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Probing Planck-scale physics with high-frequency gravitational waves

Stefano Profumo

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Phys. Rev. D 114, 036008 – Published 6 August, 2026

DOI: https://doi.org/10.1103/rgkw-h2l2

Abstract

We develop a framework for testing quantum gravity through the stochastic gravitational-wave background produced by evaporating near-Planck-mass primordial black holes. Because gravitons freestream from the emission region without rescattering, they preserve a direct spectral record of the black-hole temperature–mass relation T(M), a relation that is erased for all other Hawking-radiated species by rapid thermalization. We translate six representative phenomenological beyond-semiclassical frameworks (the generalized uncertainty principle, loop quantum gravity, noncommutative geometry, asymptotic safety, string/Hagedorn physics, and tunneling backreaction) into distinct T(M) parameterizations and compute the resulting gravitational-wave spectra numerically within a hybrid semiclassical-phenomenological framework. Modifications that suppress T(M) shift the spectral peak by up to 10 decades in frequency, in some cases into the sensitivity bands of next-generation interferometers or resonant-cavity detectors, while models imposing a hard evaporation cutoff produce distinctive peak morphologies that distinguish between phenomenological classes of modified evaporation. We further discuss the impact of different choices for post-inflationary conditions in the very early Universe and note that breaking the remaining cosmological degeneracies and connecting these signatures to specific quantum-gravity theories requires complementary observables. We find that the relative spectral displacement between the standard Hawking prediction and any modified model is cosmology-independent, and hence spectral shape rather than absolute peak frequency provides the cleanest probe of Planck-scale physics.

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Phys. Rev. D 114, 036008– Published 6 August, 2026
  • Received 5 March 2026
  • Accepted 6 July 2026
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