Hawking Radiation in the Swiss Cheese Universe

TL;DR

This paper investigates how Hawking radiation behaves in an expanding universe, specifically within a Swiss cheese universe model, revealing non-thermal spectra and increased radiation intensity due to cosmological expansion.

Contribution

It extends the analysis of Hawking radiation to a dynamical, expanding universe setting using a Swiss cheese model in two dimensions, showing non-thermal spectra and enhanced radiation.

Findings

Hawking radiation spectrum becomes non-thermal in an expanding universe.

The radiation intensity is increased by cosmological expansion.

The spectrum exhibits dumping oscillations as a function of frequency.

Abstract

The Hawking radiation forms the essential basis of the black hole thermodynamics. The black hole thermodynamics denotes a nice correspondence between black hole kinematics and the laws of ordinary thermodynamics, but has been so far considered only in an asymptotically flat case. Does such the correspondence rely strongly on the feature of the gravity vanishing at the infinity? In order to resolve this question, it should be considered for the first to extend the Hawking radiation to a case with a dynamical boundary condition like an expanding universe. Therefore the Hawking radiation in an expanding universe is discussed in this paper. As a concrete model of a black hole in an expanding universe, we use the swiss cheese universe which is the spacetime including a Schwarzschild black hole in the Friedmann-Robertson-Walker universe. Further for simplicity, our calculation is performed in two dimension. The resultant spectrum of the Hawking radiation measured by a comoving observer is generally different from a thermal one. We find that the qualitative behavior of the non-thermal spectrum is of dumping oscillation as a function of the frequency measured by the observer, and that the intensity of the Hawking radiation is enhanced by the presence of a cosmological expansion. It is appropriate to say that it a black hole with an asymptotically flat boundary condition stays in a lowest energy thermal equilibrium state, and that, once a black hole is put into an expanding universe, it is excited to a non-equilibrium state and emits its mass energy with stronger intensity than a thermal one.