A Signature of Cosmic Strings Wakes in the CMB Polarization

TL;DR

This paper predicts a distinctive polarization signature in the CMB caused by cosmic string wakes, which could be detectable at degree scales if strings have a tension near current upper limits.

Contribution

It introduces a novel polarization signature in the CMB caused by cosmic string wakes, detailing its dependence on string properties and ionization conditions.

Findings

Polarization signal can reach 0.06 μK at degree scales for high-redshift strings.

Signature is strongest for wakes formed near recombination.

The polarization pattern is scale-invariant and depends on ionization fraction.

Abstract

We calculate a signature of cosmic strings in the polarization of the cosmic microwave background (CMB). We find that ionization in the wakes behind moving strings gives rise to extra polarization in a set of rectangular patches in the sky whose length distribution is scale-invariant. The length of an individual patch is set by the co-moving Hubble radius at the time the string is perturbing the CMB. The polarization signal is largest for string wakes produced at the earliest post-recombination time, and for an alignment in which the photons cross the wake close to the time the wake is created. The maximal amplitude of the polarization relative to the temperature quadrupole is set by the overdensity of free electrons inside a wake which depends on the ionization fraction $f$ inside the wake. The signal can be as high as $0.06 {\rm \mu K}$ in degree scale polarization for a string at high redshift (near recombination) and a string tension $\mu$ given by $G \mu = 10^{-7}$.