The Nature of the Dark Matter


Most (80%) of the matter in the Universe is of an unidentified origin, a substance astronomers refer to as ``dark matter’’.  We have reasons for believing that the dark matter is a weakly interacting particle, and many beyond-the-Standard-Model theories of particle physics provide viable dark matter candidates.  I am interested in the dynamics of the dark matter and also in astrophysical methods to learn what it is.  For example, I have thought about what observations of galactic cosmic ray signals could teach us about the nature of this particle.

Several papers have also suggested that dark matter annihilations could have contributed to the heating and reionization of the early Universe via their associated radiative backgrounds.  We showed recently that for particles with masses much greater than a GeV (as occurs in the leading WIMP dark matter model) there are significant limits on their contribution: The unabsorbed photons would  overproduce the soft X-ray background even if the dark matter ionized 10% of the Universe.  This is
illustrated in the figure below, which shows the fraction of reionization that  could have occurred and not violate the X-ray background constraints as a function of the spectral index of the sources’ specific intensity.  Dark matter models predict a radiation background with spectral index of -0.5 (owing to how >>GeV electrons cool).  This figure also shows that this background constrains the contribution from a few other plausible models (high mass X-ray binaries, supernovae, and AGN).  The different black curves show the limit for different reionization redshifts (see our paper for details)