around it in a sensible way depending on the DM mass distribution. Here, we discuss the constraints that can be obtained with the orbit analysis of stars (as S2 and S16) moving inside the DM concentration with present and next generations of large telescopes. In particular, consideration of the S2 star apoastron shift may allow improving limits on the DM mass and size. We will describe severe constraints from Solar system data on parameters f (R) = Rn theories, where n = 1 corresponds to the standard general relativistic case. 1. A. F. Zakharov, A.A. Nucita, F. De Paolis, G. Ingrosso: Solar system constraints on Rn gravity, Phys. Rev. D 74, 107101, (2006). 2. A. F. Zakharov, A.A. Nucita, F. De Paolis, G. Ingrosso: Apoastron shift constraints on dark matter distribution at the Galactic Center, Phys. Rev. D 76, 062001, (2007). 3. A.F. Zakharov, S. Capozziello, F. De Paolis, G. Ingrosso, A.A. Nucita, The Role of Dark Matter and Dark Energy in Cosmological Models: Theoretical Overview, Space Sci. Rev. 148, 301-313(2009).

Zakharov, Alexander

237

In this work, we consider the cosmological constraints on the holographic Ricci dark energy proposed by Gao et al.[Phys. Rev. D 79, 043511 (2009)], by using the observational data currently available. The main characteristic of holographic Ricci dark energy is governed by a positive numerical parameter {alpha} in the model. When {alpha}<1/2, the holographic Ricci dark energy will exhibit a quintomlike behavior; i.e., its equation of state will evolve across the cosmological-constant boundary w=-1. The parameter {alpha} can be determined only by observations. Thus, in order to characterize the evolving feature of dark energy and to predict the fate of the Universe, it is of extraordinary importance to constrain the parameter {alpha} by using the observational data. In this paper, we derive constraints on the holographic Ricci dark energy model from the latest observational data including the Union sample of 307 type Ia supernovae, the shift parameter of the cosmic microwave background given by the five-year Wilkinson Microwave Anisotropy Probe observations, and the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey. The joint analysis gives the best-fit results (with 1{sigma} uncertainty): {alpha}=0.359{sub -0.025}{sup +0.024} and {omega}{sub m0}=0.318{sub -0.024}{sup +0.026}. That is to say, according to the observations, the holographic Ricci dark energy takes on the quintom feature. Finally, in light of the results of the cosmological constraints, we discuss the issue of the scalar-field dark energy reconstruction, based on the scenario of the holographic Ricci vacuum energy.

Zhang Xin [Department of Physics, College of Sciences, Northeastern University, Shenyang 110004 (China); Kavli Institute for Theoretical Physics China, Chinese Academy of Sciences, Beijing 100080 (China)

2009-05-15

238

Dynamical dark energy models dynamical system approach

NASA Astrophysics Data System (ADS)

We study the Friedmann-Robertson-Walker model with dynamical dark energy modelled in terms of the equation of state p X = w X ( a( z)) ? X in which the coefficient w X is parameterized by the scale factor a or redshift z. We use methods of qualitative analysis of differential equations to investigate the space of all admissible solutions for all initial conditions on the two-dimensional phase plane. We show advantages of representing this dynamics as a motion of a particle in the one-dimensional potential V( a). One of the features of this reduction is the possibility of investigating how typical big rip singularities are in the future evolution of the model. The properties of potential function V can serve as a tool for qualitative classification of all evolution paths. Some important features like resolution of the acceleration problem can be simply visualized as domains on the phase plane. Then one is able to see how large is the class of solutions (labelled by the inset of the initial conditions) leading to the desired property.

Szyd?owski, Marek; Hrycyna, Orest

2006-01-01

239

Dark Matter and Dark Energy from Gravitational Symmetry Breaking

We build a mechanism of gravitational symmetry breaking (GSB) of a global U(1) symmetry based on the relaxation of the equivalence principle due to the mass variation of pseudo Nambu-Goldstone dark matter (DM) particles. This GSB process is described by the modified cosmological convergence mechanism of the Abnormally Weighting Energy (AWE) Hypothesis previously introduced by the authors. Several remarkable constraints from the Hubble diagram of faraway supernovae are derived, notably on the explicit and gravitational symmetry breaking energy scales of the model. We then briefly present some consequences on neutrino masses when this mechanism is applied to the particular case of the breaking of lepton number symmetry.

Fuezfa, A. [Department of Mathematics, University of Namur (FUNDP), Rue de Bruxelles, 61, B-5000 Namur (Belgium); Center for Particle Physics and Phenomenology (CP3), Universite catholique de Louvain, Chemin du Cyclotron, 2, B-1348 Louvain-la-Neuve (Belgium); Alimi, J.-M. [CNRS, Laboratoire Univers et Theories (LUTh), UMR 8102 CNRS, Observatoire de Paris, Universite Paris Diderot, 5 Place Jules Janssen, 92190 Meudon (France)

2010-06-23

240

DESTINY: the dark energy space telescope

We have proposed the development of a low-cost space telescope, Destiny, as a concept for the NASA\\/DOE Joint Dark Energy Mission. Destiny is a 1.65m space telescope, featuring a near-infrared (0.85-1.7m) survey camera\\/spectrometer with a large flat-field Field Of View (FOV). Destiny will probe the properties of dark energy by obtaining a Hubble diagram based on Type Ia supernovae (SN)

Bert A. Pasquale; Robert A. Woodruff; Tod R. Lauer; Dominic J. Benford

2007-01-01

241

Does Dark Energy Signal a Wrong Physics?

NASA Astrophysics Data System (ADS)

The mysterious "dark energy'' needed to explain the current observations poses a serious confrontation between fundamental physics and cosmology. It is shown, in this paper, that the present crisis seems to be due to an inconsistency in the formulations of the perfect fluid which is used to model the matter content in the universe, including the dark energy. It should be noted that this aspect of the theories of gravitation has remained untested.

Vishwakarma, R. G.

2009-12-01

242

High-resolution temporal constraints on the dynamics of dark energy

We use the recent Type Ia supernova, cosmic microwave background and large-scale structure data to shed light on the temporal evolution of the dark energy equation of state w(z) out to redshift one. We constrain the most flexible parametrization of dark energy to date, and include the dark energy perturbations consistently throughout. Interpreting our results via the principal component analysis, we find no significant evidence for dynamical dark energy: the cosmological constant model is consistent with data everywhere between redshift zero and one at 95% C.L.

Zhao Gongbo [Department of Physics, Simon Fraser University, Burnaby, British Columbia, V5A 1S6 (Canada); Institute of High Energy Physics, Chinese Academy of Science, P.O. Box 918-4, Beijing 100049 (China); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan, 48109 (United States); Zhang Xinmin [Institute of High Energy Physics, Chinese Academy of Science, P.O. Box 918-4, Beijing 100049 (China)

2008-06-15

243

Evolution of perturbations in distinct classes of canonical scalar field models of dark energy

Dark energy must cluster in order to be consistent with the equivalence principle. The background evolution can be effectively modeled by either a scalar field or by a barotropic fluid. The fluid model can be used to emulate perturbations in a scalar field model of dark energy, though this model breaks down at large scales. In this paper we study evolution of dark energy perturbations in canonical scalar field models: the classes of thawing and freezing models. The dark energy equation of state evolves differently in these classes. In freezing models, the equation of state deviates from that of a cosmological constant at early times. For thawing models, the dark energy equation of state remains near that of the cosmological constant at early times and begins to deviate from it only at late times. Since the dark energy equation of state evolves differently in these classes, the dark energy perturbations too evolve differently. In freezing models, since the equation of state deviates from that of a cosmological constant at early times, there is a significant difference in evolution of matter perturbations from those in the cosmological constant model. In comparison, matter perturbations in thawing models differ from the cosmological constant only at late times. This difference provides an additional handle to distinguish between these classes of models and this difference should manifest itself in the integrated Sachs-Wolfe effect.

Jassal, H. K. [Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019 (India)

2010-04-15

244

Report of the Dark Energy Task Force

Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible.

Albrecht, Andreas; /UC, Davis; Bernstein, Gary; /Pennsylvania U.; Cahn, Robert; /LBL, Berkeley; Freedman, Wendy L.; /Carnegie Inst. Observ.; Hewitt, Jacqueline; /MIT; Hu, Wayne; /KICP, Chicago; Huth, John; /Harvard U.; Kamionkowski, Marc; /Caltech; Kolb, Edward W.; /Fermilab /Chicago U., Astron. Astrophys. Ctr.; Knox, Lloyd; /UC, Davis; Mather, John C.; /NASA, Goddard /Princeton U.

2006-09-01

245

Models where the dark matter component of the Universe interacts with the dark energy field have been proposed as a solution to the cosmic coincidence problem, since in the attractor regime both dark energy and dark matter scale in the same way. In these models the mass of the cold dark matter particles is a function of the dark energy

Luca Amendola; Gabriela Camargo Campos; Rogerio Rosenfeld

2007-01-01

246

Clarifying spherical collapse in coupled dark energy cosmologies

The spherical collapse model is often used to follow the evolution of overdensities into the nonlinear regime. We describe the correct approach to be used in coupled dark energy cosmologies, where a fifth force, different from gravity and mediated by the dark energy scalar field, influences the collapse. We reformulate the spherical collapse description by deriving it directly from the set of nonlinear hydrodynamical Navier-Stokes equations. By comparing with the corresponding relativistic equations, we show how the fifth force should be taken into account within the spherical collapse picture and clarify the problems arising when an inhomogeneous scalar field is considered within a spherical collapse picture. We then apply our method to the case of coupled quintessence, where the fifth force acts among cold dark matter particles, and to growing neutrino quintessence, where the fifth force acts between neutrinos. Furthermore, we review this method within standard cosmologies and apply our analysis to minimally coupled quintessence. We also check past results for early dark energy parametrizations.

Wintergerst, Nico [Arnold-Sommerfeld-Center, Ludwig-Maximilians-Universitaet, Theresienstrasse 37, D-80333 Muenchen (Germany); Institut fuer Theoretische Physik, Universitaet Heidelberg, Philosophenweg 16, D-69120 Heidelberg (Germany); Pettorino, Valeria [SISSA, Via Bonomea 265, 34136 Trieste (Italy); Institut fuer Theoretische Physik, Universitaet Heidelberg, Philosophenweg 16, D-69120 Heidelberg (Germany)

2010-11-15

247

Semi-analytic galaxy formation in early dark energy cosmologies

NASA Astrophysics Data System (ADS)

We study the impact of early dark energy (EDE) cosmologies on galaxy properties by coupling high-resolution numerical simulations with semi-analytic modelling (SAM) of galaxy formation and evolution. EDE models are characterized by a non-vanishing high-redshift contribution of dark energy, producing an earlier growth of structures and a modification of large-scale structure evolution. They can be viewed as typical representatives of non-standard dark energy models in which only the expansion history is modified, and hence the impact on galaxy formation is indirect. We show that in EDE cosmologies the predicted space density of galaxies is enhanced at all scales with respect to the standard ? cold dark matter (?CDM ) scenario, and the corresponding cosmic star formation history and stellar mass density are increased at high redshift. We compare these results with a set of theoretical predictions obtained with alternative SAMs applied to our reference ?CDM simulation, yielding a rough measure of the systematic uncertainty of the models. We find that the modifications in galaxy properties induced by EDE cosmologies are of the same order of magnitude as intra-SAM variations for a standard ?CDM realization (unless rather extreme EDE models are considered), suggesting that it is difficult to use such predictions alone to disentangle between different cosmological scenarios. However, when independent information on the underlying properties of host dark matter haloes is included, the SAM predictions on galaxy bias may provide important clues to the expansion history and the equation-of-state evolution.

Fontanot, Fabio; Springel, Volker; Angulo, Raul E.; Henriques, Bruno

2012-11-01

248

Impact of the matter density uncertainty on the dark energy reconstruction

In this paper we study the impact of the fractional matter density uncertainty in the reconstruction of the equation of state of dark energy. We consider both standard reconstruction methods, based on the dynamical effect that dark energy has on the expansion of the Universe, as well as nonstandard methods, in which the evolution of the dark energy equation of state with redshift is inferred through the variation of fundamental couplings such as the fine-structure constant, {alpha}, or the proton-to-electron mass ratio, {mu}. We show that the negative impact of the matter density uncertainty in the dark energy reconstruction using varying couplings may be very small compared to standard reconstruction methods. We also briefly discuss other fundamental questions which need to be answered before varying couplings can be successfully used to probe the nature of the dark energy.

Avelino, P. P. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal) and Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal)

2009-04-15

249

Can strong gravitational lensing constrain dark energy?

We discuss the ratio of the angular diameter distances from the source to the lens, D{sub ds}, and to the observer at present, D{sub s}, for various dark energy models. It is well known that the difference of D{sub s}s between the models is apparent and this quantity is used for the analysis of Type Ia supernovae. However we investigate the difference between the ratio of the angular diameter distances for a cosmological constant, (D{sub ds}/D{sub s}){sup {lambda}}, and that for other dark energy models, (D{sub ds}/D{sub s}){sup other}, in this paper. It has been known that there is lens model degeneracy in using strong gravitational lensing. Thus, we investigate the model independent observable quantity, Einstein radius ({theta}{sub E}), which is proportional to both D{sub ds}/D{sub s} and velocity dispersion squared, {sigma}{sub v}{sup 2}. D{sub ds}/D{sub s} values depend on the parameters of each dark energy model individually. However, (D{sub ds}/D{sub s}){sup {lambda}}-(D{sub ds}/D{sub s}){sup other} for the various dark energy models, is well within the error of {sigma}{sub v} for most of the parameter spaces of the dark energy models. Thus, a single strong gravitational lensing by use of the Einstein radius may not be a proper method to investigate the property of dark energy. However, better understanding to the mass profile of clusters in the future or other methods related to arc statistics rather than the distances may be used for constraints on dark energy.

Lee, Seokcheon [Institute of Physics, Academia Sinica, Taipei, Taiwan 11529 (China); Ng, K.-W. [Institute of Physics, Academia Sinica, Taipei, Taiwan 11529 (China); Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan 11529 (China)

2007-08-15

250

Dark Energy and the Accelerating Universe

The discovery ten years ago that the expansion of the Universe is\\u000aaccelerating put in place the last major building block of the present\\u000acosmological model, in which the Universe is composed of 4% baryons, 20% dark\\u000amatter, and 76% dark energy. At the same time, it posed one of the most\\u000aprofound mysteries in all of science, with deep

Joshua A. Frieman; Michael S. Turner; Dragan Huterer

2008-01-01

251

Transient and late time attractor tachyon dark energy: Can we distinguish it from quintessence?

The string inspired tachyon field can serve as a candidate of dark energy. Its equation of state parameter w varies from 0 to -1. In the case of tachyon field potential V({phi}){yields}0 slower (faster) than 1/{phi}{sup 2} at infinity, dark energy (dark matter) is a late time attractor. We investigate the tachyon dark energy models under the assumption that w is close to -1. We find that all the models exhibit unique behavior around the present epoch which is exactly the same as that of the thawing quintessence.

Ali, Amna; Sami, M.; Sen, A. A. [Centre of Theoretical Physics, Jamia Millia Islamia, New Delhi-110025 (India)

2009-06-15

252

Cosmological Bardeen-Cooper-Schrieffer condensate as dark energy

We argue that the occurrence of late-time acceleration can conveniently be described by first-order general relativity covariantly coupled to fermions. Dark energy arises as a Bardeen-Cooper-Schrieffer condensate of fermions which forms in the early universe. At late times, the gap and chemical potential evolve to have an equation of state with effective negative pressure, thus naturally leading to acceleration.

Alexander, Stephon; Biswas, Tirthabir; Calcagni, Gianluca [Department of Physics and Astronomy, Haverford College, Haverford, Pennsylvania 19041 (United States); Institute for Gravitation and the Cosmos, Department of Physics, Pennsylvania State University, 104 Davey Lab, University Park, Pennsylvania 16802 (United States)

2010-02-15

253

Topics in microlensing and dark energy

NASA Astrophysics Data System (ADS)

In this dissertation we describe two separate research projects. The first project involves the utilization and development of reddening models, color magnitude diagrams (CMDs), and microlensing population models of the Large Magellanic Cloud (LMC) to constrain the locations of micro-lensing source stars and micro-lensing objects in the Large Magellanic Cloud and the Milky Way (MW) halo using data of 13 microlensing source stars obtained by the MACHO (massive compact halo objects) collaboration with the Hubble Space Telescope. This analysis suggests that the source stars are located in the LMC disk and the lenses are located in the MW halo. For the second project, we report on the results of a Markov Chain Monte Carlo (MCMC) analysis of an inverse power law (IPL) quintessence model using the Dark Energy Task Force (DETF) simulated data models as a representation of future dark energy experiments. Simulated data sets were generated for a Lambda cold dark matter (L CDM ) background cosmology as well as a case where the dark energy is provided by a specific IPL fiducial model. The results are presented in the form of error contours generated by these two background cosmologies which are then used to consider the effects of future dark energy projects on IPL scalar field models and are able to demonstrate the power of DETF Stage 4 data sets in the context of the IPL model. We find that the respective increase in constraining power with higher quality data sets produced by our analysis gives results that are broadly consistent with the DETF results for the w 0 - w a parameterization of dark energy. Finally, using our simulated data sets constructed around a fiducial IPL model, we find that for a universe containing dark energy described by such a scalar field, a cosmological constant can be excluded by Stage 4 data at the 3s level.

Yashar, Mark

254

Distinguishing modified gravity from dark energy

The acceleration of the Universe can be explained either through dark energy or through the modification of gravity on large scales. In this paper we investigate modified gravity models and compare their observable predictions with dark energy models. Modifications of general relativity are expected to be scale independent on superhorizon scales and scale dependent on subhorizon scales. For scale-independent modifications, utilizing the conservation of the curvature scalar and a parametrized post-Newtonian formulation of cosmological perturbations, we derive results for large-scale structure growth, weak gravitational lensing, and cosmic microwave background anisotropy. For scale-dependent modifications, inspired by recent f(R) theories we introduce a parametrization for the gravitational coupling G and the post-Newtonian parameter {gamma}. These parametrizations provide a convenient formalism for testing general relativity. However, we find that if dark energy is generalized to include both entropy and shear stress perturbations, and the dynamics of dark energy is unknown a priori, then modified gravity cannot in general be distinguished from dark energy using cosmological linear perturbations.

Bertschinger, Edmund; Zukin, Phillip [Department of Physics, MIT, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States)

2008-07-15

255

Testable dark energy predictions from current data

Given a class of dark energy models, constraints from one set of cosmic acceleration observables make predictions for other observables. Here we present the allowed ranges for the expansion rate H(z), distances D(z), and the linear growth function G(z) (as well as other, derived growth observables) from the current combination of cosmological measurements of supernovae, the cosmic microwave background, baryon acoustic oscillations, and the Hubble constant. With a cosmological constant as the dark energy and assuming near-minimal neutrino masses, the growth function is already predicted to better than 2% precision at any redshift, with or without spatial curvature. Direct measurements of growth that match this precision offer the opportunity to stringently test and potentially rule out a cosmological constant. While predictions in the broader class of quintessence models are weaker, it is remarkable that they are typically only a factor of 2-3 less precise than forecasted predictions for future space-based supernovae and Planck CMB measurements. In particular, measurements of growth at any redshift, or the Hubble constant H{sub 0}, that exceed {Lambda}CDM predictions by substantially more than 2% would rule out not only a cosmological constant but also the whole quintessence class, with or without curvature and early dark energy. Barring additional systematic errors hiding in the data, such a discovery would require more exotic explanations of cosmic acceleration such as phantom dark energy, dark energy clustering, or modifications of gravity.

Mortonson, Michael J. [Center for Cosmology and AstroParticle Physics, Ohio State University, Columbus, Ohio 43210 (United States); Department of Physics, University of Chicago, Chicago, Illinois 60637 (United States); Kavli Institute for Cosmological Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States); Hu, Wayne [Kavli Institute for Cosmological Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church St, Ann Arbor, Michigan 48109-1040 (United States)

2010-03-15

256

Halo concentrations and weak-lensing number counts in dark energy cosmologies

We study the effects of a dark energy component with equation of state p=wrho with constant w>=-1 on the formation of Cold Dark Matter (CDM) haloes. We find two main effects: first, haloes form earlier as w increases, and second, the amplitude of the dark-matter power spectrum gets reduced in order to remain compatible with the large scale Cosmic Microwave

Matthias Bartelmann; Francesca Perrotta; Carlo Baccigalupi

2002-01-01

257

Probing Beyond Einstein: The Joint Dark Energy Mission

NASA Astrophysics Data System (ADS)

The discovery of the acceleration of the expansion of the universe in 1998 represents perhaps the most profound challenge to our current understanding of physics and astronomy. The observation of acceleration requires either that more than 70% of the contents of the universe be an exotic form of energy (the so-called ``dark energy'') or that there is a flaw in general relativity. The failure of present theories to convincingly explain the effect leads many experts to expect that elucidating the cause of the expansion will lead to fundamental breakthroughs that impact cosmology, astrophysics, and particle physics. The NASA/DOE Joint Dark Energy Mission (JDEM) will be the first of the Beyond Einstein probes. This mission will determine whether the acceleration of the expansion of the universe has varied over time in an attempt to determine the equation of state for dark energy or whether predictions from general relativity fail to adequately explain the acceleration. This talk will present the rationale for a space-based study of dark energy and the techniques likely to be used as part of JDEM.

Gladney, Larry

2009-03-01

258

Anisotropic perturbations due to dark energy

A variety of observational tests seem to suggest that the Universe is anisotropic. This is incompatible with the standard dogma based on adiabatic, rotationally invariant perturbations. We point out that this is a consequence of the standard decomposition of the stress-energy tensor for the cosmological fluids, and that rotational invariance need not be assumed, if there is elastic rigidity in the dark energy. The dark energy required to achieve this might be provided by point symmetric domain wall network with P/{rho}=-2/3, although the concept is more general. We illustrate this with reference to a model with cubic symmetry and discuss various aspects of the model.

Battye, Richard A.; Moss, Adam [Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL (United Kingdom)

2006-08-15

259

Dark Energy and The Dark Matter Relic Abundance

Two mechanisms by which the quintessence scalar could enhance the relic abundance of dark matter particles are discussed. These effects can have an impact on supersymmetric candidates for dark matter.

