Planck 2015 results. XIX. Constraints on primordial magnetic fields
Abstract
We compute and investigate four types of imprint of a stochastic background of primordial magnetic fields (PMFs) on the cosmic microwave background (CMB) anisotropies: the impact of PMFs on the CMB temperature and polarization spectra, which is related to their contribution to cosmological perturbations; the effect on CMB polarization induced by Faraday rotation; the impact of PMFs on the ionization history; magneticallyinduced nonGaussianities and related nonzero bispectra; and the magneticallyinduced breaking of statistical isotropy. We present constraints on the amplitude of PMFs that are derived from different Planck data products, depending on the specific effect that is being analysed. Overall, Planck data constrain the amplitude of PMFs to less than a few nanoGauss, with different bounds that depend on the considered model. In particular, individual limits coming from the analysis of the CMB angular power spectra, using the Planck likelihood, are B_{1 Mpc} < 4.4 nG (where B_{1 Mpc} is the comoving field amplitude at a scale of 1 Mpc) at 95% confidence level, assuming zero helicity. By considering the Planck likelihood, based only on parityeven angular power spectra, we obtain B_{1 Mpc} < 5.6 nG for a maximally helical field. For nearly scaleinvariant PMFs we obtain B_{1 Mpc} < 2.0 nG and B_{1 Mpc} < 0.9 nG if the impact of PMFs on the ionization history of the Universe is included in the analysis. From the analysis of magneticallyinduced nonGaussianity, we obtain three different values, corresponding to three applied methods, all below 5 nG. The constraint from the magneticallyinduced passivetensor bispectrum is B_{1 Mpc} < 2.8 nG. A search for preferred directions in the magneticallyinduced passive bispectrum yields B_{1 Mpc} < 4.5 nG, whereas the compensatedscalar bispectrum gives B_{1 Mpc} < 3 nG. The analysis of the Faraday rotation of CMB polarization by PMFs uses the Planck power spectra in EE and BB at 70 GHz and gives B_{1 Mpc} < 1380 nG. In our final analysis, we consider the harmonicspace correlations produced by Alfvén waves, finding no significant evidence for the presence of these waves. Together, these results comprise a comprehensive set of constraints on possible PMFs with Planck data.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 September 2016
 DOI:
 10.1051/00046361/201525821
 arXiv:
 arXiv:1502.01594
 Bibcode:
 2016A&A...594A..19P
 Keywords:

 magnetic fields;
 cosmic background radiation;
 early Universe;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 29 pages, 15 figures. Revised following referee report