Unveiling a Hidden Epoch: Impact of Mediator Induced Matter Domination in Freeze-in Dark Matter

Abstract

Freeze-in dark matter has recently garnered significant attention as a promising framework due to its feeble interactions, which are consistent with the null results from dark matter experiments. While previous studies have extensively investigated the production of dark matter through the decay of heavy particles, they overlook the cosmological role of the decaying mediator without justifying this assumption. We emphasize that the mediator can dominate the energy budget of the early universe during its decay, leading to an unavoidable early matter-dominated era. This intrinsic matter-dominated phase influences dark matter production in two key ways: (i) dark matter production occurs in both the early radiation and induced matter-dominated phases; specifically, considerable production occurs in the matter-dominated phase and stops when the mediator decays fully, and (ii) it causes dilution in dark matter abundance due to entropy injection before its saturation. Furthermore, this effect significantly alters the gravitational wave signature associated with the production of freeze-in dark matter through graviton emission during the mediator's decay. Specifically, it enhances the gravitational wave spectrum, making it viable for future high-frequency gravitational wave experiments.