Princeton University Astroplasmas Seminar

It has been suggested that the weak magnetic field hosted by the intergalactic medium (IGM) in voids is a relic from the early Universe. If so, the modern-day strength and coherence length of such fields could be “predicted” from reasonable assumptions about the properties that the primordial field had at its genesis, provided the evolution of the field in the intervening time were understood. Previously held theories based on magnetogenesis at the electroweak phase transition (EWPT) indicated that the primordial field would have decayed too quickly to be consistent with the observational constraints on modern fields. Thus, the “relic-field” hypothesis appeared unlikely. However, recent numerical developments have shown that these decay theories are flawed: they do not predict the “inverse transfer” of magnetic energy to larger scales that has been observed in simulations. In my talk, I present a new theory of the decay based on the conservation of the “fluctuation level” of magnetic helicity, with dynamics controlled by the reconnection of magnetic-field lines. I show that this theory explains the “inverse-transfer” phenomenon, and predicts a slower decay of primordial fields, thus restoring consistency between the relic-field hypothesis and the observational constraints. Finally, I show that the theory I present is robust, in that, unlike previous models, it does not depend on the large-scale asymptotic of the magnetic-energy spectrum at magnetogenesis.