CMB anisotropies from primordial inhomogeneous magnetic fields
Abstract
Primordial inhomogeneous magnetic fields of the right strength can leave a signature on the cosmic microwave background (CMB) temperature anisotropy and polarization. Potentially observable contributions to polarization B modes are generated by vorticity and gravitational waves sourced by the magnetic anisotropic stress. We compute the corresponding CMB transfer functions in detail, including the effect of neutrinos. The shear rapidly causes the neutrino anisotropic stress to cancel the stress from the magnetic field, suppressing the production of gravitational waves and vorticity on superhorizon scales after neutrino decoupling. A significant large scale signal from tensor modes can only be produced before neutrino decoupling, and the actual amplitude is somewhat uncertain. Plausible values suggest primordial nearly scale invariant fields with B_{λ}∼10^{10} G today may be observable from their large scale tensor anisotropy. They can be distinguished from primordial gravitational waves by their nonGaussianity. Vector mode vorticity sources Bmode power on much smaller scales with a power spectrum somewhat similar to that expected from weak lensing, suggesting amplitudes B_{λ}∼10^{9} G may be observable on small scales for a spectral index n∼2.9. In the Appendix we review the covariant equations for computing the vector and tensor CMB power spectra that we implement numerically.
 Publication:

Physical Review D
 Pub Date:
 August 2004
 DOI:
 10.1103/PhysRevD.70.043011
 arXiv:
 arXiv:astroph/0406096
 Bibcode:
 2004PhRvD..70d3011L
 Keywords:

 98.70.Vc;
 04.30.Db;
 04.40.Nr;
 Background radiations;
 Wave generation and sources;
 EinsteinMaxwell spacetimes spacetimes with fluids radiation or classical fields;
 Astrophysics;
 High Energy Physics  Phenomenology
 EPrint:
 Minor changes to match PRD accepted version. Numerical code for vector mode power spectra available at http://camb.info