The Gravitational Spectral <i>Radio Forest</i> : A Signature of Primordial Black Holes

We propose a novel gravitational signature to detect Primordial Black Hole (PBH) dark matter by treating interstellar hydrogen as a quantum sensor for spacetime curvature. Focusing on H[Formula: see text]II regions, we demonstrate that the Riemann tidal tensor of an asteroid-mass PBH induces a symmetric splitting of the [Formula: see text] state in bound hydrogen atoms. This relativistic effect redistributes [Formula: see text] absorption line into a gravitational spectral radio forest with a bandwidth [Formula: see text]. By accounting for active accretion of Hydrogen atoms and the resulting density-squared emission measure within the Bondi radius, we find a relatively enhanced absorption spectrum. This feature presents a concrete, high-contrast target for upcoming radio-surveys to constrain PBH populations in the dark matter sector.