A Tale of Two Hartle-Hawking Wave Functions: Fully Gravitational vs Partially Frozen

TL;DR

This paper compares fully gravitational and partially frozen Hartle-Hawking wave functions in AdS spacetime, analyzing their properties, one-loop corrections, and phase behaviors, revealing the impact of boundary conditions on the gravitational path integral.

Contribution

It explicitly analyzes the fully gravitational Hartle-Hawking wave function in AdS$_3$ and AdS$_2$, and compares phase behaviors with partially frozen cases, highlighting the role of boundary dynamics.

Findings

Fully gravitational wave function develops a nontrivial one-loop phase.

Partially frozen wave function remains real and positive.

Phase behavior is controlled by whether the boundary is dynamical or fixed.

Abstract

We revisit the Hartle-Hawking wave function in AdS spacetime, where natural spatial slices are open and require an additional spacetime boundary. This leads to two constructions: a fully gravitational wave function, in which the boundary configuration is integrated over, and a partially frozen one, in which it is fixed, as in AdS/CFT. To illustrate the fully gravitational construction, we explicitly analyze it in AdS$_3$ Einstein gravity and AdS$_2$ Jackiw-Teitelboim gravity. We then evaluate the one-loop correction to the hyperbolic-ball partition function in $D$-dimensional AdS Einstein gravity, expected to give the leading contribution to the wave-function norm. We demonstrate that the fully gravitational hyperbolic ball partition function, where the boundary fluctuates, develops a nontrivial one-loop phase of $(\mp i)^{D+1}$, analogous to that of the sphere partition function in dS gravity. By contrast, the partially frozen partition function, where the boundary is fixed, remains real and positive. Motivated by this AdS comparison, we conversely investigate a partially frozen dS sphere partition function where the metric on an equator is fixed, finding that its one-loop phase cancels nontrivially. Our results suggest that the phase problem is controlled by whether the gravitational path integral is fully dynamical or partially frozen.