Present bounds on the relativistic energy density in the Universe from cosmological observables

We discuss the present bounds on the relativistic energy density in the Universe parametrized in terms of the effective number of neutrinos N-v(eff) using the most recent cosmological data on cosmic microwave background (CMB) temperature anisotropies and polarization, large scale galaxy clustering from the Sloan Digital Sky Survey (SDSS) and 2dF, luminosity distances of type Ia supernovae, Lyman-alpha absorption clouds (Ly-alpha), the baryonic acoustic oscillations (BAO) detected in the luminous red galaxies of the SDSS and, finally, big bang nucleosynthesis (BBN) predictions for He-4 and deuterium abundances. We find N-v(eff) = 5.2(-2.2)(+2.7) from CMB and large scale structure data, while adding Ly-alpha and BAO we obtain Ne-v(eff) = 4.6(-1.5)(+1.6) at 95% c. l. These results show some tension with the standard value N-v(eff) = 3.046 as well as with the BBN range N-v(eff) = 3.1(-1.2)(+1.4) at 95% c. l., though the discrepancy is slightly below the 2-s level. In general, considering a smaller set of data weakens the constraints on N-v(eff). We emphasize the impact of an improved upper limit (or measurement) of the primordial value of He-3 abundance in clarifying the issue of whether the value of N(v)(ef)f at early (BBN) and more recent epochs coincide.