Kahniashvili, TinaMaravin, YuriiLavrelashvili, GeorgeKosowsky, Arthur2015-04-092015-04-092014-10-14http://hdl.handle.net/2097/18921If a primordial magnetic field in the Universe has nonzero helicity, the violation of parity symmetry results in nonzero correlations between cosmic microwave background temperature and B-mode polarization. In this paper we derive approximations to the relevant microwave background power spectra arising from a helical magnetic field. Using the cross-power spectrum between temperature and B-mode polarization from the WMAP nine-year data, we set a 95% confidence level upper limit on the helicity amplitude to be 10  nG² Gpc for helicity spectral index nH=−1.9, for a cosmological magnetic field with effective field strength of 3 nG and a power-law index nB=−2.9 near the scale-invariant value. Future microwave background polarization maps with greater sensitivity will be able to detect the helicity of an inflationary magnetic field well below the maximum value allowed by microwave background constraints on the magnetic field amplitude.en-USThis Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).Magnetic fieldMicrowave backgroundArticle (publisher version)