Cosmic Microwave Background Spectral Distortions from Cosmic String Loops

TL;DR

This paper investigates how cosmic string loops emit photon bursts from cusps, potentially causing spectral distortions in the CMB, and assesses future observational constraints on string tensions.

Contribution

It calculates the photon energy release from cosmic string cusps and predicts future CMB spectral distortion constraints on string tensions.

Findings

Current constraints are weak for string tension limits.

Future missions like PIXIE can exclude certain string tension ranges.

Intermediate-scale cosmic strings could be ruled out by upcoming observations.

Abstract

Cosmic string loops contain cusps which decay by emitting bursts of particles. A significant fraction of the released energy is in the form of photons. These photons are injected non-thermally and can hence cause spectral distortions of the Cosmic Microwave Background (CMB). Under the assumption that cusps are robust against gravitational back-reaction, we compute the fractional energy density released as photons in the redshift interval where such non-thermal photon injection causes CMB spectral distortions. Whereas current constraints on such spectral distortions are not strong enough to constrain the string tension, future missions such as the PIXIE experiment will be able to provide limits which rule out a range of string tensions between $G \mu \sim 10^{-15}$ and $G \mu \sim 10^{-12}$, thus ruling out particle physics models yielding these kind of intermediate-scale cosmic strings.