Eur. Phys. J. C (2015) 75: 60
https://doi.org/10.1140/epjc/s10052-015-3287-y

Regular Article - Theoretical Physics

Cosmic strings in gravity

¹ Department of Mathematics, University College London, Gower Street, London, WC1E 6BT, UK
² The Institute for Fundamental Study, “The Tah Poe Academia Institute”, Naresuan University, Phitsanulok, 65000, Thailand
³ Thailand Center of Excellence in Physics, Ministry of Education, Bangkok, 10400, Thailand

^* e-mail: matthewj@nu.ac.th

Received: 18 November 2014
Accepted: 24 January 2015
Published online: 7 February 2015

Abstract

We consider Kasner-type static, cylindrically symmetric interior string solutions in the theory of modified gravity. The physical properties of the string are described by an anisotropic energy-momentum tensor satisfying the condition ; that is, the energy density of the string along the -axis is equal to minus the string tension. As a first step in our study we obtain the gravitational field equations in the theory for a general static, cylindrically symmetric metric, and then for a Kasner-type metric, in which the metric tensor components have a power law dependence on the radial coordinate . String solutions in two particular modified gravity models are investigated in detail. The first is the so-called “exponential” modified gravity, in which the gravitational action is proportional to the exponential of the sum of the Ricci scalar and matter Lagrangian, and the second is the “self-consistent model”, obtained by explicitly determining the gravitational action from the field equations under the assumption of a power law dependent matter Lagrangian. In each case, the thermodynamic parameters of the string, as well as the precise form of the matter Lagrangian, are explicitly obtained.