Unified Model Of Dark Energy Research Articles

Cosmological constraints on fast transition unified dark energy and dark matter models

We explore the observational adequacy of a class of Unified Dark Energy/Matter (UDE/M) models with a fast transition. Our constraints are set using a combination of geometric probes, some low redshift ones, and some high redshift ones (CMB related included). The transition is phenomenologically modeled by two different transition functions corresponding to a fast and to an ultra-fast transition respectively. We find that the key parameters governing the transition can be well constrained, and from the statistical point of view it follows that the models cannot be discarded when compared to $\Lambda$CDM. We find the intriguing result that standard/input parameters such as $\Omega_m$ and $\Omega_b$ are far better constrained than in $\Lambda$CDM, and this is the case for the derived/output parameter measuring the deceleration value at present, $q_0$.

Observational constraints to a unified cosmological model

We propose a phenomenological unified model (UM) for dark matter and dark energy based on an equation of state parameter w that scales with the arctan of the redshift. The free parameters of the model are three constants: Ωb0, α and β. Parameter α dictates the transition rate between the matter dominated era and the accelerated expansion period. The ratio β/α gives the redshift of the equivalence between both regimes. Cosmological parameters are fixed by observational data from primordial nucleosynthesis (PN), supernovae of the type Ia (SNIa), gamma-ray bursts (GRBs) and baryon acoustic oscillations (BAOs). The calibration of the 138 GRB events is performed using the 580 SNIa of the Union2.1 data set and a new set of 79 high-redshift GRB is obtained. The various sets of data are used in different combinations to constraint the parameters through statistical analysis. The UM is compared to the ΛCDM model and their differences are emphasized.

Constraining the Generalized and Superfluid Chaplygin Gas Models with the Sandage-Loeb Test**Supported by the National Natural Science Foundation of China under Grants Nos 11175093, 11222545, 11435006, and 11375092, the Specialized Research Fund for the Doctoral Program of Higher Education under Grant No 20124306110001, and the K.C. Wong Magna Fund of Ningbo University.

The Sandage-Loeb (SL) test is a direct measurement of the cosmic expansion by probing the redshift drifts of quasi-stellar objects in the ‘redshift desert’ of 2 < z < 5. In this work, we investigate its constraints on the unified dark energy and dark matter models including the generalized Chaplygin gas and the superfluid Chaplygin gas. In addition, type Ia supernovae (SNIa) data and the distance ratios derived from the cosmic microwave background radiation and baryon acoustic oscillation observations (CMB/BAO) are also used. We find that the mock SL data gives the tightest constraints on the model parameters and it can help to reduce the parameter regions allowed by the present SNIa+CMB/BAO by about 75% when all datasets considered are combined. Thus the SL test is a worthy and long awaited measurement to probe effectively the cosmic expanding history and the properties of dark energy.

Constraints on the dark matter sound speed from galactic scales: The cases of the modified and extended Chaplygin gas

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.

Nonlinear Chaplygin gas cosmologies

We study the nonlinear regime of unified dark energy models, using generalized Chaplygin gas cosmologies as a representative example, and introduce a new parameter characterizing the level of small scale clustering in these scenarios. We show that viable generalized Chaplygin gas cosmologies, consistent with the most recent observational constraints, may be constructed for any value of the generalized Chaplygin gas parameter by considering models with a sufficiently high level of nonlinear clustering.

Extended Chaplygin gas as a unified fluid of dark components in varying gravitational constant theory

Varying gravitational constant $G(t)$ (VG) cosmology is studied in this paper, where the modified Friedmann equation and the modified energy conservation equation are given with respect to the constant-$G$ theory. Considering the extended Chaplygin gas (ECG) as background fluid (or thinking that ECG fluid is induced by the variation of $G$), the unified model of dark matter and dark energy is obtained in VG theory. The parameter spaces are investigated in the VG-ECG model by using the recent cosmic data. Constraint results show $\ensuremath{\beta}=\ensuremath{-}\frac{\stackrel{.}{G}}{HG}=\ensuremath{-}0.00{3}_{\ensuremath{-}0.020\ensuremath{-}0.055}^{+0.021+0.034}$ for the VG-GCG unified model and $\ensuremath{\beta}=\ensuremath{-}0.02{7}_{\ensuremath{-}0.032\ensuremath{-}0.066}^{+0.032+0.059}$ for the VG-MCG unified model. Equivalently, they correspond to the limits on the current variation of Newton's gravitational constant at 95.4% confidence level $|\frac{\stackrel{.}{G}}{G}{|}_{\text{today}}\ensuremath{\lesssim}4.1\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}12}\text{ }\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$ and $|\frac{\stackrel{.}{G}}{G}{|}_{\text{today}}\ensuremath{\lesssim}6.6\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}12}\text{ }\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$. And for $z\ensuremath{\le}3.5$, bounds on the variation of $\frac{\stackrel{.}{G}}{G}$ in the VG-ECG unified model are in accordance with the experiment explorations of varying $G$. In addition, in VG theory the used observational data point still cannot distinguish the VG-GCG and VG-MCG unified model from the most popular $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ cosmology. Furthermore, to see the effects of varying $G$ and physical properties for VG-ECG fluid, we discuss the evolutionary behaviors of cosmological quantities in VG theory, such as $\frac{\stackrel{.}{G}}{G}$, $\frac{\stackrel{..}{G}}{G}$ and equation of state $w$, etc. For $\ensuremath{\beta}l0$ a quintom scenario crossing over $w=\ensuremath{-}1$ can be realized in the VG-GCG model.

