The 21 cm Signature of Cosmic String Wakes

TL;DR

This paper explores how cosmic string wakes produce distinctive 21cm signals in high-redshift surveys, revealing potential observational signatures that depend on string tension and redshift, with implications for detecting cosmic strings.

Contribution

It introduces the specific 21cm signature of cosmic string wakes, detailing how tension and redshift influence the observable emission or absorption signals in 21cm surveys.

Findings

Cosmic string wakes generate wedge-shaped 21cm signals with brightness approaching 400 mK at high redshifts.

The critical string tension for detectable emission signals depends on redshift, decreasing linearly with increasing redshift.

For tensions below the critical value, cosmic strings produce observable absorption signals with similar wedge geometry.

Abstract

We discuss the signature of a cosmic string wake in 21cm redshift surveys. Since 21cm surveys probe higher redshifts than optical large-scale structure surveys, the signatures of cosmic strings are more manifest in 21cm maps than they are in optical galaxy surveys. We find that, provided the tension of the cosmic string exceeds a critical value (which depends on both the redshift when the string wake is created and the redshift of observation), a cosmic string wake will generate an emission signal with a brightness temperature which approaches a limiting value which at a redshift of $z + 1 = 30$ is close to 400 mK in the limit of large string tension. The signal will have a specific signature in position space: the excess 21cm radiation will be confined to a wedge-shaped region whose tip corresponds to the position of the string, whose planar dimensions are set by the planar dimensions of the string wake, and whose thickness (in redshift direction) depends on the string tension. For wakes created at $z_i + 1 = 10^3$, then at a redshift of $z + 1 = 30$ the critical value of the string tension $\mu$ is $G \mu = 6 \times 10^{-7}$, and it decreases linearly with redshift (for wakes created at the time of equal matter and radiation, the critical value is a factor of two lower at the same redshift). For smaller tensions, cosmic strings lead to an observable absorption signal with the same wedge geometry.