  In this note, we explore the possibility that certain high-energy holographic
CFT states correspond to black hole microstates with a geometrical
behind-the-horizon region, modelled by a portion of a second asymptotic region
terminating at an end-of-the-world (ETW) brane. We study the time-dependent
physics of this behind-the-horizon region, whose ETW boundary geometry takes
the form of a closed FRW spacetime. We show that in many cases, this
behind-the-horizon physics can be probed directly by looking at the time
dependence of entanglement entropy for sufficiently large spatial CFT
subsystems. We study in particular states defined via Euclidean evolution from
conformal boundary states and give specific predictions for the behavior of the
entanglement entropy in this case. We perform analogous calculations for the
SYK model and find qualitative agreement with our expectations.
  A fascinating possibility is that for certain states, we might have gravity
localized to the ETW brane as in the Randall-Sundrum II scenario for cosmology.
In this case, the effective description of physics beyond the horizon could be
a big bang/big crunch cosmology of the same dimensionality as the CFT. In this
case, the d-dimensional CFT describing the black hole microstate would give a
precise, microscopic description of the d-dimensional cosmological physics.
