Searching for primordial magnetism with multi-frequency CMB experiments

Bounds on the amplitude of a scale-invariant stochastic primordial magnetic field (PMF) can be significantly improved by measurements of the Faraday Rotation (FR) of CMB polarization. The mode-coupling correlations induced by FR make it possible to extract it from cross-correlations of the B-mode polarization with the E-mode and the temperature anisotropy. In this paper, we construct an estimator of the rotation measure that appropriately combines measurements of the FR from multiple frequency channels. We study the dependence of the signal-to-noise in the PMF detection on the resolution and the noise of the detectors, as well as the removal of the weak lensing contribution and the galactic FR. We show that a recently proposed space-based experiment PRISM can detect magnetic fields of 0.1 nano-Gauss strength at a 2 sigma level. Higher detection levels can be achieved by reducing the detector noise and improving the resolution or increasing the number of channels in the 30-70 GHz frequency range.Comment: 6 pages, 1 figure, 1 table; references added, Figure and the caption improved; matches the version accepted to MNRA

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

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.Comment: 5 pages, 3 figures; a reference added; matches the version published in Phys Rev Let

Interaction of Magnetic Monopoles and Domain Walls

We study the interaction of magnetic monopoles and domain walls in a model with SU(5)\times Z_2 symmetry by numerically evolving the field equations. We find that the monopoles unwind and dissipate their magnetic energy on collision with domain walls within which the full SU(5) symmetry is restored.Comment: 4 pages; included additional degree of freedom in ansatz (eq. 4), discussed time and distance scales; version to be published in PR