WHY SHOULD BARYONS AND EXOTIC RELIC PARTICLES HAVE COMPARABLE DENSITIES?

Abstract

Observations suggest that the mass density of the Universe is dominated, not by ordinary matter, but by exotic particles which are a relic of the Big Bang. In this case, a new dimensionless cosmological ratio arises, the ratio of the mass density in ordinary matter to that in exotic matter, whose value is about 0.1. A priori, it might seem remarkable that this ratio should be so close to unity. However, we point out that, for many exotic dark matter candidates, the ratio is related to the fundamental scales of particle physics. A value of order unity arises naturally provided rather simple relationships exist between these scales. If the exotic particles are of a kind whose relic abundance is determined by annihilations (e.g., the photino or a heavy neutrino), then the required relationship is already satisfied for independent, cosmological reasons.