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We discuss the cosmological degeneracy between the age of the Universe, the Hubble parameter and the effective number of relativistic particles Nₑff. We show that independent determinations of the Hubble parameter H (z) as those recently provided by Simon, Verde, Jimenez (2006), combined with other cosmological data sets can provide the most stringent constraint on Nₑff, yielding Nₑff=3. 7 (-1. 2) (+1. 1) at 95% confidence level. A neutrino background is detected with high significance: Nₑff >1. 8 at better than 99% confidence level. Constraints on the age of the universe in the framework of an extra background of relativistic particles are improved by a factor 3.
Bernardis et al. (Thu,) studied this question.