540 research outputs found
Adiabatic initial conditions for perturbations in interacting dark energy models
We present a new systematic analysis of the early radiation era solution in
an interacting dark energy model to find the adiabatic initial conditions for
the Boltzmann integration. In a model where the interaction is proportional to
the dark matter density, adiabatic initial conditions and viable cosmologies
are possible if the early-time dark energy equation of state parameter is . We find that when adiabaticity between cold dark matter, baryons,
neutrinos and photons is demanded, the dark energy component satisfies
automatically the adiabaticity condition. As supernovae Ia or baryon acoustic
oscillation data require the recent-time equation of state parameter to be more
negative, we consider a time-varying equation of state in our model. In a
companion paper [arXiv:0907.4987] we apply the initial conditions derived here,
and perform a full Monte Carlo Markov Chain likelihood analysis of this model.Comment: 12 pages. V2: Minor changes, references added, conclusions extended;
Accepted by MNRA
Non-Gaussianity of the primordial perturbation in the curvaton model
We use the delta N -formalism to investigate the non-Gaussianity of the
primordial curvature perturbation in the curvaton scenario for the origin of
structure. We numerically calculate the full probability distribution function
allowing for the non-instantaneous decay of the curvaton and compare this with
analytic results derived in the sudden-decay approximation. We also present
results for the leading-order contribution to the primordial bispectrum and
trispectrum. In the sudden-decay approximation we derive a fully non-linear
expression relating the primordial perturbation to the initial curvaton
perturbation. As an example of how non-Gaussianity provides additional
constraints on model parameters, we show how the primordial bispectrum on CMB
scales can be used to constrain variance on much smaller scales in the curvaton
field. Our analytical and numerical results allow for multiple tests of
primordial non-Gaussianity, and thus they can offer consistency tests of the
curvaton scenario.Comment: 16 pages, 6 figures. V2: minor typos corrected, references added. V3:
minor changes to match better with the PRD versio
Constraints on primordial isocurvature perturbations and spatial curvature by Bayesian model selection
We present posterior likelihoods and Bayesian model selection analysis for
generalized cosmological models where the primordial perturbations include
correlated adiabatic and cold dark matter isocurvature components. We perform
nested sampling with flat and, for the first time, curved spatial geometries of
the Universe, using data from the cosmic microwave background (CMB)
anisotropies, the Union supernovae (SN) sample and a combined measurement of
the integrated Sachs-Wolfe (ISW) effect. The CMB alone favors a 3% (positively
correlated) isocurvature contribution in both the flat and curved cases. The
non-adiabatic contribution to the observed CMB temperature variance is 0 <
alpha_T < 7% at 98% CL in the curved case. In the flat case, combining the CMB
with SN data artificially biases the result towards the pure adiabatic LCDM
concordance model, whereas in the curved case the favored level of
non-adiabaticity stays at 3% level with all combinations of data. However, the
ratio of Bayes factors, or Delta ln(evidence), is more than 5 points in favor
of the flat adiabatic LCDM model, which suggests that the inclusion of the 5
extra parameters of the curved isocurvature model is not supported by the
current data. The results are very sensitive to the second and third acoustic
peak regions in the CMB temperature angular power: therefore a careful
calibration of these data will be required before drawing decisive conclusions
on the nature of primordial perturbations. Finally, we point out that the odds
for the flat non-adiabatic model are 1:3 compared to the curved adiabatic
model. This may suggest that it is not much less motivated to extend the
concordance model with 4 isocurvature degrees of freedom than it is to study
the spatially curved adiabatic model.Comment: 15 pages, 5 figures. V2: References and future predictions added;
accepted by PR
Dark energy with non-adiabatic sound speed: initial conditions and detectability
Assuming that the universe contains a dark energy fluid with a constant
linear equation of state and a constant sound speed, we study the prospects of
detecting dark energy perturbations using CMB data from Planck,
cross-correlated with galaxy distribution maps from a survey like LSST. We
update previous estimates by carrying a full exploration of the mock data
likelihood for key fiducial models. We find that it will only be possible to
exclude values of the sound speed very close to zero, while Planck data alone
is not powerful enough for achieving any detection, even with lensing
extraction. We also discuss the issue of initial conditions for dark energy
perturbations in the radiation and matter epochs, generalizing the usual
adiabatic conditions to include the sound speed effect. However, for most
purposes, the existence of attractor solutions renders the perturbation
evolution nearly independent of these initial conditions.Comment: 16 pages, 2 figures, version accepted in JCA
Breaking parameter degeneracy in interacting dark energy models from observations
We study the interacting dark energy model with time varying dark energy
equation of state. We examine the stability in the perturbation formalism and
the degeneracy among the coupling between dark sectors, the time-dependent dark
energy equation of state and dark matter abundance in the cosmic microwave
background radiation. Further we discuss the possible ways to break such
degeneracy by doing global fitting using the latest observational data and we
get a tight constraint on the interaction between dark sectors.Comment: 8 pages, 6 figures, accepted for publication in Phys.Lett.
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