New constraint on the cosmological background of relativistic particles.pdf

a r X i v : a s t r o - p h / 0 5 1 0 5 8 2 v 2 2 5 O c t 2 0 0 5 New constraint on the cosmological background of relativistic particles Steen Hannestad Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark Abstract. We have derived new bounds on the relativistic energy density in the Universe from cosmic microwave background (CMB), large scale structure (LSS), and type Ia supernova (SNI-a) observations. In terms of the effective number of neutrino species a bound of Nν = 4.2 +1.2 ?1.7 is derived at 95% confidence. This bound is significantly stronger than previous determinations, mainly due to inclusion of new CMB and SNI-a observations. The absence of a cosmological neutrino background (Nν = 0) is now excluded at 5.4σ. The value of Nν is compatible with the value derived from big bang nucleosynthesis considerations, marking one of the most remarkable successes of the standard cosmological model. In terms of the cosmological helium abundance, the CMB, LSS, and SNI-a observations predict a value of 0.240 Y 0.281. New constraint on the cosmological background of relativistic particles 2 1. Introduction From detailed observations of the cosmic microwave background (CMB) [1–8], the large scale structure (LSS) of galaxies [9–11], and distant type Ia supernovae (SNI-a) [12– 14] many of the fundamental cosmological parameters are known quite precisely. Very interestingly the precision of the data is now also at a level where particle physics beyond the standard model can be probed. One of the prime examples of this is the total cosmological energy density in relativistic particles. In the standard model photons and neutrinos are by far the largest source of entropy in the Universe (and consequently also of energy density in the early Universe), and neutrinos are therefore the largest source of entropy in non-electromagnetically interacting species. In the early Universe neutrinos decouple at a temperature of roughly 2-3