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Area of Science:

  • Cosmology and Particle Physics
  • Early Universe Physics
  • Dark Matter Production Mechanisms

Background:

  • Ultrarelativistic freeze-out (UFO) is a mechanism for particle decoupling in the early Universe.
  • Historically, UFO was associated with hot dark matter (DM), incompatible with the standard ΛCDM model.
  • The assumption of instantaneous reheating after inflation previously limited UFO's applicability for DM.

Purpose of the Study:

  • To reexamine the viability of dark matter production via ultrarelativistic freeze-out (UFO).
  • To demonstrate how lifting the instantaneous reheating approximation allows UFO to produce cold dark matter.
  • To explore the parameter space where UFO can account for the observed relic density.

Main Methods:

  • Revisiting the theoretical framework of ultrarelativistic freeze-out.
  • Incorporating a non-instantaneous, perturbative reheating period after inflation.
  • Analyzing dark matter particle masses from sub-eV to PeV scales.
  • Investigating various interaction models between standard model particles and dark matter.

Main Results:

  • UFO can produce cold dark matter (DM) if the approximation of instantaneous reheating is lifted.
  • This mechanism accounts for the entire observed relic density in significant parameter regions.
  • DM with masses from sub-eV to PeV can be cold before structure formation via UFO.
  • The transition between weakly interacting massive particle (WIMP) and feebly interacting massive particle (FIMP) regimes can occur via UFO for specific interactions.

Conclusions:

  • Ultrarelativistic freeze-out is a viable mechanism for producing cold dark matter.
  • A simple, perturbative post-inflationary reheating period is sufficient for UFO to generate cold DM.
  • These findings reconcile the conventional hot DM production mechanism with cold DM observations within the ΛCDM framework.