  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.
