The Baby Universe is Fine and the CFT Knows It: On Holography for Closed Universes

TL;DR

This paper explores how closed universes without boundaries can be described holographically via AdS/CFT, clarifying the role of entanglement, final states, and the emergence of time's arrow, ultimately supporting the consistency of baby universes in this framework.

Contribution

It demonstrates that closed universes can be holographically encoded in CFTs with sufficient entanglement and clarifies the relationship between final states, semiclassical physics, and observable predictions.

Findings

Holographic encoding of closed universes depends on bulk entanglement.

The breakdown of encoding occurs in the absence of bulk entanglement.

Conventional effective field theory can be recovered as a coarse-grained limit.

Abstract

Big bang/big crunch closed universes can be realized in AdS/CFT, even though they lack asymptotically AdS boundaries. With enough bulk entanglement, the bulk Hilbert space of a closed universe can be holographically encoded in the CFT. We clarify the relation of this encoding to observer-clone proposals and refute recent arguments about the breakdown of semiclassical physics in such spaces. In the limit of no bulk entanglement, the holographic encoding breaks down. The oft-cited one-dimensional nature of the closed universe Hilbert space represents the limitation of the external (CFT) Hilbert space to access the quantum information in the closed universe, similar to the limitations imposed on observers outside a perfectly isolated quantum lab. We advocate that the CFT nevertheless continues to determine the physical properties of the closed universe in this regime, showing how to interpret this relationship in terms of a final state projection in the closed universe. We provide a dictionary between the final state wavefunction and CFT data. We propose a model of the emergence of an arrow of time in the universe with a given initial or final state projection. Finally, we show that the conventional EFT in the closed universe, without any projection, can be recovered as a maximally ignorant description of the final state. This conventional EFT is encoded in CFT data, and it can be probed by computing coarse-grained observables. We provide an example of one such observable. Taken together, these results amount to a clean bill of health for baby universes born of AdS/CFT.