The Enhancon, Black Holes, and the Second Law

TL;DR

This paper investigates how the enhancon mechanism in five-dimensional D-brane black holes prevents entropy reduction, thus supporting the Second Law of Thermodynamics in string theory contexts.

Contribution

It demonstrates that the enhancon mechanism enforces the Second Law by preventing entropy-decreasing configurations in certain black hole solutions.

Findings

Enhancon locus appears in D5-D1 black hole geometries.

Entropy maximized with specific D5-brane count, preventing entropy decrease.

Enhancon mechanism upholds the Second Law in these string theory black holes.

Abstract

We revisit the physics of five-dimensional black holes constructed from D5- and D1-branes and momentum modes in type IIB string theory compactified on K3. Since these black holes incorporate D5-branes wrapped on K3, an enhancon locus appears in the spacetime geometry. With a `small' number of D1-branes, the entropy of a black hole is maximised by including precisely half as many D5-branes as there are D1-branes in the black hole. Any attempts to introduce more D5-branes, and so reduce the entropy, are thwarted by the appearance of the enhancon locus above the horizon, which then prevents their approach. The enhancon mechanism thereby acts to uphold the Second Law of Thermodynamics. This result generalises: For each type of bound state object which can be made of both types of brane, we show that a new type of enhancon exists at successively smaller radii in the geometry, again acting to prevent any reduction of the entropy just when needed. We briefly explore the appearance of the enhancon in the black hole interior.