Black Hole Entropy and the Problem of Universality

Understand

To derive black hole thermodynamics in any quantum theory of gravity, one must introduce constraints that ensure that a black hole is actually present.

• For a large class of black holes, the imposition of such ``horizon constraints'' allows the use of conformal field theory methods to compute the density of states, reproducing the correct Bekenstein-Hawking entropy in a nearly model-independent manner.
• This approach may explain the ``universality'' of black hole entropy, the fact that many inequivalent descriptions of quantum states all seem to give the same thermodynamic predictions.
• It also suggests an elegant picture of the relevant degrees of freedom, as Goldstone-boson-like excitations arising from symmetry breaking by a conformal anomaly induced by the horizon constraints.