Neutrino Astrophysics at the Cross Roads

The indications for neutrino oscillations from the atmospheric and solar neutrino anomalies and from the LSND experiment are now so overwhelming that the discourse in neutrino physics has changed. One no longer asks if these particles indeed oscillate, one rather debates the most plausible pattern of masses and mixing angles, and if the existence of a sterile neutrino is required. Of course, all of the indications for oscillations are to various degrees preliminary, yet so intruiging that it is difficult to resist their charm. It is a truism that astrophysics and cosmology play a unique role in neutrino physics, and conversely, that these light, weakly interacting particles are absolutely crucial for some of the most interesting astrophysical phenomena such as core-collapse supernovae and for the universe at large. Therefore, in this brief survey of current topics in neutrino astrophysics it behoves us to discuss what astrophysics and cosmology contribute to the current debate in neutrino physics and what the future perspectives are. To this end we begin in Sec. 2 with an overview of the current indications for neutrino oscillations and possible global interpretations. Astrophysical neutrinos, i.e. those from the Sun and from cosmic-ray interactions in the upper atmosphere, play a dominant role in this context. In Sec. 3 we next turn to the cosmological arguments relevant to neutrino physics (dark matter, structure formation, cosmic microwave background, big-bang nucleosynthesis). Supernova (SN) neutrinos are the topic of Sec. 4 where we discuss the role of neutrino masses and oscillations in this environment, the interpretation of the SN 1987A neutrino burst, and what one could learn from a future galactic SN. The recent developments in high-energy neutrino astronomy are touched upon in Sec. 5, while in Sec. 6 astrophysical aspects of neutrino electromagnetic properties are briefly discussed in the light of the current evidence for oscillations. Finally, in Sec. 7 we summarize our conclusions.