Clustering of Black Hole Masses in the Pair-Instability Gap: A Signature of Modified Gravity and Extra Dimensions?

Recent gravitational wave observations from LIGO/Virgo have revealed statistically significant clustering of black hole merger remnants within the pair-instability mass gap (~50–120 M). Using Monte Carlo simulations (N=300,000), kernel density estimation, and clustering algorithms on public GWTC-2/3/4 catalogs, we identify non-random substructures peaking around ~65 M and ~95 M. We propose a theoretical framework in which these clusters are not merely the result of hierarchical mergers but are influenced or stabilized by effects arising from modified gravity or extra dimensions. This framework predicts preferred mass states within the instability region, potentially offering one of the first indirect observational signatures of new gravitational physics.