Open AccessThe isocurvature fraction after WMAP 3‐yr data
Author(s) -
Trotta Roberto
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society: letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.067
H-Index - 122
eISSN - 1745-3933
pISSN - 1745-3925
DOI - 10.1111/j.1745-3933.2006.00268.x
Subject(s) - cmb cold spot , physics , adiabatic process , cosmic microwave background , neutrino , bayes factor , fraction (chemistry) , statistical physics , model selection , bayesian probability , superposition principle , particle physics , astrophysics , anisotropy , bayesian inference , statistics , quantum mechanics , mathematics , chemistry , organic chemistry
I revisit the question of the adiabaticity of initial conditions for cosmological perturbations in view of the 3‐yr Wilkinson Microwave Anisotropy Probe ( WMAP ) data. I focus on the simplest alternative to purely adiabatic conditions, namely a superposition of the adiabatic mode and one of the three 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 cold dark matter isocurvature fraction is now constrained to be less than about 10 per cent, while the fraction in either the neutrino entropy or velocity mode is below about 20 per cent. Model comparison strongly disfavours mixed models that allow for isocurvature fractions larger than unity, while current data do not allow one to distinguish between a purely adiabatic model and models with a moderate (i.e. below about 10 per cent) 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.