Rosati, Francesca [Universita di Padova, Dipartimento di Fisica 'Galileo Galilei', via Marzolo 8, 35131 Padova (Italy); INFN - Sezione di Padova, via Marzolo 8, 35131 Padova (Italy)

2004-11-17

260

Dynamics of dark energy models and centre manifolds

NASA Astrophysics Data System (ADS)

We analyse dark energy models where self-interacting three-forms or phantom fields drive the accelerated expansion of the Universe. The dynamics of such models is often studied by rewriting the cosmological field equations in the form of a system of autonomous differential equations, or simply a dynamical system. Properties of these systems are usually studied via linear stability theory. In situations where this method fails, for instance due to the presence of zero eigenvalues in the Jacobian, centre manifold theory can be applied. We present a concise introduction and show explicitly how to use this theory in two concrete examples.

Böhmer, Christian G.; Chan, Nyein; Lazkoz, Ruth

2012-07-01

261

Dark energy from the gas of wormholes

The observed dark energy phenomenon is attributed to the presence of the zero-point fluctuations of matter fields. We show that due to the presence of the gas of virtual wormholes the zero-point energy is finite and forms the finite (of the Planckian order) value. The observed value of the cosmological constant is somewhat reduced due to the two effects. First

A. A. Kirillov; E. P. Savelova

2010-01-01

262

Dark Energy: Mystery of the Millennium

Nearly seventy per cent of the energy density in the universe is unclustered and exerts negative pressure. This conclusion - now supported by numerous observations - poses the greatest challenge for theoretical physics today. I discuss this issue with special emphasis on the cosmological constant as the possible choice for the dark energy. Several curious features of a universe with

T. Padmanabhan

2006-01-01

263

Dark energy: recent observations and future prospects.

Dark energy presents us with a challenging puzzle: understanding the new physics seen in the acceleration of the expansion of the Universe. Measurements using type-Ia supernovae (SNe) first detected this acceleration, and this approach remains the most direct route to studying the details of the Universe's expansion history that can teach us more about the nature of the dark energy. Such measurements are, however, extremely demanding in both precision and accuracy, since the different dark-energy models predict very small differences in the expansion history. While several cosmological probes may reach the required statistical uncertainties, the key measurement limit will be the systematic uncertainty. The supernova-measurement approach has the advantage of well-studied systematic uncertainties, allowing a next-generation experiment to be pursued. We briefly review the progress to date and examine the promise of future surveys with large numbers of SNe and well-bounded systematics. PMID:14667312

Perlmutter, Saul

2003-11-15

264

Dark energy: recent observations and future prospects

NASA Astrophysics Data System (ADS)

Dark energy presents us with a challenging puzzle: understanding the new physics seen in the acceleration of the expansion of the Universe. Measurements using type-Ia supernovae (SNe) first detected this acceleration, and this approach remains the most direct route to studying the details of the Universe's expansion history that can teach us more about the nature of the dark energy. Such measurements are, however, extremely demanding in both precision and accuracy, since the different dark-energy models predict very small differences in the expansion history. While several cosmological probes may reach the required statistical uncertainties, the key measurement limit will be the systematic uncertainty. The supernova-measurement approach has the advantage of well-studied systematic uncertainties, allowing a next-generation experiment to be pursued. We briefly review the progress to date and examine the promise of future surveys with large numbers of SNe and well-bounded systematics.

Perlmutter, Saul

2003-11-01

265

Observables and unobservables in dark energy cosmologies

NASA Astrophysics Data System (ADS)

The aim of this paper is to answer the following two questions: (1) Given cosmological observations of the expansion history and linear perturbations in a range of redshifts and scales as precise as is required, which of the properties of dark energy could actually be reconstructed without imposing any parameterization? (2) Are these observables sufficient to rule out not just a particular dark energy model, but the entire general class of viable models comprising a single scalar field? This paper bears both good and bad news. On one hand, we find that the goal of reconstructing dark energy models is fundamentally limited by the unobservability of the present values of the matter density ?m0, the perturbation normalization ?8 as well as the present matter power spectrum. On the other, we find that, under certain conditions, cosmological observations can nonetheless rule out the entire class of the most general single scalar-field models, i.e., those based on the Horndeski Lagrangian.

Amendola, Luca; Kunz, Martin; Motta, Mariele; Saltas, Ippocratis D.; Sawicki, Ignacy

2013-01-01

266

Constraints on the redshift dependence of the dark energy potential

We develop a formalism to characterize the shape and the redshift evolution of the dark energy potential. Our formalism makes use of quantities similar to the horizon-flow parameters in inflation and is general enough that can deal with multiscalar quintessence scenarios, exotic matter components, and higher-order curvature corrections to General Relativity. We show how the shape of the dark energy potential can be recovered nonparametrically using this formalism and we present approximations analogous to the ones relevant to slow-roll inflation. Since presently available data do not allow a nonparametric and exact reconstruction of the potential, we consider a general parametric description. This reconstruction can also be used in other approaches followed in the literature (e.g., the reconstruction of the redshift evolution of the dark energy equation of state w(z)). Using observations of passively evolving galaxies and supernova data we derive constraints on the dark energy potential shape in the redshift range 0.1

Simon, Joan; Verde, Licia; Jimenez, Raul [Dept. of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104 (United States)

2005-06-15

267

Evolution of dark energy perturbations in scalar-tensor cosmologies

NASA Astrophysics Data System (ADS)

We solve analytically and numerically the generalized Einstein equations in scalar-tensor cosmologies to obtain the evolution of dark energy and matter linear perturbations. We compare our results with the corresponding results for minimally coupled quintessence perturbations. We find that scalar-tensor dark energy density perturbations are amplified by a factor of about 104 compared to minimally coupled quintessence perturbations on scales less than about 1000h-1Mpc (sub-Hubble scales). On these scales dark energy perturbations constitute a fraction of about 10% compared to matter density perturbations. Scalar-tensor dark energy density perturbations are anticorrelated with matter linear perturbations on sub-Hubble scales. This anticorrelation of matter with negative pressure perturbations induces a mild amplification of matter perturbations by about 10% on sub-Hubble scales. The evolution of scalar field perturbations on sub-Hubble scales is scale independent and therefore corresponds to a vanishing effective speed of sound (cs?=0). We briefly discuss the observational implications of our results, which may include predictions for galaxy and cluster halo profiles that are modified compared to ?CDM. The observed properties of these profiles are known to be in some tension with the predictions of ?CDM.

Bueno Sanchez, J. C.; Perivolaropoulos, L.

2010-05-01

268

DESTINY: the dark energy space telescope

NASA Astrophysics Data System (ADS)

We have proposed the development of a low-cost space telescope, Destiny, as a concept for the NASA/DOE Joint Dark Energy Mission. Destiny is a 1.65m space telescope, featuring a near-infrared (0.85-1.7m) survey camera/spectrometer with a large flat-field Field Of View (FOV). Destiny will probe the properties of dark energy by obtaining a Hubble diagram based on Type Ia supernovae (SN) and a large-scale mass power spectrum derived from weak lensing distortions of field galaxies as a function of redshift.

Pasquale, Bert A.; Woodruff, Robert A.; Lauer, Tod R.; Benford, Dominic J.

2007-09-01

269

Dark Energy: A Crisis for Fundamental Physics

Astrophysical observations provide robust evidence that our current picture of fundamental physics is incomplete. The discovery in 1998 that the expansion of the Universe is accelerating (apparently due to gravitational repulsion between regions of empty space!) presents us with a profound challenge, at the interface between gravity and quantum mechanics. This "Dark Energy" problem is arguably the most pressing open question in modern fundamental physics. The first talk will describe why the Dark Energy problem constitutes a crisis, with wide-reaching ramifications. One consequence is that we should probe our understanding of gravity at all accessible scales, and the second talk will present experiments and observations that are exploring this issue.

Stubbs, Christopher (Harvard)

2010-04-12

270

Dark Energy: A Crisis for Fundamental Physics

Astrophysical observations provide robust evidence that our current picture of fundamental physics is incomplete. The discovery in 1998 that the expansion of the Universe is accelerating (apparently due to gravitational repulsion between regions of empty space!) presents us with a profound challenge, at the interface between gravity and quantum mechanics. This "Dark Energy" problem is arguably the most pressing open question in modern fundamental physics. The first talk will describe why the Dark Energy problem constitutes a crisis, with wide-reaching ramifications. One consequence is that we should probe our understanding of gravity at all accessible scales, and the second talk will present experiments and observations that are exploring this issue.

271

Fate of the phantom dark energy universe in semiclassical gravity

NASA Astrophysics Data System (ADS)

The fate of the phantom dark energy universe in semiclassical gravity is investigated. Quantum corrections coming from massless fields conformally coupled with gravity are considered, to see if they can lead to avoidance of the big rip singularity, which shows up in a flat Friedmann-Robertson-Walker universe, filled with phantom dark energy and modeled by an equation of state of the form p=?? with ?<-1. The dynamics of the model are discussed for all values of the two parameters, named ?>0 and ?<0, which come from quantum corrections. It is concluded that, when -1<(?)/(3?)<0, almost all solutions develop future singularities (the corresponding scale factor and energy density go down to zero in finite time). However, when -1>(?)/(3?), almost all solutions describe a universe bouncing infinitely many times (an oscillating universe).

Haro, Jaume; Amoros, Jaume; Elizalde, Emilio

2011-06-01

272

Unification of dark matter and dark energy: the inhomogeneous Chaplygin gas

We extend the world model of Kamenshchik et al. to large perturbations by formulating a Zeldovich-like approximation. We sketch how this model unifies dark matter with dark energy in a geometric setting reminiscent of M-theory.

Neven Bili?

2002-01-01

273

Advanced Dark Energy Physics Telescope (ADEPT)

In 2006, we proposed to NASA a detailed concept study of ADEPT (the Advanced Dark Energy Physics Telescope), a potential space mission to reliably measure the time-evolution of dark energy by conducting the largest effective volume survey of the universe ever done. A peer-review panel of scientific, management, and technical experts reported back the highest possible 'excellent' rating for ADEPT. We have since made substantial advances in the scientific and technical maturity of the mission design. With this Department of Energy (DOE) award we were granted supplemental funding to support specific extended research items that were not included in the NASA proposal, many of which were intended to broadly advance future dark energy research, as laid out by the Dark Energy Task Force (DETF). The proposed work had three targets: (1) the adaptation of large-format infrared arrays to a 2 micron cut-off; (2) analytical research to improve the understanding of the dark energy figure-of- merit; and (3) extended studies of baryon acoustic oscillation systematic uncertainties. Since the actual award was only for {approx}10% of the proposed amount item (1) was dropped and item (2) work was severely restricted, consistent with the referee reviews of the proposal, although there was considerable contradictions between reviewer comments and several comments that displayed a lack of familiarity with the research. None the less, item (3) was the focus of the work. To characterize the nature of the dark energy, ADEPT is designed to observe baryon acoustic oscillations (BAO) in a large galaxy redshift survey and to obtain substantial numbers of high-redshift Type Ia supernovae (SNe Ia). The 2003 Wilkinson Microwave Anisotropy Probe (WMAP) made a precise determination of the BAO 'standard ruler' scale, as it was imprinted on the cosmic microwave background (CMB) at z {approx} 1090. The standard ruler was also imprinted on the pattern of galaxies, and was first detected in 2005 in Sloan Digital Sky Survey (SDSS) data. A measurement of the BAO standard ruler as a function of time (or redshift) would provide powerful and reliable observational data to shed light on dark energy. In particular, the BAO data provide the angular diameter distance to each redshift, and directly give the expansion rate, H(z), at each redshift. The SNe measurements provide luminosity distances. A space mission is required to obtain the three-dimensional position of enormous numbers of galaxies at high redshift. As recognized by the Dark Energy Task Force, BAO systematic errors are naturally low. The following are the key findings: (1) The BAO method is robust. (2) Separation of the spectral and imaging detection focal planes vastly improves spectral identifications. (3) Prisms instead of grisms provide higher throughput and cleaner spectra. Prisms are clearly superior. (4) Lower prism dispersions improve signal-to-noise but high prism dispersions improve systematic. To ensure that the experiment is not systematic limited, a high dispersion should be used. (5) Counter-dispersion of the spectra reduces systematic errors on the redshift determination and assists in the reduction of confusion. (6) Small rolls are very effective for the reduction of confusion. (7) Interlopers can be recognized by a variety of methods, which combine to produce a sufficiently 'clean' survey data set so as not to limit the dark energy results. (8) A space mission can measure the BAO signature to the cosmic variance limit, limited only by statistics and not by systematic. (9) Density field reconstruction allows for significant BAO accuracy improvements, well beyond that assumed by the Dark Energy Task Force. (10) The BAO method is statistically powerful. It is more powerful than previously estimated, and far more powerful than high redshift Type 1a supernovae, for which the ultimate distance accuracy is limited by flux calibration accuracy. (11) The BAO technique is far simpler than the weak lensing technique and likely to produce more robust dark energy solutions.

Charles L. Bennett

2009-03-26

274

Dark energy perturbations and cosmic coincidence

While there is plentiful evidence in all fronts of experimental cosmology for the existence of a nonvanishing dark energy (DE) density {rho}{sub D} in the Universe, we are still far away from having a fundamental understanding of its ultimate nature and of its current value, not even of the puzzling fact that {rho}{sub D} is so close to the matter energy density {rho}{sub M} at the present time (i.e. the so-called 'cosmic coincidence' problem). The resolution of some of these cosmic conundrums suggests that the DE must have some (mild) dynamical behavior at the present time. In this paper, we examine some general properties of the simultaneous set of matter and DE perturbations ({delta}{rho}{sub M},{delta}{rho}{sub D}) for a multicomponent DE fluid. Next we put these properties to the test within the context of a nontrivial model of dynamical DE (the {lambda}XCDM model) which has been previously studied in the literature. By requiring that the coupled system of perturbation equations for {delta}{rho}{sub M} and {delta}{rho}{sub D} has a smooth solution throughout the entire cosmological evolution, that the matter power spectrum is consistent with the data on structure formation, and that the 'coincidence ratio' r={rho}{sub D}/{rho}{sub M} stays bounded and not unnaturally high, we are able to determine a well-defined region of the parameter space where the model can solve the cosmic coincidence problem in full compatibility with all known cosmological data.

Grande, Javier; Pelinson, Ana; Sola, Joan [High Energy Physics Group, Departament d'ECM, and Institut de Ciencies del Cosmos, Universitat de Barcelona, Avinguda Diagonal 647, E-08028 Barcelona, Catalonia (Spain)

2009-02-15

275

Dark energy from mass varying neutrinos

NASA Astrophysics Data System (ADS)

We show that mass varying neutrinos (MaVaNs) can behave as a negative pressure fluid which could be the origin of the cosmic acceleration. We derive a model independent relation between the neutrino mass and the equation of state parameter of the neutrino dark energy, which is applicable for general theories of mass varying particles. The neutrino mass depends on the local neutrino density and the observed neutrino mass can exceed the cosmological bound on a constant neutrino mass. We discuss microscopic realizations of the MaVaN acceleration scenario, which involve a sterile neutrino. We consider naturalness constraints for mass varying particles, and find that both eV cut-offs and eV mass particles are needed to avoid fine-tuning. In microscopic realizations of this scenario with a sterile neutrino, these considerations give the sterile neutrino a maximum mass today of order an eV, which could be detectable at MiniBooNE. Because the sterile neutrino was much heavier at earlier times, constraints from big bang nucleosynthesis on additional states are not problematic. We consider regions of high neutrino density and find that the most likely place today to find neutrino masses which are significantly different from the neutrino masses in our solar system is in a supernova. The possibility of different neutrino mass in different regions of the galaxy and the local group could be significant for Z-burst models of ultra-high energy cosmic rays. We also consider the cosmology of and the constraints on the 'acceleron', the scalar field which is responsible for the varying neutrino mass, and briefly discuss neutrino density dependent variations in other constants, such as the fine structure constant.

Fardon, Rob; Nelson, Ann E.; Weiner, Neal

2004-10-01

276

Zero cosmological constant and nonzero dark energy from the holographic principle

NASA Astrophysics Data System (ADS)

The first law of thermodynamics and the holographic principle applied to an arbitrary large cosmic causal horizon are shown to naturally demand a zero cosmological constant and a non-zero dynamical dark energy in the form of the holographic dark energy. A semiclassical analysis shows that the holographic dark energy has a parameter d = 1 and an equation of state comparable to current observational data if the entropy of the horizon saturates the Bekenstein-Hawking bound. This result indicates that quantum field theory should be modified on a large scale to explain the dark energy. The relations among the dark energy, the quantum vacuum energy and the entropic gravity are also discussed.

Lee, Jae-Weon

2013-09-01

277

A geometric measure of dark energy with pairs of galaxies.

Observations indicate that the expansion of the Universe is accelerating, which is attributed to a ‘dark energy’ component that opposes gravity. There is a purely geometric test of the expansion of the Universe (the Alcock–Paczynski test), which would provide an independent way of investigating the abundance (?(X)) and equation of state (W(X)) of dark energy. It is based on an analysis of the geometrical distortions expected from comparing the real-space and redshift-space shape of distant cosmic structures, but it has proved difficult to implement. Here we report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows us to determine that the Universe is flat. By alternately fixing its spatial geometry at ?(k)?0 and the dark energy equation-of-state parameter at W(X)?-1, and using the results of baryon acoustic oscillations, we can establish at the 68.3% confidence level that and -0.85>W(X)>-1.12 and 0.60

Marinoni, Christian; Buzzi, Adeline

2010-11-25

278

Comparing Cosmology Parameters: Dark Energy vs Cosmological Constant

NASA Astrophysics Data System (ADS)

The Cylindrical Radio Telescope (CRT) is designed to use the redshifted 21-cm line from HI to create a 3-D intensity map of the large scale structure of the universe. The goal is to measure the angular size of baryon acoustic oscillations (BAO) as a function of redshift. Using the angular diameter distance and redshifts the CRT can constrain the equation of state of the dark energy. In particular it can determine whether or not the dark energy is consistent with a cosmological constant. We plotted the distance-redshift relation for a variety of physically-motivated equations of state and compared them to a ?CDM (concordance) model. The graphs indicate the accuracy with which the distances must be measured to constrain dark energy models. I thank the University of Wisconsin for allowing my participation in their astrophysics REU program and to the National Science Foundation and the U.S. Department of Defense through ASSURE for the funding through NSF Award AST-1004881.

Lewis, Jeramy; Timbie, P.

2012-01-01

279

It was found that the model with interaction between cold dark matter (CDM) and dark energy (DE) proportional to the energy density of CDM $\\\\rho_m$ and constant equation of state of DE $w_d$ suffered from instabilities of the density perturbations on the supper-Hubble scales. Here we suggest a new covariant model for the energy-momentum transfer between CDM and DE. Then

Cheng-Yi Sun; Yu Song; Rui-Hong Yue

2011-01-01

280

Quantized relativistic mass energy equation

By introducing a new unit of energy, I have been able to quan tize the relativistic mass energy equation using an integer mass num ber and an integer velocity number that are discrete, additive components of the total system energy. Derived from a unique foundational model that uses a single inertial frame of reference, the quantized equations produce identical results

B D Nelson

281

The Dark Energy Survey Data Management System

The Dark Energy Survey (DES) is a project with the goal of building, installing and exploiting a new 74 CCD-camera at the Blanco telescope, in order to study the nature of cosmic acceleration. It will cover 5000 square degrees of the southern hemisphere sky and will record the positions and shapes of 300 million galaxies up to redshift 1.4. The

I. Sevilla; R. Armstrong; E. Bertin; A. Carlson; G. Daues; S. Desai; M. Gower; R. Gruendl; W. Hanlon; M. Jarvis; R. Kessler; N. Kuropatkin; H. Lin; J. Marriner; J. Mohr; D. Petravick; E. Sheldon; M. E. C. Swanson; T. Tomashek; D. Tucker; Y. Yang; B. Yanny

2011-01-01

282

Dark Energy and the Hierarchy Problem.

National Technical Information Service (NTIS)

The well-known hierarchy between the Planck scale (approx. 10(sup 19)GeV) and the TeV scale, namely a ratio of (approx) 10(sup 16) between the two, is coincidentally repeated in a inverted order between the TeV scale and the dark energy scale at (approx) ...

P. Chien

2006-01-01

283

Dark Energy Survey Data Management System.

National Technical Information Service (NTIS)

The Dark Energy Survey (DES) collaboration will study cosmic acceleration with a 5000 deg2 griZY survey in the southern sky over 525 nights from 2011-2016. The DES data management (DESDM) system will be used to process and archive these data and the resul...

C. Beldica G. E. Daues J. Mohr J. A. Darnell M. Jarvis

2008-01-01

284

An introduction to the dark energy problem

NASA Astrophysics Data System (ADS)

In this work we review briefly the origin and history of the cosmological constant and its recent reincarnation in the form of the dark energy component of the universe. We also comment on the fundamental problems associated to its existence and magnitude which require an urgent solution for the sake of the internal consistency of theoretical physics.

Dobado, Antonio; Maroto, Antonio L.

2009-04-01

285

Higher signal harmonics, LISA's angular resolution, and dark energy

It is generally believed that the angular resolution of the Laser Interferometer Space Antenna (LISA) for binary supermassive black holes (SMBH) will not be good enough to identify the host galaxy or galaxy cluster. This conclusion, based on using only the dominant harmonic of the binary SMBH signal, changes substantially when higher signal harmonics are included in assessing the parameter estimation problem. We show that in a subset of the source parameter space the angular resolution increases by more than a factor of 10, thereby making it possible for LISA to identify the host galaxy/galaxy cluster. Thus, LISA's observation of certain binary SMBH coalescence events could constrain the dark energy equation of state to within a few percent, comparable to the level expected from other dark energy missions.

