Black Holes as Fundamental Strings: Comparing the Absorption of Scalars

TL;DR

The work tests the Horowitz–Polchinski BH/string correspondence by comparing near-extremal 5D and 4D black-hole scalar absorption (greybody factors) to weakly coupled fundamental-string predictions. While entropy matching is maintained at the correspondence point, the detailed greybody spectra generally disagree, particularly in two-charge regimes, indicating the simplest BH/string mapping is insufficient to fully capture radiation properties across the explored parameter space. The discrepancy points to the need for higher-order corrections (string-loop, $\alpha'$) or more nuanced matching conditions, though some backgrounds (e.g., NS5-brane-embedded strings) exhibit exact concordance. These findings constrain the applicability of a naive BH-to-string transition for radiation spectra and motivate further investigations into regime-dependent correspondences and corrections.

Abstract

The recently proposed ``correspondence principle'' of Horowitz and Polchinski provides a concrete means to relate (among others) black holes with electric NS-NS charges to fundamental strings and correctly match their entropies. We test further this correspondence by examining the greybody factors in the absorption rates of neutral, minimally coupled scalars by a near extremal black hole. Perhaps surprisingly, the results disagree in general with the absorption by weakly coupled strings. Though this does not disprove the correspondence, it indicates that it might not be simple in this region of the black hole parameter space.