17 research outputs found
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
Can a primordial magnetic field originate large-scale anomalies in WMAP data?
Several accurate analyses of the CMB temperature maps from the Wilkinson
Microwave Anisotropy Probe (WMAP) have revealed a set of anomalous results, at
large angular scales, that appears inconsistent with the statistical isotropy
expected in the concordance cosmological model CDM. Because these
anomalies seem to indicate a preferred direction in the space, here we
investigate the signatures that a primordial magnetic field, possibly present
in the photon-baryon fluid during the decoupling era, could have produced in
the large-angle modes of the observed CMB temperature fluctuations maps. To
study these imprints we simulate Monte Carlo CMB maps, which are statistically
anisotropic due to the correlations between CMB multipoles induced by the
magnetic field. Our analyses reveal the presence of the North-South angular
correlations asymmetry phenomenon in these Monte Carlo maps, and we use these
information to establish the statistical significance of such phenomenon
observed in WMAP maps. Moreover, because a magnetic field produces planarity in
the low-order CMB multipoles, where the planes are perpendicular to the
preferred direction defined by the magnetic field, we investigate the
possibility that two CMB anomalous phenomena, namely the North-South asymmetry
and the quadrupole-octopole planes alignment, could have a common origin. Our
results, for large-angles, show that the correlations between low-order CMB
multipoles introduced by a sufficiently intense magnetic field, can reproduce
some of the large-angle anisotropic features mapped in WMAP data. We also
reconfirm, at more than 95% CL, the existence of a North-South power asymmetry
in the WMAP five-year data.Comment: 9 pages, 5 figures, to appear in MNRA