A generic $\omega_b$ tension in early-time solutions to the Hubble tension

TL;DR

Early-time solutions to the Hubble tension tend to increase the baryon density, conflicting with primordial deuterium constraints and worsening overall data fits compared to Lambda-CDM.

Contribution

The paper demonstrates that generic early-time solutions to the Hubble tension are in tension with Big Bang Nucleosynthesis constraints and degrade fit quality.

Findings

Early-time solutions increase preferred baryon density.

Including primordial deuterium data worsens model fits.

Models struggle to reconcile high H0 with BBN constraints.

Abstract

I show that early-time (pre-recombination) solutions to the Hubble tension are generically expected to increase the preferred baryon density $\omega_b$. This puts these models in tension with Big Bang Nucleosynthesis (BBN), as measurements of primordial deuterium constrain $\omega_b$ at percent level. I show that existing analyses are in tension with the BBN determination of $\omega_b$, and that including a likelihood component for primordial deuterium deters two representative models from recovering a high $H_0$, and leads to worse fits to CMB, BAO, supernova, and BBN data than $\Lambda$CDM.