Why neutrino astronomy ?

Photons have many advantages as cosmic information carriers: they are copiously produced, they are stable and electrically neutral, they are easy to detect over a wide energy range, and their spectrum carries detailed information about the chemical and physical properties of the source. 

Their disadvantage is that the hot, dense regions which form the central engines of stars, active galactic nuclei and other astrophysical energy sources are completely opaque to photons, and therefore we cannot investigate the properties of these regions by direct observation, but only by indirect inference. For example, the photons we observe from the Sun come from its photosphere, far removed from the hydrogen-fusing core. Moreover, high energy photons interact with photons of the infrared radiation background and with the cosmic microwave background to create electron-positron pairs; this is the Greisen-Zatsepin-Kuz'min effect (GZK). This effect suppresses any possibility of surveying the sky over distances greater than 100Mpc with high energy (>10 TeV) gamma rays. 

In order to observe the inner workings of the astrophysical objects and to obtain a description of the Universe over a larger range of energies, we need a probe which is electrically neutral, so that its trajectory will not be affected by magnetic fields, stable so that it will reach us from distant sources, and weakly interacting so that it will penetrate regions which are opaque to photons. The only candidate currently known to exist is the neutrino.

Top left is NE. Image width is about 1.5 degrees
© Anglo-Australian Observatory, Photograph from UK Schmidt plates by David Malin.

Astrophysical sources of high-energy neutrinos have not been observed directly, but their existence can be inferred from the properties of cosmic rays. 

Primary cosmic rays are protons, with some admixture of heavier nuclei; the energy spectrum is a power law which extends to extremely high energies, values exceeding 10²⁰ eV having been observed in recent years. Protons themselves have limited use as astrophysical information carriers because they are charged, and therefore subject to deflection by cosmic magnetic fields: only the very highest-energy cosmic rays are likely to retain any memory of the source direction. The exact source of the high-energy cosmic rays is thus unknown, although supernova remnants and active galactic nuclei have been proposed. Whatever the source, it is clear that accelerating protons to such high energies is likely to generate a large associated flux of photo-produced pions, which decay to yield gamma rays and neutrinos. These will remember the source direction, and so the existence of a general flux of very high energy cosmic-ray protons implies the existence of sources of high-energy neutrinos.

Neutrino astronomy thus offers the possibility of observing sources which correspond to the central engines of the most energetic astrophysical phenomena. It can also provides long baselines for neutrino oscillation studies, and can explore useful regions of supersymmetric parameter space in the context of dark matter. The drawback, of course, is that the weak interactions of neutrinos imply that a very massive detector with extremely good background rejection is required to observe a measurable flux.