Unitarity and quantum resolution of gravitational singularities

TL;DR

This paper investigates how the requirement of unitarity in quantum gravity affects the resolution of classical singularities, revealing that the choice of clock influences whether singularities are resolved or lead to recollapse.

Contribution

It demonstrates that unitarity constraints depend on the choice of clock, showing that singularity resolution or avoidance can be engineered by selecting different clocks in quantum gravity.

Findings

Unitarity implies quantum singularity resolution with certain clocks.

Choosing different clocks can lead to quantum recollapse instead of resolution.

General covariance conflicts with quantum unitarity, affecting singularity outcomes.

Abstract

We explore the consequences of requiring that quantum theories of gravity be unitary, mostly focusing on simple cosmological models to illustrate the main points. We show that unitarity for a clock that encounters a classical singularity at finite time implies quantum singularity resolution, but for a clock that encounters future infinity at finite time leads to a quantum recollapse. We then find that our starting point - assuming the general covariance of general relativity - is actually incompatible with general quantum unitarity: singularity resolution in quantum gravity can always be engineered by choosing the right clock, or avoided by using a different one.