Abstract
In an earlier short paper [Phys.\ Rev.\ Lett.\ 120 (2018) 101301, arXiv:1702.04439], I argued that the horizon-preserving diffeomorphisms of a generic black hole are enhanced to a larger BMS${}_3$ symmetry, which is powerful enough to determine the Bekenstein-Hawking entropy. Here I provide details and extensions of that argument, including a loosening of horizon boundary conditions and a more thorough treatment of dimensional reduction and meaning of a "near-horizon symmetry."
Highlights
The discovery by Bekenstein [1] and Hawking [2] that black holes are thermodynamic objects has led to a host of fascinating puzzles, from the information loss problem to the question of what microscopic states are responsible for black hole entropy
I focus on one particular puzzle, the “problem of universality” of black hole entropy
The same area law, with the same coefficient, holds for black holes with any charges, any spins, in any dimensions; it holds for black strings, black rings, black branes, and black Saturns; it remains true for “dirty black holes” whose horizons are distorted by nearby matter
Summary
The discovery by Bekenstein [1] and Hawking [2] that black holes are thermodynamic objects has led to a host of fascinating puzzles, from the information loss problem to the question of what microscopic states are responsible for black hole entropy. For instance, are cleanest for near-extremal black holes, while loop quantum gravity calculations may depend on a new universal constant, the Barbero-Immirzi parameter They describe very different microstates, each of these methods, within its range of validity, reproduces the standard Bekenstein-Hawking entropy. A first guess for this deeper structure is that the relevant degrees of freedom live on the horizon [1] This is not enough; while it could explain an area law for black hole entropy, there is no obvious reason why the coefficient 1=4 should be universal. Attempts to extend those results to higher dimensions soon followed [13,14] These efforts, which typically involve a search for a suitable two-dimensional group of horizon symmetries, have had significant success; see Ref. I confirm that the resulting generators satisfy a centrally extended BMS3 algebra that determines the correct BekensteinHawking entropy
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