U-Duality, D-Branes and Black Hole Emission Rates: Agreements and Disagreements

TL;DR

This paper compares spacetime and D-brane calculations of black hole emission rates, finding agreements in some regimes but discrepancies in others, especially for charged emissions and high frequencies, raising questions about the D-brane model's validity.

Contribution

It extends the calculation of black hole absorption coefficients to a larger U-duality invariant region and analyzes the regimes where spacetime and D-brane results agree or disagree.

Findings

Agreement for neutral emission in the specified parameter range.

Disagreement for charged emission in the 'Fat Black Hole' regime.

High-frequency absorption approaches unity below the string scale, contrary to D-brane predictions.

Abstract

An expression for the spacetime absorption coefficient of a scalar field in a five dimensional, near extremal black hole background is derived, which has the same form as that presented by Maldacena and Strominger, but is valid over a larger, U-duality invariant region of parameter space and in general disagrees with the corresponding D-brane result. We develop an argument, based on D-brane thermodynamics, which specifies the range of parameters over which agreement should be expected. For neutral emission, the spacetime and D-brane results agree over this range. However, for charged emission, we find disagreement in the `Fat Black Hole' regime, in which charge is quantized in smaller units on the brane, than in the bulk of spacetime. We indicate a possible problem with the D-brane model in this regime. We also use the Born approximation to study the high frequency limit of the absorption coefficient and find that it approaches unity, for large black hole backgrounds, at frequencies still below the string scale, again in disagreement with D-brane results.