Quasidecoupled state for dark matter in nonstandard thermal histories
  Dark matter drops out of kinetic equilibrium with standard model particles
when the momentum-transfer rate equals the expansion rate. In a
radiation-dominated universe, this occurs at essentially the same time as dark
matter kinetically decouples from the plasma. Dark matter may also fall out of
kinetic equilibrium with standard model particles during an early
matter-dominated era (EMDE), which occurs when the energy content of the
Universe is dominated by either a decaying oscillating scalar field or a
semistable massive particle before big bang nucleosynthesis. Until now, it has
been assumed that kinetic decoupling during an EMDE happens similarly to the
way it does in a radiation-dominated era. We show that this is not the case. By
studying the evolution of the dark matter temperature, we establish a
quasidecoupled state for dark matter in an EMDE, during which the dark matter
temperature cools faster than the plasma temperature but slower than it would
cool if the dark matter were fully decoupled. The dark matter does not fully
decouple until the EMDE ends and the Universe becomes radiation dominated. We
also extend the criteria for quasidecoupling to other nonstandard thermal
histories and consider how quasidecoupling affects the free-streaming length of
dark matter.
