Constraints on the Very Early Universe from Thermal WIMP Dark Matter

Abstract

We investigate the relic density n_χof non-relativistic long-lived or stable particles χin non-standard cosmological scenarios. We calculate the relic abundance starting from arbitrary initial temperatures of the radiation-dominated epoch, and derive the lower bound on the initial temperature T_0 \geq m_χ/23, assuming that thermally produced χparticles account for the dark matter energy density in the universe; this bound holds for all χannihilation cross sections. We also investigate cosmological scenarios with modified expansion rate. Even in this case an approximate formula similar to the standard one is capable of predicting the final relic abundance correctly. Choosing the χannihilation cross section such that the observed cold dark matter abundance is reproduced in standard cosmology, we constrain possible modifications of the expansion rate at T \sim m_χ/20, well before Big Bang nucleosynthesis.

23 pages, 8 figures, comments added