Gravitational Statistical Mechanics: A model

TL;DR

This paper models gravitational thermodynamics using quantum boundary states, deriving the Bekenstein-Hawking entropy and revealing a boundary condensate at low temperatures.

Contribution

It introduces a quantum gravitational model with boundary states that reproduces black hole entropy and analyzes temperature regimes with novel boundary condensate behavior.

Findings

At high temperatures, the model is thermodynamically stable.

At low temperatures, entropy scales with area, matching Bekenstein-Hawking entropy.

Boundary quantum geometry forms a condensate at low temperatures.

Abstract

Using the quantum Hamiltonian for a gravitational system with boundary, we find the partition function and derive the resulting thermodynamics. The Hamiltonian is the boundary term required by functional differentiability of the action for Lorentzian general relativity. In this model, states of quantum geometry are represented by spin networks. We show that the statistical mechanics of the model reduces to that of a simple non-interacting gas of particles with spin. Using both canonical and grand canonical descriptions, we investigate two temperature regimes determined by the fundamental constant in the theory, m. In the high temperature limit (kT > m), the model is thermodynamically stable. For low temperatures (kT < m) and for macroscopic areas of the bounding surface, the entropy is proportional to area (with logarithmic correction), providing a simple derivation of the Bekenstein-Hawking result. By comparing our results to known semiclassical relations we are able to fix the fundamental scale. Also in the low temperature, macroscopic limit, the quantum geometry on the boundary forms a `condensate' in the lowest energy level (j=1/2).