Xin-He Meng

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 with time-dependent viscosity or the model with variable cosmological constant. From this general EOS, a completely integrable dynamical equation to the scale factor is obtained with its solution explicitly given out. (i) In this parameterized model of cosmology, for a special choice of the parameters we can explain the late-time accelerating expansion universe in a new view. The early inflation, the median (relatively late time) deceleration, and the recently cosmic acceleration may be unified in a single equation. (ii) A generalized relation of the Hubble parameter scaling with the redshift is obtained for some cosmology interests. (iii) By using the SNe Ia data to fit the effective viscosity model we show that the case of matter described by p=0 plus with effective viscosity contributions can fit the observational gold data in an acceptable level.

Bulk viscous cosmology: Statefinder and entropy

Abstract The statefinder diagnostic pair is adopted to differentiate viscous cosmology models and it is found that the trajectories of these viscous cosmology models on the statefinder pair s–r plane are quite different from those of the corresponding non-viscous cases. Particularly for the quiessence model, the singular properties of state parameter w = − 1 are obviously demonstrated on the statefinder diagnostic pair planes. We then discuss the entropy of the viscous/dissipative cosmology system which may be more practical to describe the present cosmic observations as we understand that the perfect fluid model is just a global approximation to the complicated cosmic media in universe evolution. When the bulk viscosity takes the form of ζ = ζ 1 a ˙ / a ( ζ 1 is constant), the relationship between the entropy S and the redshift z is explicitly given out. We find that the entropy of the viscous cosmology is always increasing and consistent with the thermodynamics arrow of time for the universe evolution. With the parameter constraints by fitting to the 157 gold data of type Ia supernova observations, it is shown that this viscous cosmology model is rather well consistent to the observational data at the lower redshifts, and together with the diagnostic statefinder pair analysis it is concluded that the viscous cosmic models tend to the favored ΛCDM model in the later cosmic evolution, agreeable to lots of cosmological simulation results, especially to the fact of confidently observed current accelerating cosmic expansion.

Modified equation of state, scalar field, and bulk viscosity in Friedmann universe

Abstract A generalized dynamical equation for the scale factor of the universe is proposed to describe the cosmological evolution, of which the ΛCDM model is a special case. It also provides a general example to show the equivalence of the modified equation of state (EOS) and a scalar field model. In the mathematical aspect, the EOS, the scalar field potential V ( φ ) , and the scale factor a ( t ) all have possessed analytical solutions. Such features are due to a simple form invariance of the equation inherited which determines the Hubble parameter. From the physical point of view, this dynamical equation can be regarded as the ΛCDM model with bulk viscosity, an existence content in the universe. We employ the SNe Ia data with the parameter A measured from the SDSS data and the shift parameter R measured from WMAP data to constrain the parameters in our model. The result is that the contribution of the bulk viscosity, accumulated as an effective dark energy responsible for the current cosmic accelerating expansion, is made approximately ten percent to that of the cosmological constant.

Birkhoff’s theorem in f(T) gravity

Generalized from the so-called teleparallel gravity, which is exactly equivalent to general relativity, f(T) gravity has been proposed as an alternative gravity model to account for the dark energy phenomena. In this letter we prove that the external vacuum gravitational field for a spherically symmetric distribution of source matter in the f(T) gravity framework must be static. The conclusion is independent of the radial distribution and spherically symmetric motion of the source matter, that is, whether it is in motion or static. As a consequence, the Birkhoff’s theorem is valid in the general nonsingular f(T) theory at the un-perturbative level. We also discuss its application in the de Sitter spacetime evolution phase as preferred by present dark energy observations.

Phantom dark energy as an effect of bulk viscosity

In a homogeneous and isotropic universe bulk viscosity is the unique viscous effect that is capable of modifying the background dynamics. Effects like shear viscosity or heat conduction can only change the evolution of the perturbations. The existence of a bulk viscous pressure in a fluid-which in order to obey the second law of thermodynamics must be negative-reduces its effective pressure. We discuss the degeneracy in bulk viscous cosmologies and address the possibility that phantom dark energy cosmology could be caused by the existence of nonequilibrium pressure in any one of the cosmic components. We establish the conditions under which either viscous matter or radiation cosmologies can be mapped into the phantom dark energy scenario with constraints from multiple observational data sets.

Large scale structure formation in Eddington-inspired Born-Infeld gravity

We study the large scale structure formation in Eddington-inspired Born-Infeld (EiBI) gravity. It is found that the linear growth of scalar perturbations in EiBI gravity deviates from that in general relativity for modes with large wave numbers (k), but the deviation is largely suppressed with the expansion of the Universe. We investigate the integrated Sachs-Wolfe effect in EiBI gravity, and find that its effect on the angular power spectrum of the anisotropy of the cosmic microwave background is almost the same as that in the Lambda-cold dark matter (Lambda CDM) model. We further calculate the linear matter power spectrum in EiBI gravity and compare it with that in the Lambda CDM model. Deviation is found on small scales (k greater than or similar to 0.1h Mpc(-1)), which can be tested in the future by observations from galaxy surveys.

Effects of dark energy on the efficiency of charged AdS black holes as heat engines

In this paper, we study the heat engine where a charged AdS black hole surrounded by dark energy is the working substance and the mechanical work is done via the PdV term in the first law of black hole thermodynamics in the extended phase space. We first investigate the effects of a kind of dark energy (quintessence field in this paper) on the efficiency of the RN-AdS black holes as the heat engine defined as a rectangular closed path in the P–V plane. We get the exact efficiency formula and find that the quintessence field can improve the heat engine efficiency, which will increase as the field density

The distinctions between ΛCDM and f(T) gravity according to Noether symmetry

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