Bose-Einstein graviton condensate in a Schwarzschild black hole

TL;DR

This paper models black holes as Bose-Einstein condensates of gravitons by extending the Einstein-Hilbert action with a chemical potential, deriving equations akin to the Gross-Pitaevskii equation, and analyzing solutions within the black hole interior.

Contribution

It introduces a grand-canonical ensemble approach to graviton condensates in black holes, deriving new equations and solutions that describe the condensate's behavior inside the horizon.

Findings

Condensate vanishes outside the horizon

Non-zero condensate exists inside the black hole

Exact solution for the graviton wave function in the interior

Abstract

We analyze in detail a previous proposal by Dvali and G\'omez that black holes could be treated as consisting of a Bose-Einstein condensate of gravitons. In order to do so we extend the Einstein-Hilbert action with a chemical potential-like term, thus placing ourselves in a grand-canonical ensemble. The form and characteristics of this chemical potential-like piece are discussed in some detail. We argue that the resulting equations of motion derived from the action could be interpreted as the Gross-Pitaevskii equation describing a graviton Bose-Einstein condensate trapped by the black hole gravitational field. After this, we proceed to expand the ensuring equations of motion up to second order around the classical Schwarzschild metric so that some non-linear terms in the metric fluctuation are kept. Next we search for solutions and, modulo some very plausible assumptions, we find out that the condensate vanishes outside the horizon but is non-zero in its interior. Inspired by a linearized approximation around the horizon we are able to find an exact solution for the mean-field wave function describing the graviton Bose-Einstein condensate in the black hole interior. After this, we can rederive some of the relations involving the number of gravitons $N$ and the black hole characteristics along the lines suggested by Dvali and G\'omez.