Bogoliubov transformations for amplitudes in black-hole evaporation

TL;DR

This paper introduces a complex amplitude approach to black-hole evaporation using Bogoliubov transformations, allowing for the calculation of quantum amplitudes via Feynman's $+i\,\epsilon$ prescription, bypassing singularities in the classical boundary-value problem.

Contribution

It extends traditional methods by deriving quantum amplitudes for black-hole evaporation through analytic continuation, enabling arbitrary smooth gravitational data on final surfaces.

Findings

Quantum amplitude derived from Lorentzian limit

Probability distribution cannot be expressed via a density matrix

Analytic continuation bypasses singularities in classical solutions

Abstract

The familiar approach to quantum radiation following collapse to a black hole proceeds via Bogoliubov transformations, and yields probabilities for final outcomes. In our (complex) approach, we find quantum amplitudes, not just probabilities, by following Feynman's $+i\epsilon$ prescription. Initial and final data for Einstein gravity and (say) a massless scalar field are specified on a pair of asymptotically-flat space-like hypersurfaces $\Sigma_I$ and $\Sigma_F$; both are diffeomorphic to ${\Bbb R}^3$. Denote by $T$ the (real) Lorentzian proper-time interval between the surfaces, as measured at spatial infinity. Then rotate: $T\to{\mid}T{\mid}\exp(-i\theta),0<\theta\leq \pi/2$. The {\it classical} boundary-value problem is expected to be well-posed on a region of topology $I\times{\Bbb R}^3$, where $I$ is a closed interval. For a locally-supersymmetric theory, the quantum amplitude should be dominated by the semi-classical expression $\exp(iS_{\rm class})$, where $S_{\rm class}$ is the classical action. One finds the Lorentzian quantum amplitude from the limit $\theta\to 0_+$. In the usual approach, the only possible such final surfaces are in the strong-field region shortly before the curvature singularity. In our approach one can put arbitrary smooth gravitational data on $\Sigma_F$, provided that it has the correct mass $M$ -- the singularity is by-passed in the analytic continuation. Here, we consider Bogoliubov transformations and their possible relation to the probability distribution and density matrix in the traditional approach. We find that our probability distribution for configurations of the final scalar field cannot be expressed in terms of the diagonal elements of some non-trivial density-matrix distribution.