Physics - Mathematical and Theoretical Physics Seminar

Abstract: The gravitational path integral has sharpened our understanding of black holes. In this talk, I revisit it in de Sitter through the Hartle-Hawking no-boundary proposal. I will discuss a few puzzles that arise in regimes where the near-Nariai geometry becomes relevant. The first puzzle concerns the no-boundary wavefunction of the universe. In the expanding patch, the classical wavefunction becomes effectively insensitive to a late-time modulus and exhibits an apparent runaway, signaling a breakdown of the semiclassical approximation. We then identify precisely quantum-gravity corrections that are parametrically enhanced: a soft reparametrization (Schwarzian-like) sector becomes effectively light, and loop effects regulate the would-be divergence, making the wavefunction well-defined. The second puzzle concerns the pair creation of de Sitter black holes during inflation, and the long-debated “anti-evaporation” channel of Schwarzschild–de Sitter black holes in the near-Nariai limit. We explain why this channel is a semiclassical pathology, rather than a robust dynamical instability. Instead, the no-boundary prescription provides a clean semiclassical evolution that agrees with the expectation of a thermal Hawking flux. We also clarify why we do not expect large quantum-gravity corrections in this scenario, and how the correct interpretation is naturally framed in terms of the Hawking–Moss instanton.