Exclusive interview with Italian nuclear physicist Viviana Mossa about her team’s astonishing new deuterium-burning results
In 1948, George Gamow and Ralph Alpher predicted that the bulk of the helium in the universe was created during the hot Big Bang by means of a multi-stage process, involving, as one of the steps, elemental hydrogen (a single proton) and its isotope deuterium (a proton plus a neutron). Big Bang nucleosynthesis (BBN) was an astonishing prediction, that turned out to be right on the mark.
Similarly astounding is the precise way protons fuse with deuterium to form helium-3 (two protons plus one neutron), an energetic process called deuterium burning. Such a process happens routinely in the cores of stars, as well as in the Big Bang. There is a window of energy, called the Gamow peak, within which a proton might bash into a deuterium nucleus, cross its Coulomb barrier (electric repulsion barrier due to both nuclei being positive, and stick together. As brilliantly shown by Gamow, the process involves “quantum tunneling:” a shortcut through an otherwise insurmountable barrier. Too slow, and the protons wouldn’t be able to tunnel through. Too fast, and they would evade being trapped. The speed has to be just right for breaching the barrier and fusing.
How do we know this basic picture is correct, with regards to Big Bang nucleosynthesis? One way is to count the photons produced in the cosmic microwave background radiation, as mapped by Planck, WMAP, and other satellites. That data, in turn, tells us the baryon count in the early universe (baryons include protons and neutrons). Another means is to test the process directly in a particle collision lab. The lab has to be shielded from earth’s environment to avoid counting cosmic rays and other stray particles that have nothing to do with the experiment.
With that aim, the Laboratory for Underground Nuclear Astrophysics (LUNA) established itself deep underground, beneath Gran Sasso mountain in Italy. Accessed via a highway tunnel, the underground lab offers near-perfect shielding from cosmic-ray particles. In the experiment, protons slam into deuterons (deuterium nuclei) to produce helium-3, releasing gamma rays in the process, which are counted by detectors.
In November 2020, the LUNA team, led by Dr. Viviana Mossa, announced spectacular results, published in Nature, confirming the basic picture of Big Bang nucleosynthesis with unprecedented precision. The findings offer a giant step forward in our understanding of the early universe.
Mossa received her PhD from the University of Bari (her city of birth) in 2018, and is currently a fixed-term researcher at the University of Foggia. She graciously agreed to be interviewed about her team’s groundbreaking research.
Paul Halpern: Do you consider yourself a nuclear physicist, an astrophysicist, or someone who works at the intersection of those fields?
Viviana Mossa: Let’s say that I was born as a nuclear physicist and I have spent a lot of time studying astrophysics.
PH: How did you first become interested in science? Did your family, friends, and teachers encourage you?
VM: During my high school years there were two subjects that impassioned me: literature and physics. Both fascinating, but one the opposite of the other. In the end, physics won.
PH: How did you first learn about the conditions of the Big Bang? Did you ever read any of the popular works of George Gamow, who first proposed Big Bang nucleosynthesis? Did you read books such as “The First Three Minutes” that discuss the early universe?
VM: Actually during my classes I never delved into the subject. My interest was born shortly before I started my PhD when I was fascinated by the fact that all the matter we know, and of which we are made of, originates in the stars. We could say that we are children of the stars!
PH: What is it like to work in Gran Sasso, in a laboratory deep inside a mountain? Is it claustrophobic to be deep underground? What is the purpose of choosing such a location?
VM: I think that working in the INFN Gran Sasso National Laboratory (LNGS) is a great opportunity because it is one of the largest underground laboratory in the world. When you cross the doorway for the first time you are struck by the size of each single hall, so even if you have no windows on the external world you cannot feel claustrophobic. The 1400 meters of rock protecting the experimental halls from external radiation act as a natural shielding, putting the lab in a kind of “cosmic silence”, where , the LUNA experiment is able to recreate the processes that occurred during primordial nucleosynthesis and that still occur in stars today.
PH: There are many accelerators around the world that have much higher energies than the one you are using, such as the LHC, for example. Why is the accelerator you use ideal for deuterium burning experiments?
