7 - Massive Neutrinos in Cosmology and Astrophysics

Summary

If neutrinos have masses of several eV and if their lifetimes are comparable to, or longer than the lifetime of the universe, neutrino mass may dominate the total mass of the universe. Massive neutrinos would have a profound effect on universal expansion, primordial nucleosynthesis, and the formation and stability of structures in the universe. At the same time, cosmology and astrophysics provide important constraints on the properties of massive neutrinos. Consequently, the role of massive neutrinos in cosmology and astrophysics is an important part of the problems associated with neutrino mass. However, the various issues and techniques of cosmology and astrophysics are to a large extent beyond the scope of this book. Therefore, we present in this Chapter only a brief overview based on rather elementary considerations and refer the reader to the more rigorous derivations and discussion elsewhere, e.g., in Weinberg (72). Many problems related to the interface between cosmology and particle physics are reviewed by Steigman (79) and by Zel'dovich & Khlopov (81). The monograph by Kolb & Turner (90) treats all issues of this chapter in detail.

Cosmological Constraints on Neutrino Properties

Critical density. The simplest plausible assumption about the large scale mass and energy distribution in the universe is based on the cosmological principle, which states that all positions in the universe are equivalent and hence the universe is homogeneous and isotropic. This principle leads naturally to Friedmann's solution of Einstein's field equations, which describes uniform expansion of the cosmic fluid with time.