Inferences of $H_0$ in presence of a non-standard recombination

Understand

Measurements of the Hubble parameter from the distance ladder are in tension with indirect measurements based on the cosmic microwave background (CMB) data and the inverse distance ladder measurements at 3-4 $\sigma$ level.

• We consider phenomenological modification to the timing and width of the recombination process and show that they can significantly affect this tension.
• This possibility is appealing, because such modification affects both the distance to the last scattering surface and the calibration of the baryon acoustic oscillations (BAO) ruler.
• Moreover, because only a very small fraction of the most energetic photons keep the early universe in the plasma state, it is possible that such modification could occur without affecting the energy density budget of the universe or being incompatible with the very tight limits on the departure from the black-body spectrum of CMB.