Some attempts of easing the critical Hubble tension present in modern cosmology have resorted to using variations of fundamental constants, such as the fine-structure constant, at the time of recombination. In this article we demonstrate that there are critical hurdles to construct such viable models using scalar fields, due to the striking precision of local constraints on the fine-structure constant stability. These hurdles demonstrate that in single-field models one has to extremely fine-tune the shape of the potential and/or the initial conditions. Indeed, for single-field models in a potential that is not fine-tuned we can put a generic bound at recombination of $\mathrm{\Delta }\alpha /\alpha <5\times {10}^{-4}$ (95% CL).

• Received 5 March 2024
• Accepted 11 April 2024

DOI:https://doi.org/10.1103/PhysRevD.109.103520