Comment on Hawking radiation and trapping horizons

TL;DR

This paper investigates Hawking radiation from dynamical black holes formed by collapsing fluids, analyzing how the local Hawking temperature depends on the fluid's equation of state and horizon type, with implications for observability.

Contribution

It provides a calculation of the local Hawking temperature for dynamical horizons using the Hamilton-Jacobi method and discusses the conditions under which Hawking radiation can be observed.

Findings

Hawking temperature depends on the equation of state of the collapsing fluid.

Radiation from space-like dynamical horizons may be observable.

Hawking radiation from future trapping horizons is theoretically possible.

Abstract

We consider dynamical black hole formation from a collapsing fluid described by a symmetric and flat FRW metric. Using the Hamilton-Jacobi method the local Hawking temperature for the formed trapping/apparent horizon is calculated. The local Hawking temperature depends on the tunneling path, which we take to be along a null direction $(\Delta s=0)$. We find that the local Hawking temperature depends directly on the equation of state of the collapsing fluid. We argue that Hawking radiation by quantum tunnelling from future inner and future outer trapping horizons is possible. However, only radiation from a space-like dynamical horizon has a chance to be observed by an external observer. Some comparison to existing literature is made.