A UNIFIED DARK ENERGY MODEL FROM A VANISHING SPEED OF SOUND WITH EMERGENT COSMOLOGICAL CONSTANT

The problem of the cosmic acceleration is here revisited by using the fact that the adiabatic speed of sound can be assumed to be negligible small. Within the context of general relativity, the total energy budget is recovered under the hypothesis of a vanishing speed of sound by assuming the existence of one fluid only. We find a cosmological model which reproduces the main results of the ΛCDM paradigm at late-times, showing an emergent cosmological constant, which is not at all related with the vacuum energy term. As a consequence, the model presented here behaves as a unified dark energy (DE) model.

STUDY ON A UNIFIED MODEL OF DARK MATTER AND DARK ENERGY FROM DBI THEORY

In this paper, we study a unified model of dark matter and dark energy obtained from Dirac–Born–Infeld (DBI) action in string theory. Two accelerated expansions in universe can be unified in this action. By using the Markov Chain Monte Carlo method, we fit the current observational data to constrain the model parameters in this unified model, where various density parameters as model parameters are included, and their constraint values are: [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. In addition, the Hubble constant and cosmic age are [Formula: see text] and [Formula: see text] (Gyr), respectively. According to the constraint results on model parameters we discuss the evolutions of some cosmological quantities in structure formation, such as the density contrast and the growth variable. At last, the evolution of geometrical quantity is studied to distinguish the unified models of dark sectors with the cosmological constant model. It is shown that this unified model of dark matter and dark energy is attractive to interpret the accelerating universe.

A two measure model of dark energy and dark matter

In this work we construct a unified model of dark energy and dark matter. This is donewith the following three elements: a gravitating scalar field, ϕ with a non-conventional kinetic term, as in the string theory tachyon; an arbitrary potential, V(ϕ);two measures — a metric measure ((−g)1/2) and a non-metric measure(Φ). The model has two interesting features: (i) For potentials which are unstable and would give rise to tachyonic scalar field, this modelcan stabilize the scalar field. (ii) The form of the dark energy and dark matter that results from this modelis fairly insensitive to the exact form of the scalar field potential.

Unification of Dark fields and Inflation

We present a partial unified model for inflation, dark matter and dark energy, based on a global SU (2) × U(1) symmetry, where the corresponding cosmological scalar fields that source such components of the Universe appear as different degrees of freedom of fundamental representations. The protecting symmetry allows to build successful potentials to describe both the early and late eras of accelerated expansion of the Universe, and admit for the interpretation of Dark energy as quintessence or phantom matter. The model also provides a limit where the fields have a dual behavior.

Constraints on the unified model of dark matter and dark energy

Using recently observed data: the Constitution dataset of type supernovae Ia (SNIa), the observational Hubble data (OHD), the measurement results of baryon acoustic oscillation (BAO) from the Sloan Digital Sky Survey (SDSS) and the Two Degree Field Galaxy Redshift Survey (2dFGRS), and the current cosmic microwave background (CMB) data from the five-year Wilkinson Microwave Anisotropy Probe (WMAP), we apply the Markov Chain Monte Carlo method to investigate the observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy. For this unified model, the constraints on GCG mixture are discussed by considering the different expressions of current matter density or considering constraints as being independent of the matter quantity Ωm.

Dieterici gas as a unified model for dark matter and dark energy

The dominance of dark energy in the universe has necessitated the introduction of a repulsive gravity source to make q0 negative. The models for dark energy range from a simple lambda-term to quintessence, Chaplygin gas, etc. We look at the possibility of how change of behaviour of missing energy density, from DM to DE, may be determined by the change in the equation of state of a background fluid instead of a form of potential. The question of cosmic acceleration can be discussed within the framework of theories which do not necessarily include scalar fields.

Generalizing a unified model of dark matter, dark energy, and inflation with a noncanonical kinetic term

We study a unification model for dark energy, dark matter, and inflation with a single scalar field with non canonical kinetic term. In this model the kinetic term of the Lagrangian accounts for the dark matter and dark energy, and at early epochs a quadratic potential accounts for slow roll inflation. The present work is an extension to the work by Bose and Majumdar [1] with a more general kinetic term that was proposed by Chimento in [2]. We demonstrate that the model is viable at the background and linear perturbation levels.

