The need to refine the standard recombination theory and the Hubble tension problem

TL;DR

This paper suggests that refining the standard recombination theory by including hydrogen atom excitation processes can resolve the Hubble tension between Planck data and local measurements.

Contribution

It introduces a modification to the standard recombination theory, accounting for hydrogen excitation, which aligns cosmological parameter estimates with local Hubble constant measurements.

Findings

Recombination rate is higher than standard predictions.

Including hydrogen excitation reduces the Hubble tension.

Results improve consistency between Planck and local Hubble measurements.

Abstract

Over the past decades, cosmology has become largely based on experimental data, the most important sources of which are studies of the cosmic microwave background (CMB). CMB is present in the Universe since the very first moments of its existence, and the features of CMB recorded now reflect the fundamental processes of the evolution of the Universe. These processes cause a weak anisotropy of CMB, which is no more than 0.01 percent. The structure of anisotropy, which is interpreted on the basis of a sufficiently detailed standard theory of primary recombination, allows us to establish the most important cosmological parameters. One of these parameters is the Hubble constant (more precisely, the Hubble parameter). However, the difference between the value of the Hubble constant, obtained from the results of the Planck mission, and obtained from independent local measurements, is 10%. Such a big difference is called the Hubble tension problem, and is an important problem of cosmology. The interpretation of the Planck measurements (as well as the interpretation of WMAP results, where a close value of the Hubble constant is obtained) is critically based on the predictions of the standard recombination theory. This work shows that the recombination rate is higher than predicted by standard theory and is caused by the excitation of the kinetic degrees of freedom of hydrogen atoms, which is ignored by standard theory. The calculations performed demonstrate that taking into account this process leads to a good coincidence of the values of the Hubble constant obtained from the results of the Planck experiment and local measurements.