31 research outputs found
Topological signatures in CMB temperature anisotropy maps
We propose an alternative formalism to simulate CMB temperature maps in
CDM universes with nontrivial spatial topologies. This formalism
avoids the need to explicitly compute the eigenmodes of the Laplacian operator
in the spatial sections. Instead, the covariance matrix of the coefficients of
the spherical harmonic decomposition of the temperature anisotropies is
expressed in terms of the elements of the covering group of the space. We
obtain a decomposition of the correlation matrix that isolates the topological
contribution to the CMB temperature anisotropies out of the simply connected
contribution. A further decomposition of the topological signature of the
correlation matrix for an arbitrary topology allows us to compute it in terms
of correlation matrices corresponding to simpler topologies, for which closed
quadrature formulae might be derived. We also use this decomposition to show
that CMB temperature maps of (not too large) multiply connected universes must
show ``patterns of alignment'', and propose a method to look for these
patterns, thus opening the door to the development of new methods for detecting
the topology of our Universe even when the injectivity radius of space is
slightly larger than the radius of the last scattering surface. We illustrate
all these features with the simplest examples, those of flat homogeneous
manifolds, i.e., tori, with special attention given to the cylinder, i.e.,
topology.Comment: 25 pages, 7 eps figures, revtex4, submitted to PR
Non-adiabatic dark fluid cosmology
We model the dark sector of the cosmic substratum by a viscous fluid with an
equation of state , where is the fluid-expansion
scalar and is the coefficient of bulk viscosity for which we assume a
dependence on the energy density . The
homogeneous and isotropic background dynamics coincides with that of a
generalized Chaplygin gas with equation of state . The
perturbation dynamics of the viscous model, however, is intrinsically
non-adiabatic and qualitatively different from the Chaplygin-gas case. In
particular, it avoids short-scale instabilities and/or oscillations which
apparently have ruled out unified models of the Chaplygin-gas type. We
calculate the matter power spectrum and demonstrate that the non-adiabatic
model is compatible with the data from the 2dFGRS and the SDSS surveys. A
-analysis shows, that for certain parameter combinations the
viscous-dark-fluid (VDF) model is well competitive with the CDM model.
These results indicate that \textit{non-adiabatic} unified models can be seen
as potential contenders for a General-Relativity-based description of the
cosmic substratum.Comment: 25 pages, 9 figures, comments and references added, accepted by JCA
On general features of warm dark matter with reduced relativistic gas
Reduced Relativistic Gas (RRG) is a useful approach to describe the warm dark
matter (WDM) or the warmness of baryonic matter in the approximation when the
interaction between the particles is irrelevant. The use of Maxwell
distribution leads to the complicated equation of state of the J\"{u}ttner
model of relativistic ideal gas. The RRG enables one to reproduce the same
physical situation but in a much simpler form. For this reason RRG can be a
useful tool for the theories with some sort of a "new Physics". On the other
hand, even without the qualitatively new physical implementations, the RRG can
be useful to describe the general features of WDM in a model-independent way.
