Updated fundamental constant constraints from Planck 2018 data and possible relations to the Hubble tension

TL;DR

This study updates constraints on fundamental constants during recombination using Planck 2018 data, revealing potential links between electron mass variations and the Hubble tension.

Contribution

It provides new bounds on the variation of fundamental constants and explores their possible role in resolving the Hubble tension.

Findings

Constraints on $eta_{ m EM}$ tighten with Planck 2018 data.

CMB data alone suggests $m_e$ below local value.

Including BAO and supernova data aligns $m_e$ with fiducial and increases $H_0$.

Abstract

We present updated constraints on the variation of the fine structure constant, $\alpha_{\rm EM}$, and effective electron rest mass, $m_{\rm e}$, during the cosmological recombination era. These two fundamental constants directly affect the ionization history at redshift $z\simeq 1100$ and thus modify the temperature and polarisation anisotropies of the cosmic microwave background (CMB) measured precisely with {\it Planck }. The constraints on $\alpha_{\rm EM}$ tighten slightly due to improved {\it Planck} 2018 polarisation data but otherwise remain similar to previous CMB analysis. However, a comparison with the 2015 constraints reveals a mildly discordant behaviour for $m_{\rm e}$, which from CMB data alone is found below its local value. Adding baryon acoustic oscillation data brings $m_{\rm e}$ back to the fiducial value, $m_{\rm e}=(1.0078\pm0.0067) m_{\rm e,0}$, and also drives the Hubble parameter to $H_0=69.1\pm 1.2$ [in units of ${\rm km \, s^{-1} \, Mpc^{-1} }$]. Further adding supernova data yields $m_{\rm e}=(1.0190\pm0.0055) m_{\rm e,0}$ with $H_0=71.24\pm0.96$. We perform several comparative analyses using the latest cosmological recombination calculations to further understand the various effects. Our results indicate that a single-parameter extension allowing for a slightly increased value of $m_{\rm e}$ ($\simeq 3.5\sigma$ above $m_{\rm e,0}$) could play a role in the Hubble tension.