Researchers explain some of the lithium that is missing in our universe
PICTURE: AS A BERYLLIUM RAY COMES IN FROM THE LEFT, THE DEUTERON TROJAN HORSE CATCHES IT AT THE GOAL AND DELIVERS ITS NEUTRON SOLDIERS. THIS ALLOWS THE EXPIRED PRODUCTS OF… show more CREDIT: © 2021 HAYAKAWA ET AL.
There is a significant discrepancy between theoretical and observed amounts of lithium in our universe. This is known as the cosmological lithium problem and has preoccupied cosmologists for decades. Thanks to a new experiment on the nuclear processes responsible for lithium formation, researchers have now been able to reduce this discrepancy by around 10%. This research could point the way to a more complete understanding of the early universe.
A famous saying goes: “Theory, theory and practice are the same. In practice, they are not. ”This is true in any academic field, but most often in cosmology, the study of the entire universe where what we think we should see and what we really see are not always fits together. This is mainly because many cosmological phenomena are difficult to study due to their inaccessibility. Cosmological phenomena are usually inaccessible to us because of the extreme distances, or they often occurred before the human brain had even developed to take care of them – as is the case with the Big Bang.
Project assistant Seiya Hayakawa and lecturer Hidetoshi Yamaguchi from the Center for Nuclear Study at the University of Tokyo and her international team are particularly interested in an area of cosmology where theory and observation are very misaligned, and that is the problem of the lack of lithium, the cosmological lithium problem (CLP). In short, the theory predicts that in the minutes after the Big Bang, which created all of the matter in the cosmos, an amount of lithium should be about three times what we actually observe. However, Hayakawa and his team have explained some of this discrepancy, paving the way for research that may one day resolve it entirely.
“13.7 billion years ago, when matter merged from the energy of the Big Bang, common light elements that we all know – hydrogen, helium, lithium and beryllium – were created in a process we call Big Bang nucleosynthesis (BBN)”, said Hayakawa. “However, BBN is not a simple chain of events in which one thing successively becomes another; it is actually a complex network of processes in which a jumble of protons and neutrons builds up atomic nuclei and some of these break down into other nuclei. For example, the abundance of one form of lithium or isotope – lithium-7 – results primarily from the production and decay of beryllium-7. But it has either been overestimated in theory, under-observed in reality, or a combination of both. That has to be resolved in order to really understand what happened back then. “
Lithium-7 is the most common isotope of lithium and accounts for 92.5% of all isotopes observed. However, although the accepted BBN models predict the relative amounts of all elements involved in BBN with extreme accuracy, the expected amount of lithium-7 is about three times greater than that actually observed. This means that our knowledge of the formation of the early universe has a void. There are several theoretical and observational approaches aimed at solving this, but Hayakawa and his team simulated the conditions during the BBN using particle beams, detectors, and an observation method known as the Trojan horse.
“We examined one of the BBN reactions, in which beryllium-7 and a neutron decay into lithium-7 and a proton, more closely than ever. The resulting amount of lithium-7 was slightly lower than expected, about 10% lower, ”said Hayakawa. “This is a very difficult reaction to observe because beryllium-7 and neutrons are unstable. So we used deuteron, a hydrogen nucleus with an extra neutron, as a vessel to smuggle a neutron into a beryllium-7 beam without disturbing it. This is a unique technique developed by an Italian group we work with, where the deuteron is like the Trojan horse in Greek myth and the neutron is the soldier who sneaks into the impregnable city of Troy without the guards to tilt (the sample). Thanks to the new experimental result, we can offer future theoretical researchers a slightly less daunting task when trying to solve the CLP. “
S. Hayakawa, M. La Cognata, L. Lamia, H. Yamaguchi, D. Kahl, K. Abe, H. Shimizu, L. Yang, O. Beliuskina, SM Cha, KY Chae, S. Cherubini, P. Figuera , Z. Ge, M. Gulino, J. Hu, A. Inoue, N. Iwasa, A. Kim, D. Kim, G. Kiss, S. Kubono, M. La Commara, M. Lattuada, EJ Lee, JY Moon, S. Palmerini, C. Parascandolo, SY Park, VH Phong, D. Pierroutsakou, RG Pizzone, GG Rapisarda, S. Romano, C. Spitaleri, XD Tang, O. Trippella, A. Tumino and NT Zhang, “Restriction of the original lithium abundance: New cross-sectional measurement of the 7Be + n reactions updates the total 7Be destruction rate “Astrophysical Journal Letters,
This work was supported by JSPS KAKENHI (Grant Nos. 15K17631, 18K13556, and 19K03883). KYC and SMC were supported by the National Research Foundation of Korea (Nos. 2020R1I1A1A01065120, 2020R1A2C1005981, 2019K2A9A2A10018827, and 2016R1A5A1013277). DK would like to thank the UK STFC for the support. GGK thanks the NKFIH (NN128072) and the Janos Bolyai Research Fellowship of the Hungarian Academy of Sciences and the UNKP-20-5-DE-2 New National Excellence Program of the Hungarian Ministry of Human Resources. The authors thank Finanziamenti di Linea 2 and Starting Grant 2020 from the University of Catania.
Center for Nuclear Studies
National Institute of Nuclear Physics – Southern National Laboratories
Faculty of Physics, Sungkyunkwan University