As usual,
the Universe is still cooling and expanding. But as this happens, more and more
matter is being created by the high energy radiation. And as the Universe expands,
the matter loses less energy than does the radiation.
Eventually,
the energy density in the matter -- mostly in the newly-formed nuclei -- becomes
larger than the energy density in radiation, in massless or nearly massless
particles, mainly photons. This means that in the equations of relativity that
described cosmic expansion, the number representing the energy density of matter
becomes much bigger than the number that represents the energy density of radiation,
and we can forget about the radiation in those equations and only concentrate
on what happens to the matter. The matter then dominates in determining us how
the Universe expands from this era on.
At the
end of this process, photons scatter much more with each other than they do
with matter. As a result, the energy exchange between matter and radiation becomes
less efficient. The photons thermalize and start behaving as
thermal black body radiation. We can measure this cosmic background
radiation today.
After having
cooled off for many billions of years, the temperature of this radiation is
just a few degrees above absolute zero. But we can measure this temperature,
and we can also measure how this temperature of the cosmic background radiation
varies with direction in the Universe. This tells us important details about
the Big Bang and about particle physics as well.