2,051 research outputs found
Probing dark energy with future surveys
I review the observational prospects to constrain the equation of state parameter of dark energy and I discuss the potential of future imaging and redshift surveys. Bayesian model selection is used to address the question of the level of accuracy on the equation of state parameter that is required before explanations alternative to a cosmological constant become very implausible. I discuss results in the prediction space of dark energy models. If no significant departure from w=-1 is detected, a precision on w of order 1% will translate into strong evidence against fluid-like dark energy, while decisive evidence will require a precision of order 10^-3
Testing the paradigm of adiabaticity
We introduce the concepts of adiabatic (curvature) and isocurvature (entropy)
cosmological perturbations and present their relevance for parameter estimation
from cosmic microwave background anisotropies data. We emphasize that, while
present-day data are in excellent agreement with pure adiabaticity, subdominant
isocurvature contributions cannot be ruled out. We discuss model independent
constraints on the isocurvature contribution. Finally, we argue that the Planck
satellite will be able to do precision cosmology even if the assumption of
adiabaticity is relaxed.Comment: Proceedings of the 10th Marcel Grossmann Meeting, Rio de Janeiro,
July 2003, 5 pages, 2 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
Statistical Challenges of Global SUSY Fits
We present recent results aiming at assessing the coverage properties of Bayesian and frequentist inference methods, as applied to the reconstruction of supersymmetric parameters from simulated LHC data. We discuss the statistical challenges of the reconstruction procedure, and highlight the algorithmic difficulties of obtaining accurate profile likelihood estimates
Constraining the helium abundance with CMB data
We consider for the first time the ability of present-day cosmic microwave background (CMB) anisotropies data to determine the primordial helium mass fraction, Y_p. We find that CMB data alone gives the confidence interval 0.160 < Y_p < 0.501 (at 68% c.l.). We analyse the impact on the baryon abundance as measured by CMB and discuss the implications for big bang nucleosynthesis. We identify and discuss correlations between the helium mass fraction and both the redshift of reionization and the spectral index. We forecast the precision of future CMB observations, and find that Planck alone will measure Y_p with error-bars of 5%. We point out that the uncertainty in the determination of the helium fraction will have to be taken into account in order to correctly estimate the baryon density from Planck-quality CMB data
Reproducing Cosmic Microwave Background anisotropies with mixed isocurvature perturbations
Recently high quality data of the cosmic microwave background anisotropies
have been published. In this work we study to which extent the cosmological
parameters determined by using this data depend on assumptions about the
initial conditions. We show that for generic initial conditions, not only the
best fit values are very different but, and this is our main result, the
allowed parameter range enlarges dramatically.Comment: 4 pages, 5 figures, submitted to PRL; Major changes following
referees suggestions; the allowed cosmological parameter range enlarges
dramaticall
The isocurvature fraction after WMAP 3-year data
I revisit the question of the adiabaticity of initial conditions for
cosmological perturbations in view of the 3-year WMAP data. I focus on the
simplest alternative to pure adiabatic conditions, namely a superposition of
the adiabatic mode and one of the 3 possible isocurvature modes, with the same
spectral index as the adiabatic component.
I discuss findings in terms of posterior bounds on the isocurvature fraction
and Bayesian model selection. The Bayes factor (models likelihood ratio) and
the effective Bayesian complexity are computed for several prior ranges for the
isocurvature content. I find that the CDM isocurvature fraction is now
constrained to be less than about 10%, while the fraction in either the
neutrino entropy or velocity mode is below about 20%. Model comparison strongly
disfavours mixed models that allow for isocurvature fractions larger than
unity, while current data do not allow to distinguish between a purely
adiabatic model and models with a moderate (ie, below about 10%) isocurvature
contribution.
The conclusion is that purely adiabatic conditions are strongly favoured from
a model selection perspective. This is expected to apply in even stronger terms
to more complicated superpositions of isocurvature contributions.Comment: Expanded discussion of degeneracies, updated references, no change to
conclusions. Matches published versio
Introducing doubt in Bayesian model comparison
There are things we know, things we know we dont know, and then there are things we dont know we dont know. In this paper we address the latter two issues in a Bayesian framework, introducing the notion of doubt to quantify the degree of (dis)belief in a model given observational data in the absence of explicit alternative models. We demonstrate how a properly calibrated doubt can lead to model discovery when the true model is unknown
On prospects for dark matter indirect detection in the Constrained MSSM
In the framework of the Constrained MSSM we derive the most probable ranges of the diffuse gamma radiation flux from the direction of the Galactic center and of the positron flux from the Galactic halo due to neutralino dark matter annihilation. We find that, for a given halo model, and assuming flat priors, the 68% probability range of the integrated gamma-ray flux spans about one order of magnitude, while the 95% probability range can be much larger and extend over four orders of magnitude (even exceeding five for a tiny region at small neutralino mass). The detectability of the signal by GLAST depending primarily on the cuspiness of the halo profile. The positron flux, on the other hand, appears to be too small to be detectable by PAMELA, unless the boost factor is at least of order ten and/or the halo profile is extremely cuspy. We also briefly discuss the sensitivity of our results to the choice of priors
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