Shimmering gravitons in the gamma-ray sky

Abstract

What is the highest energy at which gravitons can be observed? We address this question by studying graviton-to-photon conversion — the inverse-Gertsenshtein effect — in the magnetic field of the Milky Way. We find that above ∼ 1 PeV the effective photon mass grows large enough to quench the conversion rate.For sub-PeV energies, the induced photon flux is comparable to the sensitivity of LHAASO to a diffuse γ-ray background, but only for graviton abundances of order Ωgw h 2 0 ∼ 1. In the future, owing to a better understanding of γ-ray backgrounds, larger effective areas and longer observation times, sub-PeV shimmering gravitons with a realistic abundance of Ωgw h 2 0 ∼ 0.01 could be detected. We show how such a large abundance is achieved in a cosmologically-motivated scenario of post-recombination superheavy dark matter decay. Therefore, the sub-PeV range might be the ultimate energy frontier at which gravitons can be observed.