Probing the Effective Number of Neutrino Species with Cosmic Microwave Background
Abstract
We discuss how much we can probe the effective number of neutrino species Nnu with cosmic microwave background alone. Using the data of WMAP, ACBAR, CBI and BOOMERANG experiments, we obtain a constraint on the effective number of neutrino species as 0.96< Nnu <7.94 at 95% C.L. for a power-law LCDM flat universe model. The limit is improved to be 1.39 < Nnu < 6.38 at 95% C.L. if we assume that the baryon density, Nnu and the helium abundance are related by the big bang nucleosynthesis theory. We also provide a forecast for the PLANCK experiment using a Markov chain Monte Carlo approach. In addition to constraining Nnu, we investigate how the big bang nucleosynthesis relation affects the estimation for these parameters and other cosmological parameters.