Null Cosmic Strings: Scattering by Black Holes, Optics and Spacetime Content

TL;DR

This paper investigates the behavior of null cosmic strings near black holes, analyzing their scattering, optical properties, and influence on black hole angular momentum within the weak field approximation.

Contribution

It provides exact solutions for null string equations of motion near black holes and introduces a stress-energy tensor to study string-induced black hole recoil and angular momentum changes.

Findings

Derived exact equations of motion for null strings near black holes.

Showed how scattering affects string momenta and black hole angular momentum.

Connected string energy evolution with optical scalar Z and gravitational wave parameters.

Abstract

Equations of motion of null cosmic strings near black holes, or other massive sources, are solved exactly in the weak field approximation. The stress-energy tensor of a null string in a curved spacetime is introduced and used to show how scattering by black holes transforms linear and angular momenta of the string. The corresponding recoil effect of a black hole and change of its angular momentum caused by a null cosmic string are calculated. For a null string, its energy $\mu$ per unit length evolves along the null direction of the string trajectory. The evolution of $\mu$ is connected with a string optical scalar $Z$. Optical properties of null strings are that their energy is concentrated on caustics, where $Z$ has poles. String parameters $\mu$ and $Z$ capture important features of the spacetime where strings move. Explicit dependence of $\mu$ and $Z$ on the strain and Bondi news tensors of gravitational wave background, mass and angular momentum aspects are established, near the future null infinity, up to the 4-th order in expansion in an inverse null parameter in asymptotically flat spacetimes.