Anisotropies in the Cosmic Neutrino Background after WMAP 5-year Data

Abstract

We search for the presence of cosmological neutrino background (CNB) anisotropies in recent WMAP 5-year data using their signature imprinted on modifications to cosmic microwave background (CMB) anisotropy power spectrum. By parametrizing the neutrino background anisotropies with the speed viscosity parameter $c_\vis$, we find that the WMAP 5-year data alone provide only a weak indication for CNB anisotropies with $c_\vis^2 > 0.06$ at the 95% confidence level. When we combine CMB anisotropy data with measurements of galaxy clustering, SN-Ia Hubble diagram, and other cosmological information, the detection increases to $c_\vis^2 > 0.16$ at the same 95% confidence level. Future data from Planck, combined with a weak lensing survey such as the one expected with DUNE from space, will be able to measure the CNB anisotropy parameter at about 10% accuracy. We discuss the degeneracy between neutrino background ansiotropies and other cosmological para meters such as the number of effective neutrinos species and the dark energy equation of state.