Below is a selection describing the process/es that led to the formation of light elements in the beginning of time. A link below is provided if you wish to download a printable copy. This is followed by Activities # 1&2: Formation of Light Elements whose download link for a printable copy is also provided.
Big Bang Nucleosynthesis: Cooking Up the First Light Elements
derived from an article
written by Achim Weiss
The
big bang models - the cosmological
models based on general relativity - tell us that the early universe was
extremely hot and dense. At the earliest stages that can be modelled using
current physical theories, the universe was filled with radiation and elementary
particles - a hot plasma in which energy was distributed evenly. During the
subsequent expansion, this plasma has progressively cooled down. By examining
how the cooling affects the matter content of the universe, one can derive one
of the most impressive testable predictions of the big bang models.
Nuclear
Physics in an Expanding Universe
As
the universe cools, the matter content changes - new particles are formed out
of the preexisting ones – the protons
and neutrons forming out of quarks. From about one second to a few minutes
cosmic time, when the temperature has fallen below 10 billion Kelvin, the
conditions are just right for protons and neutrons to combine and form certain
species of atomic nuclei. This phase is called Big Bang Nucleosynthesis.
While
the early universe is totally unlike our everyday world, the basic nuclear
physics at the appropriate energies is well within the range of laboratory
experiments. Following such experiments, the properties of the relevant nuclear
reactions are very well known. Physicists can base their calculations on solid
experimental data when they want to describe reactions like the one pictured
here:
The image illustrates two of the nuclear
reactions occurring during Big Bang Nucleosynthesis: It shows protons and neutrons combining to
form deuterium nuclei (D, containing one proton and one neutron), accompanied
by the emission of high energy photons (denoted as γ); furthermore, it shows
two deuterium nuclei fusing to produce one nucleus of helium-3 (with two
protons and one neutrons) and one free neutron.
Taking
into account a wealth of nuclear reactions similar to the ones pictured on the
left, one can then apply general statistical formula which govern the relative abundances of the different matter
constituents. What nuclei are produced, and in what amounts, is the result
of a race between the various nuclear
reactions on the one hand and the inevitable cooling that accompanies the expansion of the universe on the
other.
As
it turns out, Big Bang Nucleosynthesis strongly favors the very light elements
like hydrogen and helium - not only standard hydrogen (one proton) and helium
(two neutrons and two protons), but also the isotopes deuterium (one
proton, one neutron), tritium (one proton, two neutrons) and helium-3 (two
protons, one neutron). By mass, about a quarter of the nuclei in the universe
should be helium. Deuterium, tritium, helium-3 nuclei should occur in much
smaller, but still measurable quantities.
Trace amounts of Lithium (3 protons and 3
neutrons), Lithium-7 (3 protons and 4 neutrons), Beryllium (4 protons and 5
neutrons) and Beryllium-7 (4 protons and 3 neutrons) were also produced during
the Big Bang. However, they have relatively short half-lives and could not have
survived to the present. The Lithium and Beryllium found in present-day
universe were formed via cosmic-ray collisions, just like
Boron.
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