The Gamma-Ray Large Area Space Telescope (GLAST), to be launched in the fall of 2007, will measure the spectra of distant extragalactic sources of high energy γ-rays, particularly active galactic nuclei and γ-ray bursts. GLAST can look for energy-dependent γ-ray propagation effects from such sources as a signal of Lorentz invariance violation (LIV). These sources should also exhibit the high energy cutoffs predicted to be the result of intergalactic annihilation interactions with low energy photons having a flux level as determined by various astronomical observations. Such annihilations result in electron–positron pair production above a threshold energy given by 2me in the center-of-momentum frame of the system, assuming Lorentz invariance. If Lorentz invariance is violated, this threshold can be significantly raised, changing the predicted absorption turnover in the observed spectrum of the sources. Stecker and Glashow have shown that the existence of such absorption features in the spectra of extragalactic sources puts constraints on LIV. Such constraints have important implications for some quantum gravity and large extra dimension models. Future spaceborne detectors dedicated to measuring γ-ray polarization can look for birefringence effects as a possible signal of loop quantum gravity. As shown by Coleman and Glashow, a much smaller amount of LIV has potential implications for possibly suppressing the "GZK cutoff" predicted to be caused by the interactions of cosmic rays having multijoule energies with photons of the 2.7 K cosmic background radiation in intergalactic space. Owing to the rarity of such ultrahigh energy cosmic rays, their spectra are best studied by a UV-sensitive satellite detector which looks down on a large volume of the Earth's atmosphere to study the nitrogen fluorescence tracks of giant air showers produced by these ultrahigh energy cosmic rays. We discuss here, in particular, a two-satellite mission called OWL, which would be suited for making such studies.
Lorentz invariance violation and the observed spectrum of ultrahigh energy cosmic rays
