A reaction that occurred seconds after the Big Bang has been measured with unprecedented precision below the Italian mountain of Gran Sasso. The measure has been fed into a theoretical model that allows to compute a key cosmologic parameter, the density of ‘ordinary’ (baryonic) matter.
The result is consistent with an independent measure of the same parameter, extracted from the Cosmic Microwave Background (CMB). This means that the standard cosmologic model is sufficient to explain those early processes: there is no need for new physics, despite suggestions by some scientists.
Around one second after the Big Bang, the nuclei of the lightest elements started to form, a process called Big Bang Nucleosynthesis (BBN). Astronomic observations allow highly precise estimation of the abundance of deuterium, the first nucleus to be formed. This abundance is a key parameter of the theoretical model describing the BBN. From it, scientists can calculate the baryonic density.
However, this procedure yields a much more imprecise estimate of baryonic density as compared to CMB-based calculations. Until now, there was great uncertainty surrounding deuterium burning, a reaction in which a deuterium nucleus and a proton combine to form a helium nucleus. This process could be reproduced in surface laboratories, but its signal would be flooded with cosmic rays.
Now, researchers have managed to measure deuterium burning with the LUNA (Laboratory for Underground Nuclear Physics) particle accelerator at Laboratori Nazionali del Gran Sasso, a large underground facility where the mountain’s mass shields experiments from cosmic rays. The experiment, published in Nature1, provides a better estimate of the rate of the reaction.
The authors then fed this parameter in a BBN model and calculated baryon density, cutting the previous uncertainty by half and obtaining a value compatible with CMB-based estimates.
“It’s a real triumph for the standard model,” says Max Pettini, an astronomer at the University of Cambridge, who works on deuterium abundance and was not involved in the experiment.
Mossa, V, et al, Nature 587, 210–213 (2020).