The abundance of deuterium produced soon after the Big Bang carries precious information about the amount of ordinary matter and radiation permeating the Universe in its infancy.

In our study, we report on a new evaluation of the baryon density based on Big Bang Nucleosynthesis (BBN), now in much better agreement with that derived by PLANCK from the relic Cosmic Microwave Background. We further place constraints on the amount of dark radiation, with broad implications for the standard cosmological model. Our findings were obtained by exploiting the negligible cosmic-ray background deep underground at LUNA.

We bombarded a high-purity deuterium gas target with an intense proton beam from the LUNA 400-kV accelerator and detected the γ rays from the D(p,γ)³He nuclear reaction with a high-purity germanium detector. Our experimental results settle the most uncertain nuclear physics input to BBN calculations and substantially improve the reliability of using primordial abundances as probes of the physics of the early Universe.

The full paper is available here.