Gravitational Absorption Lines

TL;DR

This paper explores the theoretical possibility of gravitational absorption lines caused by quantum bound states, which could serve as indicators of new physics or exotic astrophysical objects, especially near the Planck scale.

Contribution

It introduces a model where non-minimal matter coupling produces gravitational absorption lines in primordial gravitational waves, suggesting a new way to probe quantum gravity effects.

Findings

Absorption cross section is approximately the Planck area.

Ordinary gravity makes gravitational absorption by bound states inefficient.

Detection of such lines would indicate new physics or exotic states.

Abstract

We consider the gravitational analogue of Lyman-alpha absorption lines in astronomical spectroscopy. If Einstein gravity with minimally coupled matter is valid up to the Planck scale, quantum bound states absorb gravitons of a specific frequency with Planckian cross section, $\sigma_{\text{abs}} \approx l_p^2$. Consequently, one can show that gravitational absorption by bound states is inefficient in ordinary gravity. If observed, gravitational absorption lines would therefore constitute a powerful smoking gun of new exotic astrophysical bound states (near extremal bound states) or new gravitational physics, as well as give direct evidence of the quantized nature of the gravitational field. We provide, as an example of new gravitational physics near the Planck scale, a non-minimal coupling of the matter fields which breaks the equivalence principle on-shell. We lay out a model in which absorption lines in the primordial gravitational wave spectrum are produced as a consequence of this coupling.