  Displaced vertices at colliders, arising from the production and decay of
long-lived particles, probe dark matter candidates produced via freeze-in. If
one assumes a standard cosmological history, these decays happen inside the
detector only if the dark matter is very light because of the relic density
constraint. Here, we argue how displaced events could very well point to
freeze-in within a non-standard early universe history. Focusing on the
cosmology of inflationary reheating, we explore the interplay between the
reheating temperature and collider signatures for minimal freeze-in scenarios.
Observing displaced events at the LHC would allow to set an upper bound on the
reheating temperature and, in general, to gather indirect information on the
early history of the universe.
