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Entangled universes

by Divij Gupta, Matthew Headrick, Martin Sasieta

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Submission summary

Authors (as registered SciPost users): Martin Sasieta
Submission information
Preprint Link: scipost_202510_00020v1  (pdf)
Date accepted: Nov. 3, 2025
Date submitted: Oct. 11, 2025, 12:21 a.m.
Submitted by: Martin Sasieta
Submitted to: SciPost Physics
Ontological classification
Academic field: Physics
Specialties:
  • High-Energy Physics - Theory
Approach: Theoretical

Abstract

We propose a generalization of the RT and HRT holographic entanglement entropy formulas to spacetimes with asymptotically Minkowski as well as asymptotically AdS regions. We postulate that such spacetimes represent entangled states in a tensor product of Hilbert spaces, each corresponding to one asymptotic region. We show that our conjectured formula has the same general properties and passes the same general tests as the standard HRT formula. We provide further evidence for it by showing that in many cases the Minkowski asymptotic regions can be replaced by AdS ones using a domain wall. We illustrate the use of our formula by calculating entanglement entropies between asymptotic regions in Brill-Lindquist spacetimes, finding phase transitions similar to those known to occur in AdS. We construct networks of universes by gluing together Brill-Lindquist spaces along minimal surfaces. Finally, we discuss a variety of possible extensions and generalizations, including to universes with asymptotically de Sitter regions; in the latter case, we identify an ambiguity in the homology condition, leading to two different versions of the HRT formula which we call ``orthodox'' and ``heterodox'', with different physical interpretations.

Author indications on fulfilling journal expectations

  • Provide a novel and synergetic link between different research areas.
  • Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
  • Detail a groundbreaking theoretical/experimental/computational discovery
  • Present a breakthrough on a previously-identified and long-standing research stumbling block

Author comments upon resubmission

We thank the referees for their careful reading of our paper and their comments on it. The report by referee 1 raises a number of interesting issues, related to: (1) the relation between anchored HRT surfaces and the unanchored ones considered in our paper; and (2) the relation, in asymptotically dS spacetimes, between the surfaces we identify and other extremal surfaces that have been proposed as encoding some form of entanglement. These are both definitely interesting and well-motivated questions for further work, although we view them as somewhat independent of the line of reasoning in our paper. We also thank the referee for pointing out the mislabelled figure as well as the missing references to work on dS extremal surfaces; we have fixed those issues in the revised version of the paper. As already noted in the text (see the last paragraph of page 44 and the caption of Fig. 23), the surfaces we study are distinct from the dS cosmological bifurcation surface, which instead plays the role of a bounce geometry in our discussion, and we appreciate the opportunity to clarify this distinction.

The report by referee 3 raises related questions about possible extensions of our conjecture to subregions and about the applicability of the Lewkowycz–Maldacena derivation in the presence of domain walls. We agree that these are interesting topics for future study, but they are not the focus of the present work. Our analysis simply assumes the validity of the RT/HRT prescription for general asymptotically AdS spacetimes, independently of the details of the Lewkowycz–Maldacena derivation.

List of changes

Corrected mislabelled figure 1.
Added missing references to work on dS extremal surfaces.
Corrected typo on top of p. 44.

Published as SciPost Phys. 19, 143 (2025)

Reports on this Submission

Report #1 by Anonymous (Referee 1) on 2025-10-18 (Invited Report)

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I agree with the authors' response, and look forward to their future studies on the questions I asked which I think may be interesting further developments to the current work. I'm happy to recommend publication.

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