Updated BBN Constraints on Ultralight Scalar Dark Matter

Ultralight scalars are an attractive dark matter candidate.  We consider the effect of such a dark matter candidate on predictions of Big Bang Nucleosynthesis. As a first study, we look at an ultralight scalar with universal couplings to all massive Standard Model particles. One striking consequence of this new coupling is a change in the predicted abundances of the light elements. In this talk, I will present updated constraints on ultralight scalar dark matter from Helium-4, taking into account the effect of the back-reaction of the Standard Model particles on the dark sector. This effect was neglected in previous works but has noticeable effects. Furthermore, I will present previously uncalculated constraints from deuterium, as well as a comparison to other probes for ultralight scalar dark matter.