Gravitational Effects of Varying Alpha Strings

TL;DR

This paper investigates how cosmic strings can cause spatial variations in the fine-structure constant and analyzes their gravitational effects, particle trajectories, and cosmological constraints within a scalar-tensor framework.

Contribution

It provides an analytical model of $eta$-varying cosmic strings, combining global and local string effects, and explores their observational implications.

Findings

Photon trajectories are scattered to infinity by $eta$-varying strings.

Massive particles can be trapped in bounded orbits around the strings.

Constraints on $eta$ variation are derived from cosmological and Equivalence Principle considerations.

Abstract

We study spatial variations of the fine-structure constant in the presence of static straight cosmic strings in the weak-field approximation in Einstein gravity. We work in the context of a generic Bekenstein-type model and consider a gauge kinetic function linear in the scalar field. We determine an analytical form for the scalar field and the string metric at large distances from the core. We show that the gravitational effects of $\alpha$-varying strings can be seen as a combination of the gravitational effects of global and local strings. We also verify that at large distances to the core the space-time metric is similar to that of a global string. We study the motion of test particles approaching from infinity and show that photons are scattered to infinity while massive particles are trapped in bounded trajectories. We also calculate an overall limit on the magnitude of the variation of $\alpha$ for a GUT string, by considering suitable cosmological constraints coming from the Equivalence Principle.