Measurement at Big Bang Conditions Confirms Lithium Problem

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The field of astrophysics has a stubborn problem
and it’s called lithium. The quantities of lithium
predicted to have resulted from the Big Bang are
not actually present in stars. But the calculations
are correct – a fact which has now been
confirmed for the first time in experiments
conducted at the underground laboratory in the
Gran Sasso mountain in Italy. As part of an
international team, researchers from the
Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
studied how much lithium forms under Big Bang
conditions. The results were published in
“Physical Review Letters ”
Lithium, aside from hydrogen and helium, is one
of the three elements that are created before the
first stars form. These three elements were –
according to the theory – already created early
on, through what is known as “primordial
nucleosynthesis.” That means that when the
universe was only a few minutes old, neutrons
and protons merged to form the nuclei of the
these elements. At the Laboratory for
Underground Nuclear Astrophysics (LUNA), the
nucleosynthesis of lithium has now been
reproduced by an international team of scientists.
Michael Anders, who earned his doctorate in the
last year at TU Dresden and HZDR on this very
topic, took a leading role on the team. Within the
framework of a project that was funded by the
German Research Foundation, he was supervised
by Dr. Daniel Bemmerer, group leader at HZDR.
In the Italian underground laboratory, the
scientists fired helium nuclei at heavy hydrogen
(known as deuterium) in order to reach energies
similar to those just after the Big Bang. The idea
was to measure how much lithium forms under
similar conditions to those during the early stages
of the universe. The result of the experiment: the
data confirmed the theoretical predictions, which
are incompatible with the observed lithium
concentrations found in the universe.
“For the first time, we could actually study the
lithium-6 production in one part of the Big Bang
energy range with our experiment," explains
Daniel Bemmerer. Lithium-6 (three neutrons,
three protons) is one of the element’s two stable
isotopes. The formation of lithium-7, which
possesses an additional neutron, was studied in
2006 by Bemmerer at LUNA.
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.“
Further research will soon be carried out in a new
underground laboratory in Dresden
What was important for the studies was the
special location of LUNA: in the mountainous
Gran Sasso d’Italia, 1400 meters of solid rock
keep the disturbance from cosmic radiation at
bay. The experimental setup is additionally
enveloped in a lead shell. Only with such good
shielding can the rare interactions between the
nuclei be precisely determined. But within the next
year, similar research will also be possible in
Dresden. TU Dresden and HZDR will put the
accelerator laboratory “Felsenkeller” into
operation. Although the solid rock shielding from
natural radiation in this former brewery cellar is
only forty-five meters, it is already sufficient for
many measurements. The new laboratory also
possesses a particle accelerator that is more than
twelve times as strong: “There we can expand our
experiments and study the formation of elements
at high energy ranges”, says Bemmerer.
Lithium ist neben Wasserstoff und Helium eines
der drei Elemente, die nicht erst innerhalb von
Sternen erzeugt werden. Stattdessen – so die
Theorie – sind sie schon früh durch die
„primordiale Nukleosynthese“ entstanden. Das
heißt: Im nur wenige Minuten alten Universum
haben sich Neutronen und Protonen zu den
Kernen der ersten drei Elemente verbunden. Am
Laboratory for Underground Nuclear Astrophysics
(LUNA) wurde die Kernentstehung von Lithium
nun von einem internationalen Forscherteam
nachgestellt. Eine führende Rolle im Team nahm
Michael Anders ein, der im vergangenen Jahr an
der TU Dresden und am HZDR zu dem Thema
promoviert hat. Im Rahmen eines von der
Deutschen Forschungsgemeinschaft geförderten
Projekts wurde er dabei von Dr. Daniel Bemmerer,
Gruppenleiter am HZDR, betreut.