10887 1810.10601 10.1007/JHEP07(2019)065 65 65 Black hole microstate cosmology Regular Article - Theoretical Physics 1 70 2019 7 12 2019 3 15 2019 6 10 2019 7 13 The Author(s) 2019 13130 2019 2019 7 7 Sean Cooper seancooper@phas.ubc.ca Moshe Rozali rozali@phas.ubc.ca Brian Swingle bswingle@umd.edu Mark Van Raamsdonk mav@phas.ubc.ca Christopher Waddell cwaddell@phas.ubc.ca David Wakeham daw@phas.ubc.ca grid.17091.3e 0000 0001 2288 9830 Department of Physics and Astronomy University of British Columbia 6224 Agricultural Road Vancouver B.C. V6T 1Z2 Canada grid.164295.d 0000 0001 0941 7177 Condensed Matter Theory Center, Maryland Center for Fundamental Physics, Joint Center for Quantum Information and Computer Science, and Department of Physics University of Maryland College Park MD 20742 U.S.A. Abstract 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. We also calculate holographic complexity for the d = 2 ETW geometries, finding that complexity-action and complexity-volume proposals give the same linear growth at late times, but differ at early times. 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. Keywords AdS-CFT Correspondence Black Holes in String Theory Conformal Field Theory Holography and condensed matter physics (AdS/CMT) ArXiv ePrint: 1810.10601