Abstract
We consider the direct $s$-channel gravitational production of dark matter during the reheating process. Independent of the identity of the dark matter candidate or its non-gravitational interactions, the gravitational process is always present and provides a minimal production mechanism. During reheating, a thermal bath is quickly generated with a maximum temperature $T_{\rm max}$, and the temperature decreases as the inflaton continues to decay until the energy densities of radiation and inflaton oscillations are equal, at $T_{\rm RH}$. During these oscillations, $s$-channel gravitational production of dark matter occurs. We show that the abundance of dark matter (fermionic or scalar) depends primarily on the combination $T_{\rm max}^4/T_{\rm RH} M_P^3$. We find that a sufficient density of dark matter can be produced over a wide range of dark matter masses: from a GeV to a ZeV.
Highlights
While we have considerable certainty in the existence of dark matter (DM), its identity and interactions with the Standard Model are entirely unknown
Particles which interact with the Standard Model primarily through gravity, such as the gravitino, never achieve equilibrium, though they are produced by the thermal bath at reheating after inflation [7,12,13,14]
We have derived the conditions for producing sufficient dark matter from inflaton scattering during reheating by s-channel graviton exchange
Summary
While we have considerable certainty in the existence of dark matter (DM), its identity and interactions with the Standard Model are entirely unknown. Particles which interact with the Standard Model primarily through gravity, such as the gravitino, never achieve equilibrium, though they are produced by the thermal bath at reheating after inflation [7,12,13,14] Very roughly, their abundance Y ∼ n3=2=nγ can. Be estimated from their production rate, Y ∼ Γp=H∼ TRH=MP, where H is the Hubble parameter, TRHpffiiffisffiffiffiffitffihffiffiffieffi reheating temperature after inflation, and MP 1⁄4 1= 8πGN is the (reduced) Planck mass This mechanism, generally referred to as freeze-in, applies to a wider class of dark matter candidates known as feebly interacting massive particles or FIMPs [15,16,17,18,19].
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