Can small-scale baryon inhomogeneities resolve the Hubble tension? An investigation with ACT DR4

The authors analyze an appealing attempt to resolve the Hubble tension that modifies the physics around the time of recombination by small-scale baryon density inhomogeneities (e.g. by primordial magnetic fields). Using the most recent data of the Atacama Cosmology Telescope (ACT), they find that baryon clumping on the CMB is severely constrained such that this mechanism most likely does not solve the Hubble tension. However, a time-dependent version of that scenario could evade these strong constraints.