Absorption of Scalars by Extended Objects

TL;DR

This paper derives a general formula for the low energy absorption cross section of scalars by various extended objects like black holes and branes, and compares the results with microscopic models, finding exact agreement in key cases.

Contribution

It provides a unified derivation of scalar absorption cross sections for a broad class of geometries, including regular and singular horizons, and validates microscopic models against geometric calculations.

Findings

Exact agreement with microscopic models for non-degenerate ground states.

Absorption through stretched horizons reproduces correct results for degenerate states.

Correct frequency and charge dependence for absorption by BPS strings and D-branes.

Abstract

We derive the low energy absorption cross section of minimal scalars for a very generic class of geometries, that includes, among others, black holes, strings, $p$-branes, as well as intersecting brane configurations. The scalar field can be absorbed across a regular surface (a horizon) of finite area, or across a zero area throat, which may be singular or regular. Then we focus on some particular cases and compare with the results obtained using microscopic models at weak coupling. Exact agreement is found for the absorption by the non-degenerate ground state of effective strings. For geometries where the throat is singular, we show that absorption through the stretched horizon yields correct results for degenerate states. For non-degenerate (ground) states we introduce a stretched throat, and show that it accounts for the correct dependence on frequency, charges, and moduli for the absorption by ground states of BPS fundamental strings. It is also shown to lead to the correct dependence on frequency for extremal D-branes.