Brillouin-Analog Scattering in Black Hole Spacetime and CMB-Mediated Signal Propagation: A Hypothesis

Existing treatments of photon-graviton interaction, including recent quantum field-theoretic frameworks that draw analogy to Raman scattering, model gravitational waves as traveling perturbations in vacuum. This paper proposes 2 related hypotheses concerning a distinct physical regime: the quasi-normal mode (QNM) ringdown of a black hole as a locally vibrating spacetime substrate, and the cosmic microwave background (CMB) as a ubiquitous photon bath through which vibrational information from that substrate propagates. First, photons passing through the photon sphere of a ringing black hole may undergo inelastic scattering off the acoustic-mode-like QNM oscillations of spacetime, producing a small but systematic frequency shift structurally analogous to Brillouin scattering - distinct from gravitational redshift and from the Raman analog already formalized for traveling gravitational waves. Second, the CMB photon bath, present throughout the region, constitutes a conducting medium for this signal in the absence of any classical acoustic medium in vacuum - analogous to bone conduction, where vibration bypasses the primary acoustic path through an alternate solid medium. Together, these hypotheses generate a testable prediction: localized CMB spectral distortions correlated spatially and temporally with black hole merger ringdown events catalogued by LIGO/Virgo. Both hypotheses are conceptual; quantitative treatment in QFT in curved spacetime is invited.