Relieving the Hubble tension with primordial magnetic fields
  The standard cosmological model determined from the accurate cosmic microwave
background measurements made by the Planck satellite implies a value of the
Hubble constant $H_0$ that is $4.2$ standard deviations lower than the one
determined from Type Ia supernovae. The Planck best fit model also predicts
higher values of the matter density fraction $\Omega_m$ and clustering
amplitude $S_8$ compared to those obtained from the Dark Energy Survey Year 1
data. Here we show that accounting for the enhanced recombination rate due to
additional small-scale inhomogeneities in the baryon density may solve both the
$H_0$ and the $S_8-\Omega_m$ tensions. The additional baryon inhomogeneities
can be induced by primordial magnetic fields present in the plasma prior to
recombination. The required field strength to solve the Hubble tension is just
what is needed to explain the existence of galactic, cluster, and extragalactic
magnetic fields without relying on dynamo amplification. Our results show clear
evidence for this effect and motivate further detailed studies of primordial
magnetic fields, setting several well-defined targets for future observations.
