Unitarity limits on thermal dark matter in (non-)standard cosmologies

A bstract Using the upper bound on inelastic reaction cross-section implied by S-matrix unitarity, we derive thermally averaged maximum dark matter (DM) annihilation rate for general k → 2 number-changing reactions, with ≥ 2, taking place either entirely within sector, or involving standard model fields. This translates to a mass of particle saturating observed DM abundance, which, dominantly s -wave annihilations, is obtained be around 130 TeV, 1 GeV, 7 MeV and 110 keV, = , 3 4 5, respectively, in radiation dominated Universe, real complex scalar stabilized minimal symmetry. For modified thermal histories pre-big bang nucleosynthesis era, an intermediate period domination, values reheating temperature higher than $$ \mathcal{O}(200) O 200 GeV 4, \mathcal{O}(1) 1 TeV \mathcal{O}(50) 50 are strongly disfavoured combined requirements unitarity relic freeze-out before reheating.