Planck Stars

§1 · The claim, in one sentence

Rovelli and Vidotto proposed in 2014 that gravitational collapse halts at Planck density due to repulsive quantum gravity effects, replacing the singularity with a Planck-scale star that eventually bounces. From outside the object looks like an ordinary black hole; from inside, matter is compressed to Planck density, held there by quantum-geometric repulsion, then re-expands. As Hawking radiation shrinks the apparent horizon, the bounce eventually exits, allowing trapped information to escape.

§2 · Why it might be true

The family stance

Something stops the gravitational collapse before infinite density is reached. The exterior of a black hole is well-described by general relativity, but the interior is not. The candidates differ on what stops it (modified equation of state, vacuum energy, string structure, quantum geometry, classical mass inflation) and on what the deep interior looks like as a result. None of the candidates has been observationally confirmed; none has been ruled out either. The family is the chapter's structural pair to the Black Hole Information Paradox family: BHIP asks where the information goes, this family asks what physically replaces the singularity.

§2.5 · Evidence

• The Loop Quantum Gravity mechanism that produces a finite-density bounce in cosmology (Bojowald 2001, Ashtekar 2006) provides the foundational physics for replacing classical singularities with quantum bounces; Planck Stars extends this to the black hole interior using the same machinery
• The Rovelli-Vidotto 2014 proposal makes specific phenomenological predictions, which is unusual for singularity-resolution proposals (most of which are mathematical only and untestable in practice)
• The Barrau-Rovelli-Vidotto 2014 link to fast radio bursts gives the model a concrete observational target. Current LIGO data, FAST telescope observations, and pulsar timing arrays already constrain parts of the predicted parameter space
• The proposal is part of a growing literature on quantum-gravity singularity resolution that overlaps with regular black holes, fuzzballs, and gravastars; see the sibling variants in this family for adjacent mechanisms

§3 · What you'd need to test it

• Gravitational collapse halts at Planck density due to repulsive quantum-geometry effects, replacing the singularity with a finite-density Planck Star inside the horizon
• The apparent event horizon eventually disappears as the bounce exits, allowing trapped information to escape with the late-stage Hawking radiation rather than being lost
• A specific phenomenological signature: at least some fast radio bursts may originate from Planck Star bounces of primordial black holes formed in the early universe, with a predicted frequency-to-distance relation
• Spectral features in late-stage Hawking radiation should encode information about the original infalling matter, in principle detectable in the right observational regime

§4 · Where it breaks

• The detailed bounce mechanism relies on the full Loop Quantum Gravity dynamics applied inside a black hole, which is computationally intractable; the bounce is asserted from analogy to loop quantum cosmology rather than derived from first principles in this setting
• Most physicists view the proposal as plausible but speculative; the empirical case rests on phenomenological signatures like fast radio bursts that have alternative astrophysical explanations (magnetar flares being the leading competing class)
• Acceptance of Planck Stars depends on broader acceptance of Loop Quantum Gravity, which remains a minority position in the quantum-gravity community relative to string theory and asymptotic safety
• Specific predictions including the fast-radio-burst link have not been observationally confirmed; the model is testable in principle but the signature has not yet been distinguished from astrophysical alternatives in actual data