Although the experiments in particle astrophysics are not directly associated with accelerators, the two disciplines have common roots and many physicists take part in both. There are some 250 experimental particle astrophysicists. The questions addressed by particle astrophysics are among the most fundamental puzzles of science today. The culprit now appears to be the neutrino. If this is confirmed, it would be of great benefit both for astrophysics, with the confirmation of one of its most basic foundations, the energy production in stars, and for particle physics, as an indication of physics beyond the Standard Model.
Particle cosmology is aimed at understanding the early universe.
A new generation of very sensitive low background experiments using novel cryogenic techniques for active background rejection is coming on line and promises to begin to test the hypothesis that the lightest supersymmetric particles constitute dark matter. Similarly, searches are in progress for the axion, another dark matter candidate. Astrophysics measurements such as the measurement of the anisotropy of the microwave background, the mapping of the large scale structure in the universe, or the measurement of space geometry have significant impact in terms of deciphering potential phase transitions in the universe induced by particle physics, and the role of quantum fluctuations in the formation of structure. Neutrino astronomy started in the 1960s when first measurements of the flux of neutrinos emitted by the sun turned out to be well below expectations. In addition to these two international collaborations, American physicists are involved in the development of a sophisticated experiment, Borexino, which will also be installed in Gran Sasso and will probe intermediate-energy solar neutrinos.