Researchers have discovered new universality classes in the critical behavior of the Kasner parameter (β) at the terminal singularity of scalarized black holes. These black holes, which spontaneously develop a scalar field in asymptotically flat spacetime, have attracted significant recent interest. Previously, it had been reported that β diverges as β∝(q/qc-1)⁻γ near the critical charge-to-mass ratio (qc), with an exponent γ=1/2 that appeared model-independent.

The new study reveals that there exist families of universality classes where the exponent γ differs from 1/2. This finding suggests a greater complexity in the internal physics of these compact objects than previously thought. The authors demonstrate that all these critical behaviors are governed by an emergent scaling symmetry at the would-be Cauchy horizon of the background Reissner-Nordström solution.

This emergent symmetry leads to a "seesaw amplification" mechanism that magnifies a small exterior scalar field, producing a limiting autonomous system that is independent of the specific details of the theory. This mechanism provides a unified picture of exterior scalarization and interior criticality, and generates sharp numerical predictions that the researchers have tested, opening new avenues for understanding the internal structure of charged black holes.