The yes boundaries wavefunctions of the universe

TL;DR

This paper develops a holographic dual framework for cosmological spacetimes with boundaries, revealing how entanglement and quantum effects shape the universe's extended structure beyond horizons.

Contribution

It introduces a microscopic holographic dual for cosmological spacetimes using doubled Hamiltonian theories and entanglement, addressing maximality of gravitational path integrals and quantum effects.

Findings

Extended spacetime arises from nearly maximally entangled states.

Lower energy levels produce tall spacetimes with overlapping causal wedges.

Quantum gravity with positive cosmological constant allows multiple consistent states.

Abstract

A generic spacetime topology contains timelike boundaries. Making use of two such boundaries, we formulate a microscopic holographic dual that captures cosmological spacetime beyond the cosmic horizon patch, including the future wedge. We build this starting from two copies of the dressed Hamiltonian quantum theories which formulate the cosmic horizon and pole patches of de Sitter. At the top level of the spectrum we obtain the extended spacetime from a nearly maximally entangled (micro-)canonical thermofield double state. This requires addressing the maximality of the unrenormalized gravitational path integral saddle in the calculation of the entanglement entropy upon tracing out one sector. We resolve this in both ensembles via explicit computations in the constrained path integral for three bulk dimensions, incorporating UV-sensitive quantum beyond-GR effects when they contribute strongly. Lower energy levels in the spectrum generate tall extended spacetimes where the boundaries' causal wedges overlap. These arise in our theory via constraints on the doubled Hilbert space, which encode the operator redundancies arising from the reconstruction of bulk operators from either boundary within the region where their causal wedges overlap. With positive cosmological constant, the tallness implies that causal wedge reconstruction is more powerful than in the AdS/CFT setting. In contrast to the special case of a closed universe, generically quantum gravity with positive cosmological constant -- including the future wedge -- is manifestly consistent with the existence of multiple states.