359 research outputs found
Unified models of the cosmological dark sector
We model the cosmological substratum by a viscous fluid that is supposed to
provide a unified description of the dark sector and pressureless baryonic
matter. In the homogeneous and isotropic background the \textit{total} energy
density of this mixture behaves as a generalized Chaplygin gas. The
perturbations of this energy density are intrinsically non-adiabatic and source
relative entropy perturbations. The resulting baryonic matter power spectrum is
shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical
analysis, using also Hubble-function and supernovae Ia data, shows that,
different from other studies, there exists a maximum in the probability
distribution for a negative present value of the
deceleration parameter. Moreover, different from other approaches, the unified
model presented here favors a matter content that is of the order of the
baryonic matter abundance suggested by big-bang nucleosynthesis.Comment: 4 pages, 1 figure, submitted to the Proceedings of the Spanish
Relativity Meeting - ERE 2010, Granada, September 6 - September 10, 201
Non-adiabatic Chaplygin gas
The split of a generalised Chaplygin gas with an equation of state p =
-A/\rho^{\alpha} into an interacting mixture of pressureless matter and a
dark-energy component with equation of state p_{\Lambda} = - \rho_{\Lambda}
implies the existence of non-adiabatic pressure perturbations. We demonstrate
that the square of the effective (non-adiabatic) sound speed c_s of the medium
is proportional to the ratio of the perturbations of the dark energy to those
of the dark matter. Since, as demonstrated explicitly for the particular case
\alpha = -1/2, dark-energy perturbations are negligible compared with
dark-matter perturbations on scales that are relevant for structure formation,
we find |c_s^2| << 1. Consequently, there are no oscillations or instabilities
which have plagued previous adiabatic Chaplygin-gas models.Comment: Version to appear in Physics Letters
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 and . 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 () of
the equation-of-state parameter 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.Comment: Latex file, 8 pages, 15 figures in eps forma
Collisional equilibrium, particle production and the inflationary universe
Particle production processes in the expanding universe are described within
a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas
with variable particle number are investigated. We find that radiation and
nonrelativistic matter may be in equilibrium at the same temperature provided
the matter particles are created at a rate that is half the expansion rate.
Using the fact that the creation of particles is dynamically equivalent to a
nonvanishing bulk pressure we calculate the backreaction of this process on the
cosmological dynamics. It turns out that the `adiabatic' creation of massive
particles with an equilibrium distribution for the latter necessarily implies
power-law inflation. Exponential inflation in this context is shown to become
inconsistent with the second law of thermodynamics after a time interval of the
order of the Hubble time.Comment: 19 pages, latex, no figures, to appear in Phys.Rev.
Generalised equilibrium of cosmological fluids in second-order thermodynamics
Combining the second-order entropy flow vector of the causal Israel-Stewart
theory with the conformal Killing-vector property of , where
is the four-velocity of the medium and T its equilibrium temperature, we
investigate generalized equilibrium states for cosmological fluids with
nonconserved particle number. We calculate the corresponding equilibrium
particle production rate and show that this quantity is reduced compared with
the results of the previously studied first-order theory. Generalized
equilibrium for massive particles turns out to be compatible with a dependence
of the fluid energy density on the scale factor a
of the Robertson-Walker metric and may be regarded as a realization of
so-called K-matter.Comment: 17 pages, iopfts.tex file, submitted to Class. Quantum Gra
Cosmological perturbation theory and conserved quantities in the large-scale limit
The linear cosmological perturbation theory of an almost homogeneous and
isotropic perfect fluid universe is reconsidered and formally simplified by
introducing new covariant and gauge-invariant variables with physical
interpretations on hypersurfaces of constant expansion, constant curvature or
constant energy density. The existence of conserved perturbation quantities on
scales larger than the Hubble scale is discussed. The quantity which is
conserved on large scales in a flat background universe may be expressed in
terms of the fractional, spatial gradient of the energy density on constant
expansion hypersurfaces or, alternatively, with the help of expansion or
curvature perturbation variables on hypersurfaces of constant energy density.
For nonvanishing background curvature the perturbation dynamics is most
suitably described in terms of energy density perturbations on hypersurfaces of
constant curvature.Comment: 12 pages, Revtex, to appear in Class.Quantum Gra
Viscous dark fluid universe
We investigate the cosmological perturbation dynamics for a universe
consisting of pressureless baryonic matter and a viscous fluid, the latter
representing a unified model of the dark sector. In the homogeneous and
isotropic background the \textit{total} energy density of this mixture behaves
as a generalized Chaplygin gas. The perturbations of this energy density are
intrinsically non-adiabatic and source relative entropy perturbations. The
resulting baryonic matter power spectrum is shown to be compatible with the
2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also
Hubble-function and supernovae Ia data, shows that, different from other
studies, there exists a maximum in the probability distribution for a negative
present value of the deceleration parameter. Moreover,
while previous descriptions on the basis of generalized Chaplygin gas models
were incompatible with the matter power spectrum data since they required a
much too large amount of pressureless matter, the unified model presented here
favors a matter content that is of the order of the baryonic matter abundance
suggested by big-bang nucleosynthesis.Comment: 19 pages, 6 figure
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