Absolute entropy and the observer's no-boundary state

2025/05/20 by Blommaert, Andreas, Kudler-Flam, Jonah, Urbach, Erez Y. · 10 citations
#FOS: Physical sciences #General Relativity and Quantum Cosmology (gr-qc) #High Energy Physics - Theory (hep-th)

paper · doi:10.48550/arxiv.2505.14771

Abstract

We investigate the no-boundary proposal for closed universes with an observer. We argue that the observer's no-boundary state is the identity operator on the physical Hilbert space, i.e., the maximum entropy state and show this explicitly in Jackiw-Teitelboim gravity. Geometrically, the no-boundary state is a bra-ket wormhole. Expectation values in the no-boundary state provide a trace for the observer's algebra, which allows one to define von Neumann entropy for observers in different universes as the relative entropy with respect to the no-boundary state. This result is consistent with all previously discussed cases of traces for invariantly defined regions.

Citations

• An intrinsic cosmological observer
• Vacua and infrared radiation in de Sitter quantum field theory
• Bras and Kets in Euclidean Path Integrals
• On observers in holographic maps
• Wormholes, branes and finite matrices in sine dilaton gravity
• Unimodular JT gravity and de Sitter quantum cosmology
• Geometric Entropies and their Hamiltonian Flows
• The gravitational path integral from an observer's point of view
• Algebras and states in super-JT gravity
• Crossed products and quantum reference frames: on the observer-dependence of gravitational entropy
• Chaos and the Emergence of the Cosmological Horizon
• The Hilbert space of de Sitter JT: a case study for canonical methods in quantum gravity
• The no boundary density matrix
• De Sitter Bra-Ket Wormholes
• A clock is just a way to tell the time: gravitational algebras in cosmological spacetimes
• Algebraic Observational Cosmology
• Gravitational entropy is observer-dependent
• On the non-perturbative bulk Hilbert space of JT gravity
• Non-perturbative de Sitter Jackiw-Teitelboim gravity
• Gauging spacetime inversions in quantum gravity
• A Background Independent Algebra in Quantum Gravity
• Axion-de Sitter wormholes
• Algebras and States in JT Gravity
• A Note On The Canonical Formalism for Gravity
• An algebra of observables for de Sitter space
• Why Does Quantum Field Theory In Curved Spacetime Make Sense? And What Happens To The Algebra of Observables In The Thermodynamic Limit?
• A Note On Complex Spacetime Metrics
• The volume of the black hole interior at late times
• Wick rotation and the positivity of energy in quantum field theory
• Dissecting the ensemble in JT gravity
• JT gravity at finite cutoff
• Replica wormholes and the black hole interior
• Emergent unitarity in de Sitter from matrix integrals
• Replica Wormholes and the Entropy of Hawking Radiation
• Late Time Correlation Functions, Baby Universes, and ETH in JT Gravity
• The Page curve of Hawking radiation from semiclassical geometry
• Entanglement Wedge Reconstruction and the Information Paradox
• Entanglement wedge reconstruction and the information paradox
• JT gravity as a matrix integral
• A semiclassical ramp in SYK and in gravity
• Conformal symmetry and its breaking in two dimensional Nearly Anti-de-Sitter space
• Quantum Extremal Surfaces: Holographic Entanglement Entropy beyond the Classical Regime
• Correlators, Feynman diagrams, and quantum no-hair in deSitter spacetime
• Group Averaging and Refined Algebraic Quantization: Where are we now?
• Positive vacuum energy and the N-bound
• Large N Field Theories, String Theory and Gravity
• The Large-N Limit of Superconformal Field Theories and Supergravity
• An introduction to quantum cosmology
• Evolution without evolution: Dynamics described by stationary observables
• Quantum field theory in de Sitter space: renormalization by point-splitting
• Topology of cosmic domains and strings
• Dynamical Structure and Definition of Energy in General Relativity
• Quantum linearization instabilities of de Sitter spacetime. I

Cited by

• Quantum Walks and Exact RG in de Sitter Space
• No boundary density matrix in elliptic de Sitter dS/ℤ₂
• Microstate counting from defects in de Sitter
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• Stringy algebras, stretched horizons, and quantum-connected wormholes
• Observers and Timekeepers: From the Page-Wootters Mechanism to the Gravitational Path Integral
• Norm of the no-boundary state
• Sphere amplitudes and observing the universe's size
• Nonperturbative Quantum Gravity in a Closed Lorentzian Universe
• Observers seeing gravitational Hilbert spaces: abstract sources for an abstract path integral

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