UC Berkeley

I show that early-time (pre-recombination) solutions to the Hubble tension are generically expected to increase the preferred baryon density ωb . This puts these models in tension with Big Bang Nucleosynthesis (BBN), as measurements of primordial deuterium constrain ωb at percent level. I show that existing analyses are in tension with the BBN determination of ωb , and that including a likelihood component for primordial deuterium deters two representative models from recovering a high H0 .