24 research outputs found
Revisiting the confrontation of the energy conditions with supernovae data
In the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach to model
the Universe the violation of the so-called energy conditions is related to
some important properties of the Universe as, for example, the current and the
inflationary accelerating expansion phases. The energy conditions are also
necessary in the formulation and proofs of Hawking-Penrose singularity
theorems. In two recent articles we have derived bounds from energy conditions
and made confrontations of these bounds with supernovae data. Here, we extend
these results in following way: first, by using our most recent statistical
procedure for calculating new q(z) estimates from the \emph{gold} and
\emph{combined} type Ia supernovae samples; second, we use these estimates to
obtain a new picture of the energy conditions fulfillment and violation for the
recent past () in the context of the standard cosmology.Comment: 5 pages. To appear in Int. J. Mod. Phys. D. Talk presented at the 3rd
International Workshop on Astronomy and Relativistic Astrophysics. V2: typos
correcte
Covariant Bardeen Perturbation Formalism
In a previous work we obtained a set of necessary conditions for the linear
approximation in cosmology. Here we discuss the relations of this approach with
the so called covariant perturbations. It is often argued in the literature
that one of the main advantages of the covariant approach to describe
cosmological perturbations is that the Bardeen formalism is coordinate
dependent. In this paper we will reformulate the Bardeen approach in a
completely covariant manner. For that, we introduce the notion of pure and
mixed tensors, which yields an adequate language to treat both perturbative
approaches in a common framework. We then stress that in the referred covariant
approach one necessarily introduces an additional hyper-surface choice to the
problem. Using our mixed and pure tensors approach, we were able to construct a
one-to-one map relating the usual gauge dependence of the Bardeen formalism
with the hyper-surface dependence inherent to the covariant approach. Finally,
through the use of this map, we define full non-linear tensors that at first
order correspond to the three known gauge invariant variables ,
and , which are simultaneously foliation and gauge invariant. We then
stress that the use of the proposed mixed tensors allows one to construct
simultaneously gauge and hyper-surface invariant variables at any order.Comment: 15 pages, no figures, revtex4-1, accepted for publication in PRD,
typos fixed, improved discussion about higher order gauge and foliation
invarianc
Cosmological constant constraints from observation-derived energy condition bounds and their application to bimetric massive gravity
Among the various possibilities to probe the theory behind the recent
accelerated expansion of the universe, the energy conditions (ECs) are of
particular interest, since it is possible to confront and constrain the many
models, including different theories of gravity, with observational data. In
this context, we use the ECs to probe any alternative theory whose extra term
acts as a cosmological constant. For this purpose, we apply a model-independent
approach to reconstruct the recent expansion of the universe. Using Type Ia
supernova, baryon acoustic oscillations and cosmic-chronometer data, we perform
a Markov Chain Monte Carlo analysis to put constraints on the effective
cosmological constant . By imposing that the cosmological
constant is the only component that possibly violates the ECs, we derive lower
and upper bounds for its value. For instance, we obtain that and within,
respectively, and confidence levels. In addition, about
30\% of the posterior distribution is incompatible with a cosmological
constant, showing that this method can potentially rule it out as a mechanism
for the accelerated expansion. We also study the consequence of these
constraints for two particular formulations of the bimetric massive gravity.
Namely, we consider the Visser's theory and the Hassan and Roses's massive
gravity by choosing a background metric such that both theories mimic General
Relativity with a cosmological constant. Using the
observational bounds along with the upper bounds on the graviton mass we obtain
constraints on the parameter spaces of both theories.Comment: 11 pages, 4 figures, 1 tabl
Quantum Cosmological Perturbations of Generic Fluids in Quantum Universes
In previous works, it was shown that the Lagrangians and Hamiltonians of
cosmological linear scalar, vector and tensor perturbations of homogeneous and
isotropic space-times with flat spatial sections containing a perfect fluid can
be put in a simple form through the implementation of canonical transformations
and redefinitions of the lapse function, without ever using the background
classical equations of motion. In this paper, we generalize this result to
general fluids, which includes entropy perturbations, and to arbitrary
spacelike hyper-surfaces through a new method together with the Faddeev-Jackiw
procedure for the constraint reduction. A simple second order Hamiltonian
involving the Mukhanov-Sasaki variable is obtained, again without ever using
the background equations of motion.Comment: 19 pages, revtex4-1, submitted to PRD, expanded discussion about our
method, fixed typo