Some features of the extended phase space approach to quantization of gravity

TL;DR

This paper discusses the extended phase space approach to quantum gravity, highlighting how it accommodates non-trivial topologies and challenges the assumptions of asymptotic states, leading to potential breakdowns in unitarity.

Contribution

It introduces a perspective on quantum gravity that considers non-trivial topologies and rejects asymptotic state assumptions, contrasting with traditional approaches.

Findings

Non-trivial topology may break unitarity in quantum gravity.

Asymptotic states are limited to asymptotically flat spacetimes.

Extended phase space approach offers a different viewpoint from Wheeler-DeWitt.

Abstract

In this paper, I emphasize those features of the extended phase space approach to quantization of gravity that distinguish it among other approaches. First of all, it is the conjecture about non-trivial topology of the Universe which was supported by Wheeler, Hawking and other founders of quantum gravity. However, this conjecture appears to be in contradiction with the assumption about asymptotic states that is used in the path integral quantization of gauge theories. The presence of asymptotic states ensures gauge invariance of the theory, but, in the case of gravity, the states exist only in asymptotically flat spacetimes, that limits possible topologies. Then we have two ways. The first way is to consider only asymptotically flat spacetimes. In fact, it reduces quantum gravity to quantum field theory on a given background. The second way is to reject the assumption about asymptotic states. In the case of non-trivial topology, one cannot cover the whole spacetime with the only coordinate system. One has to introduce various reference frames fixed by different gauge conditions in different spacetime regions. The Hamiltonian describing a gravitating system will depend on gauge conditions. It leads to the conclusion that unitary evolution may be broken down. This conclusion cannot be obtained in approaches based on the Wheeler - DeWitt equation or making use of the assumption about asymptotic states. The assessment of this conclusion is given.