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 \Delta N _ { eff } .
Within the flat \Lambda CDM scenario , the allowed range is \Delta N _ { eff } < 6 ( 95 % confidence level ) using WMAP data only , or -2.6 < \Delta N _ { eff } < 4 with the prior H _ { 0 } = 72 \pm 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 < \Delta N _ { eff } < 3.8 .
These results are in perfect agreement with the bounds from primordial nucleosynthesis .
Non-minimal cosmological models with extra relativistic degrees of freedom are now severely restricted .