Colloquium Detail

Astrophysics and Particle Physics: Complementary Tools and Unraveling a Paradox Concerning Ultra-High-Energy Cosmic Rays 

Date: 11/1/1999

Glennys Farrar
Professor, NYU

The Greisen-Zatsepin-Kuzmin (GZK) bound reflects the fact that protons with energy above about 5 x 10^{19} eV scatter off the cosmic microwave background until their energy is reduced below this level. Observations now make clear that the GZK bound is violated. This has focussed a great deal of attention on the production, propagation and identity of these Ultra-High-Energy cosmic rays (UHECRs). Proposals to explain GZK violation include postulating a new supersymmetric particle, superheavy relics of the Big Bang or cosmological defects, violation of Lorentz invariance, and a host of other ideas.

We (GRF and Tsvi Piran) recently pointed out that the existence of micro-Gauss magnetic fields in extended halos of galaxies and in regions between galaxies would remove the GZK paradox. Such fields raise interesting astronomical questions, but appear to be consistent with both direct and indirect constraints. Open questions and implications will be discussed.