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  • Open Access

Baby Universes from Thermal Pure States in the Sachdev-Ye-Kitaev Model

Martin Sasieta1,*, Brian Swingle2,†, and Alejandro Vilar López3,‡

  • *Contact author: msasieta@berkeley.edu
  • †Contact author: bswingle@brandeis.edu
  • ‡Contact author: alejandro.vilarlopez@ubc.ca
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Phys. Rev. Lett. 137, 041501 – Published 20 July, 2026

DOI: https://doi.org/10.1103/8p6x-w737

Abstract

We construct a simple two-dimensional holographic model of a closed “baby” universe. The baby universe spacetime originates from the black hole interior in Jackiw-Teitelboim gravity. The holographic description is a low-temperature thermal pure state of two Sachdev-Ye-Kitaev (SYK) models. The construction of the state relies on the insertion of a heavy matter operator, which supports the baby universe, together with a first-order thermal phase transition between a black hole phase and an empty anti–de Sitter phase. For the transition, we employ a version of the Maldacena-Qi phase transition of two coupled SYK systems. The bulk entanglement between the anti–de Sitter region and the baby universe remains O(N) below the transition, owing to the large number of light bulk fields. This entanglement admits a definition through coarse-graining over SYK couplings.

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Phys. Rev. Lett. 137, 041501– Published 20 July, 2026
  • Received 6 December 2025
  • Accepted 10 June 2026
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