Did BICEP2 see vector modes? First B-mode constraints on cosmic defects

TL;DR

This work uses the recently released BICEP2 and POLARBEAR B-mode polarization spectra to constrain properties of a wide range of different types of cosmic strings networks and finds that in order for strings to provide a satisfactory fit on their own, the effective interstring distance needs to be extremely large.

Abstract

Scaling networks of cosmic defects, such as strings and textures, actively generate scalar, vector and tensor metric perturbations throughout the history of the universe. In particular, {\em vector} modes sourced by defects are an efficient source of the CMB B-mode polarization. We use the recently released BICEP2 and POLARBEAR B-mode polarization spectra to constrain properties of a wide range of different types of cosmic strings networks. We find that in order for strings to provide a satisfactory fit on their own, the effective inter-string distance needs to be extremely large -- spectra that fit the data best are more representative of global strings and textures. When a local string contribution is considered together with the inflationary B-mode spectrum, the fit is improved. We discuss implications of these results for theories that predict cosmic defects.

Figures (3)

• Figure 1: The thick blue long dashed line is the best fit lensing+strings model ($r$=0), with the thin blue long dashed line showing the corresponding string contribution alone. The thick red short dash is the best fit lensing+strings+inflation model ($r$=0.15), with the corresponding string contribution plotted as a thin red short dashed line. The lensing contribution is shown separately with a thin black dot-dashed line. The BICEP2 best fit inflationary model ($r$=0.2) contribution is shown with a thin black dotted line, and the solid thin black line is the sum of $r$=0.2 and lensing contributions. The circles show the band powers measured by BICEP2 and the triangles are the POLARBEAR data (the third band is negative with its absolute value plotted as an inverted triangle).
• Figure 2: Marginalized likelihoods derived from the BICEP2 and POLARBEAR data for the scalar-to-tensor ratio $r$, the strength of the string contribution $f_{10}$, the inter-string distance $\xi$, and the RMS velocity $v$. The red dotted lines are for the lensing+strings+inflation model, while the blue solid lines are for the lensing+strings fit only.
• Figure 3: The marginalized joint likelihood for the tensor-to-scalar ratio $r$ and the strength of the string contribution $f_{10}$. The two different shades indicate the $68$% and the $95$% confidence regions. The vertical dashed line indicates the approximate bound on $f_{10}$ from Planck.