Testing the early universe from deep underground

Precise deep-underground measurement of a key nuclear reaction reveals properties of the universe in its infancy.

There is a key process – in the sequence of reactions known as Big Bang Nucleosynthesis – responsible for the destruction of deuterium soon after it has been created: it is a reaction in which a proton and a nucleus of deuterium fuse together to form a stable isotope of helium.

This reaction has now been studied with unprecedented precision at LUNA (Laboratory for Underground Nuclear Astrophysics), at the Gran Sasso National Laboratories of the INFN (National Institute for Nuclear Physics). Thanks to this study, it has been possible to refine theoretical calculations of primordial nucleosynthesis and to obtain an accurate determination of the density of ordinary (or 'baryonic') matter, which makes up everything we know of, including living species.

The results of the measurement conducted by the LUNA Collaboration, and their cosmological impact, were published in the journal Nature.

Gianluca Imbriani, spokesperson for the LUNA collaboration explains:

In this particular study, in addition to our longstanding expertise in the field of experimental nuclear astrophysics, we have benefited from the precious contribution of the theoretical group of astroparticle physics and theoretical cosmology of the Federico II University of Naples, to arrive at an accurate determination of the baryon density using the Parthenope code for primordial nucleosynthesis calculations. An important contribution to the description of nuclear interaction was also provided by the theoretical nuclear physics group of the University of Pisa.

About 3 minutes after the Big Bang, the temperature of the universe dropped to one billion degrees and deuterium could finally be produced by the fusion of protons and neutrons. The abundance of deuterium left over after the Big Bang carries precious information about the amount of ordinary matter and radiation permeating the Universe in its infancy. Based on experimental data of unprecedented precision obtained at LUNA (Italy), we inferred a value of matter density now in much better agreement with that derived from the relic Cosmic Microwave Background radiation still observed today. When combined with other astrophysical inputs, our results further place constraints on the amount of dark radiation not foreseen by the standard cosmological model.

In the cosmic silence of the underground Laboratories of Gran Sasso, where 1400 m of rock protect the experimental halls from external radiation, the LUNA experiment is able to recreate the processes that occurred during primordial nucleosynthesis and to bring the clock back in time to a few minutes after the birth of our Universe.

Professor Marialuisa Aliotta , from the School of Physics and Astronomy, who worked on the project said:

The fact that the theoretical primordial deuterium abundance inferred in our study is now in excellent agreement with astronomical observations provides further evidence for the validity of the standard cosmological model. This work marks the culmination of many years of intense experimental activity and represents an outstanding achievement for the entire collaboration.

LUNA is an international collaboration of about 50 scientists from Italy, Germany, Hungary and the United Kingdom. The list of collaborating institutions includes: the National Laboratories of Gran Sasso, the INFN sections and the Universities of Bari, Genoa, Milano Statale, Naples Federico II, Padua, Rome La Sapienza, Turin, and the INAF Observatory of Teramo (Italy); the Helmholtz-Zentrum Dresden-Rossendorf (Germany), the MTA-ATOMKI in Debrecen and the Konkoli Observatory of Budapest (Hungary); and the School of Physics and Astronomy of the University of Edinburgh (United Kingdom).