Semiclassical models for uniform-density Cosmic Strings and Relativistic Stars

TL;DR

This paper investigates quantum corrections in semiclassical gravity models of cosmic strings and stars, finding that non-perturbative solutions are necessary as perturbative approaches fail to produce consistent results.

Contribution

It demonstrates the limitations of perturbative solutions in semiclassical models and provides explicit non-perturbative solutions for cosmic strings, highlighting the importance of non-perturbative methods.

Findings

Perturbative solutions are inconsistent for uniform-density cosmic strings and stars.

Explicit approximate metrics for straight cosmic strings are derived.

Non-local quantum terms can potentially enable perturbative solutions.

Abstract

In this paper we show how quantum corrections, although perturbatively small, may play an important role in the analysis of the existence of some classical models. This, in fact, appears to be the case of static, uniform--density models of the interior metric of cosmic strings and neutron stars. We consider the fourth order semiclassical equations and first look for perturbative solutions in the coupling constants $\alpha$ and $\beta$ of the quadratic curvature terms in the effective gravitational Lagrangian. We find that there is not a consistent solution; neither for strings nor for spherical stars. We then look for non--perturbative solutions and find an explicit approximate metric for the case of straight cosmic strings. We finally analyse the contribution of the non--local terms to the renormalized energy--momentum tensor and the possibility of this terms to allow for a perturbative solution. We explicitly build up a particular renormalized energy--momentum tensor to fulfill that end. These state--dependent corrections are found by simple considerations of symmetry, conservation law and trace anomaly, and are chosen to compensate for the local terms. However, they are not only ad hoc, but have to depend on $\alpha$ and $\beta$, what is not expected to first perturbative order. We then conclude that non--perturbative solutions are valuable for describing certain physical situations.