AdS3's with and without BTZ's

This companion paper to arXiv:2109.00065 explores the interpretation of $AdS_3/CFT_2$ duality for $k=(R_{AdS}/\lstr)^2<1$ in terms of "non-critical little string theory". We review the underlying fivebrane structure of 1/2-BPS backgrounds for string theory in both the critical dimension and for non-critical models. D-branes bound to the fivebranes in these backgrounds are associated to nonabelian little strings, and flesh out a picture of the correspondence transition at $k=1$ wherein the nonabelian little string excitations lose their Hagedorn entropy, allowing the Hagedorn entropy of fundamental strings to dominate the asymptotic density of states for $k<1$ instead of BTZ black holes. We then discuss the application of these models as approximations to and probes of black hole dynamics and thermodynamics, as well as associated information puzzles. In particular, because the correspondence transition reveals the interior of an evaporating black hole, it serves as a sensitive probe of any proposal regarding the composition of that interior.