323 research outputs found
Constraints on Dark Energy and Modified Gravity models by the Cosmological Redshift Drift test
We study cosmological constraints on the various accelerating models of the
universe using the time evolution of the cosmological redshift of distant
sources. The important characteristic of this test is that it directly probes
the expansion history of the universe. In this work we analyze the various
models of the universe which can explain the late time acceleration, within the
framework of General Theory of Relativity (GR) (XCDM, scalar field potentials)
and beyond GR (f(R) gravity model).Comment: 7 pages, 10 figures, revised version, accepted for publication in
Physics Lett.
Observational constraints on thawing quintessence models
We use a dynamical systems approach to study thawing quintessence models,
using a multi-parameter extension of the exponential potential which can
approximate the form of typical thawing potentials. We impose observational
constraints using a compilation of current data, and forecast the tightening of
constraints expected from future dark energy surveys, as well as discussing the
relation of our results to analytical constraints already in the literature.Comment: 6 pages MNRAS style with 8 figures included. Minor updates to match
MNRAS accepted versio
The growth index of matter perturbations and modified gravity
We place tight constraints on the growth index by using the recent
growth history results of 2dFGRS, SDSS-LRG, VIMOS-VLT deep Survey (VVDS) and
{\em WiggleZ} datasets. In particular, we investigate several parametrizations
of the growth index , by comparing their cosmological evolution
using observational growth rate data at different redshifts. Utilizing a
standard likelihood analysis we find that the use of the combined growth data
provided by the 2dFGRS, SDSS-LRG, VVDS and {\em WiggleZ} galaxy surveys, puts
the most stringent constraints on the value of the growth index. As an example,
assuming a constant growth index we obtain that for the
concordance CDM expansion model. Concerning the
Dvali-Gabadadze-Porrati gravity model, we find which is
lower, and almost away, from the theoretically predicted value of
. Finally, based on a time varying growth index we
also confirm that the combined growth data disfavor the DGP gravity.Comment: 8 pages, 5 figures. Revised version accepted in MNRAS. arXiv admin
note: text overlap with arXiv:1202.163
Revisiting the parametrization of Equation of State of Dark Energy via SNIa Data
In this paper, we revisit the parameterizations of the equation of state of
dark energy and point out that comparing merely the of different
fittings may not be optimal for choosing the "best" parametrization. Another
figure of merit for evaluating different parametrizations based on the area of
the band is proposed. In light of the analysis of some two-parameter
parameterizations and models based on available SNIa data, the area of
band seems to be a good figure of merit, especially in the situation that the
value of for different parametrizations are very close.
Therefore, we argue that both the area of the band and should be synthetically considered for choosing a better parametrization
of dark energy in the future experiments.Comment: 7 pages, contains 5 figures and 2 tables, accepted for publication in
MNRA
Limitations of Bayesian Evidence Applied to Cosmology
There has been increasing interest by cosmologists in applying Bayesian
techniques, such as Bayesian Evidence, for model selection. A typical example
is in assessing whether observational data favour a cosmological constant over
evolving dark energy. In this paper, the example of dark energy is used to
illustrate limitations in the application of Bayesian Evidence associated with
subjective judgements concerning the choice of model and priors. An analysis of
recent cosmological data shows a statistically insignificant preference for a
cosmological constant over simple dynamical models of dark energy. It is argued
that for nested problems, as considered here, Bayesian parameter estimation can
be more informative than computing Bayesian Evidence for poorly motivated
physical models.Comment: 8 pages 4 figures MNRAS accepted. Substantially revised and extende
Composite dark energy: cosmon models with running cosmological term and gravitational coupling
In the recent literature on dark energy (DE) model building we have learnt
that cosmologies with variable cosmological parameters can mimic more
traditional DE pictures exclusively based on scalar fields (e.g. quintessence
and phantom). In a previous work we have illustrated this situation within the
context of a renormalization group running cosmological term, Lambda. Here we
analyze the possibility that both the cosmological term and the gravitational
coupling, G, are running parameters within a more general framework (a variant
of the so-called ``LXCDM models'') in which the DE fluid can be a mixture of a
running Lambda and another dynamical entity X (the ``cosmon'') which may behave
quintessence-like or phantom-like. We compute the effective EOS parameter, w,
of this composite fluid and show that the LXCDM can mimic to a large extent the
standard LCDM model while retaining features hinting at its potential composite
nature (such as the smooth crossing of the cosmological constant boundary
w=-1). We further argue that the LXCDM models can cure the cosmological
coincidence problem. All in all we suggest that future experimental studies on
precision cosmology should take seriously the possibility that the DE fluid can
be a composite medium whose dynamical features are partially caused and
renormalized by the quantum running of the cosmological parameters.Comment: LaTeX, 17 pages, 4 figures. Extended discussion, references added.
