Vector dark matter from inflationary fluctuations

Abstract

We calculate the production of a massive vector boson by quantum fluctuations\nduring inflation. This gives a novel dark-matter production mechanism quite\ndistinct from misalignment or thermal production. While scalars and tensors are\ntypically produced with a nearly scale-invariant spectrum, surprisingly the\nvector is produced with a power spectrum peaked at intermediate wavelengths.\nThus dangerous, long-wavelength, isocurvature perturbations are suppressed.\nFurther, at long wavelengths the vector inherits the usual adiabatic, nearly\nscale-invariant perturbations of the inflaton, allowing it to be a good dark\nmatter candidate. The final abundance can be calculated precisely from the mass\nand the Hubble scale of inflation, H_I. Saturating the dark matter abundance we\nfind a prediction for the mass m = 10^-5 eV (10^14 GeV/H_I)^4. High-scale\ninflation, potentially observable in the CMB, motivates an exciting mass range\nfor recently proposed direct detection experiments for hidden photon dark\nmatter. Such experiments may be able to reconstruct the distinctive, peaked\npower spectrum, verifying that the dark matter was produced by quantum\nfluctuations during inflation and providing a direct measurement of the scale\nof inflation. Thus a detection would not only be the discovery of dark matter,\nit would also provide an unexpected probe of inflation itself.\n