Cosmic-void observations reconciled with primordial magnetogenesis
  It has been suggested that the weak magnetic field hosted by the
intergalactic medium (IGM) in voids could be a relic from the early Universe.
However, accepted models of turbulent magnetohydrodynamic (MHD) decay predict
that the present-day strength of fields generated at the electroweak phase
transition (EWPT) should be too low to explain the observed scattering of
$\gamma$-rays from TeV blazars. Here, we apply recent theoretical developments
-- namely, the discovery of the Saffman helicity invariant and of the role of
magnetic reconnection in setting the decay rate -- to show that the accepted
models greatly underpredict the present-day strength of relic fields. Our
estimates restore the consistency of the EWPT-relic hypothesis with
observational constraints; moreover, we find that efficient magnetogenesis at
the EWPT could produce relics with the strength that is believed sufficient to
resolve the Hubble tension and explain galaxy-cluster magnetic fields without
invoking dynamo amplification.