Arun, K. G. [LAL, Universite Paris-Sud, IN2P3/CNRS, Orsay (France); GReCO, Institut d'Astrophysique de Paris-C.N.R.S., Paris (France); Iyer, Bala R. [Raman Research Institute, Bangalore, 560 080 (India); Sathyaprakash, B. S.; Broeck, Chris van den [School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff, CF24 3YB (United Kingdom); Sinha, Siddhartha [Raman Research Institute, Bangalore, 560 080 (India); Department of Physics, Indian Institute of Science, Bangalore, 560 012 (India)

2007-11-15

286

CONSTRAINING PERTURBATIVE EARLY DARK ENERGY WITH CURRENT OBSERVATIONS

In this work, we study a class of early dark energy (EDE) models, in which, unlike in standard dark energy models, a substantial amount of dark energy exists in the matter-dominated era. We self-consistently include dark energy perturbations, and constrain these models using current observations. We consider EDE models in which the dark energy equation of state is at least w{sub m} {approx_gt} -0.1 at early times, which could lead to an EDE density of up to {Omega}{sub DE}(z{sub CMB})= 0.03{Omega}{sub m}(z{sub CMB}). Our analysis shows that marginalizing over the non-DE parameters such as {Omega}{sub m}, H{sub 0}, andn{sub s} , current CMB observations alone can constrain the scale factor of transition from EDE to late-time dark energy to a{sub t} {approx_gt} 0.44 and width of transition to {Delta}{sub t} {approx_lt} 0.37. The equation of state at present is somewhat weakly constrained to w{sub 0} {approx_lt} -0.6, if we allow H{sub 0} < 60 km s{sup -1} Mpc{sup -1}. Taken together with other observations, such as SNe, Hubble Space Telescope, and Sloan Digital Sky Survey luminous red galaxies, w{sub 0} is constrained much more tightly to w{sub 0} {approx_lt} -0.9, while redshift of transition and width of transition are also tightly constrained to a{sub t} {approx_lt} 0.19 and{Delta}{sub t} {approx_lt} 0.21. The evolution of the equation of state for EDE models is thus tightly constrained to {Lambda}CDM-like behavior at low redshifts. Incorrectly assuming dark energy perturbations to be negligible leads to different constraints on the equation of state parameters-w{sub 0} {approx_lt} -0.8, a{sub t} {approx_lt} 0.33, and{Delta}{sub t} {approx_lt} 0.31, thus highlighting the necessity of self-consistently including dark energy perturbations in the analysis. If we allow the spatial curvature to be a free parameter, then the constraints are relaxed to w{sub 0} {approx_lt} -0.77, a{sub t} {approx_lt} 0.35, and{Delta}{sub t} {approx_lt} 0.35 with -0.014 < {Omega}{sub {kappa}} < 0.031 for CMB + other observations. For perturbed EDE models, the 2{sigma} lower limit on {sigma}{sub 8} ({sigma}{sub 8} {>=} 0.59) is much lower than that in {Lambda}CDM ({sigma}{sub 8} {>=} 0.72), thus raising the interesting possibility of discriminating EDE from {Lambda}CDM using future observations such as halo mass functions or the Sunyaev-Zeldovich power spectrum.

Alam, Ujjaini [ISR-1, ISR Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

2010-05-10

287

Cosmological observations are normally fit under the assumption that the dark sector can be decomposed into dark matter and dark energy components. However, as long as the probes remain purely gravitational, there is no unique decomposition and observations can only constrain a single dark fluid; this is known as the dark degeneracy. We use observations to directly constrain this dark fluid in a model-independent way, demonstrating, in particular, that the data cannot be fit by a dark fluid with a single constant equation of state. Parametrizing the dark fluid equation of state by a variety of polynomials in the scale factor a, we use current kinematical data to constrain the parameters. While the simplest interpretation of the dark fluid remains that it is comprised of separate dark matter and cosmological constant contributions, our results cover other model types including unified dark energy/matter scenarios.

Kunz, Martin; Liddle, Andrew R.; Parkinson, David [Astronomy Centre, University of Sussex, Brighton BN1 9QH (United Kingdom); Gao Changjun [National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012 (China)

2009-10-15

288

Cosmological evolution for dark energy models in f( T) gravity

NASA Astrophysics Data System (ADS)

In this paper, we investigate the behavior of equation of state parameter and energy density for dark energy in the framework of f( T) gravity. For this purpose, we use anisotropic LRS Bianchi type I universe model. The behavior of accelerating universe is discussed for some well-known f( T) models. It is found that the universe takes a transition between phantom and non-phantom phases for f( T) models except exponential and logarithmic models. We conclude that our results are relativity analogous to the results of FRW universe.

Sharif, M.; Azeem, Sehrish

2012-12-01

289

Holographic Dark Energy (DE) Cosmological Models with Quintessence in Bianchi Type-V Space Time

NASA Astrophysics Data System (ADS)

In this paper, author studied homogeneous and anisotropic Bianchi type-V universe filled with matter and holographic dark energy (DE) components. The exact solutions to the corresponding Einstein's field equations are obtained for exponential and power-law volumetric expansion. The holographic dark energy (DE) EoS parameter behaves like constant, i.e. ? ? =-1, which is mathematically equivalent to cosmological constant (?) for exponential expansion of the model, whereas the holographic dark energy (DE) EoS parameter behaves like quintessence for power-law expansion of the model. A correspondence between the holographic dark energy (DE) models with the quintessence dark energy (DE) is also established. Quintessence potential and dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe. The statefinder diagnostic pair {r,s} is adopted to characterize different phases of the universe.

Samanta, G. C.

2013-09-01

290

Distance measurements from supernovae and dark energy constraints

Constraints on dark energy from current observational data are sensitive to how distances are measured from Type Ia supernova (SN Ia) data. We find that flux averaging of SNe Ia can be used to test the presence of unknown systematic uncertainties, and yield more robust distance measurements from SNe Ia. We have applied this approach to the nearby+SDSS+ESSENCE+SNLS+HST set of 288 SNe Ia, and the 'Constitution' set of 397 SNe Ia. Combining the SN Ia data with cosmic microwave background anisotropy data from Wilkinson Microwave Anisotropy Probe 5 yr observations, the Sloan Digital Sky Survey baryon acoustic oscillation measurements, the data of 69 gamma-ray bursts (GRBs) , and the Hubble constant measurement from the Hubble Space Telescope project SHOES, we measure the dark energy density function X(z){identical_to}{rho}{sub X}(z)/{rho}{sub X}(0) as a free function of redshift (assumed to be a constant at z>1 or z>1.5). Without the flux averaging of SNe Ia, the combined data using the Constitution set of SNe Ia seem to indicate a deviation from a cosmological constant at {approx}95% confidence level at 0 < or apporx. z < or approx. 0.8; they are consistent with a cosmological constant at {approx}68% confidence level when SNe Ia are flux averaged. The combined data using the nearby+SDSS+ESSENCE+SNLS+HST data set of SNe Ia are consistent with a cosmological constant at 68% confidence level with or without flux averaging of SNe Ia, and give dark energy constraints that are significantly more stringent than that using the Constitution set of SNe Ia. Assuming a flat Universe, dark energy is detected at >98% confidence level for z{<=}0.75 using the combined data with 288 SNe Ia from nearby+SDSS+ESSENCE+SNLS+HST, independent of the assumptions about X(z{>=}1). We quantify dark energy constraints without assuming a flat Universe using the dark energy figure of merit for both X(z) and a dark energy equation-of-state linear in the cosmic scale factor.

Wang Yun [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W Brooks Street, Norman, Oklahoma 73019 (United States)

2009-12-15

291

Cosmic probes of the physics of dark matter and dark energy

Starting around 2014, data from the Large Synoptic Survey Telescope (LSST) will be analyzed for a wide range of phenomena. The nature of dark matter can be constrained by measuring the scales on which it clumps. The nature of dark energy can be constrained by measuring the time evolution of cosmic dark matter structures and through measurements of the distribution

Tony Tyson

2008-01-01

292

Interacting Holographic Polytropic Gas Model of Dark Energy

NASA Astrophysics Data System (ADS)

In this work, we establish a correspondence between the holographic dark energy model and polytropic gas model of dark energy in the FRW universe. This correspondence allows us to reconstruct the potential and the dynamics for the scalar field of the polytropic model according to the evolution of holographic dark energy in the FRW universe.

Taji, M.; Malekjani, M.

2013-10-01

293

Cosmological consequences of a Chaplygin gas dark energy

A combination of recent observational results has given rise to what is currently known as the dark energy problem. Although several possible candidates have been extensively discussed in the literature, the nature of this dark energy component is not well understood at present. In this paper we investigate some cosmological implications of another dark energy candidate: an exotic fluid known

Abha Dev; J. S. Alcaniz; Deepak Jain

2003-01-01

294

Current cosmological observations show a strong signature of the existence of a dark energy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equation of state wX=p\\/rho=-1), which, however, raises several theoretical difficulties. This has led to models for a dark energy component that evolves with time. We discuss certain questions

T. Padmanabhan; T. Roy Choudhury

2003-01-01

295

Symmetron dark energy in laboratory experiments.

The symmetron scalar field is a matter-coupled dark energy candidate which effectively decouples from matter in high-density regions through a symmetry restoration. We consider a previously unexplored regime, in which the vacuum mass ?~2.4×10(-3) eV of the symmetron is near the dark energy scale, and the matter coupling parameter M~1 TeV is just beyond standard model energies. Such a field will give rise to a fifth force at submillimeter distances which can be probed by short-range gravity experiments. We show that a torsion pendulum experiment such as Eöt-Wash can exclude symmetrons in this regime for all self-couplings ? is < or approximately equal to 7.5. PMID:23373910

Upadhye, Amol

2013-01-18

296

Probing neutrino dark energy with extremely high energy cosmic neutrinos

Recently, a new non-standard-model neutrino interaction mediated by a light scalar field was proposed, which renders the big bang relic neutrinos of the cosmic neutrino background a natural dark energy candidate, the so-called neutrino dark energy. As a further consequence of this interaction, the neutrino masses become functions of the neutrino energy densities and are thus promoted to dynamical, time\\/redshift

Andreas Ringwald; Lily Schrempp

2006-01-01

297

NASA Astrophysics Data System (ADS)

In this work the collapsing process of a spherically symmetric star, made of dust cloud, in the background of dark energy is studied for two different gravity theories separately, i.e., DGP Brane gravity and Loop Quantum gravity. Two types of dark energy fluids, namely, Modified Chaplygin gas and Generalised Cosmic Chaplygin gas are considered for each model. Graphs are drawn to characterize the nature and the probable outcome of gravitational collapse. A comparative study is done between the collapsing process in the two different gravity theories. It is found that in case of dark matter, there is a great possibility of collapse and consequent formation of Black hole. In case of dark energy possibility of collapse is far lesser compared to the other cases, due to the large negative pressure of dark energy component. There is an increase in mass of the cloud in case of dark matter collapse due to matter accumulation. The mass decreases considerably in case of dark energy due to dark energy accretion on the cloud. In case of collapse with a combination of dark energy and dark matter, it is found that in the absence of interaction there is a far better possibility of formation of black hole in DGP brane model compared to Loop quantum cosmology model.

Rudra, Prabir; Biswas, Ritabrata; Debnath, Ujjal

2012-12-01

298

Ten scenarios from early radiation to late time acceleration with a minimally coupled dark energy

NASA Astrophysics Data System (ADS)

We consider General Relativity with matter, radiation and a minimally coupled dark energy defined by an equation of state w. Using dynamical system method, we find the equilibrium points of such a theory assuming an expanding Universe and a positive dark energy density. Two of these points correspond to classical radiation and matter dominated epochs for the Universe. For the other points, dark energy mimics matter, radiation or accelerates Universe expansion. We then look for possible sequences of epochs describing a Universe starting with some radiation dominated epoch(s) (mimicked or not by dark energy), then matter dominated epoch(s) (mimicked or not by dark energy) and ending with an accelerated expansion. We find ten sequences able to follow this Universe history without singular behaviour of w at some saddle points. Most of them are new in dark energy literature. To get more than these ten sequences, w has to be singular at some specific saddle equilibrium points. This is an unusual mathematical property of the equation of state in dark energy literature, whose physical consequences tend to be discarded by observations. This thus distinguishes the ten above sequences from an infinity of ways to describe Universe expansion.

Fay, Stéphane

2013-09-01

299

Anisotropic dark energy and CMB anomalies

We investigate the breaking of global statistical isotropy caused by a dark energy component with an energy-momentum tensor which has point symmetry, that could represent a cubic or hexagonal crystalline lattice. In such models Gaussian, adiabatic initial conditions created during inflation can lead to anisotropies in the cosmic microwave background whose spherical harmonic coefficients are correlated, contrary to the standard assumption. We develop an adaptation of the line of sight integration method that can be applied to models where the background energy-momentum tensor is isotropic, but whose linearized perturbations are anisotropic. We then show how this can be applied to the cases of cubic and hexagonal symmetry. We compute quantities which show that such models are indistinguishable from isotropic models even in the most extreme parameter choices, in stark contrast to models with anisotropic initial conditions based on inflation. The reason for this is that the dark energy based models contribute to the CMB anisotropy via the integrated Sachs-Wolfe effect, which is only relevant when the dark energy is dominant, that is, on the very largest scales. For inflationary models, however, the anisotropy is present on all scales.

Battye, Richard; Moss, Adam [Jodrell Bank Center for Astrophysics, University of Manchester, Manchester, M13 9PL (United Kingdom); Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1 (Canada)

2009-07-15

300

Dark Fluid: A Unified Framework for Modified Newtonian Dynamics, Dark Matter, and Dark Energy

NASA Astrophysics Data System (ADS)

Empirical theories of dark matter (DM) like modified Newtonian dynamics (MOND) gravity and of dark energy (DE) like f(R) gravity were motivated by astronomical data. But could these theories be branches rooted from a more general and hence generic framework? Here we propose a very generic Lagrangian of such a framework based on simple dimensional analysis and covariant symmetry requirements, and explore various outcomes in a top-down fashion. The desired effects of quintessence plus cold DM particle fields or MOND-like scalar field(s) are shown to be largely achievable by one vector field only. Our framework preserves the covariant formulation of general relativity, but allows the expanding physical metric to be bent by a single new species of dark fluid flowing in spacetime. Its non-uniform stress tensor and current vector are simple functions of a vector field with variable norm, not coupled with the baryonic fluid and the four-vector potential of the photon fluid. The dark fluid framework generically branches into a continuous spectrum of theories with DE and DM effects, including the f(R) gravity, tensor-vector-scalar-like theories, Einstein-Aether, and ?? theories as limiting cases. When the vector field degenerates into a pure scalar field, we obtain the physics for quintessence. Choices of parameters can be made to pass Big Bang nucleosynthesis, parameterized post-Newtonian, and causality constraints. In this broad setting we emphasize the non-constant dynamical field behind the cosmological constant effect, and highlight plausible corrections beyond the classical MOND predictions.

Zhao, HongSheng; Li, Baojiu

2010-03-01

301

NASA Astrophysics Data System (ADS)

In this work, we study the power-law and the logarithmic entropy corrected versions of the Ricci Dark Energy (RDE) model in the framework of the Brans-Dicke cosmology non-minimally coupled with a chameleon scalar field ?. Considering the presence of interaction between Dark Energy (DE) and Dark Matter (DM), we derived the expressions of some relevant cosmological parameters, i.e. the equation of state parameter ? D , the deceleration parameter q and the evolution of the energy density parameter \\varOmega'D.

Pasqua, Antonio; Assaf, Khudhair A.; Aly, Ayman A.

2013-10-01

302

Dark Energy Science Constraints on Calibration: Design of the SNAP Calibration System

SNAP's primary science goal is investigating dark energy properties, and thereby distinguishing amongst the families of theoretical models. This places requirements on the precision of the determination of dark energy equation of state parameters (w(z) =w0 + wa(1+ z), where w = -rho\\/p (density\\/pressure) ), corresponding to uncertainties on w0 to 0.05 and wa to 0.3 or better. In this

Susana E. Deustua; S. Allam; R. Bohlin; S. Kent; M. L. Lampton; N. Mostek; S. L. Mufson; M. Richmond; J. A. Smith; D. Tucker; B. Woodgate; G. Smadja

2006-01-01

303

Viscous ghost dark energy with a varying gravitational constant

NASA Astrophysics Data System (ADS)

We study the ghost model of dark energy with a varying gravitational constant, G, in the presence of viscosity. In an isotropic and homogeneous Friedmann-Robertson-Walker (FRW) universe, the dissipative effects arise from the presence of bulk viscosity in cosmic fluids. We present the equation of state and the deceleration parameters for interacting viscous ghost dark energy (GDE) in a non-flat universe. We also obtain the dynamical equation governing the evolution of viscous GDE with a time-variable gravitational constant. Our study shows that in the case of a varying gravitational constant, the equation of state parameter of GDE can cross the phantom line (wD ? - 1) in the late time where ?D ? 1, whereas at the present time it always satisfies wD > -1. We also find that at the present time, the deceleration parameter varies in the range -0.44 < q - ?0.38 as G varies between 0 < G?/G ? 0.07.

Sheykhi, A.

2012-04-01

304

Interacting dark sector with variable vacuum energy

NASA Astrophysics Data System (ADS)

We examine a cosmological scenario where dark matter is coupled to a variable vacuum energy while baryons and photons are two decoupled components for a spatially flat Friedmann-Robertson-Walker spacetime. We apply the ?2 method to the updated observational Hubble data for constraining the cosmological parameters and analyze the amount of dark energy in the radiation era. We show that our model fulfills the severe bound of ?x(z?1100)<0.009 at the 2? level, so it is consistent with the recent analysis that includes cosmic microwave background anisotropy measurements from the Planck survey, the Atacama Cosmology Telescope, and the South Pole Telescope along with the future constraints achievable by the Euclid and CMBPol experiments, and fulfills the stringent bound ?x(z?1010)<0.04 at the 2? level in the big-bang nucleosynthesis epoch.

Chimento, Luis P.; Richarte, Martín G.; García, Iván E. Sánchez

2013-10-01

305

NASA Astrophysics Data System (ADS)

Recent observations on Type-Ia supernovae and low density (? m =0.3) measurement of matter including dark matter suggest that the present-day universe consists mainly of repulsive-gravity type ‘exotic matter’ with negative-pressure often said ‘dark energy’ (? x =0.7). But the nature of dark energy is mysterious and its puzzling questions, such as why, how, where and when about the dark energy, are intriguing. In the present paper the authors attempt to answer these questions while making an effort to reveal the genesis of dark energy and suggest that ‘the cosmological nuclear binding energy liberated during primordial nucleo-synthesis remains trapped for a long time and then is released free which manifests itself as dark energy in the universe’. It is also explained why for dark energy the parameter w=-2/3 . Noting that w=1 for stiff matter and w=1/3 for radiation; w=-2/3 is for dark energy because “-1” is due to ‘deficiency of stiff-nuclear-matter’ and that this binding energy is ultimately released as ‘radiation’ contributing “ +1/3 ”, making w=-1+1/3=-2/3 . When dark energy is released free at Z=80, w=-2/3 . But as on present day at Z=0 when the radiation-strength-fraction ( ?), has diminished to ??0, the w=-1+?1/3=-1 . This, almost solves the dark-energy mystery of negative pressure and repulsive-gravity. The proposed theory makes several estimates/predictions which agree reasonably well with the astrophysical constraints and observations. Though there are many candidate-theories, the proposed model of this paper presents an entirely new approach (cosmological nuclear energy) as a possible candidate for dark energy.

Gupta, R. C.; Pradhan, Anirudh

2010-04-01

306

Modified GBIG scenario as an alternative for dark energy

NASA Astrophysics Data System (ADS)

We construct a DGP-inspired braneworld model where induced gravity on the brane is modified in the spirit of f(R) gravity and stringy effects are taken into account by incorporation of the Gauss-Bonnet term in the bulk action. We explore cosmological dynamics of this model and we show that this scenario is a successful alternative for dark energy proposal. Interestingly, it realizes the phantom-like behavior without introduction of any phantom field on the brane and the effective equation of state parameter crosses the cosmological constant line naturally in the same way as observational data suggest.

Nozari, Kourosh; Rashidi, Narges

2009-09-01

307

Viscous Dark Energy in f(T) Gravity

NASA Astrophysics Data System (ADS)

We study the bulk viscosity taking dust matter in the generalized teleparallel gravity. We consider different dark energy (DE) models in this scenario along with a time-dependent viscous model to construct the viscous equation of state (EoS) parameter for these DE models. We discuss the graphical representation of this parameter to investigate the viscosity effects on the accelerating expansion of the universe. It is mentioned here that the behavior of the universe depends upon the viscous coefficients showing the transition from decelerating to accelerating phase. It leads to the crossing of phantom divide line and becomes phantom dominated for specific ranges of these coefficients.

Sharif, M.; Rani, Shamaila

2013-09-01

308

Dark energy domination in the Virgocentric flow

NASA Astrophysics Data System (ADS)

Context. The standard ?CDM cosmological model implies that all celestial bodies are embedded in a perfectly uniform dark energy background, represented by Einstein's cosmological constant, and experience its repulsive antigravity action. Aims: Can dark energy have strong dynamical effects on small cosmic scales as well as globally? Continuing our efforts to clarify this question, we now focus on the Virgo Cluster and the flow of expansion around it. Methods: We interpret the Hubble diagram from a new database of velocities and distances of galaxies in the cluster and its environment, using a nonlinear analytical model, which incorporates the antigravity force in terms of Newtonian mechanics. The key parameter is the zero-gravity radius, the distance at which gravity and antigravity are in balance. Results: 1. The interplay between the gravity of the cluster and the antigravity of the dark energy background determines the kinematical structure of the system and controls its evolution. 2. The gravity dominates the quasi-stationary bound cluster, while the antigravity controls the Virgocentric flow, bringing order and regularity to the flow, which reaches linearity and the global Hubble rate at distances ?15 Mpc. 3. The cluster and the flow form a system similar to the Local Group and its outflow. In the velocity-distance diagram, the cluster-flow structure reproduces the group-flow structure with a scaling factor of about 10; the zero-gravity radius for the cluster system is also 10 times larger. Conclusions: The phase and dynamical similarity of the systems on the scales of 1-30 Mpc suggests that a two-component pattern may be universal for groups and clusters: a quasi-stationary bound central component and an expanding outflow around it, caused by the nonlinear gravity-antigravity interplay with the dark energy dominating in the flow component.

Chernin, A. D.; Karachentsev, I. D.; Nasonova, O. G.; Teerikorpi, P.; Valtonen, M. J.; Dolgachev, V. P.; Domozhilova, L. M.; Byrd, G. G.

2010-09-01

309

Probing dark energy with Constellation-X

NASA Astrophysics Data System (ADS)

Con-X will carry out two powerful and independent sets of tests of dark energy based on X-ray observations of galaxy clusters. The first group of tests will measure the absolute distances to clusters, primarily using measurements of the X-ray gas mass fraction in the largest, dynamically relaxed clusters, but with additional constraining power provided by follow-up observations of the Sunyaev-Zel'dovich (SZ) effect. As with supernovae studies, such data determine the transformation between redshift and true distance, d(z), allowing cosmic acceleration to be measured directly. The second, independent group of tests will use the spectroscopic capabilities of Con-X to determine scaling relations between X-ray observables and mass. Together with theoretical models for the mass function and X-ray and SZ cluster surveys, these data will help to constrain the growth of structure, which is also a strong function of cosmological parameters. Con-X data will constrain dark energy with comparable accuracy and in a beautifully complementary manner to the best other techniques available circa 2018. For example, with a modest ˜ 10-15% (10-15Ms) investment of the available observing time over the first 5 years of the Con-X mission, we will be able to measure the X-ray gas mass fraction (or predict the Compton y-parameter) to 5% or 3.5% accuracy for 500 or 250 clusters, respectively, with a median redshift z ˜1. When combined with CMB data, the predicted dark energy constraints from Con-X X-ray gas mass fraction data are comparable to those projected by the SNAP collaboration for CMB+SNAP data as well as the predictions for future galaxy redshift surveys. Only by combining such independent and complementary methods can a precise understanding of the nature of dark energy be expected to be achieved.

