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, σabs≈lp2. 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 nonminimal 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.
Physical Review D
- Pub Date:
- February 2020
- High Energy Physics - Theory;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- General Relativity and Quantum Cosmology
- 33 pages, 2 figures. Added one reference and an additional section in the appendix. Version accepted for publication in PRD