Constraining Axion Dark Matter with Big Bang Nucleosynthesis

Understand

We show that Big Bang Nucleosynthesis (BBN) significantly constrains axion-like dark matter.

• The axion acts like an oscillating QCD $\theta$ angle that redshifts in the early universe, increasing the neutron-proton mass difference at neutron freeze-out.
• An axion-like particle that couples too strongly to QCD results in the underproduction of 4He during BBN and is thus excluded.
• The BBN bound overlaps with much of the parameter space that would be covered by proposed searches for time-varying neutron EDMs.