5,639 research outputs found
Reconstruction of interacting dark energy models from parameterizations
Models with interacting dark energy can alleviate the cosmic coincidence
problem by allowing dark matter and dark energy to evolve in a similar fashion.
At a fundamental level, these models are specified by choosing a functional
form for the scalar potential and for the interaction term. However, in order
to compare to observational data it is usually more convenient to use
parameterizations of the dark energy equation of state and the evolution of the
dark matter energy density. Once the relevant parameters are fitted it is
important to obtain the shape of the fundamental functions. In this paper I
show how to reconstruct the scalar potential and the scalar interaction with
dark matter from general parameterizations. I give a few examples and show that
it is possible for the effective equation of state for the scalar field to
cross the phantom barrier when interactions are allowed. I analyze the
uncertainties in the reconstructed potential arising from foreseen errors in
the estimation of fit parameters and point out that a Yukawa-like linear
interaction results from a simple parameterization of the coupling.Comment: 6 pages, 8 figure
Observational Constraints on Silent Quartessence
We derive new constraints set by SNIa experiments (`gold' data sample of
Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of
the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan
Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the
quartessence Chaplygin model. We consider both adiabatic perturbations and
intrinsic non-adiabatic perturbations such that the effective sound speed
vanishes (Silent Chaplygin). We show that for the adiabatic case, only models
with equation of state parameter are allowed: this
means that the allowed models are very close to \LambdaCDM. In the Silent case,
however, the results are consistent with observations in a much broader range,
-0.3<\alpha<0.7.Comment: 7 pages, 12 figures, to be submitted to JCA
Holographic dark energy described at the Hubble length
We consider holographic cosmological models of dark energy in which the
infrared cutoff is set by the Hubble's radius. We show that any interacting
dark energy model with a matter like term able to alleviate the coincidence
problem (i.e., with a positive interaction term, regardless of its detailed
form) can be recast as a noninteracting model in which the holographic
parameter evolves slowly with time. Two specific cases are analyzed. First, the
interacting model presented in [1] is considered, and its corresponding
noninteracting version found. Then, a new noninteracting model, with a specific
expression of the time-dependent holographic parameter, is proposed and
analyzed along with its corresponding interacting version. We constrain the
parameters of both models using observational data, and show that they can be
told apart at the perturbative level.Comment: 15 pages, 6 figure
Applications of Bayesian model selection to cosmological parameters
Bayesian model selection is a tool to decide whether the introduction of a
new parameter is warranted by data. I argue that the usual sampling statistic
significance tests for a null hypothesis can be misleading, since they do not
take into account the information gained through the data, when updating the
prior distribution to the posterior. On the contrary, Bayesian model selection
offers a quantitative implementation of Occam's razor.
I introduce the Savage-Dickey density ratio, a computationally quick method
to determine the Bayes factor of two nested models and hence perform model
selection. As an illustration, I consider three key parameters for our
understanding of the cosmological concordance model. By using WMAP 3-year data
complemented by other cosmological measurements, I show that a non-scale
invariant spectral index of perturbations is favoured for any sensible choice
of prior. It is also found that a flat Universe is favoured with odds of 29:1
over non--flat models, and that there is strong evidence against a CDM
isocurvature component to the initial conditions which is totally
(anti)correlated with the adiabatic mode (odds of about 2000:1), but that this
is strongly dependent on the prior adopted.
These results are contrasted with the analysis of WMAP 1-year data, which
were not informative enough to allow a conclusion as to the status of the
spectral index. In a companion paper, a new technique to forecast the Bayes
factor of a future observation is presented.Comment: v2 to v3: minor changes, matches accepted version by MNRAS. v1 to v2:
major revision. New results using WMAP 3-yr data, scale-invariant spectrum
now disfavoured with moderate evidence. New benchmark test for the accuracy
of the method. Bayes factor forecast methodology (PPOD, formerly called ExPO)
expanded and now presented in a companion paper (astro-ph/0703063
General CMB and Primordial Trispectrum Estimation
We present trispectrum estimation methods which can be applied to general
non-separable primordial and CMB trispectra. We present a general optimal
estimator for the connected part of the trispectrum, for which we derive a
quadratic term to incorporate the effects of inhomogeneous noise and masking.
We describe a general algorithm for creating simulated maps with given
arbitrary (and independent) power spectra, bispectra and trispectra. We propose
a universal definition of the trispectrum parameter , so that the
integrated bispectrum on the observational domain can be consistently compared
between theoretical models. We define a shape function for the primordial
trispectrum, together with a shape correlator and a useful parametrisation for
visualizing the trispectrum. We derive separable analytic CMB solutions in the
large-angle limit for constant and local models. We present separable mode
decompositions which can be used to describe any primordial or CMB bispectra on
their respective wavenumber or multipole domains. By extracting coefficients of
these separable basis functions from an observational map, we are able to
present an efficient estimator for any given theoretical model with a
nonseparable trispectrum. The estimator has two manifestations, comparing the
theoretical and observed coefficients at either primordial or late times. These
mode decomposition methods are numerically tractable with order
operations for the CMB estimator and approximately order for the general
primordial estimator (reducing to order in both cases for a special class
of models). We also demonstrate how the trispectrum can be reconstructed from
observational maps using these methods.Comment: 38 pages, 9 figures. In v2 Figures 4-7 are altered slightly and some
extra references are included in the bibliography. v3 matches version
submitted to journal. Includes discussion of special case
Scaling solutions in general non-minimal coupling theories
A class of generalized non-minimal coupling theories is investigated, in
search of scaling attractors able to provide an accelerated expansion at the
present time. Solutions are found in the strong coupling regime and when the
coupling function and the potential verify a simple relation. In such cases,
which include power law and exponential functions, the dynamics is independent
of the exact form of the coupling and the potential. The constraint from the
time variability of , however, limits the fraction of energy in the scalar
field to less than 4% of the total energy density, and excludes accelerated
solutions at the present.Comment: 10 pages, 3 figures, accepted for publication in Phys. Rev.
Black holes in scalar-tensor gravity
Hawking has proven that black holes which are stationary as the endpoint of
gravitational collapse in Brans--Dicke theory (without a potential) are no
different than in general relativity. We extend this proof to the much more
general class of scalar-tensor and f(R) gravity theories, without assuming any
symmetries apart from stationarity.Comment: v1: 4 pages; v2: typos corrected, published versio
Instabilities in tensorial nonlocal gravity
We discuss the cosmological implications of nonlocal modifications of general
relativity containing tensorial structures. Assuming the presence of standard
radiation- and matter-dominated eras, we show that, except in very particular
cases, the nonlocal terms contribute a rapidly growing energy density. These
models therefore generically do not have a stable cosmological evolution.Comment: 10 pages, 2 figures. v2: version published in PR
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