$T\overline{T}$ and the black hole interior

Abstract: There is ample evidence that the bulk dual of a $T\overline{T}$ deformed holographic CFT is a gravitational system with a finite area cutoff boundary. For states dual to black holes, the finite cutoff surface cannot be moved beyond the event horizon. We overcome this by considering an extension of the $T\overline{T}$ deformation with a boundary cosmological constant and a prescription for a sequence of flows that successfully pushes the cutoff boundary past the event horizon and arbitrarily close to the black hole singularity. We show how this sequence avoids the complexification of the deformed boundary energies. The approach to the singularity is reflected on the boundary by the approach of the deformed energies to an accumulation point in the limit of arbitrarily large distance in deformation space. We argue that this sequence of flows is automatically implemented by the gravitational path integral given only the values of the initial ADM charges and the area of the finite cutoff surface, suggesting a similar automatic boundary mechanism that keeps all the deformed energies real at arbitrary values of the deformation parameter. This leads to a natural definition of a deformed boundary canonical ensemble partition function that sums over the entire spectrum and remains real for any value of the deformation parameter. We find that this partition function displays Hagedorn growth at the scale set by the deformation parameter, which we associate to the region near the inner horizon in the bulk dual.