Quasi-thermal Universe and its implications for gravitino production, baryogenesis and dark matter

Abstract

Under general circumstances full thermal equilibrium may not be established for a long period after perturbative or non-perturbative decay of the inflaton has completed. One can instead have a distribution of particles which is in kinetic equilibrium and evolves adiabatically during this period. Number-violating reactions which are required to establish chemical equilibrium can become efficient only at much later times. We highlight some of the striking consequences of such a quasi-thermal Universe. In particular, thermal gravitino production yields {\it no bound} on the maximum temperature of the primordial thermal bath alone. As another consequence, the lower bound on the mass of the lightest right-handed (s)neutrino from thermal leptogenesis can be $\gg 10^9$ GeV. Depending on the phase, a Wino or Higgsino considerably lighter than TeV, or a Bino in the bulk region, can be a viable {\it thermal} dark matter candidate. Finally the electroweak symmetry may never be restored in the early Universe, therefore weakening any hopes of realizing a successful electroweak baryogenesis.

Title changed. Upon requests the role of SUSY flat directions in thermalization is emphasized. Minor changes and added comments for better clarification. 5 revtex pages

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