Rapetti, D. A.; Allen, S. W.; Con-X Facility Science Team

2005-12-01

310

Present and future evidence for evolving dark energy

We compute the Bayesian evidences for one- and two-parameter models of evolving dark energy, and compare them to the evidence for a cosmological constant, using current data from Type Ia supernova, baryon acoustic oscillations, and the cosmic microwave background. We use only distance information, ignoring dark energy perturbations. We find that, under various priors on the dark energy parameters, {lambda}CDM is currently favored as compared to the dark energy models. We consider the parameter constraints that arise under Bayesian model averaging, and discuss the implication of our results for future dark energy projects seeking to detect dark energy evolution. The model selection approach complements and extends the figure-of-merit approach of the Dark Energy Task Force in assessing future experiments, and suggests a significantly-modified interpretation of that statistic.

Liddle, Andrew R.; Mukherjee, Pia; Parkinson, David [Astronomy Centre, University of Sussex, Brighton BN1 9QH (United Kingdom); Wang Yun [Department of Physics and Astronomy, University of Oklahoma, Norman, Oklahoma 73019 (United States)

2006-12-15

311

Probing dark energy dynamics from current and future cosmological observations

We report the constraints on the dark energy equation-of-state w(z) using the latest 'Constitution' SNe sample combined with the WMAP5 and Sloan Digital Sky Survey data. Assuming a flat Universe, and utilizing the localized principal component analysis and the model selection criteria, we find that the {Lambda}CDM model is generally consistent with the current data, yet there exists a weak hint of the possible dynamics of dark energy. In particular, a model predicting w(z)<-1 at z is an element of [0.25,0.5) and w(z)>-1 at z is an element of [0.5,0.75), which means that w(z) crosses -1 in the range of z is an element of [0.25,0.75), is mildly favored at 95% confidence level. Given the best fit model for current data as a fiducial model, we make future forecast from the joint data sets of Joint Dark Energy Mission, Planck, and Large Synoptic Survey Telescope, and we find that the future surveys can reduce the error bars on the w bins by roughly a factor of 10 for a 5-w-bin model.

Zhao Gongbo [Institute of Cosmology and Gravitation, Dennis Sciama Building, Burnaby Road, Portsmouth, PO1 3FX (United Kingdom); Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6 (Canada); Zhang Xinmin [Theoretical Physics Division, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918-4, Beijing 100049 (China); Theoretical Physics Center for Science Facilities (TPCSF), Chinese Academy of Sciences (China)

2010-02-15

312

Perturbed dark energy: Classical scalar field versus tachyon

The evolution of scalar linear perturbations is studied in gauge-invariant approach for 2-component models with nonrelativistic matter and minimally coupled scalar fields, the potentials of which were constructed for either constant dark energy equation of state parameter w or its adiabatic sound speed c{sub a}{sup 2} equal to zero. The numerical solutions show that such fields are almost smoothed out on subhorizon scales. However, they cause the scale dependent suppression of the nonrelativistic matter density perturbations and the decay of gravitational potential, which can be used for choice of the dark energy model. We discuss two types of the Lagrangian: classical and tachyonic ones. As our results show, the fields with w=const are almost indistinguishable, while for fields with c{sub a}{sup 2}=0 the difference of dark energy effective sound speeds c{sub s}{sup 2}, which is caused by the shape of Lagrangian, affects the evolution of perturbations significantly. We present also the transfer functions for both components.

Sergijenko, Olga; Novosyadlyj, Bohdan [Astronomical Observatory of Ivan Franko National University of Lviv, Kyryla i Methodia str., 8, Lviv, 79005 (Ukraine)

2009-10-15

313

The Dark Energy Survey Camera (DECam)

NASA Astrophysics Data System (ADS)

The Dark Energy Survey (DES) is a next generation optical survey aimed at understanding the expansion rate of the universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. To perform the survey, the DES Collaboration is building the Dark Energy Camera (DECam), a 3 square degree, 520 Megapixel CCD camera which will be mounted at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. The survey will cover 5000 square-degrees of the southern galactic cap with 5 filters (g, r, i, z, Y). DECam will be comprised of 74 250 micron thick fully depleted CCDs: 62 2k x 4k CCDs for imaging and 12 2k x 2k CCDs for guiding and focus. Construction of DECam is nearing completion. In order to verify that the camera meets technical specifications for the Dark Energy Survey and to reduce the time required to commission the instrument on the telescope, we have constructed a full sized "Telescope Simulator" and are performing full system testing and integration prior to shipping to CTIO. An overview of the DECam design and the status of the construction and integration tests will be presented.

Honscheid, Klaus

2011-04-01

314

Cosmic probes of the physics of dark matter and dark energy

NASA Astrophysics Data System (ADS)

Starting around 2014, data from the Large Synoptic Survey Telescope (LSST) will be analyzed for a wide range of phenomena. The nature of dark matter can be constrained by measuring the scales on which it clumps. The nature of dark energy can be constrained by measuring the time evolution of cosmic dark matter structures and through measurements of the distribution of galaxies and the cosmic ``shear'' of their apparent shapes. A sample of three billion galaxies will enable maps of dark matter and several independent cross-checking probes of the nature of dark energy. These and other probes of dark energy involving the deep wide-area survey will be described. By separately tracing the development of mass structure and rate of expansion of the universe, these data will address the physics of dark matter and dark energy, the possible existence of modified gravity on large scales, the neutrino mass, and possible self interaction of dark matter particles. Images of dark matter from our current survey will be shown, and the status of the LSST project will be reviewed.

Tyson, Tony

2008-10-01

315

Limits on dark radiation, early dark energy, and relativistic degrees of freedom

Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. This component is often parametrized as an excess of the effective neutrino number N{sub eff} over the standard value of 3.046. The excess relativistic energy could be an indication for an extra (sterile) neutrino, but early dark energy and barotropic dark energy also contribute to the relativistic degrees of freedom. We examine the capabilities of current and future data to constrain and discriminate between these explanations, and to detect the early dark energy density associated with them. We find that while early dark energy does not alter the current constraints on N{sub eff}, a dark radiation component, such as that provided by barotropic dark energy models, can substantially change current constraints on N{sub eff}, bringing its value back to agreement with the theoretical prediction. Both dark energy models also have implications for the primordial mass fraction of Helium Y{sub p} and the scalar perturbation index n{sub s}. The ongoing Planck satellite mission will be able to further discriminate between sterile neutrinos and early dark energy.

Calabrese, Erminia; Melchiorri, Alessandro [Physics Department and INFN, Universita' di Roma ''La Sapienza'', Ple Aldo Moro 2, 00185, Rome (Italy); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church St, Ann Arbor, Michigan 48109 (United States); Linder, Eric V. [Berkeley Lab and University of California, Berkeley, California 94720, USA. (United States); Institute for the Early Universe WCU, Ewha Womans University, Seoul (Korea, Republic of); Pagano, Luca [Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena California 91109 (United States)

2011-06-15

316

Limits on dark radiation, early dark energy, and relativistic degrees of freedom

NASA Astrophysics Data System (ADS)

Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. This component is often parametrized as an excess of the effective neutrino number Neff over the standard value of 3.046. The excess relativistic energy could be an indication for an extra (sterile) neutrino, but early dark energy and barotropic dark energy also contribute to the relativistic degrees of freedom. We examine the capabilities of current and future data to constrain and discriminate between these explanations, and to detect the early dark energy density associated with them. We find that while early dark energy does not alter the current constraints on Neff, a dark radiation component, such as that provided by barotropic dark energy models, can substantially change current constraints on Neff, bringing its value back to agreement with the theoretical prediction. Both dark energy models also have implications for the primordial mass fraction of Helium Yp and the scalar perturbation index ns. The ongoing Planck satellite mission will be able to further discriminate between sterile neutrinos and early dark energy.

Calabrese, Erminia; Huterer, Dragan; Linder, Eric V.; Melchiorri, Alessandro; Pagano, Luca

2011-06-01

317

Repulsive gravity model for dark energy

We construct a multimetric gravity theory containing N{>=}3 copies of standard model matter and a corresponding number of metrics. In the Newtonian limit, this theory generates attractive gravitational forces within each matter sector and repulsive forces of the same strength between matter from different sectors. This result demonstrates that the recently proven no-go theorem that forbids gravity theories of this type in N=2 cannot be extended beyond the bimetric case. We apply our theory to cosmology and show that the repulsion between different types of matter may induce the observed accelerating expansion of the universe. In this way dark energy can be explained simply by dark copies of the well-understood standard model.

Hohmann, Manuel; Wohlfarth, Mattias N. R. [Zentrum fuer Mathematische Physik und II. Institut fuer Theoretische Physik, Universitaet Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)

2010-05-15

318

In this paper, we investigate the integrated Sachs-Wolfe effect in the quintessence cold dark matter model with constant equation of state and constant speed of sound in dark energy rest frame, including dark energy perturbation and its anisotropic stress. Comparing with the {Lambda}CDM model, we find that the integrated Sachs-Wolfe (ISW)-power spectrums are affected by different background evolutions and dark energy perturbation. As we change the speed of sound from 1 to 0 in the quintessence cold dark matter model with given state parameters, it is found that the inclusion of dark energy anisotropic stress makes the variation of magnitude of the ISW source uncertain due to the anticorrelation between the speed of sound and the ratio of dark energy density perturbation contrast to dark matter density perturbation contrast in the ISW-source term. Thus, the magnitude of the ISW-source term is governed by the competition between the alterant multiple of (1+3/2xc-circumflex{sub s}{sup 2}) and that of {delta}{sub de}/{delta}{sub m} with the variation of c-circumflex{sub s}{sup 2}.

Wang, Y. T.; Xu, L. X.; Gui, Y. X. [School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian, Liaoning 116024 (China)

2010-10-15

319

NASA Astrophysics Data System (ADS)

In this paper, we investigate the integrated Sachs-Wolfe effect in the quintessence cold dark matter model with constant equation of state and constant speed of sound in dark energy rest frame, including dark energy perturbation and its anisotropic stress. Comparing with the ?CDM model, we find that the integrated Sachs-Wolfe (ISW)-power spectrums are affected by different background evolutions and dark energy perturbation. As we change the speed of sound from 1 to 0 in the quintessence cold dark matter model with given state parameters, it is found that the inclusion of dark energy anisotropic stress makes the variation of magnitude of the ISW source uncertain due to the anticorrelation between the speed of sound and the ratio of dark energy density perturbation contrast to dark matter density perturbation contrast in the ISW-source term. Thus, the magnitude of the ISW-source term is governed by the competition between the alterant multiple of (1+3/2×c^s2) and that of ?de/?m with the variation of c^s2.

Wang, Y. T.; Xu, L. X.; Gui, Y. X.

2010-10-01

320

NASA Astrophysics Data System (ADS)

Using ESO's Very Large Telescope Interferometer, astronomers have probed the inner parts of the disc of material surrounding a young stellar object, witnessing how it gains its mass before becoming an adult. ESO PR Photo 03/08 ESO PR Photo 03a/08 The disc around MWC 147 (Artist's Impression) The astronomers had a close look at the object known as MWC 147, lying about 2,600 light years away towards the constellation of Monoceros ('the Unicorn'). MWC 147 belongs to the family of Herbig Ae/Be objects. These have a few times the mass of our Sun and are still forming, increasing in mass by swallowing material present in a surrounding disc. MWC 147 is less than half a million years old. If one associated the middle-aged, 4.6 billion year old Sun with a person in his early forties, MWC 147 would be a 1-day-old baby [1]. The morphology of the inner environment of these young stars is however a matter of debate and knowledge of it is important to better understand how stars and their cortège of planets form. The astronomers Stefan Kraus, Thomas Preibisch, and Keiichi Ohnaka have used the four 8.2-m Unit Telescopes of ESO's Very Large Telescope to this purpose, combining the light from two or three telescopes with the MIDI and AMBER instruments. "With our VLTI/MIDI and VLTI/AMBER observations of MWC147, we combine, for the first time, near- and mid-infrared interferometric observations of a Herbig Ae/Be star, providing a measurement of the disc size over a wide wavelength range [2]," said Stefan Kraus, lead-author of the paper reporting the results. "Different wavelength regimes trace different temperatures, allowing us to probe the disc's geometry on the smaller scale, but also to constrain how the temperature changes with the distance from the star." The near-infrared observations probe hot material with temperatures of up to a few thousand degrees in the innermost disc regions, while the mid-infrared observations trace cooler dust further out in the disc. The observations show that the temperature changes with radius are much steeper than predicted by the currently favoured models, indicating that most of the near-infrared emission emerges from hot material located very close to the star, that is, within one or two times the Earth-Sun distance (1-2 AU). This also implies that dust cannot exist so close to the star, since the strong energy radiated by the star heats and ultimately destroys the dust grains. ESO PR Photo 03/08 ESO PR Photo 03b/08 The Region Around MWC 147 "We have performed detailed numerical simulations to understand these observations and reached the conclusion that we observe not only the outer dust disc, but also measure strong emission from a hot inner gaseous disc. This suggests that the disc is not a passive one, simply reprocessing the light from the star," explained Kraus. "Instead, the disc is active, and we see the material, which is just transported from the outer disc parts towards the forming star." ESO PR Photo 03/08 ESO PR Photo 03c/08 Close-up on MWC 147 The best-fit model is that of a disc extending out to 100 AU, with the star increasing in mass at a rate of seven millionths of a solar mass per year. "Our study demonstrates the power of ESO's VLTI to probe the inner structure of discs around young stars and to reveal how stars reach their final mass," said Stefan Kraus. More Information The authors report their results in a paper in the Astrophysical Journal ("Detection of an inner gaseous component in a Herbig Be star accretion disk: Near- and mid-infrared spectro-interferometry and radiative transfer modeling of MWC 147", by Stefan Kraus, Thomas Preibisch, Keichii Ohnaka").

2008-01-01

321

This program is concerned with developing and verifying the validityof observational methods for constraining the properties of dark matter and dark energy in the universe. Excellent progress has been made in comparing observational projects involving weak gravitational lensing using both ground and space-based instruments, in further constraining the nature of dark matter via precise measures of its distribution in clusters of galaxies using strong gravitational lensing, in demonstrating the possible limitations of using distant supernovae in future dark energy missions, and in investigating the requirement for ground-based surveys of baryonic acoustic oscillations.

Ellis, Richard, S.

2008-02-01

322

Dark Energy and the Hierarchy Problem

The well-known hierarchy between the Planck scale (>> 1019GeV) and the TeV scale, namely a ratio of {approx} 10{sup 16} between the two, is coincidentally repeated in a inverted order between the TeV scale and the dark energy scale at {approx} 10{sup -3} eV implied by the observations. We argue that this is not a numerical coincidence. The same brane-world setups to address the first hierarchy problem may also in principle address this second hierarchy issue. Specifically, we consider supersymmetry in the bulk and its breaking on the brane and resort to the Casimir energy induced by the bulk graviton-gravitino mass-shift on the brane as the dark energy. For the ADD model we found that our notion is sensible only if the number of extra dimension n = 2. We extend our study to the Randall-Sundrum model. Invoking the chirality-flip on the boundaries for SUSY-breaking, the zero-mode gravitino contribution to the Casimir energy does give rise to the double hierarchy. Unfortunately since the higher Kaluza-Klein modes acquire relative mass-shifts at the TeV level, the zero-mode contribution to Casimir energy is overshadowed.

Chen, Pisin

2006-12-06

323

Dark energy and the hierarchy problem

NASA Astrophysics Data System (ADS)

The well-known hierarchy between the Planck scale (˜10GeV) and the TeV scale, namely a ratio of ˜10 between the two, is coincidentally repeated in a inverted order between the TeV scale and the dark energy scale at ˜10eV implied by the observations. We argue that this is not a numerical coincidence. The same brane-world setups to address the first hierarchy problem may also in principle address this second hierarchy issue. Specifically, we consider supersymmetry in the bulk and its breaking on the brane and resort to the Casimir energy induced by the bulk graviton-gravitino mass-shift on the brane as the dark energy. For the ADD model we found that our notion is sensible only if the number of extra dimension n=2. We extend our study to the Randall-Sundrum model. Invoking the chirality-flip on the boundaries for SUSY-breaking, the zero-mode gravitino contribution to the Casimir energy does give rise to the double hierarchy. Unfortunately since the higher Kaluza-Klein modes acquire relative mass-shifts at the TeV level, the zero-mode contribution to Casimir energy is overshadowed.

Chen, Pisin

2009-03-01

324

Dynamics of Dirac-Born-Infeld dark energy interacting with dark matter

NASA Astrophysics Data System (ADS)

We study the dynamics of Dirac-Born-Infeld (DBI) dark energy interacting with dark matter. The DBI dark energy model considered here has a scalar field with a nonstandard kinetic energy term, and has potential and brane tension that are power-law functions. The new feature considered here is an interaction between the DBI dark energy and dark matter through a phenomenological interaction between the DBI scalar field and the dark matter fluid. We analyze two different types of interactions between the DBI scalar field and the dark matter fluid. In particular we study the phase-space diagrams of and look for critical points of the phase space that are both stable and lead to accelerated, late-time expansion. In general we find that the interaction between the two dark components does not appear to give rise to late-time accelerated expansion. However, the interaction can make the critical points in the phase space of the system stable. Whether such stabilization occurs or not depends on the form of the interaction between the two dark components.

Kaeonikhom, Chakkrit; Singleton, Douglas; Sushkov, Sergey V.; Yongram, N.

2012-12-01

325

Statefinder Diagnostic for Born-Infeld Type Dark Energy Model

Using a new method--statefinder diagnostic which can differ one dark energy model from the others, we investigate in this letter the dynamics of Born-Infeld(B-I) type dark energy model. The evolutive trajectory of B-I type dark energy with Mexican hat potential model with respect to $e-folding$ time $N$ is shown in the $r(s)$ diagram. When the parameter of noncanonical kinetic energy

Z. G. Huang; H. Q. Lu

2008-01-01

326

Dark Energy: As A Fact of Matter

NASA Astrophysics Data System (ADS)

At the current time Dark Energy has been attributed to a property of space itself, ie. the vacuum energy of space. This explanation does not account for the inflationary expansion of the early universe nor does it explain the flatness of the universe. If however, Dark Energy is considered to be a property of matter, these issues are resolved. The Space Production Model of Gravity proposes that matter is converted to and emits space. The amount of space emitted per unit time is proportionate to mass by the formula: 4?(2Gm/c^2)^2(c) Where: G = gravitational constant m = mass c = speed of light Using this formula and the observed Hubble constant one can calculate the mass of the universe to be 5.441x10^53kg. One can also calculate the size of the universe at a given age. 1.36x10^20 cubic meters of new space is produced per planck time. Therefore one planck time after the Big Bang the radius of the universe was 3164 kilometers; after 1 second 88515 light years; and after 13.7 billion years 67 billion light years. The ``Flatness'' problem is also resolved. More massive gravitationally bound objects will produce more volume per unit time than less massive objects. This means that the rate of expansion of the universe is dependent upon the local mass density, the higher the density the greater the rate of expansion. Over time this produces a universe that is extremely homogenous. If the Space Production Model is true, the current Dark Energy survey should reveal that more massive gravitationally bound objects such as galaxy super-clusters recede slightly faster than much less massive lone galaxies at the same distance.

Bowen, Richard

2013-04-01

327

Reconciling dark energy models with f(R) theories

NASA Astrophysics Data System (ADS)

Higher-order theories of gravity have recently attracted a lot of interest as alternative candidates to explain the observed cosmic acceleration without the need of introducing any scalar field. A critical ingredient is the choice of the function f(R) of the Ricci scalar curvature entering the gravity Lagrangian and determining the dynamics of the Universe. We describe an efficient procedure to reconstruct f(R) from the Hubble parameter H depending on the redshift z. Using the metric formulation of f(R) theories, we derive a third order linear differential equation for f[R(z)] which can be numerically solved after setting the boundary conditions on the basis of physical considerations. Since H(z) can be reconstructed from the astrophysical data, the method we present makes it possible to determine, in principle, what is the f(R) theory which best reproduces the observed cosmological dynamics. Moreover, the method allows to reconcile dark energy models with f(R) theories finding out what is the expression of f(R) which leads to the same H(z) of the given quintessence model. As interesting examples, we consider “quiessence” (dark energy with constant equation of state) and the Chaplygin gas.

Capozziello, S.; Cardone, V. F.; Troisi, A.

2005-02-01

328

Remarks on dynamical dark energy measured by the conformal age of the universe

NASA Astrophysics Data System (ADS)

We elaborate on a model of conformal dark energy (dynamical dark energy measured by the conformal age of the universe) recently proposed in [H. Wei and R. G. Cai, arXiv:0708.0884] where the presentday dark energy density was taken to be ?q?3?2mP2/?2, where ? is the conformal time and ? is a numerical constant. In the absence of an interaction between the ordinary matter and dark energy field q, the model may be adjusted to the present values of the dark energy density fraction ?q?0.73 and the equation of state parameter wq<-0.78, if the numerical constant ? takes a reasonably large value, ??2.6. However, in the presence of a nontrivial gravitational coupling of q-field to matter, say Q˜, the model may be adjusted to the values ?q?0.73 and wq?-1, even if ?˜O(1), given that the present value of Q˜ is large. Unlike for the model in [R. G. Cai, arXiv:0707.4049], the bound ?q<0.1 during big bang nucleosynthesis (BBN) may be satisfied for almost any value of ?. Here we discuss some other limitations of this proposal as a viable dark energy model. The model draws some parallels with the holographic dark energy; we also briefly comment on the latter model.

Neupane, Ishwaree P.

2007-12-01

329

Quasinormal modes of black holes absorbing dark energy

NASA Astrophysics Data System (ADS)

We study perturbations of black holes absorbing dark energy. Due to the accretion of dark energy, the black hole mass changes. We observe distinct perturbation behaviors for absorption of different forms of dark energy onto the black holes. This provides the possibility of extracting information whether dark energy lies above or below the cosmological constant boundary w=-1. In particular, we find in the late time tail analysis that, differently from the other dark energy models, the accretion of phantom energy exhibits a growing mode in the perturbation tail. The instability behavior found in this work is consistent with the Big Rip scenario, in which all of the bound objects are torn apart with the presence of the phantom dark energy.

