Early recombination as a solution to the H0 tension

We show that the H-0 tension can be resolved by making recombination occur earlier, keeping the fit to cosmic microwave background (CMB) data almost intact. We provide a suite of general necessary conditions to give a good fit to CMB data while realizing a high value of H-0 suggested by local measurements. As a concrete example for a successful scenario with early recombination, we demonstrate that a model with a time-varying m(e) can indeed satisfy all of the conditions. We further show that such a model can also be well fitted to low-z distance measurements of baryon acoustic oscillations (BAO) and type Ia supernovae (SNeIa) with a simple extension of the model. A time-varying m(e) in the framework of Omega(k)Lambda CDM is found to be a sufficient and excellent example of a solution to the H-0 tension, yielding H-0 = 72.3(-)(2.8)(+2.7) km/sec /Mpc from the combination of CMB, BAO, and SNeIa data even without incorporating any direct local H-0 measurements. Employing the Bayesian posterior predictive distribution, we find that this model can reduce the H-0 tension in the reference ACDM model from 4.8 sigma down to 2.2 sigma. Apart from the H-0 tension, this model is also favored from the viewpoint of the CMB lensing anomaly.

Automated summary

This study shows that resolving the H-0 tension by early recombination while fitting well to CMB data is possible. Models with time-varying m(e) can satisfy conditions and fit low-z distance measurements effectively. Time-varying m(e) in the framework of Omega(k)Lambda CDM provides a sufficient solution to the H-0 tension, reducing it significantly from 4.8 sigma to 2.2 sigma.