Matter power spectrum for the generalized Chaplygin gas model: The relativistic case

The generalized Chaplygin gas (GCG) model is the prototype of a unified model of dark energy (DE) and dark matter (DM). It is characterized by equation-of-state (EoS) parameters $A$ and $\alpha$. We use a statistical analysis of the 2dFGRS data to constrain these parameters. In particular, we find that very small (close to zero) and very large values ($\alpha\gg 1$) of the equation-of-state parameter $\alpha$ are preferred. To test the validity of this type of unification of the dark sector we admit the existence of a separate DM component in addition to the Chaplygin gas and calculate the probability distribution for the fractional contributions of both components to the total energy density. This analysis favors a model for which the Universe is nearly entirely made up of the separate DM component with an almost negligible Chaplygin gas part. This confirms the results of a previous Newtonian analysis.

Observational constraints on a unified dark matter and dark energy model based on generalized Chaplygin gas

We study a generalized version of Chaplygin gas as unified model of dark matter and dark energy. Using realistic theoretical models and the currently available observational data from the age of the universe, the expansion history based on the type Ia supernovae, the matter power spectrum, the cosmic microwave background radiation anisotropy power spectra, and the perturbation growth factor we put the unified model under observational test. As the model has only two free parameters in the flat Friedmann background [$\Lambda$CDM (cold dark matter) model has only one free parameter] we show that the model is already tightly constrained by currently available observations. The only parameter space extremely close to the $\Lambda$CDM model is allowed in this unified model.

Bose-Einstein condensation in the early universe

By introducing the Bose-Einstein condensation dynamics (BEC), we develop a unified model of dark energy and dark matter. The two phases of BEC (= dark energy) and normal gas (= dark matter) transform with each other through the phase transition. We study the present mild acceleration, the inflation in the early universe, and the reduction of the cosmological constant. We find the stagflation regime when the cosmic expansion stops and the uniform mode of BEC becomes unstable. We also comment on the possibility that BEC generation/decay series might have continued all the time in the cosmic history from the inflation to present.

Bose‐Einstein condensation in the early universe

AbstractBy introducing the Bose‐Einstein condensation dynamics (BEC), we develop a unified model of dark energy and dark matter. The two phases of BEC (= dark energy) and normal gas (= dark matter) transform with each other through the phase transition. We study the present mild acceleration, the inflation in the early universe, and the reduction of the cosmological constant. We find the stagflation regime when the cosmic expansion stops and the uniform mode of BEC becomes unstable. We also comment on the possibility that BEC generation/decay series might have continued all the time in the cosmic history from the inflation to present.

Stagflation: Bose-Einstein condensation in the early universe

Our universe experienced the accelerated expansion at least twice; an extreme inflationary acceleration in the early universe and the recent mild acceleration. By introducing the Bose-Einstein condensation (BEC) phase of a boson field, we have been developing a unified model of dark energy (DE) and dark matter (DM) for the later mild acceleration. In this scenario, two phases of BEC (=DE) and normal gas (=DM) transform with each other through BEC phase transition. This unified model has successfully explained the mild acceleration as an attractor. We extend this BEC cosmology to the early universe without introducing new ingredients. In this scenario, the inflation is naturally initiated by the condensation of the bosons in the huge vacuum energy. This inflation and even the cosmic expansion eventually terminates exactly at zero energy density. We call this stage as stagflation. At this stagflation era, particle production and the decay of BEC take place. The former makes the universe turn into the standard hot big bang stage and the latter makes the cosmological constant vanishingly small after the inflation. Furthermore, we calculate the density fluctuations produced in this model, which turns out to be in the range allowed by the present observational data. We also show that the stagflation is quite robust and easily appears when one allows negative region of the potential. Further, we comment on the possibility that BEC generation/decay series might have continued all the time in the cosmic history from the inflation to present.

MODIFIED CHAPLYGIN TRAVERSABLE WORMHOLES

Modified Chaplygin gas (MCG) is a strong candidate for the unified model of dark matter and dark energy. The equation of state of this modified model is valid from the radiation era to the ΛCDM model. In the early epoch (when ρ was large), dark matter had the dominant role, while at later stages (when ρ is small), the MCG model behaves as dark energy. In this work, we have found an exact solution to static, spherically symmetric Einstein equations describing a wormhole for an inhomogeneous distribution of MCG. For the existence of a wormhole solution, there are some restrictions concerning the parameters in the equation of state for MCG and the throat radius of the wormhole. Physical properties and characteristics of such modified Chaplygin wormholes are analyzed in detail.

Inflation and accelerated universe based on Bose-Einstein condensation

A new cosmology based on the Bose-Einstein condensation is proposed. This is a unified model of Dark Energy and Dark Matter, and predicts several collapses of BEC, followed by the final acceleration which successfully describes the recent observational results. Furthermore, this model can be extended to the early inflationary regime, and explains natural initiation of the inflation, autonomous termination of the inflation, inevitable initiation of the reheating process, autonomous adjustment of the cosmological constant to zero, and acceptable generation of density fluctuations.