He, Xi; Wang, Bin; Wu, Shao-Feng; Lin, Chi-Yong

2009-03-01

330

Interacting new agegraphic dark energy in nonflat Brans-Dicke cosmology

We construct a cosmological model of late acceleration based on the new agegraphic dark energy model in the framework of Brans-Dicke cosmology where the new agegraphic energy density {rho}{sub D}=3n{sup 2}m{sub p}{sup 2}/{eta}{sup 2} is replaced with {rho}{sub D}=3n{sup 2{phi}2}/(4{omega}{eta}{sup 2}). We show that the combination of the Brans-Dicke field and agegraphic dark energy can accommodate a w{sub D}=-1 crossing for the equation of state of noninteracting dark energy. When an interaction between dark energy and dark matter is taken into account, the transition of w{sub D} to the phantom regime can be more easily accounted for than when we resort to the Einstein field equations. In the limiting case {alpha}=0 ({omega}{yields}{infinity}), all previous results of the new agegraphic dark energy in Einstein gravity are restored.

Sheykhi, Ahmad [Department of Physics, Shahid Bahonar University, P.O. Box 76175, Kerman (Iran, Islamic Republic of); Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), Maragha (Iran, Islamic Republic of)

2010-01-15

331

NASA Astrophysics Data System (ADS)

In this paper, the resonant nonlinear Schrödinger's equation is studied with five forms of nonlinearity. This equation is also considered with time-dependent coefficients and additionally time-dependent linear attenuation is considered. The ansatz method approach is used to carry out the integration. Both bright and dark soliton solutions are obtained in this paper. The constraint conditions for the existence of soliton solutions are also given.

Triki, Houria; Hayat, T.; Aldossary, Omar M.; Biswas, Anjan

2012-10-01

332

NASA Astrophysics Data System (ADS)

In this paper, we propose new efficient and accurate numerical methods for computing dark solitons and review some existing numerical methods for bright and/or dark solitons in the nonlinear Schrödinger equation (NLSE), and compare them numerically in terms of accuracy and efficiency. We begin with a review of dark and bright solitons of NLSE with defocusing and focusing cubic nonlinearities, respectively. For computing dark solitons, to overcome the nonzero and/or non-rest (or highly oscillatory) phase background at far field, we design efficient and accurate numerical methods based on accurate and simple artificial boundary conditions or a proper transformation to rest the highly oscillatory phase background. Stability and conservation laws of these numerical methods are analyzed. For computing interactions between dark and bright solitons, we compare the efficiency and accuracy of the above numerical methods and different existing numerical methods for computing bright solitons of NLSE, and identify the most efficient and accurate numerical methods for computing dark and bright solitons as well as their interactions in NLSE. These numerical methods are applied to study numerically the stability and interactions of dark and bright solitons in NLSE. Finally, they are extended to solve NLSE with general nonlinearity and/or external potential and coupled NLSEs with vector solitons.

Bao, Weizhu; Tang, Qinglin; Xu, Zhiguo

2013-02-01

333

Cosmological constraints on holographic dark energy models under the energy conditions

NASA Astrophysics Data System (ADS)

We study the holographic and agegraphic dark energy models without interaction using the latest observational Hubble parameter data , the Union2.1 compilation of type Ia supernovae , and the energy conditions. Scenarios of dark energy are distinguished by the cutoff of cosmic age, conformal time, and event horizon. The best-fit value of matter density for the three scenarios almost steadily located at ?m0=0.26 by the joint constraint. For the agegraphic models, they can be recovered to the standard cosmological model when the constant c which presents the fraction of dark energy approaches to infinity. Absence of the upper limit of c by the joint constraint demonstrates the recovery possibility. Using the fitted result, we also reconstruct the current equation of state of dark energy at different scenarios, respectively. Employing the model criteria ?min?2/dof, we find that conformal time model is the worst, but they cannot be distinguished clearly. Comparing with the observational constraints, we find that the strong energy condition is fulfilled at redshift 0.2?z?0.3 with 1? confidence level. We also find that the null energy condition gives a meaningful constraint for the event horizon cutoff model, especially compared with observational Hubble parameter data only. We note that the energy condition maybe could play an important role in the interacting models because of different degeneracy between ?m and constant c.

Zhang, Ming-Jian; Ma, Cong; Zhang, Zhi-Song; Zhai, Zhong-Xu; Zhang, Tong-Jie

2013-09-01

334

Comparison of perturbations in fluid and scalar field models of dark energy

We compare perturbations in a fluid model of dark energy with those in a scalar field. As compared to the {lambda}CDM model, large scale matter power spectrum is suppressed in fluid model as well as in a generic quintessence dark energy model. To check the efficacy of fluid description of dark energy in emulating a scalar field, we consider a potential which gives the same background evolution as a fluid with a constant equation of state. We show that for sub-Hubble scales, a fluid model effectively emulates a scalar field model. At larger scales, where dark energy perturbations may play a significant role, the fluid analogy breaks down and the evolution of matter density contrast depends on individual scalar field models.

Jassal, H. K. [Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019 (India)

2009-06-15

335

Exploring parameter constraints on quintessential dark energy: The Albrecht-Skordis model

NASA Astrophysics Data System (ADS)

We consider the effect of future dark energy experiments on “Albrecht-Skordis” (AS) models of scalar field dark energy using the Monte Carlo Markov chain method. We deal with the issues of parametrization of these models, and have included spatial curvature as a parameter, finding it to be important. We use the Dark Energy Task Force (DETF) simulated data to represent future experiments and report our results in the form of likelihood contours in the chosen parameter space. Simulated data is produced for cases where the background cosmology has a cosmological constant, as well as cases where the dark energy is provided by the AS model. The latter helps us demonstrate the power of DETF Stage 4 data in the context of this specific model. Though the AS model can produce equations of state functions very different from what is possible with the w0-wa parametrization used by the DETF, our results are consistent with those reported by the DETF.

Barnard, Michael; Abrahamse, Augusta; Albrecht, Andreas; Bozek, Brandon; Yashar, Mark

2008-05-01

336

Can the Existence of Dark Energy be Directly Detected?

The majority of astronomers and physicists accept the reality of dark energy and also believe that it can only be studied indirectly through observation of the motions of stars and galaxies. In this paper I open the experimental question of whether it is possible to directly detect dark energy through the presence of dark energy density. Two thirds of this paper outlines the major aspects of dark energy density as now comprehended by the astronomical and physics community. The final third summarizes various proposals for direct detection of dark energy density or its possible effects. At this time I do not have a fruitful answer to the question: Can the Existence of Dark Energy Be Directly Detected?

Perl, Martin L.; /SLAC /KIPAC, Menlo Park

2011-11-23

337

Aetherizing Lambda: Barotropic fluids as dark energy

NASA Astrophysics Data System (ADS)

We examine the class of barotropic fluid models of dark energy, in which the pressure is an explicit function of the density, p=f(?). Through general physical considerations we constrain the asymptotic past and future behaviors and show that this class is equivalent to the sum of a cosmological constant and a decelerating perfect fluid, or “aether,” with wAE?0. Barotropic models give substantially disjoint predictions from quintessence, except in the limit of ?CDM. They are also interesting in that they simultaneously can ameliorate the coincidence problem and yet “predict” a value of w?-1.

Linder, Eric V.; Scherrer, Robert J.

2009-07-01

338

Dark energy properties in DBI theory

The Dirac-Born-Infeld (DBI) action from string theory provides several new classes of dark energy behavior beyond quintessence due to its relativistic kinematics. We constrain parameters of natural potentials and brane tensions with cosmological observations as well as showing how to design these functions for a desired expansion history. We enlarge the attractor solutions, including new ways of obtaining cosmological constant behavior, to the case of generalized DBI theory with multiple branes. An interesting novel signature of DBI attractors is that the sound speed is driven to zero, unlike for quintessence where it is the speed of light.

Ahn, Changrim; Kim, Chanju [Institute for the Early Universe and Department of Physics, Ewha Womans University, Seoul 120-750 (Korea, Republic of); Linder, Eric V. [Institute for the Early Universe and Department of Physics, Ewha Womans University, Seoul 120-750 (Korea, Republic of); Berkeley Center for Cosmological Physics and Berkeley Lab, University of California, Berkeley, California 94720 (United States)

2009-12-15

339

Alternative explanations of ``dark energy'' in cosmology

NASA Astrophysics Data System (ADS)

I consider alternative explanations for the data suggesting detection of dark energy (or a cosmological constant ?). Apart from alternative gravity theories and some form of `quintessence', one proposal to explain a small non-zero value of ? is a multiverse, but this is not testable in the usual sense. Other possibilities are that it may at least in part be due to the backreaction from and observational effects of small scale inhomogeneity, or it might be completely explained by large scale spatial inhomogeneity with zero ?. The latter proposal can be observationally tested in a number of ways.

Ellis, George F. R.

2010-12-01

340

The Dark Energy Survey CCD imager design

The Dark Energy Survey is planning to use a 3 sq. deg. camera that houses a {approx} 0.5m diameter focal plane of 62 2kx4k CCDs. The camera vessel including the optical window cell, focal plate, focal plate mounts, cooling system and thermal controls is described. As part of the development of the mechanical and cooling design, a full scale prototype camera vessel has been constructed and is now being used for multi-CCD readout tests. Results from this prototype camera are described.

Cease, H.; DePoy, D.; Diehl, H.T.; Estrada, J.; Flaugher, B.; Guarino, V.; Kuk, K.; Kuhlmann, S.; Schultz, K.; Schmitt, R.L.; Stefanik, A.; /Fermilab /Ohio State U. /Argonne

2008-06-01

341

Dark energy from primordial inflationary quantum fluctuations.

We show that current cosmic acceleration can be explained by an almost massless scalar field experiencing quantum fluctuations during primordial inflation. Provided its mass does not exceed the Hubble parameter today, this field has been frozen during the cosmological ages to start dominating the Universe only recently. By using supernovae data, completed with baryonic acoustic oscillations from galaxy surveys and cosmic microwave background anisotropies, we infer the energy scale of primordial inflation to be around a few TeV, which implies a negligible tensor-to-scalar ratio of the primordial fluctuations. Moreover, our model suggests that inflation lasted for an extremely long period. Dark energy could therefore be a natural consequence of cosmic inflation close to the electroweak energy scale. PMID:20867625

Ringeval, Christophe; Suyama, Teruaki; Takahashi, Tomo; Yamaguchi, Masahide; Yokoyama, Shuichiro

2010-09-13

342

Dissipative or conservative cosmology with dark energy?

All evolutional paths for all admissible initial conditions of FRW cosmological models with dissipative dust fluid (described by dark matter, baryonic matter and dark energy) are analyzed using dynamical system approach. With that approach, one is able to see how generic the class of solutions leading to the desired property-acceleration-is. The theory of dynamical systems also offers a possibility of investigating all possible solutions and their stability with tools of Newtonian mechanics of a particle moving in a one-dimensional potential which is parameterized by the cosmological scale factor. We demonstrate that flat cosmology with bulk viscosity can be treated as a conservative system with a potential function of the Chaplygin gas type. We characterize the class of dark energy models that admit late time de Sitter attractor solution in terms of the potential function of corresponding conservative system. We argue that inclusion of dissipation effects makes the model more realistic because of its structural stability. We also confront viscous models with SNIa observations. The best fitted models are obtained by minimizing the {chi}{sup 2} function which is illustrated by residuals and {chi}{sup 2} levels in the space of model independent parameters. The general conclusion is that SNIa data supports the viscous model without the cosmological constant. The obtained values of {chi}{sup 2} statistic are comparable for both the viscous model and {lambda}CDM model. The Bayesian information criteria are used to compare the models with different power-law parameterization of viscous effects. Our result of this analysis shows that SNIa data supports viscous cosmology more than the {lambda}CDM model if the coefficient in viscosity parameterization is fixed. The Bayes factor is also used to obtain the posterior probability of the model.

Szydlowski, Marek [Astronomical Observatory, Jagiellonian University, Orla 171, 30-244 Krakow (Poland); Marc Kac Complex Systems Research Center, Jagiellonian University, Reymonta 4, 30-059 Krakow (Poland)], E-mail: uoszydlo@cyf-kr.edu.pl; Hrycyna, Orest [Department of Theoretical Physics, Faculty of Philosophy, John Paul II Catholic University of Lublin, Al. Raclawickie 14, 20-950 Lublin (Poland)], E-mail: hrycyna@kul.lublin.pl

2007-12-15

343

An accelerating cosmology without dark energy

NASA Astrophysics Data System (ADS)

The negative pressure accompanying gravitationally-induced particle creation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996 [1]) without requiring the presence of dark energy or a cosmological constant. In a recent study, Lima et al. 2008 [2] (LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations [3] of the recent transition from a decelerating to an accelerating Universe, without the need for Dark Energy. Here we consider a class of such models where the particle creation rate is assumed to be of the form ? = ?H+?H0, where H is the Hubble parameter and H0 is its present value. The evolution of such models is tested at low redshift by the latest SNe Ia data provided by the Union compilation [4] and at high redshift using the value of zeq, the redshift of the epoch of matter — radiation equality, inferred from the WMAP constraints on the early Integrated Sachs-Wolfe (ISW) effect [5]. Since the contributions of baryons and radiation were ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM are constrained at widely-separated epochs (zeq approx 3000 and z approx 0) in the evolution of the Universe. The comparison of the parameter values, {?, ?}, determined at these different epochs reveals a tension between the values favored by the high redshift CMB constraint on zeq from the ISW and those which follow from the low redshift SNIa data, posing a potential challenge to this class of models. While for ? = 0 this conflict is only at lesssim 2?, it worsens as ? increases from zero.

Steigman, G.; Santos, R. C.; Lima, J. A. S.

2009-06-01

344

Will multiple probes of dark energy find modified gravity?

One of the most pressing issues in cosmology is whether general relativity (GR) plus a dark sector is the underlying physical theory or whether a modified gravity model is needed. Upcoming dark energy experiments designed to probe dark energy with multiple methods can address this question by comparing the results of the different methods in constraining dark energy parameters. Disagreement would signal the breakdown of the assumed model (GR plus dark energy). We study the power of this consistency test by projecting constraints in the w{sub 0}-w{sub a} plane from the four different techniques of the Dark Energy Survey in the event that the underlying true model is modified gravity. We find that the standard technique of looking for overlap has some shortcomings, and we propose an alternative, more powerful Multidimensional Consistency Test. We introduce the methodology for projecting whether a given experiment will be able to use this test to distinguish a modified gravity model from GR.

Shapiro, Charles [Institute of Cosmology and Gravitation, Portsmouth, PO1 3FX (United Kingdom); Dodelson, Scott [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Kavli Institute for Cosmological Physics, Chicago, Illinois 60637 (United States); Hoyle, Ben [Institut de Ciencies del Cosmos, Barcelona (Spain); Samushia, Lado [Institute of Cosmology and Gravitation, Portsmouth, PO1 3FX (United Kingdom); National Abastumani Astrophysical Observatory, Ilia State University, 2A Kazbegi Ave, GE-0160 Tbilisi (Georgia); Flaugher, Brenna [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States)

2010-08-15

345

(m, n)-TYPE Holographic Dark Energy Models

NASA Astrophysics Data System (ADS)

We construct (m, n)-type holographic dark energy models at a phenomenological level, which can be viewed as a generalization of agegraphic models with the conformal-like age as the holographic characteristic size. For some values of (m, n) the holographic dark energy can automatically evolve across ? = -1 into a phantom phase even without introducing an interaction between the dark energy and background matter. Our construction is also applicable to the holographic dark energy with generalized future event horizon as the characteristic size. Finally, we address the issue on the stability of our model and show that they are generally stable under the scalar perturbation.

Ling, Yi; Pan, Wen-Jian

2013-10-01

346

The effective field theory of dark energy

NASA Astrophysics Data System (ADS)

We propose a universal description of dark energy and modified gravity that includes all single-field models. By extending a formalism previously applied to inflation, we consider the metric universally coupled to matter fields and we write in terms of it the most general unitary gauge action consistent with the residual unbroken symmetries of spatial diffeomorphisms. Our action is particularly suited for cosmological perturbation theory: the background evolution depends on only three operators. All other operators start at least at quadratic order in the perturbations and their effects can be studied independently and systematically. In particular, we focus on the properties of a few operators which appear in non-minimally coupled scalar-tensor gravity and galileon theories. In this context, we study the mixing between gravity and the scalar degree of freedom. We assess the quantum and classical stability, derive the speed of sound of fluctuations and the renormalization of the Newton constant. The scalar can always be de-mixed from gravity at quadratic order in the perturbations, but not necessarily through a conformal rescaling of the metric. We show how to express covariant field-operators in our formalism and give several explicit examples of dark energy and modified gravity models in our language. Finally, we discuss the relation with the covariant EFT methods recently appeared in the literature.

Gubitosi, Giulia; Piazza, Federico; Vernizzi, Filippo

2013-02-01

347

Measuring Dark Energy With The Wigglez Survey

NASA Astrophysics Data System (ADS)

We present new measurements of the cosmic expansion history and growth history over the last 7 billion years, using data from the WiggleZ Dark Energy Survey of 200,000 galaxy redshifts. We have used baryon acoustic oscillations (BAOs) in the galaxy distribution as a standard ruler to measure the distance-redshift relation up to z=0.73, and present a BAO "Hubble diagram" which provides a powerful cross-check of the use of Type Ia supernovae as standard candles. We additionally use redshift-space distortions in the galaxy clustering pattern to determine the cosmic growth rate with 10% accuracy in redshift bins up to z=0.9. We show that a cosmological constant model of dark energy is able to simultaneously fit both the expansion and growth data. Finally, we measure Alcock-Paczynski distortions in the clustering pattern to reconstruct the expansion history in a non-parametric manner, demonstrating the reality of accelerating cosmic expansion.

Blake, Chris; Dark Energy Survey, WiggleZ

2012-01-01

348

The Dark Energy Survey Camera (DECam)

The Dark Energy Survey (DES) is a next generation optical survey aimed at understanding the expansion rate of the Universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. To perform the survey, the DES Collaboration is building the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera that will be mounted at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. CCD production has finished, yielding roughly twice the required 62 2k x 4k detectors. The construction of DECam is nearly finished. Integration and commissioning on a 'telescope simulator' of the major hardware and software components, except for the optics, recently concluded at Fermilab. Final assembly of the optical corrector has started at University College, London. Some components have already been received at CTIO. 'First-light' will be sometime in 2012. This oral presentation concentrates on the technical challenges involved in building DECam (and how we overcame them), and the present status of the instrument.

Diehl, H.Thomas; /Fermilab

2011-09-09

349

The Dark Energy Camera readout system

NASA Astrophysics Data System (ADS)

The Dark Energy Camera (DECam) was developed for use by the Dark Energy Survey (DES). The camera will be installed in the Blanco 4M telescope at the Cerro Tololo Inter-American Observatory (CTIO) and be ready for observations in the second half of 2012. The focal plane consists of 62 2×4K and 12 2×2K fully depleted CCDs. The camera provides a 3 sq. degree view and the survey will cover a 5000 sq. degree area. The camera cage and corrector have already been installed. The development of the electronics to readout the focal plane was a collaborative effort by multiple institutions in the United States and in Spain. The goal of the electronics is to provide readout at 250 kpixels/second with less than 15erms noise. Integration of these efforts and initial testing took place at Fermi National Accelerator Laboratory. DECam currently resides at CTIO and further testing has occurred in the Coudé room of the Blanco. In this paper, we describe the development of the readout system, test results and the lessons learned.

Shaw, Theresa; Ballester, Otger; Cardiel-Sas, Laia; Castilla, Javier; Chappa, Steve; de Vicente, Juan; Holm, Scott; Huffman, Dave; Kozlovsky, Mark; Martínez, Gustavo; Moore, Todd; Olsen, Jamieson; Simaitis, Vaidas; Stuermer, Walter

2012-07-01

350

Dynamical evolution of quintessence dark energy in collapsing dark matter halos

In this paper, we analyze the dynamical evolution of quintessence dark energy induced by the collapse of dark matter halos. Different from other previous studies, we develop a numerical strategy which allows us to calculate the dark energy evolution for the entire history of the spherical collapse of dark matter halos, without the need of separate treatments for linear, quasilinear, and nonlinear stages of the halo formation. It is found that the dark energy perturbations evolve with redshifts, and their specific behaviors depend on the quintessence potential as well as the collapsing process. The overall energy density perturbation is at the level of 10{sup -6} for cluster-sized halos. The perturbation amplitude decreases with the decrease of the halo mass. At a given redshift, the dark energy perturbation changes with the radius to the halo center, and can be either positive or negative depending on the contrast of {partial_derivative}{sub t}{phi}, {partial_derivative}{sub r}{phi}, and {phi} with respect to the background, where {phi} is the quintessence field. For shells where the contrast of {partial_derivative}{sub r}{phi} is dominant, the dark energy perturbation is positive and can be as high as about 10{sup -5}.

Wang Qiao; Fan Zuhui [Department of Astronomy, School of Physics, Peking University, Beijing, 100871 (China)

2009-06-15

351

Final state and thermodynamics of a dark energy universe

As it follows from the classical analysis, the typical final state of a dark energy universe where a dominant energy condition is violated is a finite-time, sudden future singularity (a big rip). For a number of dark energy universes (including scalar phantom and effective phantom theories as well as specific quintessence models) we demonstrate that quantum effects play the dominant

Shin'ichi Nojiri; Sergei D. Odintsov

2004-01-01

352

Bright-dark solitons and their collisions in mixed N -coupled nonlinear Schrödinger equations

NASA Astrophysics Data System (ADS)

Mixed-type (bright-dark) soliton solutions of the integrable N -coupled nonlinear Schrödinger (CNLS) equations with mixed signs of focusing- and defocusing-type nonlinearity coefficients are obtained by using Hirota’s bilinearization method. Generally, for the mixed N -CNLS equations the bright and dark solitons can be split up in (N-1) ways. By analyzing the collision dynamics of these coupled bright and dark solitons systematically we point out that for N>2 , if the bright solitons appear in at least two components, nontrivial effects, such as onset of intensity redistribution, amplitude-dependent phase shift, and change in relative separation distance take place in the bright solitons during collision. However their counterparts, the dark solitons, undergo elastic collision but experience the same amplitude-dependent phase shift as that of bright solitons. Thus, in the mixed CNLS system, there is a coexisting shape-changing collision of bright solitons and elastic collision of dark solitons with amplitude-dependent phase shift, thereby influencing each other mutually in an intricate way.

