A New Picture of Cosmic String Evolution and Anisotropic Stochastic Gravitational-Wave Background

TL;DR

This paper explores how cosmic strings formed during early universe symmetry breaking produce anisotropic gravitational-wave backgrounds, with frequency-dependent anisotropies that could be detected by future pulsar timing arrays.

Contribution

It introduces a model of cosmic string evolution influenced by primordial fluctuations, predicting large anisotropies in the SGWB and their frequency dependence, which can explain recent observational hints.

Findings

SGWB anisotropies depend on frequency and are detectable in multiband GW observations.

Cosmic strings can explain the common-spectrum process observed by NANOGrav.

Predicted angular power spectrum at large scales is within reach of upcoming pulsar timing array experiments.

Abstract

We investigate the anisotropies of the stochastic gravitational-wave background (SGWB) produced by cosmic strings associated with the spontaneous U(1) symmetry breaking of Grand Unified Theory, which happens at the onset of inflation. The string network evolution is determined by primordial fluctuations and never reaches the scaling regime. The string loops are inhomogeneously distributed in large scale regions, resulting in large anisotropies in the SGWB. We find that the angular power spectrum of SGWB anisotropies depends on frequency, which is testable in multiband observations of GWs. In particular, GWs from the cosmic strings can appropriately interpret the common-spectrum process reported by NANOGrav collaboration, and the angular power spectrum in the nanohertz band, $\mathtt{l}(\mathtt{l}+1)C_{\mathtt{l}}=5.6\times 10^{-2}$ at large scales, is expected to be detectable by pulsar timing array experiments in the near future.