Accepted in Physics Letters
Thawing Versus. Tracker Behaviour: Observational Evidence
Currently there is a variety of scalar field models to explain the late time
acceleration of the Universe. This includes the standard canonical and
non-canonical scalar field models together with recently proposed Galileon
scalar field models. One can divide all these scalar field models into two
broad categories, namely the thawing and the tracker class. In this work we
investigate the evidence for these models with the presently available
observational data using the Bayesian approach. We use the Generalized
Chaplygin Gas (GCG) parametrization for dark energy equation of state (EoS) as
it gives rise to both the thawing and tracking behaviours for different values
of the parameters. Analysis of the observational data does not give any clear
evidence for either thawing or tracking behaviour within the context of
background cosmology, However, if we consider the evolution of inhomogenities
and analyze the data in this context then there is a significant evidence in
favour of thawing behaviour.Comment: 6 Pages, three eps figures, new material added, new references added.
Conclusion changed. Accepted for publication MNRA
Growth Index of DGP Model and Current Growth Rate Data
Recently, some efforts focus on differentiating dark energy and modified
gravity with the growth function . In the literature, it is useful
to parameterize the growth rate
with the growth index . In this note, we consider the general DGP model
with any . We confront the growth index of DGP model with currently
available growth rate data and find that the DGP model is still consistent with
it. This implies that more and better growth rate data are required to
distinguish between dark energy and modified gravity.Comment: 12 pages, 1 table, 2 figures, Latex2e; v2: discussions added, Phys.
Lett. B in press; v3: published versio
The thawing dark energy dynamics: Can we detect it?
We consider different classes of scalar field models including quintessence,
and tachyon scalar fields with a variety of generic potential belonging to
thawing type. Assuming the scalar field is initially frozen at , we
evolve the system until the present time. We focus on observational quantities
like Hubble parameter, luminosity distance as well as quantities related to the
Baryon Acoustic Oscillation measurement. Our study shows that with present
state of observations, one can not distinguish amongst various models which in
turn can not be distinguished from cosmological constant. This lead us to a
conclusion that there is a thin chance to observe the dark energy metamorphosis
in near future.Comment: 7 pages, Revtex Style, 6 eps figures, replaced with revised version,
some figures are modified, minor changes, conclusions remain the same,
Accepted for publication in Physics Letters
Spherical collapse model in dark energy cosmologies
We study the spherical collapse model for several dark energy scenarios using
the fully nonlinear differential equation for the evolution of the density
contrast within homogeneous spherical overdensities derived from Newtonian
hydrodynamics. While mathematically equivalent to the more common approach
based on the differential equation for the radius of the perturbation, this
approach has substantial conceptual as well as numerical advantages. Among the
most important are that no singularities at early times appear, which avoids
numerical problems in particular in applications to cosmologies with dynamical
and early dark energy, and that the assumption of time-reversal symmetry can
easily be dropped where it is not strictly satisfied. We use this approach to
derive the two parameters characterising the spherical-collapse model, i.e.~the
linear density threshold for collapse and the virial
overdensity , for a broad variety of dark-energy models and
to reconsider these parameters in cosmologies with early dark energy. We find
that, independently of the model under investigation, and
are always very close to the values obtained for the
standard CDM model, arguing that the abundance of and the mean density
within non-linear structures are quite insensitive to the differences between
dark-energy cosmologies. Regarding early dark energy, we thus arrive at a
different conclusion than some earlier papers, including one from our group,
and we explain why.Comment: 11 pages, 7 figures, accepted for publications on MNRA
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