Cosmological magnetic fields from primordial helical seeds

Most early universe scenarios predict negligible magnetic fields on cosmological scales if they are unprocessed during subsequent expansion of the universe. We present a new numerical treatment of the evolution of primordial fields and apply it to weakly helical seeds as they occur in certain early universe scenarios. If seed fields created during the electroweak phase transition have close to thermal strength and coherence lengths a few orders of magnitude below the horizon scale, initial helicities not much larger than the baryon to photon number can lead to fields of ~10^-13G at scales up to 100 parsec today.