22 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
CMB and matter power spectra with non-linear dark-sector interactions
An interaction between dark matter and dark energy, proportional to the
product of their energy densities, results in a scaling behavior of the ratio
of these densities with respect to the scale factor of the Robertson-Walker
metric. This gives rise to a class of cosmological models which deviate from
the standard model in an analytically tractable way. In particular, it becomes
possible to quantify the role of potential dark-energy perturbations. We
investigate the impact of this interaction on the structure formation process.
Using the (modified) CAMB code we obtain the CMB spectrum as well as the linear
matter power spectrum. It is shown that the strong degeneracy in the parameter
space present in the background analysis is considerably reduced by considering
\textit{Planck} data. Our analysis is compatible with the CDM model at
the confidence level with a slightly preferred direction of the
energy flow from dark matter to dark energy.Comment: 22 pages, 8 figures, analysis enlarged, comments and references
added, accepted for publication in JCA
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
Viscous dark fluid Universe: a unified model of the dark sector?
The Universe is modeled as consisting of pressureless baryonic matter and a
bulk viscous fluid which is supposed to represent a unified description 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 nonadiabatic 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, the unified model presented here favors a matter content that is of
the order of the baryonic matter abundance suggested by big-bang
nucleosynthesis. A problem of simple bulk viscous models, however, is the
behavior of the gravitational potential and the reproduction of the CMB power
spectrum.Comment: 12 pages, 3 figures, contributed paper to 8th Friedmann Seminar, 30
May to 3 June 2011, Rio de Janeiro, Brazi
Perturbations for transient acceleration
According to the standard CDM model, the accelerated expansion of
the Universe will go on forever. Motivated by recent observational results, we
explore the possibility of a finite phase of acceleration which asymptotically
approaches another period of decelerated expansion. Extending an earlier study
on a corresponding homogeneous and isotropic dynamics, in which interactions
between dark matter and dark energy are crucial, the present paper also
investigates the dynamics of the matter perturbations both on the Newtonian and
GR levels and quantifies the potential relevance of perturbations of the
dark-energy component. In the background, the model is tested against the
Supernova type Ia (SNIa) data of the Constitution set and on the perturbative
level against growth rate data and the data of the 2dFGRS project. Our results
indicate, that a transient phase of accelerated expansion is not excluded by
current observations.Comment: 31 pages, 10 figures, 2 tables, material and references adde