Self-interacting Dark Matter in Non-Standard Cosmology

Abstract

Discrepancies of the $\Lambda {\rm CDM}$ model with small-scale cosmological observations and stringent constraints from direct search experiments cast doubts over typical weak scale cold dark matter candidates e.g. WIMPs. Self-interacting dark matter (SIDM) is a very promising alternative to WIMP, which not only alleviates the small-scale anomalies of the $\Lambda {\rm CDM}$ model, but also matches with the highly accurate large-scale $\Lambda {\rm CDM}$ predictions. The small-scale anomalies can be resolved with a self-scattering cross-section $\sigma/m_{DM} \sim 1 cm^2/gm$. Such large cross-sections can be realised in models of DM with a light MeV scale mediator and a sizeable coupling. We assume the DM to be Dirac fermion and the mediator to be either a light scalar or vector boson. In a standard cosmological history, one major issue with such models is to realise the correct relic density of dark matter via thermal freeze-out as the DM annihilates efficiently to the light mediators and ends up with an under-abundant relic. However, we show that, if the expansion rate of the universe is not radiation-dominated (RD) during the epoch of SIDM freeze-out, its relic abundance is enhanced significantly. We assume a non-standard expansion history of the universe by introducing a non-radiation like component in the early universe. In such a scenario, DM freezes out at an earlier epoch resulting in enhanced DM abundance, which can be matched with the correct relic density with suitable model parameters. The light mediator can also couple to an SM mediator and pave a way to detect SIDM at terrestrial laboratories. The mixing between the mediators can be constrained by data from direct search experiments. We find out the viable parameter space for a generic SIDM model taking into account the relevant phenomenological and experimental constraints.