Particle production and Higgs reheating

TL;DR

The paper investigates how the Universe can transition from inflation to a hot, radiation-dominated state when the inflaton couples only through gravity. It develops a Higgs-like inflation scenario with a non-minimal coupling to curvature $ξ φ^2 R$ and analyzes both geometric reheating and gravitational particle production of spectator fields, using a Bogoliubov framework to track particle creation. The results show that rapid curvature oscillations after inflation act as a gravitational pump, transferring energy to radiation and exciting spectator fields; the efficiency of this process is controlled by $ξ$ and by the post-inflation equation of state $ω_φ$, with RG running allowing moderate $ξ$ (e.g., $ξ∼10$) to achieve successful reheating. Spectator fields can behave as late-forming dark matter (LFDM), potentially altering the sound horizon and offering a route to address the Hubble tension, while there are observable implications such as gravitational waves from geometric reheating. Overall, the work provides a minimal, robust mechanism connecting inflation to the standard thermal history through curvature effects, with testable consequences for dark matter and cosmological observations.

Abstract

Reheating is essential for transforming the cold, vacuum dominated Universe at the end of inflation into the hot thermal bath required by the Standard Model. In many well motivated inflationary models, however, the inflaton has no direct couplings to other fields, raising the question of how the Universe becomes repopulated with particles. We address this question within the framework of geometric reheating, where energy transfer occurs purely through gravitational effects. Focusing on a Higgs inflationary scenario with a non-minimal curvature coupling $ξφ^2 R$, we derive the post-inflationary dynamics and compute particle production using the Bogoliubov formalism. We show that the rapid, oscillatory evolution of the curvature scalar after inflaton acts as a time dependent gravitational pump, creating scalar spectator particles even in the absence of explicit interactions. This curvature driven production mechanism provides a natural and efficient route to reheating, demonstrating that gravity alone can initiate the standard thermal history and bridge inflation with radiation domination in minimal, coupling free models of the early Universe.

Figures (4)

• Figure 1: $\xi=0.02$ (blue), $\xi=0.01$ (orange) and $\xi=1$ (green).
• Figure 2: $\xi=0.02$ (blue), $\xi=0.01$ (orange) and $\xi=1$ (green).
• Figure 3: $n_k$ against $k$ in the limit $\eta\rightarrow -\infty$, for different values of case of $\xi$.
• Figure :