  Recently it was proposed that microscopic models of braneworld cosmology
could be realized in the context of AdS/CFT using black hole microstates
containing an end-of-the-world brane. Motivated by a desire to establish the
microscopic existence of such microstates, which so far have been discussed
primarily in bottom-up models, we have studied similar microstates in a simpler
version of AdS/CFT. On one side, we define and study boundary states in the
charged Sachdev-Ye-Kitaev model and show that these states typically look
thermal with a certain pattern of symmetry breaking. On the other side, we
study the dimensional reduction of microstates in Einstein-Maxwell theory
featuring an end-of-the-world brane and show that they have an equivalent
description in terms of 2D Jackiw-Teitelboim gravity coupled to an
end-of-the-world particle. In particular, the same pattern of symmetry breaking
is realized in both sides of the proposed duality. These results give
significant evidence that such black hole microstates have a sensible
microscopic realization.
