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.
Highlights
When gravitational waves travel through a medium, they are generally absorbed and reemitted by the intervening matter
The absorption of gravitational waves in a cosmological setting was first studied by Hawking [1], who calculated the absorption rate of gravitational radiation by viscous matter
The absorption processes so far considered in the literature all involve the interaction between a graviton and a scattering state, that is a quantum state of matter with a continuous energy spectrum
Summary
When gravitational waves travel through a medium, they are generally absorbed and reemitted by the intervening matter. All bound states absorb gravitons with the same probability, independently of their internal structure, such as their mass or coupling This includes purely gravitational atoms [5], which consist of near Planckian particles bound together by gravity. We show that gravitational absorption can be marginally efficient (the optical depth reaches order one) if the absorbing material is a maximally dense condensate of a bosonic field with discrete energy levels We believe it will be very difficult to have a consistent effective quantum field theory description of Nature below the Planck scale without quantizing gravity and introducing a graviton, gravitational absorption lines would provide a way to discriminate between classical and quantum gravity at the observational level. Perhaps more importantly, they would confirm that the gravitational field is quantized at low energies, effectively proving the existence of gravitons
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
