B-modes from Cosmic Strings

TL;DR

Cosmic strings can produce B-mode polarization in the CMB with a spectrum that may dominate over inflationary signals, offering a way to probe post-inflationary physics or constrain string tensions.

Contribution

This paper analyzes the physical mechanisms and model dependence of B-modes generated by cosmic strings, highlighting their potential observational signatures.

Findings

String-sourced B-modes can be more prominent than inflationary ones.

Detection would probe GUT-scale physics.

Non-detection constrains cosmic string tension to Gμ < 10^{-7}.

Abstract

Detecting the parity-odd, or B-mode, polarization pattern in the cosmic microwave background radiation due to primordial gravity waves is considered to be the final observational key to confirming the inflationary paradigm. The search for viable models of inflation from particle physics and string theory has (re)discovered another source for B-modes: cosmic strings. Strings naturally generate as much vector mode perturbation as they do scalar, producing B-mode polarization with a spectrum distinct from that expected from inflation itself. In a large set of models, B-modes arising from cosmic strings are more prominent than those expected from primordial gravity waves. In light of this, we study the physical underpinnings of string-sourced B-modes and the model dependence of the amplitude and shape of the $C_l^{BB}$ power spectrum. Observational detection of a string-sourced B-mode spectrum would be a direct probe of post-inflationary physics near the GUT scale. Conversely, non-detection would put an upper limit on a possible cosmic string tension of G\mu < 10^{-7} within the next three years.