ABSTRACT
Neutrinos and astrophysics, including cosmology, have three main areas of interface. The first is the standard role of neutrinos as radiation in the early universe, in ordinary stars and in core-collapse supernovae. Sometimes they play a dominant dynamical role (early universe, supernovae) or simply carry away energy (ordinary stars). In addition, they play a crucial role for nucleosynthesis in some of these environments because they participate in beta interactions of the type νe+n↔ p+e− and ν¯e+ p↔ n+e+. Their broad role as radiation is possible because of their small masses. Actually, in a typical stellar plasma or in the early universe, the dispersive photon mass far exceeds that of neutrinos, allowing for the plasmon decay process γ → νν¯ , i.e., in many astrophysical situations, neutrinos are effectively the lowest-mass particles except for gravitons. Of course, neutrinos do have small vacuum masses and therefore contribute a small fraction of the cosmic dark matter. Moreover, their small masses may be responsible for creating the cosmic matter-antimatter asymmetry by virtue of the leptogenesis mechanism.
