Dark matters are Inert, or FIMPy, or WIMPy or UFOy: An inflationary gravitational particle production

Abstract

In this letter, we explore the phenomenological impact of inflationary gravitational particle production in the physics of Dark Matter (DM). Large-scale DM fluctuations generated during inflation behave as gravitational particles upon their post-inflationary horizon reentry and alter the conventional Boltzmann dynamics of DM with a non-conserving source term, thereby producing significant phenomenological consequences. Within this framework, we analyze four distinct types of DM classified according to their production mechanisms. Dark matter may be completely non-interacting with the thermal bath, behaving as Inert Dark Matter. Alternatively, depending on the strength of its interactions with bath particles, DM may exhibit WIMPy, UFOy, or FIMPy behavior, sharing characteristics with their conventional counterparts. The late-time enhancement of the DM number density, driven by the successive horizon reentry of gravitationally produced low-momentum modes, enlarges the viable parameter space for both thermal and non-thermal DM scenarios. Remarkably, this expanded parameter space remains consistent with current constraints from $\Delta N_{\rm eff}$ and Lyman-$\alpha$ bound.