VM: LUNA aims to study the thermonuclear fusion reactions that take place during the Big Bang and in the core of the stars where the elements making up matter are produced. At typical astrophysical temperatures, nuclear fusion reactions take place in a narrow energy window, called the Gamow peak, often far below the Coulomb repulsion barrier. So in order to recreates in the laboratory the energy of nuclei at the center of stars we need energies from tens to several hundred keV. Consequently, the energies reached by the LUNA 400 kV accelerator are suitable for performing astrophysical measurements. In particular, referring to the measurement related to the primordial deuterium abundance, the BBN energy range (Ecm ≈ 30−300 keV) is well covered by our accelerator. Let me anticipate that LUNA experiment will continue its scientific activity over the next decade with the LUNA-MV project, focused on the study of key reactions important to understand the chemical composition of the universe and the nucleosynthesis of the heavy elements requiring higher proton energies.
PH: How are the conditions of your experiment similar to those of the Big Bang? How do they differ?
VM:The aim of the LUNA experiment was to measure with high precision the cross section of the D(p,gamma)3He reaction to the energies of interest for the BBN. For this reason we have exploited the full dynamic range of the 440 kV accelerator, corresponding to Ecm ~30–300 keV and thus covering a broad energy region around Ecm=130 keV, where the predicted abundance of primordial deuterium is most sensitive to the D(p,gamma) reaction
PH: Whenever there is an experiment simulating Big Bang conditions, sometimes a few nervous members of the public warn of the danger of creating an explosion, or even something like a new universe. How would you re-assure them?
VM: The fear can probably arise from not knowing actually what is happening in a lab. We have simply hit a gas target of deuterium at appropriate pressure and temperature with a beam of protons having an energy within the BBN range. The study of the reaction in which a deuterium nucleus and a proton combine to yield a helium-3 nucleus and a photon allowed us to achieve the important cosmological results published in Nature. No explosions, no new universes, only gamma rays detected by means of a High Purity Germanium detector!
PH: How would you explain “primordial baryon density” to the general public?
VM: We can interpret it as the density of the ordinary matter that consists of neutrons and protons, essentially anything in the periodic table.
PH. How did your results match predictions found by analysing the cosmic microwave background?
VM: Until now, among the reactions involved in the production and destruction of deuterium during BBN, the deuterium-burning D(p,gamma)3He reaction has the largest uncertainty and limits the prediction of theoretical estimates of primordial deuterium abundance. At LUNA we have measured with unprecedented precision this reaction cross section allowing thus to refine the calculations of the primordial nucleosynthesis and to obtain an accurate determination of the density of baryonic matter. 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. This estimation at the 1.6 percent level is in excellent agreement with the value derived independently from CMB measurements. This agreement represents an important goal for the basic theoretical framework of cosmology because it ensures that the use of the known laws of physics together with the observations of the Universe can move backwards the movie of the cosmos to when the Universe was just one second old and show that it started with a hot Big Bang.
PH: How do your results support the general picture that most of the helium in the universe was made in the Big Bang, rather than stars?
VM: When BBN started, the Universe was a hot soup of particles in which neutrons and protons were swarmed by photons and neutrinos. With the Universe expansion and cooling, neutrons and protons combined forming at fist a heavy isotope of hydrogen, the deuterium. It was then transformed by a series of reactions into 3He and 4He nuclei. After about three minutes, the Universe consisted of about 75% ordinary hydrogen nuclei and 25% 4He, with deuterium, 3He and 7Li present only in residual parts. The Big Bang was thus the origin of the two most abundant elements in the Universe (hydrogen and helium), and made only light elements. Cosmologists and astronomers observe the light elements in the Universe and infer their primordial abundances, to test the theoretical models of BBN. Such observations have confirmed that the primordial abundance of 4He was 25%.
PH: What would you say to someone who believed that the Big Bang didn’t exist, and the universe lasted forever?
VM: Good question. I can only say that trying to gain knowledge of the universe is an ambitious task. Science tries to contribute brick by brick by sharing with the whole community its victories and failures, but always
ready to justify, demonstrate and motivate every single statement. We have theorized the Big Bang model, we are performing a lot of calculations, we are making many measurements and at the moment no experimental evidence seems to disagree. Only the debate with those who have different ideas and opinions from yours allow you to grow. What is important is to be always able to motivate everything with study and work.
PH: What is the future direction for research about primordial nucleosynthesis?
VM: The results we obtained in primordial deuterium abundance estimation allow a sharper determination of the baryonic content of the Universe, about 4% of the total density today. We can infer that the remaining 96% consists of invisible dark matter and dark energy, the identities of which are unknown. If these dark components influenced the abundances of light elements produced during BBN, they will also need to be accounted for correctly in BBN models to ensure that predictions from those models agree with observations. Studies of BBN can therefore help to inform theories of the dark side of the Universe.