has now expanded and cooled to the point where something incredible happens
to the quarks and gluons that are popping around at high speed by themselves.
They undergo an enormous Universe-wide phase transformation where all of the
quarks and gluons in the Universe become confined together
inside mesons such as the pi meson and baryons such as the proton and neutron.
Prior to this era, protons and neutrons and mesons don't exist, there is just
a hot soup of quarks and gluons in their place.
to be honest, particle physicists have only measured quarks and gluons that
are trapped inside baryons and mesons. Nobody has ever measured a quark or a
gluon zipping around freely on its own. In the theory of quarks and gluons,
called Quantum Chromodynamics (QCD for short),
it is believed there is a phase transition at high temperature where quarks
and gluons become deconfined and can and do zip around freely
of this deconfinement transition are still not well understood, even in the
theory. However, judging by the past successes of theoretical particle physics
in predicting phenomena that were later observed, it's probably a safe bet to
say that as the Universe cooled to a temperature below the deconfinement
temperature of QCD, quarks and gluons were no longer able to zip around
on their own and became confined together into the mesons and
baryons that produced the Universe we see today.