Vijayajayanthi, M.; Kanna, T.; Lakshmanan, M.

2008-01-01

353

Phase space analysis of interacting dark energy in f( T) cosmology

NASA Astrophysics Data System (ADS)

In this paper, we examine the interacting dark energy model in f( T) cosmology. We assume dark energy as a perfect fluid and choose a specific cosmologically viable form f( T) = ?? T. We show that there is one attractor solution to the dynamical equation of f( T) Friedmann equations. Further we investigate the stability in phase space for a general f( T) model with two interacting fluids. By studying the local stability near the critical points, we show that the critical points lie on the sheet u* = ( c - 1) v* in the phase space, spanned by coordinates ( u, v, ?, T). From this critical sheet, we conclude that the coupling between the dark energy and matter c ? (-2, 0).

Jamil, Mubasher; Yesmakhanova, Kuralay; Momeni, Davood; Myrzakulov, Ratbay

2012-10-01

354

No evidence for dark energy dynamics from a global analysis of cosmological data

We use a variant of principal component analysis to investigate the possible temporal evolution of the dark energy equation of state, w(z). We constrain w(z) in multiple redshift bins, utilizing the most recent data from type Ia supernovae, the cosmic microwave background, baryon acoustic oscillations, the integrated Sachs-Wolfe effect, galaxy clustering, and weak lensing data. Unlike other recent analyses, we find no significant evidence for evolving dark energy; the data remain completely consistent with a cosmological constant. We also study the extent to which the time evolution of the equation of state would be constrained by a combination of current- and future-generation surveys, such as Planck and the Joint Dark Energy Mission.

Serra, Paolo; Cooray, Asantha [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Holz, Daniel E. [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Melchiorri, Alessandro [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Physics Department and Sezione INFN, University of Rome, 'La Sapienza', Piazzale Aldo Moro 2, 00185 Rome (Italy); Pandolfi, Stefania [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Physics Department and International Centre for Relativistic Astrophysics, University of Rome, 'La Sapienza', Piazzale Aldo Moro 2, 00185 Rome (Italy); Sarkar, Devdeep [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Physics Department, University of Michigan, Ann Arbor, Michigan 48109 (United States)

2009-12-15

355

Exact numerical solutions for dark waves on the discrete nonlinear Schrödinger equation.

In this paper we study numerically existence and stability of exact dark waves on the (nonintegrable) discrete nonlinear Schrödinger equation for a finite one-dimensional lattice. These are solutions that bifurcate from stationary dark modes with constant background intensity and zero intensity at a site, and whose initial state translates exactly one site each period of the internal oscillations. We show that exact dark waves are characterized by an oscillatory background whose wavelength is closely related with the velocity. Faster dark waves require smaller wavelengths. For slow enough velocity dark waves are linearly stable, but when trying to continue numerically a solution towards higher velocities bifurcations appear, due to rearrangements in the oscillatory tail in order to make possible a decreasing of the wavelength. However, in principle, one might control the stability of an exact dark wave adjusting a phase factor which plays the role of a discreteness parameter. In addition, we also study the regimes of existence and stability for stationary discrete gray modes, which are exact solutions with phase-twisted constant-amplitude background and nonzero minimum intensity. Also such solutions develop envelope oscillations on top of the homogeneous background when continued into moving phase-twisted solutions. PMID:15903625

Sánchez-Rey, Bernardo; Johansson, Magnus

2005-03-30

356

Supernovae constraints on models of dark energy reexamined

We use the Type Ia Supernova gold sample data of Riess et al in order to constrain three models of dark energy. We study the Cardassian model, the Dvali-Turner gravity modified model, and the generalized Chaplygin gas model of dark energy-dark matter unification. In our best-fit analysis for these three dark energy proposals we consider the flat model and the nonflat model priors. We also discuss the degeneracy of the models with the XCDM model through the computation of the so-called jerk parameter.

Bento, M.C.; Santos, N.M.C.; Sen, A.A. [Departamento de Fisica and Centro de Fisica Teorica de Particulas, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal); Bertolami, O. [Departamento de Fisica, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal)

2005-03-15

357

BOOK REVIEW Dark Energy: Theory and Observations Dark Energy: Theory and Observations

NASA Astrophysics Data System (ADS)

The 1998 discovery of what seems an acceleration of the cosmic expansion was made using type Ia supernovae and was later confirmed by other cosmological observations. It has made a huge impact on cosmology, prompting theoreticians to explain the observations and introducing the concept of dark energy into modern physics. A vast literature on dark energy and its alternatives has appeared since then, and this is the first comprehensive book devoted to the subject. This book is addressed to an advanced audience comprising graduate students and researchers in cosmology. Although it contains forty four fully solved problems and the first three chapters are rather introductory, they do not constitute a self-consistent course in cosmology and this book assumes graduate level knowledge of cosmology and general relativity. The fourth chapter focuses on observations, while the rest of this book addresses various classes of models proposed, including the cosmological constant, quintessence, k-essence, phantom energy, coupled dark energy, etc. The title of this book should not induce the reader into believing that only dark energy models are addressed—the authors devote two chapters to discussing conceptually very different approaches alternative to dark energy, including ƒ(R) and Gauss-Bonnet gravity, braneworld and void models, and the backreaction of inhomogeneities on the cosmic dynamics. Two chapters contain a general discussion of non-linear cosmological perturbations and statistical methods widely applicable in cosmology. The final chapter outlines future perspectives and the most likely lines of observational research on dark energy in the future. Overall, this book is carefully drafted, well presented, and does a good job of organizing the information available in the vast literature. The reader is pointed to the essential references and guided in a balanced way through the various proposals aimied at explaining the cosmological observations. Not all classes of models are treated in great detail, as expected from a volume covering an estimated four thousand papers. This much needed volume fills a gap in the literature and is a must-have in the library of young and seasoned researchers alike.

Faraoni, Valerio

2011-02-01

358

NASA Astrophysics Data System (ADS)

In this work we consider the evolution of the interactive dark fluids in the background of homogeneous and isotropic FRW model of the universe. The dark fluids consist of a warm dark matter and a dark energy and both are described as perfect fluid with barotropic equation of state. The dark species interact non-gravitationally through an additional term in the energy conservation equations. An autonomous system is formed in the energy density spaces and fixed points are analyzed. A general expression for the deceleration parameter has been obtained and it is possible to have more than one zero of the deceleration parameter. Finally, vanishing of the deceleration parameter has been examined with some examples.

Pan, Supriya; Chakraborty, Subenoy

2013-09-01

359

CONSTRAINTS ON DARK ENERGY FROM BARYON ACOUSTIC PEAK AND GALAXY CLUSTER GAS MASS MEASUREMENTS

We use baryon acoustic peak measurements by Eisenstein et al. and Percival et al., together with the Wilkinson Microwave Anisotropy Probe (WMAP) measurement of the apparent acoustic horizon angle, and galaxy cluster gas mass fraction measurements of Allen et al., to constrain a slowly rolling scalar field dark energy model, phiCDM, in which dark energy's energy density changes in time. We also compare our phiCDM results with those derived for two more common dark energy models: the time-independent cosmological constant model, LAMBDACDM, and the XCDM parameterization of dark energy's equation of state. For time-independent dark energy, the Percival et al. measurements effectively constrain spatial curvature and favor a close to the spatially flat model, mostly due to the WMAP cosmic microwave background prior used in the analysis. In a spatially flat model the Percival et al. data less effectively constrain time-varying dark energy. The joint baryon acoustic peak and galaxy cluster gas mass constraints on the phiCDM model are consistent with but tighter than those derived from other data. A time-independent cosmological constant in a spatially flat model provides a good fit to the joint data, while the alpha parameter in the inverse power-law potential phiCDM model is constrained to be less than about 4 at 3sigma confidence level.

Samushia, Lado; Ratra, Bharat, E-mail: lado@phys.ksu.ed, E-mail: ratra@phys.ksu.ed [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, KS 66506 (United States)

2009-10-01

360

Constraints on Dark Energy from Baryon Acoustic Peak and Galaxy Cluster Gas Mass Measurements

NASA Astrophysics Data System (ADS)

We use baryon acoustic peak measurements by Eisenstein et al. and Percival et al., together with the Wilkinson Microwave Anisotropy Probe (WMAP) measurement of the apparent acoustic horizon angle, and galaxy cluster gas mass fraction measurements of Allen et al., to constrain a slowly rolling scalar field dark energy model, phiCDM, in which dark energy's energy density changes in time. We also compare our phiCDM results with those derived for two more common dark energy models: the time-independent cosmological constant model, ?CDM, and the XCDM parameterization of dark energy's equation of state. For time-independent dark energy, the Percival et al. measurements effectively constrain spatial curvature and favor a close to the spatially flat model, mostly due to the WMAP cosmic microwave background prior used in the analysis. In a spatially flat model the Percival et al. data less effectively constrain time-varying dark energy. The joint baryon acoustic peak and galaxy cluster gas mass constraints on the phiCDM model are consistent with but tighter than those derived from other data. A time-independent cosmological constant in a spatially flat model provides a good fit to the joint data, while the ? parameter in the inverse power-law potential phiCDM model is constrained to be less than about 4 at 3? confidence level.

Samushia, Lado; Ratra, Bharat

2009-10-01

361

Chameleon dark energy models with characteristic signatures

In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable chameleon potentials consistent with a number of recent observational and experimental bounds. A novel chameleon field potential, motivated by f(R) gravity, is constructed where observable cosmological signatures are present both at the background evolution and in the growth rate of the perturbations. We study the evolution of matter density perturbations on low redshifts for this potential and show that the growth index today {gamma}{sub 0} can have significant dispersion on scales relevant for large scale structures. The values of {gamma}{sub 0} can be even smaller than 0.2 with large variations of {gamma} on very low redshifts for the model parameters constrained by local gravity tests. This gives a possibility to clearly distinguish these chameleon models from the {Lambda}-cold-dark-matter ({Lambda}CDM) model in future high-precision observations.

Gannouji, Radouane [IUCAA, Post Bag 4, Ganeshkhind, Pune 411 007 (India); Department of Physics, Faculty of Science, Tokyo University of Science, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan); Moraes, Bruno; Polarski, David [Laboratoire de Physique Theorique et Astroparticules, CNRS Universite Montpellier II (France); Mota, David F.; Winther, Hans A. [Institute of Theoretical Astrophysics University of Oslo (Norway); Tsujikawa, Shinji [Department of Physics, Faculty of Science, Tokyo University of Science, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)

2010-12-15

362

Neutrino mass, dark energy, and the linear growth factor

We study the degeneracies between neutrino mass and dark energy as they manifest themselves in cosmological observations. In contradiction to a popular formula in the literature, the suppression of the matter power spectrum caused by massive neutrinos is not just a function of the ratio of neutrino to total mass densities f{sub {nu}}={omega}{sub {nu}}/{omega}{sub m}, but also each of the densities independently. We also present a fitting formula for the logarithmic growth factor of perturbations in a flat universe, f(z,k;f{sub {nu}},w,{omega}{sub DE}){approx_equal}[1-A(k){omega}{sub DE}f{sub {nu}}+B(k)f{sub {nu}}{sup 2}-C(k)f{sub {nu}}{sup 3}]{omega}{sub m}{sup {alpha}}(z), where {alpha} depends on the dark energy equation of state parameter w. We then discuss cosmological probes where the f factor directly appears: peculiar velocities, redshift distortion, and the integrated Sachs-Wolfe effect. We also modify the approximation of Eisenstein and Hu [Astrophys. J. 511, 5 (1999)] for the power spectrum of fluctuations in the presence of massive neutrinos and provide a revised code [http://www.star.ucl.ac.uk/{approx}lahav/nu{sub m}atter{sub p}ower.f].

Kiakotou, Angeliki; Lahav, Ofer [Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT (United Kingdom); Elgaroey, Oystein [Institute of Theoretical Astrophysics, University of Oslo, Box 1029, 0315 Oslo (Norway)

2008-03-15

363

Observational constraints on a variable dark energy model

We study the effect of a phenomenological parameterized quintessence model on low, intermediate and high redshift observations. At low and intermediate redshifts, we use the Gold sample of supernova Type Ia (SNIa) data and recently observed size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), to put constraint on the parameters of the quintessence model. At the high redshift, the same fitting procedure is done using WAMP data, comparing the location of acoustic peak with that obtain from the dark energy model. As a complementary analysis in a flat universe, we combine the results from the SNIa, CMB and SDSS. The best fit values for the model parameters are {omega}{sub m}=0.27{sub -0.02}{sup +0.02} (the present matter content) and w{sub 0}=-1.45{sub -0.60}{sup +0.35} (dark energy equation of state). Finally we calculate the age of universe in this model and compare it with the age of old stars and high redshift objects.

Movahed, M. Sadegh [Department of Physics, Sharif University of Technology, P.O.Box 11365-9161, Tehran (Iran, Islamic Republic of); Institute for Studies in theoretical Physics and Mathematics, P.O.Box 19395-5531, Tehran (Iran, Islamic Republic of); Iran Space Agency, P.O.Box 199799-4313, Tehran (Iran, Islamic Republic of); Rahvar, Sohrab [Department of Physics, Sharif University of Technology, P.O.Box 11365-9161, Tehran (Iran, Islamic Republic of); Institute for Studies in theoretical Physics and Mathematics, P.O.Box 19395-5531, Tehran (Iran, Islamic Republic of)

2006-04-15

364

Advanced Dark Energy Physics Telescope (ADEPT). Final Report.

National Technical Information Service (NTIS)

In 2006, we proposed to NASA a detailed concept study of ADEPT (the Advanced Dark Energy Physics Telescope), a potential space mission to reliably measure the time-evolution of dark energy by conducting the largest effective volume survey of the universe ...

C. L. Bennett

2009-01-01

365

CONSTRAINING DARK ENERGY WITH GAMMA-RAY BURSTS

We use the measurement of gamma-ray burst (GRB) distances to constrain dark energy cosmological model parameters. We employ two methods for analyzing GRB data-fitting luminosity relation of GRBs in each cosmology and using distance measures computed from binned GRB data. Current GRB data alone cannot tightly constrain cosmological parameters and allow for a wide range of dark energy models.

Samushia, Lado; Ratra, Bharat, E-mail: lado@phys.ksu.ed, E-mail: ratra@phys.ksu.ed [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, KS 66506 (United States)

2010-05-10

366

Low energy antideuterons: shedding light on dark matter

Low energy antideuterons suffer a very low secondary and tertiary astrophysical background, while they can be abundantly synthesized in dark matter pair annihilations, therefore providing a privileged indirect dark matter detection technique. The recent publication of the first upper limit on the low energy antideuteron flux by the BESS Collaboration, a new evaluation of the standard astrophysical background, and remarkable

Howard Baer; Stefano Profumo

2005-01-01

367

Status of the Dark Energy Survey Camera (DECam) Project.

National Technical Information Service (NTIS)

The Dark Energy Survey Collaboration has completed construction of the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera which will be mounted on the Blanco 4-meter telescope at CTIO. DECam will be used to perform the 5000 sq. deg. D...

2012-01-01

368

Age problem in the holographic dark energy model

In this paper, we test the original holographic dark energy model with some old high redshift objects. The main idea is very simple: the universe cannot be younger than its constituents. We find that the original holographic dark energy model can be ruled out, unless a lower Hubble constant is taken.

Wei Hao; Zhang Shuangnan [Department of Physics and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084 (China) and Department of Physics and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084 (China) and Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) and Physics Department, University of Alabama in Huntsville, Huntsville, Alabama 35899 (United States)

2007-09-15

369

Dark solitons of the power-energy saturation model: application to mode-locked lasers

NASA Astrophysics Data System (ADS)

The generation and dynamics of dark solitons in mode-locked lasers is studied within the framework of a nonlinear Schrödinger equation which incorporates power-saturated loss, as well as energy-saturated gain and filtering. Mode-locking into single dark solitons and multiple dark pulses are found by employing different descriptions for the energy and power of the system defined over unbounded and periodic (ring laser) systems. Treating the loss, gain and filtering terms as perturbations, it is shown that these terms induce an expanding shelf around the soliton. The dark soliton dynamics are studied analytically by means of a perturbation method that takes into regard the emergence of the shelves and reveals their importance.

Ablowitz, M. J.; Nixon, S. D.; Horikis, T. P.; Frantzeskakis, D. J.

2013-03-01

370

We generalize the small scale dynamics of the Universe by taking into account models with an equation of state which evolves with time, and provide a complete formulation of the cluster virialization attempting to address the nonlinear regime of structure formation. In the context of the current dark energy models, we find that galaxy clusters appear to form at z{approx}1-2, in agreement with previous studies. Also, we investigate thoroughly the evolution of spherical matter perturbations, as the latter decouple from the background expansion and start to 'turn around' and finally collapse. Within this framework, we find that the concentration parameter depends on the choice of the considered dark energy (homogeneous or clustered). In particular, if the distribution of the dark energy is clustered then we produce more concentrated structures with respect to the homogeneous dark energy. Finally, comparing the predicted concentration parameter with the observed concentration parameter, measured for four massive galaxy clusters, we find that the scenario which contains a pure homogeneous dark energy is unable to reproduce the data. The situation becomes somewhat better in the case of an inhomogeneous (clustered) dark energy.

Basilakos, Spyros; Sanchez, Juan Carlos Bueno; Perivolaropoulos, Leandros [Academy of Athens, Research Center for Astronomy and Applied Mathematics, Soranou Efesiou 4, 11527 Athens (Greece); Department of Physics, University of Ioannina, 45110 Ioannina (Greece)

2009-08-15

371

Constraining dark energy evolution with gravitational lensing by large scale structures

NASA Astrophysics Data System (ADS)

We study the sensitivity of weak lensing by large scale structures as a probe of the evolution of dark energy. We explore a two-parameters model of dark energy evolution, inspired by tracking quintessence models. To this end, we compute the likelihood of a few fiducial models with varying and nonvarying equation of states. For the different models, we investigate the dark energy parameter degeneracies with the mass power spectrum shape ?, normalization ?8, and with the matter mean density ?M. We find that degeneracies are such that weak lensing turns out to be a good probe of dark energy evolution, even with limited knowledge on ?, ?8, and ?M. This result is a strong motivation for performing large scale structure simulations beyond the simple constant dark energy models, in order to calibrate the nonlinear regime accurately. Such calibration could then be used for any large scale structure tests of dark energy evolution. Prospective for the Canada France Hawaii Telescope Legacy Survey and Super-Novae Acceleration Probe are given. These results complement nicely the cosmic microwave background and supernovae constraints.

Benabed, Karim; van Waerbeke, Ludovic

2004-12-01

372

Explaining the dark energy, baryon and dark matter coincidence via domain-dependent random densities

NASA Astrophysics Data System (ADS)

The dark energy, dark matter and baryon densities in the Universe are observed to be similar, with a factor of no more than 20 between the largest and smallest densities. We show that this coincidence can be understood via superhorizon domains of randomly varying densities when the baryon density at initial collapse of galaxy-forming perturbations is determined by anthropic selection. The baryon and dark matter densities are assumed to be dependent on random variables ?d and ?b according to ?dmpropto?d? and ?bpropto?b?, while the effectively constant dark energy density is dependent upon a random variable phiQ according to ?QproptophiQn. The ratio of the baryon density to the dark energy density at initial collapse, rQ, and the baryon-to-dark matter ratio, r, are then determined purely statistically, with no dependence on the anthropically-preferred baryon density. We compute the probability distribution for rQ and r and show that the observed values of rQ and r can be naturally understood within this framework. In particular, for the case ? = 2, ? = 1 and n = 4, which can be physically realized via a combination of axion dark matter, Affleck-Dine baryogenesis and frozen quintessence with a phiQ4 potential, the range of rQ and r which corresponds to the observed Universe is a quite natural, with a probability which is broadly similar to other ranges of rQ and r.

McDonald, John

2013-05-01

373

Dark energy as an elastic strain fluid

NASA Astrophysics Data System (ADS)

The origin of the accelerated expansion of the Universe is still unclear and new physics is needed on cosmological scales. We propose and test a novel interpretation of dark energy as originated by an elastic strain due to a cosmic defect in an otherwise Euclidean space-time. The strain modifies the expansion history of the universe. This new effective contribution tracks radiation at early times and mimics a cosmological constant at late times. The theory is tested against observations, from nucleosynthesis to the cosmic microwave background and formation and evolution of large-scale structure to supernovae. Data are very well reproduced with Lamé parameters of the order of 10-52 m-2.

Radicella, N.; Sereno, M.; Tartaglia, A.

2013-02-01

374

Linear and nonlinear instabilities in unified dark energy models

We revisit the paradigm of unified dark energy discussing in detail the averaging problem in this type of scenario, highlighting the need for a full nonlinear treatment. We also address the question of if and how models with one or several dark fluids can be observationally distinguished. Simpler and physically clearer derivations of some key results, most notably on the relation between the generalized Chaplygin gas and the standard ({lambda}CDM) 'concordance' model and on a Jeans-type small-scale instability of some coupled dark energy/dark matter models are presented.

Avelino, P. P.; Beca, L. M. G. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Martins, C. J. A. P. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Centro de Astrofisica, Universidade do Porto, Rua das Estrelas s/n, 4150-762 Porto (Portugal); Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)

2008-03-15

375

NASA Astrophysics Data System (ADS)

Context: Clusters are potentially powerful tools for cosmology provided their observed properties, such as the Sunyaev-Zel'dovich (SZ) or X-ray signals, can be translated into physical quantities like mass and temperature. Scaling relations are the appropriate means to perform this translation. It is, therefore, important to understand their evolution and their modifications with respect to the physics and to the underlying cosmology. Aims: In this spirit, we investigate the effect of dark energy on the X-ray and SZ scaling relations. The study is based on the first hydro-simulations of cluster formation for diferent models of dark energy. We present results for four dark-energy models which differ from each other by their equations-of-state parameter, w. Namely, we use a cosmological constant model w=-1 (as a reference), a perfect fluid with constant equation of state parameter w=-0.8 and one with w = -1.2 and a scalar field model (or quintessence) with varying w. Methods: We generate N-body/hydrodynamic simulations that include radiative cooling with the public version of the Hydra code, modified to consider an arbitrary dark-energy component. We produce cluster catalogues for the four models and derive the associated X-ray and SZ scaling relations. Results: We find that dark energy has little effect on scaling laws, making it safe to use the standard ?CDM scalings for conversion of observed quantities into cluster temperatures and masses.

Aghanim, N.; da Silva, A. C.; Nunes, N. J.

