Is the Universe dominated by relativistic particles?

Abstract

The decaying dark matter (DM) cosmology1–3 attempts to reconcile the theoretical prejudice that the total energy density of the Universe, Ω=1 with the observations that the matter clustered on scales up to a few megaparsecs amounts only to4,5 Ωcl=0.1–0.3. Decaying DM postulates that a heavy-elementary-particle species X first drives the formation of galaxies and clusters, and then decays nonradiatively, providing a smooth, undetected background of relativistic particles that at present contribute Ωr to the total energy density of the Universe, Ω. We consider here the effects of decaying DM on the radial distribution of mass in spiral galaxies, assumed to have formed through dissipative collapse inside a gravitationally induced protogalaxy consisting initially of a homogeneous mixture of dissipationless DM and a small fraction of dissipative baryonic material6. The baryonic inner parts of galaxies are self gravitating, but mass loss from X decay causes the rotation velocity in the outer parts to decrease. We find that the observed flat rotation curves cannot be obtained in these decaying DM models. Thus, a relativistic, weakly interacting decay product cannot be dominant.