Holography of Tensionless Strings

This thesis investigates black hole solutions and microstate geometries in string theory in order to provide a concrete setting to explore the conjectured holographic correspondence between strings in curved backgrounds and conformal field theories (CFTs). After reviewing core concepts in conformal field theory and string theory, we discuss the string dynamics on backgrounds with an AdS factor and explore how, in a certain limit, it can be equivalently described by a two-dimensional CFT. Specifically, we examine the AdS₃ × S³ × T⁴ geometry with n₅ units of NS–NS flux arising from the NS5–F1 system in type IIB string theory, which, in the tensionless limit n₅=1, admits an exact description via the psu(1,1|2)₁ WZW model coupled to a topologically twisted internal CFT. This worldsheet theory, expressible in a free-field realization within the hybrid formalism, is holographically dual to the symmetric product orbifold CFT of T⁴. Recent developments suggest that this duality can be extended to linear-dilaton backgrounds through marginal current–current deformations in the bulk, which correspond to single-trace TT̅-deformed symmetric-orbifold theories on the boundary. This thesis aims to recast such marginally deformed models within the framework of a null-gauged coset WZW model, offering more control over the worldsheet theory and potentially enabling a more comprehensive understanding of holography in non-AdS spacetimes, including both microstate geometries and black hole solutions.