Consistency of Planck, ACT and SPT constraints on magnetically assisted recombination and forecasts for future experiments

Understand

Primordial magnetic fields can change the recombination history of the universe by inducing clumping in the baryon density at small scales.

• They were recently proposed as a candidate model to relieve the Hubble tension.
• We investigate the consistency of the constraints on a clumping factor parameter $b$ in a simplistic model, using the latest CMB data from Planck, ACT DR4 and SPT-3G 2018.
• For the combined CMB data alone, we find no evidence for clumping being different from zero, though when adding a prior on $H_0$ based on the latest distance-ladder analysis of the SH0ES team, we report a weak detection of $b$.