Irreducible Graviton Floor from Reheating

TL;DR

Reheating after inflation produces an unavoidable stochastic gravitational-wave background from graviton bremsstrahlung, with a predictable spectrum fixed by Weinberg's soft-graviton theorem, potentially detectable at GHz frequencies.

Contribution

This work demonstrates that the soft part of graviton emission during reheating creates an irreducible GW background with a fixed spectrum, independent of decay details.

Findings

The spectrum's infrared branch is fixed by Weinberg's theorem, proportional to frequency.

Maximum GW energy density scales as 2/n for n-body inflaton decays.

Predicted GW background could reach 10^{-17} at GHz frequencies.

Abstract

Inflaton decay inevitably emits gravitons through bremsstrahlung during reheating. We show that the soft part of this emission amplitude, fixed by Weinberg's soft-graviton theorem, becomes an irreducible stochastic gravitational-wave (GW) background after accounting for cosmological evolution. The theorem fixes the infrared branch of the spectrum, $\Omega_{\rm GW}\propto f$, independently of the microscopic operator responsible for inflaton decay, while the normalization is controlled by the hard inflaton decay rate and by a phase-space factor. We carry this out for inflaton $n$-body decays, including the phase-space integrals, finding that the maximum of the spectrum scales as $2/n$ relative to the $n=2$ case. The signal can reach $\Omega_{\rm GW}h^2\sim \mathcal O(10^{-17})$ at frequencies above the GHz scale. This predicts a stochastic graviton floor from perturbative reheating: a larger signal would require either other processes beyond perturbative bremsstrahlung or inflationary scenarios beyond conventional single-field slow roll.