Absorption of Scalars by Extended Objects

TL;DR

The paper derives a universal low-energy absorption cross section for minimal scalars impinging on a broad class of extended-object geometries, encompassing black holes, strings, p-branes, and their intersections. By formulating the problem in terms of near-surface geometry characterized by exponents $a$ and $b$ and scales $S$ and $T$, the author obtains a general cross-section formula that reduces to the horizon area for $a=b$ and exhibits distinct frequency dependences depending on the throat structure. The work then analyzes throat anatomy, distinguishing regular throats (including AdS-like regions) from singular ones, and shows how regular throats yield universal $\sigma \propto \omega^p$ behavior, while singular cases require stretched-horizon or stretched-throat regularizations to reproduce microscopic results. Across several subsectors, including effective strings and BPS fundamental strings, the paper demonstrates exact or consistent agreement with weakly coupled microscopic models, highlighting a deep link between classical absorption and string/brane dynamics. The results provide a practical framework to compare macroscopic gravity calculations with microscopic brane descriptions and offer insights into how singularities are resolved in the absorption process.

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.