WMAP 5-year constraints on lepton asymmetry and radiation energy density: Implications for Planck

TL;DR

This study uses WMAP 5-year data combined with other cosmological measurements to constrain the universe's radiation content and neutrino properties, highlighting the impact of lepton asymmetry and predicting improvements with Planck data.

Contribution

It provides the first comprehensive bounds on neutrino degeneracy and relativistic degrees of freedom using combined CMB and BBN constraints, considering lepton asymmetry.

Findings

Strong bounds on helium mass fraction and neutrino degeneracy parameter.

Correlation between matter density and redshift of matter-radiation equality.

Future Planck data will significantly improve parameter constraints.

Abstract

In this paper we set bounds on the radiation content of the Universe and neutrino properties by using the WMAP-5 year CMB measurements complemented with most of the existing CMB and LSS data (WMAP5+All),imposing also self-consistent BBN constraints on the primordial helium abundance. We consider lepton asymmetric cosmological models parametrized by the neutrino degeneracy parameter and the variation of the relativistic degrees of freedom, due to possible other physical processes occurred between BBN and structure formation epochs. We find that WMAP5+All data provides strong bounds on helium mass fraction and neutrino degeneracy parameter that rivals the similar bounds obtained from the conservative analysis of the present data on helium abundance. We also find a strong correlation between the matter energy density and the redshift of matter-radiation equality, z_re, showing that we observe non-zero equivalent number of relativistic neutrinos mainly via the change of the of z_re, rather than via neutrino anisotropic stress claimed by the WMAP team. We forecast that the CMB temperature and polarization measurements observed with high angular resolutions and sensitivities by the future Planck satellite will reduce the errors on these parameters down to values fully consistent with the BBN bounds.