28 research outputs found
Finite-time singularities in f(R, T) gravity and the effect of conformal anomaly
We investigate gravity models ( is the curvature scalar and
is the trace of the stress-energy tensor of ordinary matter) that are able to
reproduce the four known types of future finite-time singularities. We choose a
suitable expression for the Hubble parameter in order to realise the cosmic
acceleration and we introduce two parameters, and , which
characterise each type of singularity. We address conformal anomaly and we
observe that it cannot remove the sudden singularity or the type IV one, but,
for some values of , the big rip and the type III singularity may be
avoided. We also find that, even without taking into account conformal anomaly,
the big rip and the type III singularity may be removed thanks to the presence
of the contribution of the theory.Comment: 18 pages; Accepted for publication in Canadian Journal of Physics
(CJP
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
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
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
CMB-Galaxy correlation in Unified Dark Matter Scalar Field Cosmologies
We present an analysis of the cross-correlation between the CMB and the
large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar
field cosmologies. We work out the predicted cross-correlation function in UDM
models, which depends on the speed of sound of the unified component, and
compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and
SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of
the speed of sound and perform a likelihood analysis, finding that the UDM
model is as likely as the LambdaCDM, and is compatible with observations for a
range of values of c_\infinity (the value of the sound speed at late times) on
which structure formation depends. In particular, we obtain an upper bound of
c_\infinity^2 \leq 0.009 at 95% confidence level, meaning that the LambdaCDM
model, for which c_\infinity^2 = 0, is a good fit to the data, while the
posterior probability distribution peaks at the value c_\infinity^2=10^(-4) .
Finally, we study the time dependence of the deviation from LambdaCDM via a
tomographic analysis using a mock redshift distribution and we find that the
largest deviation is for low-redshift sources, suggesting that future low-z
surveys will be best suited to constrain UDM models.Comment: Slightly revised version accepted for publication in JCAP, with a few
added references; 26 pages, 8 figure
Unified Dark Matter models with fast transition
We investigate the general properties of Unified Dark Matter (UDM) fluid
models where the pressure and the energy density are linked by a barotropic
equation of state (EoS) and the perturbations are adiabatic. The
EoS is assumed to admit a future attractor that acts as an effective
cosmological constant, while asymptotically in the past the pressure is
negligible. UDM models of the dark sector are appealing because they evade the
so-called "coincidence problem" and "predict" what can be interpreted as
, but in general suffer the effects of a non-negligible
Jeans scale that wreak havoc in the evolution of perturbations, causing a large
Integrated Sachs-Wolfe effect and/or changing structure formation at small
scales. Typically, observational constraints are violated, unless the
parameters of the UDM model are tuned to make it indistinguishable from
CDM. Here we show how this problem can be avoided, studying in detail
the functional form of the Jeans scale in adiabatic UDM perturbations and
introducing a class of models with a fast transition between an early
Einstein-de Sitter CDM-like era and a later CDM-like phase. If the
transition is fast enough, these models may exhibit satisfactory structure
formation and CMB fluctuations. To consider a concrete case, we introduce a toy
UDM model and show that it can predict CMB and matter power spectra that are in
agreement with observations for a wide range of parameter values.Comment: 30 pages, 15 figures, JHEP3 style, typos corrected; it matches the
published versio
Stability analysis for the background equations for inflation with dissipation and in a viscous radiation bath
The effects of bulk viscosity are examined for inflationary dynamics in which
dissipation and thermalization are present. A complete stability analysis is
done for the background inflaton evolution equations, which includes both
inflaton dissipation and radiation bulk viscous effects. Three representative
approaches of bulk viscous irreversible thermodynamics are analyzed: the Eckart
noncausal theory, the linear and causal theory of Israel-Stewart and a more
recent nonlinear and causal bulk viscous theory. It is found that the causal
theories allow for larger bulk viscosities before encountering an instability
in comparison to the noncausal Eckart theory. It is also shown that the causal
theories tend to suppress the radiation production due to bulk viscous
pressure, because of the presence of relaxation effects implicit in these
theories. Bulk viscosity coefficients derived from quantum field theory are
applied to warm inflation model building and an analysis is made of the effects
to the duration of inflation. The treatment of bulk pressure would also be
relevant to the reheating phase after inflation in cold inflation dynamics and
during the radiation dominated regime, although very little work in both areas
has been done, the methodology developed in this paper could be extended to
apply to these other problems.Comment: 27 pages, 14 figures, Published version JCA
The extreme limit of the generalized 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 . We discuss the implications on
the background evolution of the Universe if large values of are
considered. In this case, the Universe expansion mimics a matter-dominated
phase abruptly followed by a de Sitter one at the transition redshift .
Thanks to an analysis of the type Ia supernovae Constitution set we are able to
place .Comment: 17 pages, 4 figures, corrected typos, references added, JHEP3 class;
it matches the published version
Revisiting Generalized Chaplygin Gas as a Unified Dark Matter and Dark Energy Model
In this paper, we revisit generalized Chaplygin gas (GCG) model as a unified
dark matter and dark energy model. The energy density of GCG model is given as
,
where and are two model parameters which will be constrained by
type Ia supernova as standard candles, baryon acoustic oscillation as standard
rulers and the seventh year full WMAP data points. In this paper, we will not
separate GCG into dark matter and dark energy parts any more as adopted in the
literatures. By using Markov Chain Monte Carlo method, we find the result:
and .Comment: 6 pages, 4 figure