Can topological defects mimic the BICEP2 B-mode signal?

TL;DR

The paper investigates whether topological defects could mimic the B-mode polarization signal observed by BICEP2. Focusing on cosmic strings in the Abelian Higgs model (and other defect classes), it compares their B-mode spectra to BICEP2 and Planck TT constraints, finding that defects alone cannot reproduce the data without violating temperature anisotropy limits, since matching the low-$\ell$ signal would demand $f_{10}$ values ($\sim 0.3$) that overshoot at higher multipoles and exceed $f_{10}\lesssim 0.03$–$0.055$. A modest admixture of defects on top of inflationary tensors (for example $f_{10}\sim 0.04$ with $r\approx 0.15$) can slightly improve the BICEP2 fit around $\ell\simeq 200$, but larger defect contributions are disfavored and the TT constraints remain competitive. The study thus rules out defects as the sole source of the BICEP2 signal while highlighting a limited role for defects in lowering the inferred $r$ and improving the overall fit, pending more detailed numerical analyses and future data.

Abstract

We show that the B-mode polarization signal detected at low multipoles by BICEP2 cannot be entirely due to topological defects. This would be incompatible with the high-multipole B-mode polarization data and also with existing temperature anisotropy data. Adding cosmic strings to a model with tensors, we find that B-modes on their own provide a comparable limit on the defects to that already coming from Planck satellite temperature data. We note that strings at this limit give a modest improvement to the best-fit of the B-mode data, at a somewhat lower tensor-to-scalar ratio of $r \simeq 0.15$.

Figures (4)

• Figure 1: The CMB temperature and polarization power spectra contributions from inflationary scalar modes (black solid), inflationary tensor modes (black dashed), and cosmic strings (blue dot-dashed) Bevis:2007qz. The inflationary tensors have $r=0.2$ while the string contribution has $f_{10}=0.03$.
• Figure 2: B-mode polarization power spectra for textures (solid red), semilocal strings (dashed black), and Abelian Higgs strings (dot-dash blue). All the curves are normalized to make the temperature spectra match the Planck$\ell=10$ value. We see that all these types of topological defects predict similar shapes in the BICEP2 data range $30 \lesssim \ell \lesssim 300$, though they become different for $\ell>300$.
• Figure 3: Temperature (upper panel) and B-mode polarization (lower panel) power spectra compared to the Planck temperature and the BICEP2 B-mode polarization data. The black curve in the upper panel is the best-fit $\Lambda$CDM model and the blue dashed lines show the contribution from strings for $f_{10} = 0.3$, $0.15$, $0.06$, and $0.03$. The green-dotted curves in the lower panel show the combined contribution from strings and the lensing of the scalar perturbations, for the same values of $f_{10}$ as in the upper panel. The lowest dotted curve, for $f_{10} = 0.03$, shows roughly the maximal allowed contribution from strings to the temperature power spectrum, given the Planck data. The highest dotted curve, $f_{10} = 0.3$, matches the BICEP2 B-mode polarization at $\ell = 80$. The grey dashed line is the sum of the $f_{10} = 0.3$ string prediction with the Planck best-fit $\Lambda$CDM model. The thin solid red line in the lower panel shows the combined contribution from the lensing of scalar perturbations and textures, normalized to match the $\ell=80$ BICEP2 data point.
• Figure 4: A contribution from strings (blue dot-dashed) is added to the prediction from $r = 0.15$ plus scalar lensing (solid black) to give a total spectrum shown in grey. The data points are from BICEP2. From bottom to top the string fractions are 0.015, 0.03, 0.04 (highlighted in red), and 0.06. A marginal improvement to the overall data fit is given for a string fraction around 0.04, which is about the maximum permitted by current constraints from Planck .