Asymptotic behaviour of cylindrical waves interacting with spinning strings

TL;DR

This paper analyzes the asymptotic behavior of cylindrical gravitational waves interacting with spinning cosmic strings, establishing boundary conditions for well-defined metrics and revealing how wave content affects the deficit angle.

Contribution

It introduces boundary conditions ensuring consistent Hamiltonian dynamics for cylindrical spacetimes with angular momentum, linking wave behavior to cosmic string properties.

Findings

Metrics exhibit conical geometry with time dislocation near the axis.

Wave content increases the deficit angle at spatial infinity.

Spacetimes represent cylindrical gravitational waves around spinning cosmic strings.

Abstract

We consider a family of cylindrical spacetimes endowed with angular momentum that are solutions to the vacuum Einstein equations outside the symmetry axis. This family was recently obtained by performing a complete gauge fixing adapted to cylindrical symmetry. In the present work, we find boundary conditions that ensure that the metric arising from this gauge fixing is well defined and that the resulting reduced system has a consistent Hamiltonian dynamics. These boundary conditions must be imposed both on the symmetry axis and in the region far from the axis at spacelike infinity. Employing such conditions, we determine the asymptotic behaviour of the metric close to and far from the axis. In each of these regions, the approximate metric describes a conical geometry with a time dislocation. In particular, around the symmetry axis the effect of the singularity consists in inducing a constant deficit angle and a timelike helical structure. Based on these results and on the fact that the degrees of freedom in our family of metrics coincide with those of cylindrical vacuum gravity, we argue that the analysed set of spacetimes represent cylindrical gravitational waves surrounding a spinning cosmic string. For any of these spacetimes, a prediction of our analysis is that the wave content increases the deficit angle at spatial infinity with respect to that detected around the axis.