Absorption vs Decay of Black holes in string theory and T-symmetry

TL;DR

The paper investigates how classical absorption by black holes can emerge from a time-reversal symmetric D-brane microstate model. By analyzing Hawking decay and its time-reversed absorption, it shows that the degeneracy factor $\Omega$ enhances absorption via $\Omega/\Omega'$, yielding a nonzero classical cross-section. In the classical limit, Hawking decay vanishes while absorption remains finite, and a dedicated classical wave analysis demonstrates that the absorption cross-section equals the horizon area $A_h$ in the low-frequency regime. This work strengthens the link between microstate counting and horizon physics, hinting at the robustness of horizon-driven phenomena under strong coupling and offering a route to connect microscopic calculations with macroscopic classical results, e.g. $S = \frac{1}{4} A$ and $\sigma_A = A_h$.

Abstract

Classically a black hole can absorb but not emit energy. We discuss how this T-asymmetric property of black holes arises in the recently proposed (T-symmetric) microscopic models of black holes based on bound states of D-branes. In these string theory based models, the nonvanishing classical absorption is made possible essentially by the exponentially increasing degeneracy of quantum states with mass of the black hole. The classical limit of the absorption crosssection computed in the microscopic model agrees with the result obtained from a classical analysis of a wave propagating in the background metric of the corresponding black hole (upto a numerical factor).