What’s inside a black hole?

The frozen star model proposes an alternative to the general relativity description of black holes, suggesting that astrophysical black holes may instead be horizon-scale compact objects with an internal structure that prevents their further collapse and the formation of singularities.  The Frozen Star geometry differs from the Schwarzschild or Kerr geometries on horizon scales rather than at the Planck scale and is sourced by exotic matter beyond the Standard Model.
A key prediction of the Frozen Star model concerns the gravitational waves emitted when two such objects collide. In addition to the ringdown modes of black holes, associated with the decay of the surrounding spacetime to equilibrium, frozen stars exhibit an additional branch of ringdown modes, with a longer decay time and lower amplitude than the prompt signal. This branch is associated with the decay of their internal fluid-like modes, excited during the merger. Additionally, the internal structure implies a large, negative Love number. These observational signatures provide a potential method for distinguishing frozen stars from General Relativity black holes by their gravitational-wave signatures, thus potentially confirming their existence.