131 research outputs found
Cosmological singularities and modified theories of gravity
We consider perturbative modifications of the Friedmann equations in terms of density corresponding to modified theories of gravity proposed as an alternative route to comply with the observed accelerated expansion of the universe. Assuming that the present matter content of the universe is a pressureless fluid, the possible singularities that may arise as the final state of the universe are surveyed. It is shown that, at most, two coefficients of the perturbative expansion of the Friedman equations are relevant for the analysis. Some examples of application of the perturbative scheme are included
Equation of state and singularities in FLRW cosmological models
We consider FLRW cosmological models with standard Friedmann equations, but leaving free the equation of state. We assume that the dark energy content of the universe is encoded in an equation of state p = f(ρ), which is expressed with most generality in the form of a power expansion. The inclusion of this expansion in Friedmann equations allows us to construct a perturbative solution and to relate the coefficients of the equation of state with the formation of singularities of different types
Tracking solutions in tachyon cosmology
We perform a thorough phase-plane analysis of the flow defined by the
equations of motion of a FRW universe filled with a tachyonic fluid plus a
barotropic one. The tachyon potential is assumed to be of inverse square form,
thus allowing for a two-dimensional autonomous system of equations. The
Friedmann constraint, combined with a convenient choice of coordinates, renders
the physical state compact. We find the fixed-point solutions, and discuss
whether they represent attractors or not. The way the two fluids contribute at
late-times to the fractional energy density depends on how fast the barotropic
fluid redshifts. If it does it fast enough, the tachyonic fluid takes over at
late times, but if the opposite happens, the situation will not be completely
dominated by the barotropic fluid; instead there will be a residual
non-negligible contribution from the tachyon subject to restrictions coming
from nucleosynthesis.Comment: 5 pages, 4 figure
Revisiting a model-independent dark energy reconstruction method
Model independent reconstructions of dark energy have received some
attention. The approach that addresses the reconstruction of the dimensionless
coordinate distance and its two first derivatives using a polynomial fit in
different redshift windows is well developed
\cite{DalyDjorgovski1,DalyDjorgovski2,DalyDjorgovski3}. In this work we offer
new insights into the problem by focusing on two types of observational probes:
SNeIa and GRBs. Our results allow to highlight some of the intrinsic weaknesses
of the method. One of the directions we follow is to consider updated
observational samples. Our results indicate than conclusions on the main dark
energy features as drawn from this method are intimately related to the
features of the samples themselves (which are not quite ideal). This is
particularly true of GRBs, which manifest themselves as poor performers in this
context. In contrast to original works, we conclude they cannot be used for
cosmological purposes, and the state of the art does not allow to regard them
on the same quality basis as SNeIa. The next direction we contribute to is the
question of how the adjusting of some parameters (window width, overlap,
selection criteria) affect the results. We find again there is a considerable
sensitivity to these features. Then, we try to establish what is the current
redshift range for which one can make solid predictions on dark energy
evolution. Finally, we strengthen the former view that this model is modest in
the sense it provides only a picture of the global trend. But, on the other
hand, we believe it offers an interesting complement to other approaches given
that it works on minimal assumptions.Comment: revtex4-1, 17 page
Observational constraints on phantom-like braneworld cosmologies
We investigate a simple braneworld model in which the universe contains only
cold dark matter and a cosmological constant, but the effective dark energy is
phantom-like because of extra-dimensional gravity effects. Modified gravity
screens the cosmological constant Lambda, allowing for a larger Lambda. In
practice, observations do not favour any significant screening. We use
supernova data, the cosmic microwave background shift parameter, and the baryon
oscillation peak in the galaxy distribution to constrain the model. We find the
mean value of Omega_m with 68% confidence limits, and an upper limit on
Omega_Lambda at the 68% confidence level. The best-fit model is very close to a
standard LCDM model, but the LCDM model provides a better fit since it has one
less parameter.Comment: small improvements; matches the version to appear in Phys Rev
- …