CONSTRAINTS ON AXIONS FROM THE EXTRAGALACTIC BACKGROUND LIGHT

We consider the effect of dark matter in the form of multi-Ev axions o n the extragalactic background light. Our treatment differs from that of other workers in that we assume axions to be clustered in Galactic halos, with nonzero velocity dispersions. We also approach the problem in a fully general relativistic manner, treating the axion halos as l uminous elements of a pressure-free perfect fluid in a standard Friedm ann-Robertson-Walker universe. We find that the ultraviolet extragalac tic background light places a firm upper limit of 9 eV on the axion re st energy, and that this drops to 4 eV for the simplest axion models, all but closing the multi-eV axion window (which begins at 3 eV). Thes e results are close to earlier upper limits of 5 and 8 eV derived from the extragalactic background by Turner and Ressell, respectively. Alt hough our methods differ somewhat from theirs, our findings support th eir conclusion that axions, if they exist, are likely to have rest ene rgies well below the eV range.