Unified Dark Energy Research Articles

A modified Chaplygin gas model with interaction

A modified Chaplygin gas (MCG) model of unifying dark energy and dark matter is considered in this paper, in which dark energy interacts with dark matter. Concretely, the evolution of such a unified dark sectors model is studied and the statefinder diagnostic to the MCG model is performed in our model. By analysis, it is shown that the effective equation of state (EoS) parameter of dark energy can cross the so-called phantom divide ω = −1, the behavior of MCG will be like ΛCDM in the future and therefore our Universe will not end up with Big Rip in the future. Furthermore, we plot the evolution trajectories of the MCG model in the statefinder parameter r–s plane and illustrate the discrimination between this scenario and the generalized Chaplygin gas (GCG) model.

Constraints on the unified dark energy–dark matter model from latest observationaldata

The generalized Chaplygin gas (GCG) is studied in this paper by usingthe latest observational data including 182 gold sample type Ia supernovae(Sne Ia) data, the ESSENCE Sne Ia data, the distance ratio fromz = 0.35 to 1089 (the redshift of decoupling), the cosmic microwave background shift parameter andthe Hubble parameter data. Our results rule out the standard Chaplygin gas model(α = 1) at the99.7% confidence level,but allow for the λCDM model (α = 0) at the68.3% confidence level.At a 95.4% confidencelevel, we obtain w = −0.74−0.09+0.10 and α = −0.14−0.19+0.30. In addition, we find that the phase transition from deceleration to acceleration occurs at redshiftzq = 0∼0.78−0.89 ata 1σ confidence level for the GCG model.

Chaplygin gas in light of recent integrated Sachs–Wolfe effect data

We investigate the possibility of constraining Chaplygin dark energy models with current integrated Sachs–Wolfe effect data. In the case of a flat universe we found that generalized Chaplygin gas models must have an energy density such that Ωc > 0.55 and an equation of state w < −0.6 at 95% c.l. We also investigate the recently proposed silent Chaplygin models, constraining Ωc > 0.55 and w < −0.65 at 95% c.l. Better measurements of the CMB–LSS correlation will be possible with the next generation of deep redshift surveys. This will provide independent and complementary constraints on unified dark energy models such as the Chaplygin gas.

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.

Unifying dark energy and dark matter with a scalar field

The standard model of cosmology considers the existence of two components of unknown nature, ``dark matter'' and ``dark energy'', which determine the cosmological evolution. Their nature remains unknown, and other models can also be considered. In particular, it may be possible to reinterpret the recent cosmological observations so that the Universe does not contain two fluids of unknown natures, but only one fluid with particular properties. After a brief review of constraints on this unifying ``dark fluid'', we will discuss a specific model of dark fluid based on a complex scalar fluid.

Toy models unifying dark energy and dark matter

It is very common to find numerical studies of dark energy and dark matter. Among these, one can find the interesting proposal of unifying dark matter and dark energy with the use of a single component with an 'exotic' equation of state. However, there is a certain shortage of analyses involving exact analytic models following this proposal. Therefore, here are presented examples of simple exact cosmological models which can reproduce some of the desired properties of an unified dark matter/energy fluid.

Cosmic Microwave Background Radiation Constraints on aModified Chaplygin Gas Model

A modified Chaplygin gas model of unifying darkenergy and dark matter with the exotic equation of state p = Bρ−(A/ρα), which can also explain therecent expansion of the universe, is investigated bymeans of constraining the location of the peak of the cosmic microwavebackground radiation spectrum. We find that the result of CMBRmeasurements does not exclude the nonzero value of parameter B, butallows it in the range −0.35≲ B ≲0.025.

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 $\ensuremath{\delta}\ensuremath{\sim}a$ behavior.

Unified dark energy models: A phenomenological approach

A phenomenological approach is proposed to the problem of the accelerated expansion of the Universe and of the nature of dark energy. A general class of models is introduced whose energy density depends on the redshift z in such a way that a smooth transition among the three main phases of the evolution of the Universe (radiation era, matter domination, asymptotical de Sitter state) is naturally achieved. We use the estimated age of the Universe, the Hubble diagram of type-Ia supernovae, and the angular size--redshift relation for compact and ultracompact radio structures, to test whether the model is in agreement with astrophysical observation and to constrain its main parameters. Although phenomenologically motivated, the model may be straightforwardly interpreted as a two-fluid scenario in which the quintessence is generated by a suitably chosen scalar field potential. On the other hand, the same model may also be read in the context of unified dark energy models or in the framework of modified Friedmann equation theories.