Probing the Effective Number of Neutrino Species with Cosmic Microwave Background

TL;DR

This paper assesses how well current and future cosmic microwave background data can constrain the effective number of neutrino species, revealing broad limits with existing data and tighter bounds with theoretical assumptions.

Contribution

It provides the first comprehensive analysis of N_nu constraints from CMB data alone and forecasts improvements with the PLANCK experiment, including the impact of big bang nucleosynthesis assumptions.

Findings

Current data constrains 0.96 < N_nu < 7.94 at 95% C.L.

Assuming BBN relations tightens constraints to 1.39 < N_nu < 6.38.

Forecasts indicate PLANCK will significantly improve N_nu measurements.

Abstract

We discuss how much we can probe the effective number of neutrino species N_nu 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< N_nu <7.94 at 95% C.L. for a power-law LCDM flat universe model. The limit is improved to be 1.39 < N_nu < 6.38 at 95% C.L. if we assume that the baryon density, N_nu 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 N_nu, we investigate how the big bang nucleosynthesis relation affects the estimation for these parameters and other cosmological parameters.