Challenges for describing unitary evolution in nontrivial geometries: pictures and representations

TL;DR

This paper investigates the difficulties in describing unitary evolution in complex spacetimes, highlighting issues with state representations, and explores potential resolutions relevant to quantum gravity and cosmology.

Contribution

It introduces a new approach to defining physical state equivalence classes in evolving spacetimes, extending previous methods and analyzing their implications for quantum gravity.

Findings

Identifies the non-unitary nature of state evolution in nontrivial geometries.

Proposes a local limit analysis to address state representation issues.

Discusses concrete examples involving cosmological and black hole evolution.

Abstract

Description of evolution between spatial slices in a general spacetime suffers from a significant difficulty: the states on the slices, in a given basis, are not related by a unitary transformation. This problem, which occurs in spacetime dimensions above two, is directly related to the infinite number of inequivalent representations of the canonical commutators, and in particular will arise for interacting theories in time-dependent spacetimes. We connect different facets of this issue, and discuss its possible resolution. It is directly related to discussions of failure of a standard Schr\"odinger picture of evolution, and of evolution via "many-fingered time." One requires a condition specifying a physical unitary equivalence class of states; in general this equivalence class evolves with time, and an important question is how it is determined. One approach to this in free theories is by imposing a Hadamard condition on the two point function. We explore a different approach, which also may be helpful for interacting theories, analyzing the structure of the state in a local limit, and relate these approaches. We also elucidate the non-Hadamard behavior of unphysical vacua, and discuss concrete examples of these approaches involving cosmological and black hole evolution. The issues are extended in the context of quantum dynamical geometry, and raise important questions for the proper description of the wavefunction of the universe and for the role of the Wheeler-DeWitt equation.