Every reheating era leaves an unavoidable graviton background

When the inflaton decays to reheat the universe, it inevitably radiates gravitons via bremsstrahlung. The infrared shape of the resulting gravitational-wave spectrum is pinned by Weinberg's soft-graviton theorem regardless of the decay mechanism, giving a universal $\Omega_{\rm GW}\propto f$ slope. Working through the phase-space integrals for general n-body inflaton decays, the peak signal reaches $\Omega_{\rm GW}h^2\sim10^{-17}$ above GHz frequencies. Any detected signal exceeding this floor would demand physics beyond standard perturbative reheating or single-field slow-roll inflation.