Dark matter transient annihilations in the early Universe
  The cosmological evolution can modify the dark matter (DM) properties in the
early Universe to be vastly different from the properties today. Therefore, the
relation between the relic abundance and the DM constraints today needs to be
revisited. We propose novel \textit{transient} annihilations of DM which helps
to alleviate the pressure from DM null detection results. As a concrete
example, we consider the vector portal DM and focus on the mass evolution of
the dark photon. When the Universe cools down, the gauge boson mass can
increase monotonically and go across several important thresholds; opening new
transient annihilation channels in the early Universe. Those channels are
either forbidden or weakened at the late Universe which helps to evade the
indirect searches. In particular, the transient resonant channel can survive
direct detection (DD) without tuning the DM to be half of the dark photon mass
and can be soon tested by future DD or collider experiments. A feature of the
scenario is the existence of a light dark scalar.
