BBN constraints on dark radiation isocurvature

Peter Adshead, Gilbert Holder, Pranjal Ralegankar

Research output: Contribution to journalArticlepeer-review


The existence of dark radiation that is completely decoupled from the standard model in the early Universe leaves open the possibility of an associated dark radiation isocurvature mode. We show that the presence of dark radiation isocurvature leads to spatial variation in the primordial abundances of helium and deuterium due to spatial variation in Neff during Big Bang nucleosynthesis. We use the result to constrain the existence of such an isocurvature mode on scales down to ∼ 1 Mpc scales. By measuring the excess variance in the primordial helium to hydrogen and deuterium to hydrogen ratio in different galaxies, we constrain the variance in average isocurvature in a galaxy to be less than 0.13/Δ eff at 95% confidence. Here Δ eff is the spatially averaged increase in Neff due to the additional dark radiation component.

Original languageEnglish (US)
Article number016
JournalJournal of Cosmology and Astroparticle Physics
Issue number9
StatePublished - Sep 2020


  • big bang nucleosynthesis
  • cosmology of theories beyond the SM
  • initial conditions and eternal universe
  • physics of the early universe

ASJC Scopus subject areas

  • Astronomy and Astrophysics

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