Ultraviolet Freeze-in and Non-Standard Cosmologies
  A notable feature of UV freeze-in is that the relic density is strongly
dependent on the highest temperatures of the thermal bath, and a common
assumption is that the relevant 'highest temperature' should be the reheating
temperature after inflation $T_\text{RH}$. However, the temperature of the
thermal bath can be significantly higher in certain scenarios, reaching a value
denoted T max , a fact which is only apparent away from the instantaneous decay
approximation. Interestingly, it has been shown that if the operators are of
sufficiently high mass dimension then the dark matter abundance can be enhanced
by a 'boost factor' depending on ($T_\text{max}/T_\text{RH}$) relative to naive
estimates assuming instantaneous reheating. We highlight here that in
non-standard cosmological histories the critical mass dimension of the operator
above at which the instantaneous decay approximation breaks down, and the
exponent of the boost factor, depend on the equation of state $\omega$ prior to
reheating. We highlight four examples in which the dark matter abundance
receives a significant enhancement in the context of gravitino dark matter, the
moduli portal, the Higgs portal, and the spin-2 portal (as might arise in
bimetric gravity models). We comment on the transition from kination domination
to radiation domination as a motivated example of non-standard cosmologies.
