Probing pre-BBN era with Scale Invariant FIMP

Understand

Detecting dark matter (DM) relic via freeze-in is difficult in laboratories due to smallness of the couplings involved.

• However, a non-standard cosmological history of the Universe, prior to Big Bang Nucleosynthesis (BBN), can dramatically change this scenario.
• In this context, we study the freeze-in production of dark matter in classically scale invariant $U(1)_X$ gauge extension of the Standard Model (SM), recently dubbed as the \textit{Scale Invariant FIMP Miracle}.
• We assume an additional species dominates the energy density of the Universe at early times, causing the expansion rate at a given temperature to be larger than that in the standard radiation-dominated case.