Jacques Wagstaff (Hamburg University, Germany)
We calculate the CMB spectral distortions due to the decay of causally generated magnetic fields at the electroweak and QCD phase transitions. We show that the decay of non-helical magnetic fields generated at either the electroweak or QCD scale produce μ and y-type distortions below 10^−8 which are probably not detectable by a future PIXIE-like experiment. We show that magnetic fields generated at the electroweak scale must have a helicity fraction f∗>10^−4 in order to produce detectable μ-type distortions. Hence a positive detection coming from the decay of magnetic fields would rule out non-helical primordial magnetic fields and provide a lower bound on the magnetic helicity.