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Islands and mixed states in closed universes

Fallows, Seamus and Ross, Simon F. (2021) 'Islands and mixed states in closed universes.', Journal of high energy physics., 2021 (7). 022.


We investigate the appearance of islands when a closed universe with gravity is entangled with a non-gravitating quantum system. We use braneworlds in three-dimensional multiboundary wormhole geometries as a model to explore what happens when the non-gravitating system has several components. The braneworld can be either completely contained in the entanglement wedge of one of the non-gravitating systems or split between them. In the former case, entanglement with the other system leads to a mixed state in the closed universe, unlike in simpler setups with a single quantum system, where the closed universe was necessarily in a pure state. We show that the entropy of this mixed state is bounded by half of the coarse-grained entropy of the effective theory on the braneworld.

Item Type:Article
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Publisher statement:Open Access . This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.
Date accepted:17 June 2021
Date deposited:21 October 2021
Date of first online publication:06 July 2021
Date first made open access:21 October 2021

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