
Comments on the Hartle-Hawking State and Observers
Ying Zhao, a physicist at MIT, presents this talk at the Institute for Advanced Study's workshop on Quantum Aspects of Black Holes and Spacetime. She addresses a puzzle in holography: any fixed holographic theory appears to contain only one closed universe state, which fails to reproduce ordinary semi-classical physics. A proposed fix introduces a classical observer living inside the universe, and earlier work examined this idea for closed universes tied to asymptotic Euclidean boundaries. Zhao extends the analysis to the Hartle-Hawking state, where the dominant Euclidean topology is a sphere rather than those earlier boundary conditions, and shows that the physics changes in important ways. She closes by discussing what this might mean for understanding de Sitter space. The work is done jointly with Daniel Harlow. The talk assumes familiarity with holography and quantum gravity and runs about 48 minutes, recorded in Wolfensohn Hall.