In this sense one can see, in particular, to which extent the cosmological
manifestations of WDM may be dependent on its Particle Physics background. In
the present work RRG is used as a complementary approach to derive the main
observational exponents for the WDM in a model-independent way. The only
assumption concerns a non-negligible velocity for dark matter particles
which is parameterized by the warmness parameter . The relatively high
values of ( ) erase the radiation (photons and
neutrinos) dominated epoch and cause an early warm matter domination after
inflation. Furthermore, RRG approach enables one to quantify the lack of power
in linear matter spectrum at small scales and in particular, reproduces the
relative transfer function commonly used in context of WDM with accuracy of
. A warmness with (equivalent to ) does not alter significantly the CMB power spectrum and is in
agreement with the background observational tests.Comment: 15 pages, 8 figures. Essential improvements in style and presentatio
Cosmological framework for renormalization group extended gravity at the action level
General relativity (GR) extensions based on renormalization group (RG) flows
may lead to scale-dependent couplings with nontrivial effects at large distance
scales. Here we develop further the approach in which RG effects at large
distance scales are fully encoded in an effective action and we apply it to
cosmology. In order to evaluate the cosmological consequences, our main
assumption is the use of a RG scale such that the (infrared) RG effects only
appear at perturbative order (not at the background level). The emphasis here
is on analytical results and qualitative understanding of the implied
cosmology. We employ commonly used parametrizations for describing modified
gravity in cosmology (as the slip parameter). From them, we describe the
dynamics of the first order perturbations and estimate bounds on the single
dimensionless parameter () introduced by this framework. Possible impacts
on dark matter and dark energy are discussed. It is also shown here that the
parameter effects to are stronger at low redshifts (),
while different values for do not appreciably change at
higher redshifts, thus opening a window to alleviate an issue that is currently
faced by CDM.Comment: v3: 13 pages, 1 figure. Added analysis and text
improvements. Version to appear in EPJ
Bulk viscous cosmology with causal transport theory
We consider cosmological scenarios originating from a single imperfect fluid
with bulk viscosity and apply Eckart's and both the full and the truncated
M\"uller-Israel-Stewart's 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\"uller-Israel-Stewart's, both in its full and truncated forms) is taken into
account instead of Eckart's non-causal theory. To this purpose, we find
numerical solutions for the gravitational potential and compare its behaviour
with the corresponding LambdaCDM case. Eckart's and the full causal theory seem
to be disfavoured, whereas the truncated theory leads to results similar to
those of the LambdaCDM model for a bulk viscous speed in the interval 10^{-11}
<< c_b^2 < 10^{-8}. Tentatively relating such value to a square propagation
velocity of the order of T/m of perturbations in a non-relativistic gas of
particles with mass m at the epoch of matter-radiation equality, this may be
compatible with a mass range 0.1 GeV < m << 100 GeV.Comment: 23 pages, 7 figure
Note on the Evolution of the Gravitational Potential in Rastall Scalar Field Theories
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 , which is
the General Relativity limit. On the other hand, the addition of another
canonical fluid component allows also to consider the case .Comment: 16 pages, 3 figures, Sections 2 and 5 enlarged, accepted for
publication in Physics Letters
Observational constraints on Rastall's cosmology
Rastall's theory is a modification of General Relativity, based on the
non-conservation of the stress-energy tensor. The latter is encoded in a
parameter such that restores the usual law. We test Rastall's theory in cosmology, on a flat
Robertson-Walker metric, investigating a two-fluid model and using the type Ia
supernovae Constitution dataset. One of the fluids is pressureless and obeys
the usual conservation law, whereas the other is described by an equation of
state , with constant. The Bayesian analysis of the
Constitution set does not strictly constrain the parameter and prefers
values of close to -1. We then address the evolution of small
perturbations and show that they are dramatically unstable if and
, i.e. General Relativity is the favored configuration. The only
alternative is , for which the dynamics becomes independent from
.Comment: Latex file, 14 pages, 6 figures in eps format. Substantial
modifications performed, main conclusions change
Mapping the large-scale anisotropy in the WMAP data
Analyses of recent cosmic microwave background (CMB) observations have
provided increasing indications for the existence of large scale anisotropy in
the universe. Given the far reaching consequences of such an anisotropy for our
understanding of the universe, it is important to employ alternative indicators
in order to determine whether the reported anisotropy is cosmological in
origin, and if so extract further information that may be helpful for
identifying its causes. Here we propose a new directional indicator, based on
angular-separation histograms of pairs of pixels with similar temperatures in
the CMB map, which provides a measure of departure from statistical isotropy.
The main advantage of this indicator is that it can be used to generate a sky
map of large-scale anisotropies in the CMB temperature map, thus allowing a
possible additional window into their causes. Using this indicator, we find a
statistically significant (at 95% CL) preferred direction in the CMB data and
discuss how it compares with other such axes recently reported. We also show
that our findings are robust with respect to both the details of the method
used, and the choice of the WMAP CMB maps employed, including the three-year
CMB data released recently.Comment: 8 pages, 5 figures. v4.: Version to match the one to be published in
A&A. 3-year WMAP data used. Presentation improved. New figures added. Results
unchange