Comparing early dark energy and extra radiation solutions to the Hubble tension with BBN

A shorter sound horizon scale at the recombination epoch, arising from introducing extra energy components such as extra radiation or early dark energy (EDE), is a simple approach to resolving the socalled Hubble tension. We compare EDE models, an extra radiation model, and a model in which EDE and extra radiation coexist, paying attention to the fit to big bang nucleosynthesis (BBN). We find that the fit to BBN in EDE models is somewhat poorer than that in the Lambda CDM model, because the increased inferred baryon asymmetry leads to a smaller deuterium abundance. We find that an extra radiation-EDE coexistence model gives the largest present Hubble parameter H-0 among the models studied. We also examine the differences between the results obtained with and without consideration of the BBN. The difference in the extra radiation model is 3.22 < N-eff < 3.49 (68%) for data sets without BBN and 3.16 < N-eff < 3.40 (68%) for data sets with BBN, which is so large that the 1s border of the larger side becomes the 2 sigma border.

Automated summary

Introducing extra energy components can help alleviate the “Hubble tension,” but there are still some shortcomings in terms of BBN fit, with the extra radiation-EDE coexistence model giving the largest present Hubble parameter H0.