Imprints of cosmic strings on the cosmological gravitational wave background

TL;DR

This paper investigates how cosmic strings influence the evolution of gravitational waves in the universe, leading to distortions in the gravitational wave spectrum across a wide frequency range due to their effect on the effective potential.

Contribution

It introduces a model of gravitational wave evolution in a universe with cosmic strings, showing how non-oscillatory modes affect the current gravitational wave spectrum.

Findings

Cosmic strings cause a distortion in the gravitational wave spectrum.

The spectrum deviation occurs in the frequency range 10^{-16} Hz to 10^5 Hz.

Abstract

The equation which governs the temporal evolution of a gravitational wave (GW) in curved space-time can be treated as the Schrodinger equation for a particle moving in the presence of an effective potential. When GWs propagate in an expanding Universe with constant effective potential, there is a critical value (k_c) of the comoving wave-number which discriminates the metric perturbations into oscillating (k > k_c) and non-oscillating (k < k_c) modes. As a consequence, if the non-oscillatory modes are outside the horizon they do not freeze out. The effective potential is reduced to a non-vanishing constant in a cosmological model which is driven by a two-component fluid, consisting of radiation (dominant) and cosmic strings (sub-dominant). It is known that the cosmological evolution gradually results in the scaling of a cosmic-string network and, therefore, after some time (\Dl \ta) the Universe becomes radiation-dominated. The evolution of the non-oscillatory GW modes during \Dl \ta (while they were outside the horizon), results in the distortion of the GW power spectrum from what it is anticipated in a pure radiation-model, at present-time frequencies in the range 10^{-16} Hz < f < 10^5 Hz.