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4.4 Nuclear Astrophysics
look at the range of facilities which will be needed in
future years to address the key questions in each of the
areas of nucleosynthesis.
Nuclear astrophysics does not, however, simply involve
the measurement of reactions. In some cases technical
or physics considerations mean we cannot accelerate
the nuclei we need, or the reaction rates are too low for
us to measure. In addition, effects due to finite temperature
and the presence of the stellar plasma cannot be
reproduced in the laboratory. In some astrophysical sites
the evolution is determined by enormous networks of
reactions – too many for us to measure – while in other
sites (e.g. neutron stars) the extreme conditions prevailing
in their interior are very far from those encountered
in laboratory experiments. For this reason the field also
requires advanced theoretical study, developing nuclear
models which can predict the key nuclear properties
(nuclear levels, masses, optical potentials, decay rates,
equation of state of dense matter etc.) and theories to
enable us to calculate reaction probabilities. Often this
work is carried out at the interface with astrophysics
theorists who model the astrophysical sites. In a later
section we identify where the key theoretical advances
are required and what computational resources are
required to support this.
4.4.2 State of the Art
and Future Directions
Big Bang nucleosynthesis
The Big-Bang model is supported by three pieces of
observational evidence: the expansion of the Universe,
the Cosmic Microwave Background (CMB) radiation
and the Primordial or Big-Bang Nucleosynthesis (BBN).
The latter evidence comes from the primordial abundances
of the “light elements”: 4 He, D, 3 He and 7 Li.
Historically the comparison between their BBN calculated
abundances and those deduced from observations
in primitive astrophysical sites were used to determine
the baryonic density of the Universe Ω B . The latter is now
more precisely deduced from the observations of the
anisotropies of the CMB radiation. Hence, all the parameters
of the Standard BBN (Ω B , the number of neutrino
families and the nuclear reactions) are now known and
the BBN is now used as a probe of the early Universe.
Indeed, deviations from the standard BBN results may
be hints of non-standard physics.
Most of the nuclear reactions responsible for the
production of 4 He, D, 3 He and 7 Li in the Standard BBN
model have been measured in the laboratory, usually at
Figure 3. The relative abundances of the light elements as
predicted by BB nucleosynthesis model are shown as a function
of the baryonic matter density in the universe. The horizontal bands
show the experimentally observed abundance measurements
(and uncertainty). The values are, except for Li, in agreement
with the value expected from measurements of the CMB radiation
(yellow line).
the relevant energies. Despite the fact that the primordial
abundances of these light isotopes span nine orders of
magnitude, the agreement between the BBN calculations
and the observations is good for helium and excellent for
deuterium. However, the calculated 7 Li abundance is a
factor of ≈5 above the value deduced from observations
of low metallicity stars in the halo of our galaxy where
the Li abundance is found to be almost independent of
metallicity. This discrepancy is surprising and its origin
remains an open question. The small scatter of values
around this “Spite plateau” is an indication that in situ
stellar depletion may not have been very effective. It
is thus essential to determine precisely the absolute
cross sections important for 7 Li nucleosynthesis, e.g.,
2
H(p,γ) 3 He and 3 He(α,γ) 7 Be need to be measured with
greater precision. That would allow for a better determination
of the required 7 Li depletion factor in stellar
model calculations, or better limits on non-standard
BBN models.
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