In 1973, Jacob Bekenstein proposed that black hole possess entropy — a measure of hidden disorder. Stephen Hawking later confirmed this by adding radiation, but the microscopic origin of entropy remained obscure. New research offers an answer: inside a black hole there is no singularity, but a dynamic de Sitter core — a tiny universe that expands or contracts. This core acts as a thermodynamic compass.
The compass needle is the sign of the expansion of the universe \(H\), which measures the expansion rate of space. If \(H > 0\), the core expands — the compass points to positive entropy. If \(H < 0\), the core contracts — the needle turns, and entropy becomes negative. Thus, a single sign determines the thermodynamic 'polarity' of the entire interior region.
The opposite scenario (black hole) contains a contracting core. Here, the compass points to negative entropy, which exactly compensates the outer horizon's entropy: the total entropy of the object is zero. This does not violate second law of thermodynamics — over time, the negative contribution disappears and entropy increases.
Both pictures, expansion and contraction, satisfy thermodynamics, but are separated by a classical barrier. When collapse reaches the 'Minkowski violation' point (disappearance of flat space at the center), the classical description breaks down. The subsequent transition between phases is a quantum jump, and its probability is exponentially small due to the huge entropy difference. The integer nature of the parameter \(n\) hints at a quantized entropy spectrum: \(\Delta S\) is quantized, like steps on a staircase.
Thus, the Hubble compass not only points the way but turns collapse into a story of quantum leaps. Perhaps gravitational waves from black hole mergers will one day capture these stepwise releases, confirming that inside the horizon lies not a point, but an entire universe with its own thermodynamic arrow.
For an astrophysical black hole of 10 M☉, the semiclassical description of collapse breaks down at n ~ 0.01—well before the classical Minkowski violation (n=0). In this regime, the inner horizon’s temperature surpasses the Planck temperature, and the geometry turns into a quantum foam.