Constraints on the spectral signatures of superconducting cosmic strings

TL;DR

This paper investigates the electromagnetic energy injection from superconducting cosmic strings and updates constraints on their properties using CMB spectral distortions, ionization history, and 21cm signals, highlighting the potential of future measurements.

Contribution

It provides a refined analytic formalism and comprehensive analysis combining spectral distortions, ionization history, and 21cm signals to constrain superconducting cosmic string models.

Findings

CMB spectral distortions set strong constraints on string parameters.

Future PIXIE-like missions could explore new parameter regions.

Soft photon heating significantly affects 21cm constraints.

Abstract

If they exist, networks of superconducting cosmic strings are capable of injecting copious amounts of electromagnetic energy into the background over a broad range of frequencies. We study this injection both analytically, as well as numerically using the thermalization code CosmoTherm. With our refined analytic formalism, we update constraints from CMB spectral distortions by following the injection of entropy, as well as energy, on the amplitude of the $\mu$-distortion, leading to a significant improvement in those limits. Furthermore, we utilize the full shape of the distorted spectrum from CosmoTherm to include constraints from non-$\mu$, non-$y$ type distortions. Additionally, we use the outputs for the ionization history and global 21cm signal to derive and update constraints on string model parameters using measurements from other datasets. Analysis of CMB anisotropies provides the most stringent constraints, though with a slightly modified shape and strength when compared to previous results. Modifications of the reionization history provide new bounds in the high current domain, and we also find that the observations of the low-frequency radio background probe a small region of parameter space not explored by other datasets. We also analyze global $21$-cm constraints, and find that the inclusion of soft photon heating plays a crucial role, essentially removing any constraints in the considered parameter domain. Spectral distortion measurements from COBE/FIRAS are covered by other constraints, but our conservative forecast shows that a PIXIE-type satellite would probe important unexplored regions of parameter space.