Distinguishing Primordial Magnetic Fields from Inflationary Tensor
  Perturbations in the Cosmic Microwave Background
  A claimed detection of cosmological tensor perturbations from inflation via
B-mode polarization of the cosmic microwave background requires distinguishing
other possible B-mode sources. One such potential source of confusion is
primordial magnetic fields. For sufficiently low-amplitude B-mode signals, the
microwave background temperature and polarization power spectra from power-law
tensor perturbations and from a power-law primordial magnetic field are
indistinguishable. However, we show that such a magnetic field will induce a
small-scale Faraday rotation which is detectable using four-point statistics
analogous to gravitational lensing of the microwave background. The Faraday
rotation signal will distinguish a magnetic-field induced B-mode polarization
signal from tensor perturbations for effective tensor-scalar ratios larger than
0.001, detectable in upcoming polarization experiments.
