Astrophysical sources of high energy neutrinos

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I give a brief critical review of the predicted intensity of diffuse high energy neutrinos of astrophysical origin over the energy range from 10¹2 eV to 10²4 eV. Neutrinos from interactions of galactic cosmic rays with interstellar matter are guaranteed, and the intensity can be reliably predicted to within a factor of about 2 up to 10¹7 eV. Somewhat less certain are intensities in the same energy range from cosmic rays escaping from normal galaxies or active galactic nuclei (AGN) and interacting with intracluster gas. At higher energies, neutrinos will be produced by interactions of extragalactic cosmic rays with the microwave background. Other sources, such as AGN, in particular blazars, and topological defects, are more speculative. However, searches for neutrinos from all of these potential sources should be made because their observation would have important implications for high energy astrophysics and cosmology.