How Dark is 'Dark'? Electromagnetic Interactions in the Dark Sector

Abstract

We review the physical and cosmological consequences of two possible electromagnetic couplings to the dark sector: (i) a neutral lightest dark-matter particle (LDP) with nonzero electric and/or magnetic dipole moments and (ii) a charged next-to-lightest dark-matter particle (NLDP) which decays to a neutral LDP. For scenario (i) we find that a relatively light particle with mass between a few MeV and a few GeV and an electric or magnetic dipole as large as $\sim 3\times 10^{-16}e$ cm (roughly $1.6\times10^{-5} μ_B$) satisfies experimental and observational bounds. In scenario (ii), we show that charged-particles decaying in the early Universe result in a suppression of the small-scale matter power spectrum on scales that enter the horizon prior to decay. This leads to either a cutoff in the matter power spectrum, or if the charged fraction is less than unity, an effect in the power spectrum that might resemble a running (scale-dependent) spectral index in small-scale data.

12 pages, 7 figures. Enlarged figures, minor changes, and references added. Matches version to be published in the proceedings of DARK 2004, the Fifth International Heidelberg Conference on Dark Matter in Astro and Particle Physics, held October 3-9, 2004 at Texas A&M University, College Station, TX, USA