Gravitational ultra-relativistic freeze-out during general reheating

We investigate ultrarelativistic freeze-out (UFO) in the context of generic reheating scenarios. While the standard WIMP dark matter paradigm has been extensively studied, UFO has so far only been analyzed within the specific reheating channel phi to f. Unlike in the standard WIMP mechanism, where dark matter can only be diluted after freeze-out at T FO about m chi/O(10), UFO dark matter can undergo freeze-in like phases following the initial freeze-out, driven by the non-trivial temperature evolution. The exact temperature evolution then needs to be accounted for, as a change in the temperature scaling can modify the IR/UV nature of UFO, impacting the relic abundance. We first generalize UFO to an arbitrary temperature profile T about a-xi, making explicit the UV and IR regimes for a thermally averaged cross section sigma v about T n / n+2. Then, as a concrete example, we consider the minimal scenario in which gravitational particle production at the onset of reheating sources an initial radiation abundance, and show that this early hot bath changes the UFO parameter space. We refer to this effect as GUFO. Specializing to n = 2, we find that matter-like reheating (V about phi squared) accommodates dark matter masses up to 10 7 GeV for 10 9 GeV as thermalization becomes less stringent, while radiation-like reheating (V about phi 4) is compatible with GUFO across all reheating channels only if gravitational processes are taken into account.