One of the primary successes of the Big Bang theory is its explanation for
the chemical composition of the universe. Recall that the universe is mostly
hydrogen and helium, with very small amounts of heavier elements. How does this
relate to the Big Bang?
Well, a long time ago, the universe was hot and dense. When the
temperature is high enough (a few thousand degrees), atoms lose all their
electrons; we call this state of matter, a mix of nuclei and electrons, a
fully-ionized plasma. If the temperature is even higher (millions of
degrees), then the nuclei break up into fundamental particles, and one is left
with a "soup" of fundamental particles:
protons
neutrons
electrons
Now, if the "soup" is very dense, then these particles will collide with each
other frequently. Occasionally, groups of protons and neutrons will stick
together to form nuclei of light elements ... but under extremely high pressure
and temperature, the nuclei are broken up by subsequent collisions. The Big Bang
theory postulates that the entire universe was so hot at one time that it was
filled with this proton-neutron-electron "soup."
But the Big Bang theory then states that, as the universe expanded, both the
density and temperature dropped. As the temperature and density fell, collisions
between particles became less violent, and less frequent. There was a brief
"window of opportunity" when protons and neutrons could collide hard enough to
stick together and form light nuclei, yet not suffer so many subsequent
collisions that the nuclei would be destroyed. This "window" appeared about
three minutes after the starting point, and lasted for a bit less than a minute.
Which nuclei would form under these conditions? Experiments with particle
colliders have shown us that most of the possible nuclei are unstable,
meaning they break up all by themselves, or fragile, meaning they are
easily broken by collisions.
Helium (the ordinary sort, with 2 protons and 2 neutrons) is by far the most
stable and robust compound nucleus. Deuterium (one proton and one neutron) is
easily destroyed, and so is helium-3 (2 protons, one neutron).
So, it seems that this period of hot, dense plasma would create a lot of
helium. Could it create other, heavier elements, too?
Detailed models of Big Bang nucleosynthesis predict that the brief
"window of opportunity" lasted
only a minute or
two. After that, about three and a half minutes after the starting point,
the temperature and density dropped so much that collisions between particles
were rare, and of such low energy that the electric forces of repulsion between
positively-charged nuclei prevented fusion. The result is
