As space expanded, temperatures dropped below those required to sustain fusion, and as a result nucleosynthesis only lasted for about three minutes. A third of the atomic hydrogen was converted into helium and no elements heavier than lithium could synthesize.

On the nucleosynthesis of lithium, beryllium, and boron

The lightest elements (hydrogen, helium, deuterium, lithium) were produced in the Big Bang nucleosynthesis. According to the Big Bang theory, the temperatures in the early universe were so high that fusion reactions could take place. This resulted in the formation of light elements: hydrogen, deuterium, helium (two isotopes), lithium and trace amounts of beryllium.

Nucleosynthesis of lithium. Homework Academic Service

Big‐Bang Nucleosynthesis: lithium problems and scalar‐tensor theories of gravity

With these new results, what is known as the "lithium problem" remains a hard nut to crack: on the one hand, now all laboratory results of the astrophysicists suggest that the theory of primordial nucleosynthesis is correct. On the other hand, many observations of astronomers show that the oldest stars in our Milky Way contain only half as much lithium-7 as predicted. Sensational reports by Swedish researchers, who discovered clearly more lithium-6 in such stars than predicted, must also likely be checked again based on the new LUNA data. Bemmerer says, "Should unusual lithium concentrations be observed in the future, we know, thanks to the new measurements, that it cannot be due to the primordial nucleosynthesis."