Impact of a primordial magnetic field on cosmic microwave background \(B\) modes with weak lensing

We present results from an analysis of all data taken by the BICEP2 & Keck Array CMB polarization experiments up to and including the 2014 observing season. This includes the first Keck Array observations at 95 GHz. The maps reach a depth of 50 nK deg in Stokes \(Q\) and \(U\) in the 150 GHz band and 127 nK deg in the 95 GHz band. We take auto- and cross-spectra between these maps and publicly available maps from WMAP and Planck at frequencies from 23 GHz to 353 GHz. An excess over lensed-LCDM is detected at modest significance in the 95x150 \(BB\) spectrum, and is consistent with the dust contribution expected from our previous work. No significant evidence for synchrotron emission is found in spectra such as 23x95, or for correlation between the dust and synchrotron sky patterns in spectra such as 23x353. We take the likelihood of all the spectra for a multi-component model including lensed-LCDM, dust, synchrotron and a possible contribution from inflationary gravitational waves (as parametrized by the tensor-to-scalar ratio \(r\)), using priors on the frequency spectral behaviors of dust and synchrotron emission from previous analyses of WMAP and Planck data in other regions of the sky. This analysis yields an upper limit \(r_{0.05}<0.09\) at 95% confidence, which is robust to variations explored in analysis and priors. Combining these \(B\)-mode results with the (more model-dependent) constraints from Planck analysis of CMB temperature plus BAO and other data, yields a combined limit \(r_{0.05}<0.07\) at 95% confidence. These are the strongest constraints to date on inflationary gravitational waves.

Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound \(B\ge 3\times 10^{-16}\)~gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma-ray in intergalactic medium. The bound improves as \(\lambda_B^{-1/2}\) if magnetic field correlation length, \(\lambda_B\), is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.