Superheavy Dark Matter A

In the standard dark matter scenarios, WIMPs are usually considered to have once been in local thermodynamic equilibrium (LTE), and their present abundance is determined by their self-annihilation cross section. In that case, unitarity and the lower bound on the age of the universe constrains the mass of the relic to be less than 500 TeV. 1 On the other hand, if the DM particles never attained LTE in the past, self-annihilation cross section does not determine their abundance. For example, axions, which may never have been in LTE, can have their abundance determined by the dynamics of the phase transition associated with the breaking of U(1)PQ. These nonthermal relics (ones that never obtained LTE) are typically light. However, there are mechanisms that can produce superheavy (many orders of magnitude greater than the weak scale) nonthermal relics. Some of this is reviewed in Ref. . Although not known at the time when this talk was given, it is now known that if the DM fields are coupled to the inflaton field, then the mass of the DM particles that can be naturally produced in significant abundance after inflation can be as large as 10−3MPl (paper in preparation). In this article, I discuss the gravitational production mechanism 3 which is a generic consequence of any large field inflationary phase ending. As Ref. 3 shows, the nonadiabatic change in the way that the spacetime expands at the end of any large field inflationary model induces superheavy particle production gravitationally with sufficient efficiency as to render those superheavy DM to be a significant component of the energy density in the universe today. To turn this around, if stable superheavy WIMPs within the mass range 0.04 − 2 × 10−6MPl exist in the mass spectrum of any particle physics models, then those