Gravitational Waves from Reheating beyond Instantaneous Thermalization

Kyohei Mukaida; Tenta Tsuji

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arXiv · 2609.01480

Gravitational Waves from Reheating beyond Instantaneous Thermalization

Kyohei Mukaida·Tenta Tsuji

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Abstract

We study gravitational-wave production during perturbative reheating without assuming instantaneous thermalization of the inflaton decay products. The injected energetic particles thermalize through an in-medium cascade of nearly collinear splittings subject to the Landau-Pomeranchuk-Migdal effect and are ultimately isotropized by elastic scatterings near the thermal scale. The non-thermal hard population present before complete thermalization produces an additional gravitational-wave component through hard-soft scatterings, with $\Omega_{\mathrm{GW}}\propto f^{1/2}$ below the injection-scale turnover. The same isotropization erases the directional information underlying the vacuum $1/k$ bremsstrahlung soft pole, changing the deep-infrared spectrum from $\Omega_{\mathrm{GW}}\propto f$ to $\Omega_{\mathrm{GW}}\propto f^3$.

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Kyohei Mukaida, Tenta Tsuji. 2026-09-01. Gravitational Waves from Reheating beyond Instantaneous Thermalization. https://arxiv.org/abs/2609.01480

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