Signatures of primordial helicity in the CMBR
  We have studied ways in which helical primordial magnetic fields could be constrained by measurements of the cosmic microwave background radiation (CMBR). If there were helical flows in the primordial plasma at the time of recombination, they would produce parity violating temperature-polarization correlations ($C_l^{TB}$ and $C_l^{EB}$). However, the magnitude of helical flows induced by helical magnetic fields is unobservably small. We discuss an alternate scheme for extracting the helicity of a stochastically homogeneous and isotropic primordial magnetic field using Faraday rotation measure maps of the CMBR and the power spectrum of B-type polarization ($C_l^{BB}$). 
Levon Pogosian, Tanmay Vachaspati and Serge Winitzki