Neutrinos in Astrophysics and Cosmology

Abstract

Since its existence was postulated in 1932, the neutrino has been an elusive particle and until recently its theoretical importance has been to salvage the conservation laws of physics (1). As far back as 1939, Gamow & Schonberg (2) recognized the role played by neutrinos in the course of the evolution of stars, the fundamental building blocks of the universe. How­ ever, the study of the relation between neutrinos and the theory of stellar evolution has become more active only in recent years during which theories of both the weak interaction and the stellar interior have undergone great advances. Laboratory detection and study of the neutrino have been successfully carried out in a number of cases (3), and neutrinos from cosmic sources have recently been detected (4). An experiment to detect solar neutrinos is also under way and is expected to yield results in a few years (5). However, neutrino astronomy is likely to remain a theoretical subject for a number of years to come. The same fact that makes neutrinos important in astro­ p hysical processes on a cosmic scale also makes cosmic neutrinos exceedingly hard to detect. This fact is that the neutrino interacts little with matter regardless of energy. The interaction cross section (I of neutrinos with matter is approximately 10-44 x2 cm2, where x is the neutrino momentum in meV Ie. At very high energy (",1 BeV), fT approaches a few times 10-38 Cm2, and it is believed that it does not increase at higher energies. The mean free path of a 1 meV neutrino in water is around 1021 cm, many times greater than the dimension of a star, which is from 109 to 1013 cm. Thus, once a neutrino is produced in the interior of a star, it can escape easily, carrying energy away. Neutrino emission processes are therefore important as a separate mecha­ nism for dissipating stellar energy by means other than the usual surface radiation. The properties of the neutrino, the relation of the neutrino with other fundamental particles, and the theory of weak interaction have been dis­ cussed in detail by Lee & Wu (6). In this article we shall be mainly concerned with the role of neutrinos in stellar evolution and in cosmological processes. Because of limitation in space, a number of topics are omitted from dis1 The survey of literature for this review was concluded in June 1966.