point in the expansion and cooling of the Universe, the average temperature
is low enough so that neutrons and protons can stick together and make nuclei
of the lighter elements such as hydrogen, helium and lithium. Physicists call
this process nucleosynthesis, and it had to occur before the
structures we observed today, such as atoms and molecules, could exist.
and protons only attract each other at very short distances, less than 10-13
centimeters. The strong nuclear force that holds them together is confined
and cancels out at larger distances. So in order to form nuclei, neutrons and
protons have to spend some time in very close proximity to one another. This
can't happen if the temperature is too high, because then the protons and neutrons
will be moving too fast to spend much time near one another.
of the neutrons in the Universe wind up stuck in combinations of two protons,
two neutrons, in the helium nucleus. A few neutrons contribute to lithium, with
three protons and three neutrons, and the leftovers wind up in deuterium, which
is an isotope of hydrogen with one proton and one neutron.
sets the stage for the formation of atoms and then galaxies and stars.