Cosmic Strings and Chronology Protection

TL;DR

This paper investigates the structure of a spacetime with cosmic strings that contains closed timelike curves, analyzing quantum effects and suggesting that backreaction prevents the formation of time machines, supporting Hawking's Chronology Protection Conjecture.

Contribution

It provides a detailed analysis of the global structure of a spacetime with cosmic strings and demonstrates that quantum backreaction likely prevents the formation of closed timelike curves.

Findings

Vacuum expectation value diverges weakly on the Chronology horizon

Stronger divergence occurs on polarised hypersurfaces

Backreaction becomes significant before quantum gravity effects, supporting Chronology Protection

Abstract

A space consisting of two rapidly moving cosmic strings has recently been constructed by Gott that contains closed timelike curves. The global structure of this space is analysed and is found that, away from the strings, the space is identical to a generalised Misner space. The vacuum expectation value of the energy momentum tensor for a conformally coupled scalar field is calculated on this generalised Misner space. It is found to diverge very weakly on the Chronology horizon, but more strongly on the polarised hypersurfaces. The divergence on the polarised hypersurfaces is strong enough that when the proper geodesic interval around any polarised hypersurface is of order the Planck length squared, the perturbation to the metric caused by the backreaction will be of order one. Thus we expect the structure of the space will be radically altered by the backreaction before quantum gravitational effects become important. This suggests that Hawking's `Chronology Protection Conjecture' holds for spaces with non-compactly generated Chronology horizon.