Abstract
We have derived new bounds on the relativistic energy density in the Universe from cosmicmicrowave background (CMB), large scale structure (LSS), and type Ia supernova (SNI-a)observations. In terms of the effective number of neutrino species, a bound ofNν = 4.2−1.7+1.2 is derived at 95% confidence. This bound is significantly stronger thanprevious determinations, mainly due to the inclusion of new CMB andSNI-a observations. The absence of a cosmological neutrino background(Nν = 0) is nowexcluded at 5.4σ.The value of Nν is compatible with the value derived from big bang nucleosynthesis considerations, markingone of the most remarkable successes of the standard cosmological model. In terms of thecosmological helium abundance, the CMB, LSS, and SNI-a observations predict a value of0.240 < Y < 0.281.
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