2009-03-01

376

The CHASE laboratory search for chameleon dark energy

A scalar field is a favorite candidate for the particle responsible for dark energy. However, few theoretical means exist that can simultaneously explain the observed acceleration of the Universe and evade tests of gravity. The chameleon mechanism, whereby the properties of a particle depend upon the local environment, is one possible avenue. We present the results of the Chameleon Afterglow Search (CHASE) experiment, a laboratory probe for chameleon dark energy. CHASE marks a significant improvement other searches for chameleons both in terms of its sensitivity to the photon/chameleon coupling as well as its sensitivity to the classes of chameleon dark energy models and standard power-law models. Since chameleon dark energy is virtually indistinguishable from a cosmological constant, CHASE tests dark energy models in a manner not accessible to astronomical surveys.

Steffen, Jason H.; /Fermilab

2010-11-01

377

Baryon acoustic oscillation intensity mapping of dark energy.

The expansion of the Universe appears to be accelerating, and the mysterious antigravity agent of this acceleration has been called "dark energy." To measure the dynamics of dark energy, baryon acoustic oscillations (BAO) can be used. Previous discussions of the BAO dark energy test have focused on direct measurements of redshifts of as many as 10(9) individual galaxies, by observing the 21 cm line or by detecting optical emission. Here we show how the study of acoustic oscillation in the 21 cm brightness can be accomplished by economical three-dimensional intensity mapping. If our estimates gain acceptance they may be the starting point for a new class of dark energy experiments dedicated to large angular scale mapping of the radio sky, shedding light on dark energy. PMID:18352692

Chang, Tzu-Ching; Pen, Ue-Li; Peterson, Jeffrey B; McDonald, Patrick

2008-03-05

378

Mega-masers, Dark Energy and the Hubble Constant

Powerful water maser emission (water mega-masers) can be found in accretion disks in the nuclei of some galaxies. Besides providing a measure of the mass at the nucleus, such mega-masers can be used to determine the distance to the host galaxy, based on a kinematic model. We will explain the importance of determining the Hubble Constant to high accuracy for constraining the equation of state of Dark Energy and describe the Mega-maser Cosmology Project that has the goal of determining the Hubble Constant to better than 3%. Time permitting, we will also present the scientific capabilities of the current and future NRAO facilities: ALMA, EVLA, VLBA and GBT, for addressing key astrophysical problems

379

A back-reaction approach to dark energy

NASA Astrophysics Data System (ADS)

This thesis is mainly about how to set up and carry out in a physically meaningful way the idea of back-reaction, according to which dark energy could be an effective source. There are, broadly speaking, two distinct approaches. One is focused on how cosmological observables are affected by inhomogeneities, while the other is focused on a theoretical description of the inhomogeneous universe by means of a mean-field description. Both approaches, however, share the idea of smoothing out inhomogeneities. We developed this duality in the interpretation of the back-reaction by means of toy models based on the Lemaitre-Tolman-Bondi solution of Einstein's equations. In particular we focused on voids expanding faster than the background solution.

Marra, Valerio

2008-03-01

380

Controlling for the Effects of Baryons in Cosmic Shear Constraints on Dark Energy

NASA Astrophysics Data System (ADS)

The uncertainty introduced by the effects of baryons on the power spectrum of the convergence field is a significant theoretical error limiting forthcoming gravitational lensing surveys. A proposed method to account for baryonic effects is to include parameters that characterize dark matter halos, and to fit lensing data to these halo parameters concurrently with the cosmological parameters. We test this proposal by using this technique to model convergence power spectrum predictions from a set of cosmological simulations. We estimate biases in dark energy parameters that would be incurred if one were to fit the spectra predicted by the simulations either with no model for baryons, or with the proposed method. Neglecting baryonic effects leads to biases in dark energy parameters that are several times the statistical errors of surveys like the Dark Energy Survey. The proposed method to correct for baryonic effects renders the residual biases in dark energy equation of state parameters smaller than the statistical errors. These results suggest that this mitigation method may be applied to analyze convergence spectra from a survey like the Dark Energy Survey. For significantly larger surveys, such as the LSST or Euclid, the biases introduced by baryonic effects remain significant.

Zentner, Andrew

2013-04-01

381

We characterize the full family of soliton solutions sitting over a background plane wave and ruled by the cubic-quintic nonlinear Schroedinger equation in the regime where a quintic focusing term represents a saturation of the cubic defocusing nonlinearity. We discuss the existence and properties of solitons in terms of catastrophe theory and fully characterize bistability and instabilities of the dark-antidark pairs, revealing mechanisms of decay of antidark solitons into dispersive shock waves.

Crosta, M. [Dept. of Physics, Sapienza University of Rome, I-00185, Rome (Italy); Fratalocchi, A. [PRIMALIGHT, Faculty of Electrical Engineering, Applied Mathematics and Computational Science, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900 (Saudi Arabia); Trillo, S. [Dipartimento di Ingegneria, Universita di Ferrara, Via Saragat 1, I-44122 Ferrara (Italy)

2011-12-15

382

Joint galaxy-lensing observables and the dark energy

Deep multicolor galaxy surveys with photometric redshifts will provide a large number of two-point correlation observables: galaxy-galaxy angular correlations, galaxy-shear cross correlations, and shear-shear correlations between all redshifts. These observables can potentially enable a joint determination of the dark-energy-dependent evolution of the dark matter and distances as well as the relationship between galaxies and dark matter halos. With recent cosmic

Wayne Hu; Bhuvnesh Jain

2004-01-01

383

Measuring Time Dependence of Dark Energy Density from Type IA Supernova Data

Observations of high-redshift supernovae imply an accelerating universe that can only be explained by an unusual energy component such as vacuum energy or quintessence. To assess the ability of current and future supernova data to constrain the properties of the dark energy, we allow its density to have arbitrary time dependence, rhoX(z). This leads to an equation of state for

Yun Wang; Peter M. Garnavich

2001-01-01

384

Entropy and universality of the Cardy-Verlinde formula in a dark energy universe

We study the entropy of a FRW universe filled with dark energy (cosmological\\u000aconstant, quintessence or phantom). For general or time-dependent equation of\\u000astate $p=w\\\\rho$ the entropy is expressed in terms of energy, Casimir energy,\\u000aand $w$. The correspondent expression reminds one about 2d CFT entropy only for\\u000aconformal matter. At the same time, the cosmological Cardy-Verlinde formula\\u000arelating three

Iver Brevik; Sergei D. Odintsov; Luciano Vanzo; Edifici Nexus

2004-01-01

385

Generalized holographic dark energy model in the Hubble length

NASA Astrophysics Data System (ADS)

We generalize the holographic dark energy model described in Hubble length IR cutoff by assuming a slowly time varying function for holographic parameter c 2. We calculate the evolution of EoS parameter and the deceleration parameter as well as the evolution of dark energy density parameter of the model in flat FRW universe. We show that in this model the phantom line is crossed from quintessence regime to phantom regime which is in agreement with observation. The evolution of deceleration parameter of the model indicates the transition from decelerated to accelerated expansion consistently with observation. Eventually, we show that the holographic dark energy model with Hubble horizon IR cutoff can interpret the pressureless dark matter era at the early time and dark energy dominated phase later. The singularity of the model is also calculated.

Malekjani, Mohammad

2013-10-01

386

NASA Astrophysics Data System (ADS)

The abundance and distribution of collapsed objects such as galaxy clusters will become an important tool to investigate the nature of dark energy and dark matter. Number counts of very massive objects are sensitive not only to the equation of state of dark energy, which parametrizes the smooth component of its pressure, but also to the sound speed of dark energy, which determines the amount of pressure in inhomogeneous and collapsed structures. Since the evolution of these structures must be followed well into the nonlinear regime, and a fully relativistic framework for this regime does not exist yet, we compare two approximate schemes: the widely used spherical collapse model and the pseudo-Newtonian approach. We show that both approximation schemes convey identical equations for the density contrast, when the pressure perturbation of dark energy is parametrized in terms of an effective sound speed. We also make a comparison of these approximate approaches to general relativity in the linearized regime, which lends some support to the approximations.

Abramo, L. R.; Batista, R. C.; Liberato, L.; Rosenfeld, R.

2009-01-01

387

The abundance and distribution of collapsed objects such as galaxy clusters will become an important tool to investigate the nature of dark energy and dark matter. Number counts of very massive objects are sensitive not only to the equation of state of dark energy, which parametrizes the smooth component of its pressure, but also to the sound speed of dark energy, which determines the amount of pressure in inhomogeneous and collapsed structures. Since the evolution of these structures must be followed well into the nonlinear regime, and a fully relativistic framework for this regime does not exist yet, we compare two approximate schemes: the widely used spherical collapse model and the pseudo-Newtonian approach. We show that both approximation schemes convey identical equations for the density contrast, when the pressure perturbation of dark energy is parametrized in terms of an effective sound speed. We also make a comparison of these approximate approaches to general relativity in the linearized regime, which lends some support to the approximations.

Abramo, L. R.; Batista, R. C. [Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo (Brazil); Liberato, L.; Rosenfeld, R. [Instituto de Fisica Teorica, Universidade Estadual Paulista, R. Pamplona 145, 01405-900, Sao Paulo (Brazil)

2009-01-15

388

Chemical potential and the nature of dark energy: The case of a phantom field

The influence of a possible nonzero chemical potential {mu} on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state, p={omega}{rho} ({omega}<0, constant). The entropy condition, S{>=}0, implies that the possible values of {omega} are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For {mu}>0, the {omega} parameter must be greater than -1 (vacuum is forbidden) while for {mu}<0 not only the vacuum but even a phantomlike behavior ({omega}<-1) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, {mu}/T={mu}{sub 0}/T{sub 0}. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons {mu} is always negative and the extended Wien's law allows only a dark component with {omega}<-1/2, which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for {mu}<0. However, fermionic particles with {mu}>0 are permitted only if -1<{omega}<-1/2. The thermodynamics and statistical arguments constrain the equation-of-state parameter to be {omega}<-1/2, a result surprisingly close to the maximal value required to accelerate a Friedmann-Robertson-Walker-type universe dominated by matter and dark energy ({omega} < or approx. -10/21)

Lima, J. A. S.; Pereira, S. H. [Departamento de Astronomia, Universidade de Sao Paulo, Rua do Matao, 1226 - 05508-900, Sao Paulo, Sao Paulo (Brazil)

2008-10-15

389

Unified model of k-inflation, dark matter, and dark energy

We present a k-essence model where a single scalar field is responsible for the early expansion of the Universe through the process of k inflation and at appropriate subsequent stages acts both as dark matter and dark energy. The Lagrangian contains a potential for the scalar field as well as a noncanonical kinetic term, and is of the form F(X)V({phi}) which has been widely used as a k-essence Lagrangian. After the period of inflation is over the model can be approximated as purely kinetic k essence, generating dark matter and dark energy at late times. We show how observational results are used to put constraints on the parameters of this model.

Bose, Nilok; Majumdar, A. S. [S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Calcutta 700098 (India)

2009-11-15

390

Testing dark energy paradigms with weak gravitational lensing

NASA Astrophysics Data System (ADS)

Any theory invoked to explain cosmic acceleration predicts consistency relations between the expansion history, structure growth, and all related observables. Currently there exist high-quality measurements of the expansion history from type Ia supernovae, the cosmic microwave background temperature and polarization spectra, and baryon acoustic oscillations. We can use constraints from these data sets to predict what future probes of structure growth should observe. We apply this method to predict what range of cosmic shear power spectra would be expected if we lived in a ?CDM universe, with or without spatial curvature, and what results would be inconsistent and therefore falsify the model. Though predictions are relaxed if one allows for an arbitrary quintessence equation of state -1?w(z)?1, we find that any observation that rules out ?CDM due to excess lensing will also rule out all quintessence models, with or without early dark energy. We further explore how uncertainties in the nonlinear matter power spectrum, e.g. from approximate fitting formulas such as Halofit, warm dark matter, or baryons, impact these limits.

Vanderveld, R. Ali; Mortonson, Michael J.; Hu, Wayne; Eifler, Tim

2012-05-01

391

A Possible Solution to the Smallness Problem of Dark Energy

The smallness of the dark energy density has been recognized as the most crucial difficulty in understanding dark energy and also one of the most important questions in the new century. In a recent paper[1], we proposed a new dark energy model in which the smallness of the cosmological constant is naturally achieved by invoking the Casimir energy in a supersymmetry-breaking brane-world. In this paper we review the basic notions of this model. Various implications, perspectives, and subtleties of this model are briefly discussed.

Chen, Pisin; /SLAC; Gu, Je-An; /Taiwan, Natl. Taiwan U.

2005-07-08

392

NEW LIMITS ON EARLY DARK ENERGY FROM THE SOUTH POLE TELESCOPE

We present new limits on early dark energy (EDE) from the cosmic microwave background (CMB) using data from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite on large angular scales and South Pole Telescope on small angular scales. We find a strong upper limit on the EDE density of {Omega}{sub e} < 0.018 at 95% confidence, a factor of three improvement over WMAP data alone. We show that adding lower-redshift probes of the expansion rate to the CMB data improves constraints on the dark energy equation of state, but not the EDE density. We also explain how small-scale CMB temperature anisotropy constrains EDE.

Reichardt, C. L. [Department of Physics, University of California, Berkeley, CA 94720 (United States); De Putter, R. [IFIC, Universidad de Valencia-CSIC, Valencia 46071 (Spain); Zahn, O. [Berkeley Center for Cosmological Physics, Department of Physics, University of California, and Lawrence Berkeley National Labs, Berkeley, CA 94720 (United States); Hou, Z., E-mail: cr@bolo.berkeley.edu [Department of Physics, University of California, Davis, CA 95616 (United States)

2012-04-10

393

In this paper, we give a proof of the existence of stationary dark soliton solutions or heteroclinic orbits of nonlinear equations of Schroedinger type with periodic inhomogeneous nonlinearity. The result is illustrated with examples of dark solitons for cubic and photorefractive nonlinearities.

Belmonte-Beitia, J. [Departamento de Matematicas, E. T. S. de Ingenieros Industriales and Instituto de Matematica Aplicada a la Ciencia y la Ingenieria (IMACI), Universidad de Castilla-La Mancha, 13071 Ciudad Real (Spain); Cuevas, J. [Grupo de Fisica No Lineal, Departamento de Fisica Aplicada I, Escuela Politecnica Superior, Universidad de Sevilla, C/ Virgen de Africa, 7, 41011 Sevilla (Spain)

2011-03-15

394

Planck constraints on holographic dark energy

NASA Astrophysics Data System (ADS)

We perform a detailed investigation on the cosmological constraints on the holographic dark energy (HDE) model by using the Plank data. We find that HDE can provide a good fit to the Plank high-l (l gtrsim 40) temperature power spectrum, while the discrepancy at l simeq 20-40 found in the ?CDM model remains unsolved in the HDE model. The Plank data alone can lead to strong and reliable constraint on the HDE parameter c. At the 68% confidence level (CL), we obtain c = 0.508 ± 0.207 with Plank+WP+lensing, favoring the present phantom behavior of HDE at the more than 2? CL. By combining Plank+WP with the external astrophysical data sets, i.e. the BAO measurements from 6dFGS+SDSS DR7(R)+BOSS DR9, the direct Hubble constant measurement result (H0 = 73.8 ± 2.4 kms-1Mpc-1) from the HST, the SNLS3 supernovae data set, and Union2.1 supernovae data set, we get the 68% CL constraint results c = 0.484 ± 0.070, 0.474 ± 0.049, 0.594 ± 0.051, and 0.642 ± 0.066, respectively. The constraints can be improved by 2%-15% if we further add the Plank lensing data into the analysis. Compared with the WMAP-9 results, the Plank results reduce the error by 30%-60%, and prefer a phantom-like HDE at higher significant level. We also investigate the tension between different data sets. We find no evident tension when we combine Plank data with BAO and HST. Especially, we find that the strong correlation between ?mh3 and dark energy parameters is helpful in relieving the tension between the Plank and HST measurements. The residual value of ?2Plank+WP+HST-?2Plank+WP is 7.8 in the ?CDM model, and is reduced to 1.0 or 0.3 if we switch the dark energy to w model or the holographic model. When we introduce supernovae data sets into the analysis, some tension appears. We find that the SNLS3 data set is in tension with all other data sets; for example, for the Plank+WP, WMAP-9 and BAO+HST, the corresponding ??2 is equal to 6.4, 3.5 and 4.1, respectively. As a comparison, the Union2.1 data set is consistent with these three data sets, but the combination Union2.1+BAO+HST is in tension with Plank+WP+lensing, corresponding to a large ??2 that is equal to 8.6 (1.4% probability). Thus, combining internal inconsistent data sets (SNIa+BAO+HST with Plank+WP+lensing) can lead to ambiguous results, and it is necessary to perform the HDE data analysis for each independent data sets. Our tightest self-consistent constraint is c = 0.495 ± 0.039 obtained from Plank+WP+BAO+HST+lensing.

Li, Miao; Li, Xiao-Dong; Ma, Yin-Zhe; Zhang, Xin; Zhang, Zhenhui

2013-09-01

395

Dark Energy and Dark Matter from the same Vacuum Condensate

The micro-quantum Dirac negative energy electron Fermi sphere with Planck scale cutoff is unstable to the formation of off-mass-shell Cooper pairs of virtual electrons and positrons from their static Coulomb attraction. The resulting virtual BEC complex macro-quantum coherent local order parameter (0|e+e-|0) gives rise to both spin 2 gravity guv and spin 0 quintessence \\/ from the Goldstone and Higgs

Jack Sarfatti

2003-01-01

396

Scalar field dark energy perturbations and their scale dependence

We estimate the amplitude of perturbation in dark energy at different length scales for a quintessence model with an exponential potential. It is shown that on length scales much smaller than Hubble radius, perturbation in dark energy is negligible in comparison to that in dark matter. However, on scales comparable to the Hubble radius ({lambda}{sub p}>1000 Mpc) the perturbation in dark energy in general cannot be neglected. As compared to the {lambda}CDM model, the large-scale matter power spectrum is suppressed in a generic quintessence dark energy model. We show that on scales {lambda}{sub p}<1000 Mpc, this suppression is primarily due to different background evolution compared to the {lambda}CDM model. However, on much larger scales perturbation in dark energy can affect the matter power spectrum significantly. Hence this analysis can act as a discriminator between the {lambda}CDM model and other generic dark energy models with w{sub de}{ne}-1.

Unnikrishnan, Sanil; Seshadri, T. R. [Department of Physics and Astrophysics, University of Delhi, Delhi 110007 (India); Jassal, H. K. [Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019 (India)

2008-12-15

397

On the spherical collapse model in dark energy cosmologies

We study the spherical collapse model in dark energy cosmologies, in which\\u000adark energy is modelled as a minimally coupled scalar field. We first follow\\u000athe standard assumption that dark energy does not cluster on the scales of\\u000ainterest. Investigating four different popular potentials in detail, we show\\u000athat the predictions of the spherical collapse model depend on the potential

David F. Mota; Carsten van de Bruck

2004-01-01

398

Large format filter changer mechanism for the dark energy survey

NASA Astrophysics Data System (ADS)

The Dark Energy Survey is a Stage III Dark Energy Experiment that will obtain cosmological parameters by combining four observational techniques; Galaxy Clusters, Weak Lensing, Type Ia Supernovae and Baryon Acoustic Oscillations. The observations will be performed with a new wide field camera (DECam) that will be placed on the Blanco 4 m telescope at CTIO. Here we describe the large format (600 mm clear aperture) Filter Changer Mechanism (FCM) for the Dark Energy Survey Camera (DECam). The FCM, based on the Pan-STARRS design, is the largest ever constructed. Fabrication of the filter changer has been completed and it has been tested under realistic conditions.

Tarlé, G.; Bigelow, B.; Boprie, D.; Cooper, C.; Dede, E.; Lorenzon, W.; Nord, B.; Schubnell, M.; Weaverdyck, C.

2010-07-01

399

Neutrino dark energy with more than one neutrino species

The mass varying neutrino scenario is a model that successfully explains the origin of dark energy while at the same time solves the coincidence problem. The model is, however, heavily constrained by its stability towards the formation of neutrino bound states when the neutrinos become nonrelativistic. We discuss these constraints and find that natural, adiabatic, stable models with the right amount of dark energy today do not exist. Second, we explain why using the lightest neutrino, which is still relativistic, as an explanation for dark energy does not work because of a feedback mechanism from the heavier neutrinos.

Bjaelde, Ole Eggers; Hannestad, Steen [Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark and Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003 (United States); Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C (Denmark)

2010-03-15

400

"CosmoMicroPhysics" Approach to Study the Dark Matter and Dark Energy

NASA Astrophysics Data System (ADS)

In 2007-2009 the Complex Research Program of the NAS of Ukraine titled "Study of the Structure of the Universe, Dark Matter and Dark Energy" (CosmoMicroPhysics) was con-ducted with the aim to join efforts of the Ukrainian scientists for resolving this actual task (http://www.nas.gov.ua/ResearchActivities/ComplexProgram/Pages/17.aspx). Our research team is presented by the scientists and post-graduated students from 15 institutes and univer-sities of Ukraine ()about 70 persons) working in the different fields (astrophysics, mathematics, theoretical physics, and nuclear physics). The main scientific goals, which were put forwards on the observational and theoretical revelations of dark matter/dark energy, were the follow-ing: -Observational base of the astronomical revelations of dark matter and dark energy as well as candidates to the different baryonic components of the hidden mass of the Universe; -Observational base of the earlier evolution of the Universe and properties of the large-scale structure; -Theoretical support for such observational data and creation of the cosmological models; -Experimental search of the WIMPs and study of the neutrino properties as one of the main components of a dark matter; -Theoretical research of the classical and quantum fields in astrophysics and cosmology. We will discuss the main results obtained by our team as the essential contribution to resolve this problem: * Observations, data analysis, and estimation as regarding the various LMS components of the Universe, at the first turn as the candidates to the dark matter (AGNs, black holes in double stars, halo of galaxies and galaxy groups/clusters, mass-to-luminosity estimation for isolated galaxies and galaxies in clusters/groups, brawn dwarfs etc.); * Gravitational lenses as the sources of the mass distribution data in the Universe; *Theoretical models of the Universe with cosmological fields, Dark energy models, and research of the dark energy impact on the evolution of the Universe; *Experimental limits for the WIMPs cross-section with nuclei, as well as a study of the neutrino properties in the experiments on the double beta-decay; *Development of the modifications of the Relativity Theory, including in spaces with the additional dimensions; *Study of the role of quantum effects in cosmology and astrophysics, as well as the cosmological and astrophysical revelations at the phase transitions in super dense matter. The most part of these researches is conducting in the tight international cooperation. Focusing an attention at the NASU "CosmoMicroPhysics" program, we will discuss also our proposals for the further cooperation, including the experiments for space missions..

