Relic neutrino clustering and implications for their detection

The standard big bang theory predicts the existence of 10 neutrinos per flavour in the visible universe. This is an enormous abundance unrivalled by any other known form of matter, falling second only to the cosmic microwave background (CMB) photon. Yet, unlike the CMB which boasts its first detection in the 1960s and which has since been observed and its properties measured to high accuracy in a series of experiments, the relic neutrino continues to be elusive in the laboratory. The chief reason for this is of course the feebleness of the weak interaction. The smallness of the neutrino mass also makes momentum-transfer-based detection methods highly impractical. At present, the only evidence for the relic neutrino comes from inferences from other cosmological measurements, such as big bang nucleosynthesis, CMB and large scale structure (LSS) data (e.g., [2]). Nevertheless, it is difficult to accept that these neutrinos will never be detected in a more direct way. In order to design feasible direct, scattering-based detection methods, a precise knowledge of the relic neutrino phase space distribution is indispensable. In this connection, it is important note that an oscillation interpretation of the atmospheric and solar neutrino data (e.g., [3]) implies that at least two of the neutrino mass eigenstates are nonrelativistic today. These neutrinos are subject to gravitational clustering on existing cold dark matter (CDM) and baryonic structures, possibly causing the local neutrino number density to depart from the standard value of n̄ν = n̄ν̄ ≃ 56 cm, and the momentum distribution to deviate from the relativistic Fermi–Dirac function. In this talk, we describe a method that allows us to study the gravitational clustering of relic neutrinos onto CDM/baryonic structures. We calculate the present day neutrino overdensities in general CDM halos and in the Milky