The local B-polarization of the CMB: a very sensitive probe of cosmic defects

TL;DR

This paper proposes that local B-polarization of the CMB is a highly sensitive method for detecting cosmic defects, surpassing previous techniques and potentially constraining defect models with unprecedented precision.

Contribution

It introduces a new signature in local B-polarization for cosmic defects, showing it is more sensitive than previous methods and can significantly improve constraints on defect models.

Findings

Local B-polarization signal exceeds inflationary gravitational wave signals for small defect contributions.

Upcoming experiments can detect or tightly constrain cosmic defects with high sensitivity.

Planck and future missions will improve defect constraints by at least an order of magnitude.

Abstract

We present a new and especially powerful signature of cosmic strings and other topological or non-topological defects in the polarization of the cosmic microwave background (CMB). We show that even if defects contribute 1% or less in the CMB temperature anisotropy spectrum, their signature in the local $\tilde{B}$-polarization correlation function at angular scales of tens of arc minutes is much larger than that due to gravitational waves from inflation, even if the latter contribute with a ratio as big as $r\simeq 0.1$ to the temperature anisotropies. We show that when going from non-local to local $\tilde{B}$-polarization, the ratio of the defect signal-to-noise with respect to the inflationary value increases by about an order of magnitude. Proposed B-polarization experiments, with a good sensitivity on arcminute scales, may either detect a contribution from topological defects produced after inflation or place stringent limits on them. Even Planck should be able to improve present constraints on defect models by at least an order of magnitude, to the level of $\ep <10^{-7}$. A future full-sky experiment like CMBpol, with polarization sensitivities of the order of $1\mu$K-arcmin, will be able to constrain the defect parameter $\ep=Gv^2$ to a few $\times10^{-9}$, depending on the defect model.