Measuring the cosmological background of relativistic particles with the Wilkinson Microwave Anisotropy Probe

We show that the first year results of the Wilkinson Microwave Anisotropy Probe (WMAP) constrain very efficiently the energy density in relativistic particles in the Universe. We derive new bounds on additional relativistic degrees of freedom expressed in terms of an excess in the effective number of light neutrinos DeltaN(eff). Within the flat LambdaCDM scenario, the allowed range is DeltaN(eff)<6 (95% confidence level) using WMAP data only, or -2.6<DeltaN(eff)<4 with the prior H-0=72+/-8 km s(-1) Mpc(-1). When other cosmic microwave background and large scale structure experiments are taken into account, the window shrinks to -1.6<DeltaN(eff)<3.8. These results are in perfect agreement with the bounds from primordial nucleosynthesis. Nonminimal cosmological models with extra relativistic degrees of freedom are now severely restricted.