Graviton production in the scaling of a long-cosmic-string network

TL;DR

This paper explores how the transition of a cosmic-string network during the early universe affects gravitational wave production and the resulting power spectrum, with implications for observational cosmology.

Contribution

It introduces a detailed analysis of graviton creation during the transition to scaling of cosmic strings and its impact on the gravitational wave spectrum.

Findings

Discontinuous changes in scalar curvature trigger graviton creation.

The gravitational wave power spectrum is distorted by cosmic string dynamics.

Transition effects could be observable in future gravitational wave data.

Abstract

In a previous paper [1] we considered the possibility that (within the early-radiation epoch) there has been (also) a short period of a significant presence of cosmic strings. During this radiation-plus-strings stage the Universe matter-energy content can be modelled by a two-component fluid, consisting of radiation (dominant) and a cosmic-string fluid (subdominant). It was found that, during this stage, the cosmological gravitational waves (CGWs) - that had been produced in an earlier (inflationary) epoch - with comoving wave-numbers below a critical value (which depends on the physics of the cosmic-string network) were filtered, leading to a distorsion in the expected (scale-invariant) CGW power spectrum. In any case, the cosmological evolution gradually results in the scaling of any long-cosmic-string network and, hence, after a short time-interval, the Universe enters into the late-radiation era. However, along the transition from an early-radiation epoch to the late-radiation era through the radiation-plus-strings stage, the time-dependence of the cosmological scale factor is modified, something that leads to a discontinuous change of the corresponding scalar curvature, which, in turn, triggers the quantum-mechanical creation of gravitons. In this paper we discuss several aspects of such a process, and, in particular, the observational consequences on the expected gravitational-wave (GW) power spectrum.