Oliver F. Piattella

Gauge-invariant analysis of perturbations in Chaplygin gas unified models of dark matter and dark energy

We exploit the gauge-invariant formalism to analyse the perturbative behaviour of two cosmological models based on the generalized Chaplygin gas describing both dark matter and dark energy in the present universe. In the first model we consider the generalized Chaplygin gas alone, while in the second one we add a baryon component to it. We extend our analysis also into the parameter range α>1, where the generalized Chaplygin gas sound velocity can be larger than that of light. In the first model we find that the matter power spectrum is compatible with the observed one only for α<10−5, which makes the generalized Chaplygin gas practically indistinguishable from ΛCDM. In the second model we study the evolution of inhomogeneities of the baryon component. The theoretical power spectrum is in good agreement with the observed one for almost all values of α. However, the growth of inhomogeneities seems to be particularly favoured either for sufficiently small values of α or for . Thus, it appears that the viability of the generalized Chaplygin gas as a cosmological model is stronger when its sound velocity is superluminal. We show that in this case the generalized Chaplygin gas equation of state can be changed in an unobservable region in such a way that its equivalent k-essence microscopical model has no problems with causality.

Rastall Cosmology and the \Lambda CDM Model

Rastalls theory is based on the non-conservation of the energy-momentum tensor. We show that, in this theory, if we introduce a two-fluid model, one component representing vacuum energy whereas the other pressureless matter (e.g. baryons plus cold dark matter), the cosmological scenario is the same as for the \Lambda CDM model, both at background and linear perturbative levels, except for one aspect: now dark energy may cluster. We speculate that this can lead to a possibility of distinguishing the models at the non-linear perturbative level.

The extreme limit of the generalised Chaplygin gas

Unified Dark Matter models describe Dark Matter and Dark Energy as a single entity which is, in the simplest case, embodied in a perfect barotropic fluid. It is a well-established fact that small adiabatic perturbations of Unified Dark Matter have an evolution characterised by oscillations and decay which provide predictions on the Cosmic Background Radiation anisotropies which are in poor agreement with observation. In this paper we investigate the generalised Chaplygin gas and we find that the Integrated Sachs-Wolfe effect excludes the model for 10−3 < α < 350. We discuss the implications on the background evolution of the Universe if large values of α are considered. In this case, the Universe expansion mimics a matter-dominated phase abruptly followed by a de Sitter one at the transition redshift ztr. Thanks to an analysis of the type Ia supernovae Constitution set we are able to place ztr = 0.22.

Bulk viscous cosmology with causal transport theory

We consider cosmological scenarios originating from a single imperfect fluid with bulk viscosity and apply Eckarts and both the full and the truncated M?ller-Israel-Stewarts theories as descriptions of the non-equilibrium processes. Our principal objective is to investigate if the dynamical properties of Dark Matter and Dark Energy can be described by a single viscous fluid and how such description changes when a causal theory (M?ller-Israel-Stewarts, both in its full and truncated forms) is taken into account instead of Eckarts non-causal one. To this purpose, we find numerical solutions for the gravitational potential and compare its behaviour with the corresponding ?CDM case. Eckarts and the full causal theory seem to be disfavoured, whereas the truncated theory leads to results similar to those of the ?CDM model for a bulk viscous speed in the interval 10?11 cb2 10?8.

More about the Tolman-Oppenheimer-Volkoff equations for the generalized Chaplygin gas

We investigate the Tolman-Oppenheimer-Volkoff equations for the generalized Chaplygin gas with the aim of extending the findings of V. Gorini, U. Moschella, A. Y. Kamenshchik, V. Pasquier, and A. A. Starobinsky [Phys. Rev. D 78, 064064 (2008)]. We study both the standard case, where we reproduce some previous results, and the phantom case. In the phantom case we show that even a superluminal group velocity arising for

Note on the evolution of the gravitational potential in Rastall scalar field theories

\ensuremath{\alpha}g1

Unified Dark Matter scalar field models with fast transition

cannot prevent the divergence of the pressure at a finite radial distance. Finally, we investigate how a modification of the generalized Chaplygin gas equation of state, required by causality arguments at densities very close to

The Brans–Dicke–Rastall theory

\ensuremath{\Lambda}

Scalar models for the generalized Chaplygin gas and the structure formation constraints

, affects the results found so far.

Reconstructing a

Abstract We investigate the evolution of the gravitational potential in Rastall scalar field theories. In a single component model a consistent perturbation theory, formulated in the Newtonian gauge, is possible only for γ = 1 , which is the General Relativity limit. On the other hand, the addition of another canonical fluid component allows to consider the case γ ≠ 1 .