Detectability of nGauss primordial magnetic fields in the B-mode
  polarization of CMB
  The origin of large-scale magnetic fields in the Universe is still unknown.
Observations suggest the presence of nGauss primordial magnetic fields at the
last scattering surface. The presence of such fields will affect the evolution
of cosmological perturbations and potentially leave an imprint on the CMB
anisotropies. Here, we show that the B-mode power spectrum carries a clear
signature of the stochastic primordial magnetic fields up to a few nGauss.
Specifically, the presence of nGauss primordial magnetic fields changes the BB
power spectrum at all scales. At large scales, the tensor modes contribute to
the B-mode spectrum, while non-vanishing vector modes contribute at small
scales. We show that the B-mode of CMB carry a distinct signature of the
primordial magnetic fields. To validate our prediction, we use BICEP2 and
POLARBEAR data and find that B-mode observations from the experiments are
consistent with non-zero primordial fields. We also use the BICEP2 observations
to constrain the primordial magnetic field. We provide a detailed analysis of
the B-mode polarization of the CMB with primordial magnetic fields and
investigate the implications for the upcoming CMB missions.