Vavilova, Iryna; Shulga, Valery M.

401

Stable large-scale perturbations in interacting dark-energy model

NASA Astrophysics Data System (ADS)

It is found that the evolutions of density perturbations on the super-Hubble scales are unstable in the models with dark-sector interaction Q proportional to the energy density of cold dark matter (CDM) and constant equation of state parameter of dark energy (DE). In this paper, to avoid the instabilities, we suggest a new covariant model for the energy-momentum transfer between DE and CDM. Then we show that the the large-scale instabilities of curvature perturbations can be avoided in a universe filled only by DE and CDM. Furthermore, by including the additional components of radiation and baryons, we calculate the dominant non-adiabatic modes in the radiation era and find that the modes grow in the power law with exponent at the order of unit.

Sun, Cheng-Yi; Yue, Rui-Hong

2013-08-01

402

Phantom instability of viscous dark energy in anisotropic space-time

NASA Astrophysics Data System (ADS)

Phantom dark energy is a proposal that explains the current observations that mildly favor the equation of state of dark energy ? de crossing -1 at 68 % confidence level. However, phantom fields are generally ruled out by ultraviolet quantum instabilities. To overcome this discrepancy, in this paper we propose a mechanism to show that how the presence of bulk viscosity in the cosmic fluid can temporarily drive the fluid into the phantom region ( ?<-1). As time is going on, phantom decays and ultimately ? de approaches to -1. Then we show these quintessence and phantom descriptions of non-viscous and viscous dark energy and reconstruct the potential of these two scalar fields. Also a diagnostic for these models are performed by using the statefinder pairs { s, r}. All results are obtained in an anisotropic space-time which is a generalization of FLRW universe.

Amirhashchi, Hassan

2013-06-01

403

NASA Astrophysics Data System (ADS)

We present a best-fit analysis on the single-parameter holographic dark energy model characterized by the conformal-age-like length, L = (1)/(a4(t))? 0^tdt^' a3(t^'). Based on the Union2 compilation of 557 supernova Ia (SNIa) data, the baryon acoustic oscillation (BAO) results from the Sloan Digital Sky Survey data release 7 (SDSS DR7) and the cosmic microwave background radiation (CMB) data from the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7), we show that the model gives the minimal ?2\\min = 546.273, which is comparable to ?2{? CDM} = 544.616 for the ?CDM model. The single parameter d concerned in the model is found to be d = 0.232±0.006±0.009. Since the fractional density of dark energy ?de d2a2 at a ? 1, the fraction of dark energy is naturally negligible in the early universe, ?de ? 1 at a ? 1. The resulting constraints on the present fractional energy density of matter and the equation of state are ? m0 = 0.286+0.019-0.018{}+0.032-0.028 and w de 0 = -1.240+0.027-0.027{}+0.045-0.044 respectively. We also provide a systematic analysis on the cosmic evolutions of the fractional energy density of dark energy, the equation of state of dark energy, the deceleration parameter and the statefinder. It is noticed that the equation of state crosses from wde > -1 to wde < -1, the universe transits from decelerated expansion (q > 0) to accelerated expansion (q < 0) recently, and the statefinder may serve as a sensitive diagnostic to distinguish the CHDE model with the ?CDM model.

Huang, Zhuo-Peng; Wu, Yue-Liang

2012-09-01

404

Black Hole Universe Model and Dark Energy

NASA Astrophysics Data System (ADS)

Considering black hole as spacetime and slightly modifying the big bang theory, the author has recently developed a new cosmological model called black hole universe, which is consistent with Mach principle and Einsteinian general relativity and self consistently explains various observations of the universe without difficulties. According to this model, the universe originated from a hot star-like black hole and gradually grew through a supermassive black hole to the present universe by accreting ambient material and merging with other black holes. The entire space is infinitely and hierarchically layered and evolves iteratively. The innermost three layers are the universe that we lives, the outside space called mother universe, and the inside star-like and supermassive black holes called child universes. The outermost layer has an infinite radius and zero limits for both the mass density and absolute temperature. All layers or universes are governed by the same physics, the Einstein general relativity with the Robertson-Walker metric of spacetime, and tend to expand outward physically. When one universe expands out, a new similar universe grows up from its inside black holes. The origin, structure, evolution, expansion, and cosmic microwave background radiation of black hole universe have been presented in the recent sequence of American Astronomical Society (AAS) meetings and published in peer-review journals. This study will show how this new model explains the acceleration of the universe and why dark energy is not required. We will also compare the black hole universe model with the big bang cosmology.

Zhang, Tianxi

2011-01-01

405

Cooling the dark energy camera instrument

DECam, camera for the Dark Energy Survey (DES), is undergoing general design and component testing. For an overview see DePoy, et al in these proceedings. For a description of the imager, see Cease, et al in these proceedings. The CCD instrument will be mounted at the prime focus of the CTIO Blanco 4m telescope. The instrument temperature will be 173K with a heat load of 113W. In similar applications, cooling CCD instruments at the prime focus has been accomplished by three general methods. Liquid nitrogen reservoirs have been constructed to operate in any orientation, pulse tube cryocoolers have been used when tilt angles are limited and Joule-Thompson or Stirling cryocoolers have been used with smaller heat loads. Gifford-MacMahon cooling has been used at the Cassegrain but not at the prime focus. For DES, the combined requirements of high heat load, temperature stability, low vibration, operation in any orientation, liquid nitrogen cost and limited space available led to the design of a pumped, closed loop, circulating nitrogen system. At zenith the instrument will be twelve meters above the pump/cryocooler station. This cooling system expected to have a 10,000 hour maintenance interval. This paper will describe the engineering basis including the thermal model, unbalanced forces, cooldown time, the single and two-phase flow model.

Schmitt, R.L.; Cease, H.; /Fermilab; DePoy, D.; /Ohio State U.; Diehl, H.T.; Estrada, J.; Flaugher, B.; /Fermilab; Kuhlmann, S.; /Ohio State U.; Onal, Birce; Stefanik, A.; /Fermilab

2008-06-01

406

Cooling the Dark Energy Camera instrument

NASA Astrophysics Data System (ADS)

DECam, camera for the Dark Energy Survey (DES), is undergoing general design and component testing. For an overview see DePoy, et al in these proceedings. For a description of the imager, see Cease, et al in these proceedings. The CCD instrument will be mounted at the prime focus of the CTIO Blanco 4m telescope. The instrument temperature will be 173K with a heat load of 113W. In similar applications, cooling CCD instruments at the prime focus has been accomplished by three general methods. Liquid nitrogen reservoirs have been constructed to operate in any orientation, pulse tube cryocoolers have been used when tilt angles are limited and Joule-Thompson or Stirling cryocoolers have been used with smaller heat loads. Gifford-MacMahon cooling has been used at the Cassegrain but not at the prime focus. For DES, the combined requirements of high heat load, temperature stability, low vibration, operation in any orientation, liquid nitrogen cost and limited space available led to the design of a pumped, closed loop, circulating nitrogen system. At zenith the instrument will be twelve meters above the pump/cryocooler station. This cooling system expected to have a 10,000 hour maintenance interval. This paper will describe the engineering basis including the thermal model, unbalanced forces, cooldown time, the single and two-phase flow model.

Schmitt, R. L.; Cease, H.; DePoy, D.; Diehl, H. T.; Estrada, J.; Flaugher, B.; Kuhlmann, S.; Onal, Birce; Stefanik, A.

2008-08-01

407

The Dark Energy Survey Data Management System

The Dark Energy Survey (DES) is a project with the goal of building, installing and exploiting a new 74 CCD-camera at the Blanco telescope, in order to study the nature of cosmic acceleration. It will cover 5000 square degrees of the southern hemisphere sky and will record the positions and shapes of 300 million galaxies up to redshift 1.4. The survey will be completed using 525 nights during a 5-year period starting in 2012. About O(1 TB) of raw data will be produced every night, including science and calibration images. The DES data management system has been designed for the processing, calibration and archiving of these data. It is being developed by collaborating DES institutions, led by NCSA. In this contribution, we describe the basic functions of the system, what kind of scientific codes are involved and how the Data Challenge process works, to improve simultaneously the Data Management system algorithms and the Science Working Group analysis codes.

Sevilla, I.; /Madrid, CIEMAT; Armstrong, R.; Jarvis, M.; /Pennsylvania U.; Bertin, E.; /Paris, Inst. Astrophys.; Carlson, A.; Desai, S.; Mohr, J.; /Munich U.; Daues, G.; Gower, M.; Gruendl, R.; Petravick, D.; /Illinois U., Urbana /Illinois U., Urbana /Chicago U. /Fermilab /Brookhaven /Harvard-Smithsonian Ctr. Astrophys.

2011-09-01

408

Dark Viscous Fluid Described by a Unified Equation of State in Cosmology

NASA Astrophysics Data System (ADS)

We generalize the ?CDM model by introducing a unified EOS to describe the Universe contents modeled as dark viscous fluid, motivated by the fact that a single constant equation of state (EOS) p = -p0 (p0 > 0) reproduces the ?CDM model exactly. This EOS describes the perfect fluid term, the dissipative effect, and the cosmological constant in a unique framework and the Friedmann equations can be analytically solved. Especially, we find a relation between the EOS parameter and the renormalizable condition of a scalar field. We develop a completely numerical method to perform a ?2 minimization to constrain the parameters in a cosmological model directly from the Friedmann equations, and employ the SNe data with the parameter {A} measured from the SDSS data to constrain our model. The result indicates that the dissipative effect is rather small in the late-time Universe.

Ren, Jie; Meng, Xin-He

409

Revisit of the interaction between holographic dark energy and dark matter

NASA Astrophysics Data System (ADS)

In this paper we investigate the possible direct, non-gravitational interaction between holographic dark energy (HDE) and dark matter. Firstly, we start with two simple models with the interaction terms Qpropto?dm and Qpropto?de, and then we move on to the general form Qpropto?m??de?. The cosmological constraints of the models are obtained from the joint analysis of the present Union2.1+BAO+CMB+H0 data. We find that the data slightly favor an energy flow from dark matter to dark energy, although the original HDE model still lies in the 95.4% confidence level (CL) region. For all models we find c < 1 at the 95.4% CL. We show that compared with the cosmic expansion, the effect of interaction on the evolution of ?dm and ?de is smaller, and the relative increment (decrement) amount of the energy in the dark matter component is constrained to be less than 9% (15%) at the 95.4% CL. By introducing the interaction, we find that even when c < 1 the big rip still can be avoided due to the existence of a de Sitter solution at z?-1. We show that this solution can not be accomplished in the two simple models, while for the general model such a solution can be achieved with a large ?, and the big rip may be avoided at the 95.4% CL.

Zhang, Zhenhui; Li, Song; Li, Xiao-Dong; Zhang, Xin; Li, Miao

2012-06-01

410

Remarks on dynamical dark energy measured by the conformal age of the universe

We elaborate on a model of conformal dark energy (dynamical dark energy measured by the conformal age of the universe) recently proposed in [H. Wei and R. G. Cai, arXiv:0708.0884] where the presentday dark energy density was taken to be {rho}{sub q}{identical_to}3{alpha}{sup 2}m{sub P}{sup 2}/{eta}{sup 2}, where {eta} is the conformal time and {alpha} is a numerical constant. In the absence of an interaction between the ordinary matter and dark energy field q, the model may be adjusted to the present values of the dark energy density fraction {omega}{sub q}{approx_equal}0.73 and the equation of state parameter w{sub q}<-0.78, if the numerical constant {alpha} takes a reasonably large value, {alpha} > or approx. 2.6. However, in the presence of a nontrivial gravitational coupling of q-field to matter, say Q-tilde, the model may be adjusted to the values {omega}{sub q}{approx_equal}0.73 and w{sub q}{approx_equal}-1, even if {alpha}{approx}O(1), given that the present value of Q-tilde is large. Unlike for the model in [R. G. Cai, arXiv:0707.4049], the bound {omega}{sub q}<0.1 during big bang nucleosynthesis (BBN) may be satisfied for almost any value of {alpha}. Here we discuss some other limitations of this proposal as a viable dark energy model. The model draws some parallels with the holographic dark energy; we also briefly comment on the latter model.

Neupane, Ishwaree P. [Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch 8020 (New Zealand)

2007-12-15

411

Effective action approach to cosmological perturbations in dark energy and modified gravity

NASA Astrophysics Data System (ADS)

In light of upcoming observations modelling perturbations in dark energy and modified gravity models has become an important topic of research. We develop an effective action to construct the components of the perturbed dark energy momentum tensor which appears in the perturbed generalized gravitational field equations, ?G?? = 8?G?T??+?U?? for linearized perturbations. Our method does not require knowledge of the Lagrangian density of the dark sector to be provided, only its field content. The method is based on the fact that it is only necessary to specify the perturbed Lagrangian to quadratic order and couples this with the assumption of global statistical isotropy of spatial sections to show that the model can be specified completely in terms of a finite number of background dependent functions. We present our formalism in a coordinate independent fashion and provide explicit formulae for the perturbed conservation equation and the components of ?U?? for two explicit generic examples: (i) the dark sector does not contain extra fields, Script L = Script L(g??) and (ii) the dark sector contains a scalar field and its first derivative Script L = Script L(g??,phi,??phi). We discuss how the formalism can be applied to modified gravity models containing derivatives of the metric, curvature tensors, higher derivatives of the scalar fields and vector fields.

Battye, Richard A.; Pearson, Jonathan A.

2012-07-01

412

Topics in cosmology: Structure formation, dark energy and recombination

NASA Astrophysics Data System (ADS)

The field of theoretical cosmology consists of numerous, inter-related branches, whose ambitious goal is to uncover the history of the universe from its beginning to its future. Achieving this, no doubt, requires a deep understanding of many areas of physics. In this thesis I touch upon a few of these areas in which I worked during my PhD studies. Chapter (2) describes our work in finding the accretion and merger history of dark matter halos. Dark matter halos are the collapsed dark matter structures in the late time evolution of the universe, whose existence is vital for the formation of galaxies in the Universe as they act as the potential wells where normal matter (collectively called Baryons) can accumulate, cool, and form stars. It is then no surprise that the properties of galaxies depends on the properties of the dark matter halo in which it resides, including its merger history, i.e. the number of times it merged with other halos. Even though these merger rates can be calculated theoretically for infinitesimal time steps, in order to find the merger history over an extended period of time one had to use either Monte-Carlo simulations to build up the total rates of merging and accreting from the infinitesimal rates or use N-body simulations. In chapter (2) we show how we used random walk formalism to write down an analytical (integral) equation for the merger history of halos. We have solved this equation numerically and find very good agreement with Monte-Carlo simulations. This work can be used in theories of galaxy formation and evolution. We then switch from the overdense regions of the Universe, halos, to the underdense ones, voids. These structures have not attracted as much attention from cosmologists as their overdense counterparts in probing the cosmological models. We show here that the shapes of voids as a probe can be of use for future surveys to pin down the equation of state of the dark energy, i.e. the ratio of its pressure to its energy density. As first approximation, voids can be considered to be ellipsoids whose axis ratio evolution depends on the cosmological parameters. This, together with the fact that the initial distribution of the axis ratios is known (because the intial density field is Gaussian) can be used to infer the equation of state of the dark energy statistically from the observation of voids at different redshifts and with different sizes. The standard method of Fisher matrices is then used to forecast how well a future survey can measure the equation of state. We find promising results with constraints coming from void ellipticity measurements comparable to those of other standard methods. Chapter (4) goes farther back in the history of the Universe. During the recombination era, when the Universe was around a thousandth of its present size, it became cool enough that free electrons got captured by free protons to make hydrogen atoms. Consequently, the Thompson scattering of photons off of free electrons dropped dramatically and the Universe became transparent to photon propagation. The Cosmic Microwave Background (CMB) is a remnant from this epoch, consisting of photons last scattered off of a free electron. A wealth of information is contained in the statistical properties of the CMB field. However, in order to take full advantage of this probe one needs to know the recombination history, i.e. the evolution of the number density of free electrons as a function of time, to sub-percent level accuracy during this era. There are a plethora of phenomena, from radiative transfer effects to atomic and molecular ones, that have the potential to change the recombination history to this level. Our work was to calculate the effect that the formation of hydrogen molecules will have on the recombination history. Even though the abundance of hydrogen molecules is very small, they still have the potential to change the recombination history by reshuffling photons from the blue side of the Ly-alpha line to its red side and vise-versa. To find the magnitude of the effect, we solve the appropri

Alizadeh, Esfandiar

413

Difficulties distinguishing dark energy from modified gravity via redshift distortions

The bulk motion of galaxies induced by the growth of cosmic structure offers a rare opportunity to test the validity of general relativity across cosmological scales. However, modified gravity can be degenerate in its effect with the unknown values of cosmological parameters. More seriously, even the 'observed' value of the redshift-space distortions used to measure the fluctuation growth rate depends on the assumed cosmological parameters (the Alcock-Paczynski effect). We give a full analysis of these issues, showing how to combine redshift-space distortions with baryon acoustic oscillations and CMB data, in order to obtain joint constraints on deviations from general relativity and on the equation of state of dark energy while allowing for factors such as nonzero curvature. In particular we note that the evolution of {Omega}{sub m}(z), along with the Alcock-Paczynski effect, produces a degeneracy between the equation of state w and the modified growth parameter {gamma}. Typically, the total marginalized error on either of these parameters will be larger by a factor {approx_equal}2 compared to the conditional error where one or the other is held fixed. We argue that future missions should be judged by their figure of merit as defined in the w{sub p}-{gamma} plane, and note that the inclusion of spatial curvature can degrade this value by an order of magnitude.

Simpson, Fergus; Peacock, John A. [SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)

2010-02-15

414

Nonlinear growth in modified gravity theories of dark energy

Theoretical differences in the growth of structure offer the possibility that we might distinguish between modified gravity theories of dark energy and {lambda}CDM. A significant impediment to applying current and prospective large scale galaxy and weak lensing surveys to this problem is that, while the mildly nonlinear regime is important, there is a lack of numerical simulations of nonlinear growth in modified gravity theories. A major question exists as to whether existing analytical fits, created using simulations of standard gravity, can be confidently applied. In this paper we address this, presenting results of N-body simulations of a variety of models where gravity is altered including the Dvali, Gabadadze, and Porrati model. We consider modifications that alter the Poisson equation and also consider the presence of anisotropic shear stress that alters how particles respond to the gravitational potential gradient. We establish how well analytical fits of the matter power spectrum by Peacock and Dodds and Smith et al. are able to predict the nonlinear growth found in the simulations from z=50 up to today, and also consider implications for the weak lensing convergence power spectrum. We find that the analytical fits provide good agreement with the simulations, being within 1{sigma} of the simulation results for cases with and without anisotropic stress and for scale-dependent and independent modifications of the Poisson equation. No strong preference for either analytical fit is found.

Laszlo, Istvan; Bean, Rachel [Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York, 14853 (United States)

2008-01-15

415

Cosmological viability conditions for f(T) dark energy models

NASA Astrophysics Data System (ADS)

Recently f(T) modified teleparallel gravity where T is the torsion scalar has been proposed as the natural gravitational alternative for dark energy. We perform a detailed dynamical analysis of these models and find conditions for the cosmological viability of f(T) dark energy models as geometrical constraints on the derivatives of these models. We show that in the phase space exists two cosmologically viable trajectory which (i) The universe would start from an unstable radiation point, then pass a saddle standard matter point which is followed by accelerated expansion de sitter point. (ii) The universe starts from a saddle radiation epoch, then falls onto the stable matter era and the system can not evolve to the dark energy dominated epoch. Finally, for a number of f(T) dark energy models were proposed in the more literature, the viability conditions are investigated.

Setare, M. R.; Mohammadipour, N.

2012-11-01

416

NASA's Galaxy Evolution Explorer Helps Confirm Nature of Dark Energy

A five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, has led to one of the best independent confirmations that dark energy is driving our universe apart at accelerating speeds.

417

Determination of Dark Matter Properties at High-Energy Colliders.

National Technical Information Service (NTIS)

If the cosmic dark matter consists of weakly-interacting massive particles, these particles should be produced in reactions at the next generation of high-energy accelerators. Measurements at these accelerators can then be used to determine the microscopi...

E. A. Baltz M. Battaglia M. E. Peskin T. Wizansky

2006-01-01

418

NASA - NASA Telescope Helps Confirm Nature of Dark Energy

419

Interacting viscous matter with a dark energy fluid

NASA Astrophysics Data System (ADS)

We study a cosmological model composed of a dark energy fluid interacting with a viscous matter fluid in a spatially flat Universe. The matter component represents the baryon and dark matter and it is taken into account, through a bulk viscosity, the irreversible process that the matter fluid undergoes because of the accelerated expansion of the universe. The bulk viscous coefficient is assumed to be proportional to the Hubble parameter. The radiation component is also taken into account in the model. The model is constrained using the type Ia supernova observations, the shift parameter of the CMB, the acoustic peak of the BAO and the Hubble expansion rate, to constrain the values of the barotropic index of dark energy and the bulk viscous coefficient. It is found that the bulk viscosity is constrained to be negligible (around zero) from the observations and that the barotropic index for the dark energy to be negative and close to zero too, indicating a phantom energy.

Avelino, Arturo

2012-08-01

420

An ecological approach to problems of Dark Energy, Dark Matter, MOND and Neutrinos

NASA Astrophysics Data System (ADS)

Modern astronomical data on galaxy and cosmological scales have revealed powerfully the existence of certain dark sectors of fundamental physics, i.e., existence of particles and fields outside the standard models and inaccessible by current experiments. Various approaches are taken to modify/extend the standard models. Generic theories introduce multiple de-coupled fields A, B, C, each responsible for the effects of DM (cold supersymmetric particles), DE (Dark Energy) effect, and MG (Modified Gravity) effect respectively. Some theories use adopt vanilla combinations like AB, BC, or CA, and assume A, B, C belong to decoupled sectors of physics. MOND-like MG and Cold DM are often taken as antagnising frameworks, e.g. in the muddled debate around the Bullet Cluster. Here we argue that these ad hoc divisions of sectors miss important clues from the data. The data actually suggest that the physics of all dark sectors is likely linked together by a self-interacting oscillating field, which governs a chameleon-like dark fluid, appearing as DM, DE and MG in different settings. It is timely to consider an interdisciplinary approach across all semantic boundaries of dark sectors, treating the dark stress as one identity, hence accounts for several "coincidences" naturally.

Zhao, Hong Sheng

2008-11-01