Subsequent Evolution

The most important events of the big bang happened rapidly, in the first three minutes. The evolution following the first three minutes was much more sedate.
Time ~ 35 Minutes
The temperature has now dropped to about 300 million K and the Universe consists of protons, the excess electrons that did not annihilate with the positrons, helium-4, photons, neutrinos, and antineutrinos. There are no hydrogen atoms yet because the temperature still is far too high for the protons and electrons to bind together. The hot but rapidly cooling Universe is extremely opaque to light because all the free electrons interact strongly with the photons.

Time ~ 1000 years
The adjacent figure illustrates the energy density residing in radiation and that residing in matter as a function of the scale factor of the Universe (which is plotted in a logarithmic scale). The energy of the photons and the mass of the matter have been related by the Einstein energy-mass relation. The Universe is radiation-dominated as long as the energy density comes primarily from radiation and matter-dominated if it comes primarily from matter (we ignore the role of the dark energy for the moment).

The radiation density and matter density lines cross at a time of about 1000 years after the big bang, when the temperature had dropped to about 10 eV or about 100,000 K. Thus, for approximately its first thousand years the Universe was dominated by radiation, but since that time it has been dominated by matter (and increasingly, dark energy). At the time of the transition the logarithm of the scale factor was about -4 or -5, implying that the scale factor was about 0.0001 or 0.00001 of the present value. Since the scale parameter measures the relative size of the Universe, at the time of the transition from radiation domination to matter domination, the Universe was about 10,000 - 100,000 times smaller than its present size. This in turn implies that the redshift corresponding to the transition from radiation to matter domination was z ~ 10,000 - 100,000.

Time ~ 300,000 years
The temperature has fallen to several thousand K, which is sufficiently low that electrons and protons can hold together to begin forming hydrogen atoms. This is called the recombination transition, though this is a misnomer because the electrons and protons had never been combined before so they could not "recombine". Nevertheless, this is the term that has been used for too long to change it now!

Until this point, matter and radiation have been in thermal equilibrium, but now they decouple. As the free electrons are bound up in atoms the main process leading to the interaction of photons with matter (interaction with the free electrons) is removed and the Universe, which has been very opaque until this point, becomes transparent. Light can now travel large distances before being absorbed because its interaction with hydrogen atoms is much weaker than its interaction with free electrons.

Summary: History of the Early Universe
The overall history of the early Universe is summarized in the following diagram. The portion we have described to this point extends only back to the time marked "Quark-Gluon Confinement". We shall address some of the even earlier instants of the Universe shortly.

A somewhat more detailed description of this history is given in the following table. Times are measured since the big bang and temperatures are given in both kelvins and in GeV.

A Brief History of the Universe
Time* Temp (GeV)** Temp (K)** Characteristics
10-43 s 1.2 x 1019 1.4 x 1032 Planck scale; quantum gravity; superstrings; spacetime foam?
10-35 s 1.0 x 1014 1.2 x 1027 Begin brief inflationary epoch; end of grand unified era
10-32 s 1.0 x 1014 1.2 x 1027 End of inflationary epoch; beginning of standard big bang evolution; origin of matter-antimatter asymmetry
10-11 s 1.0 x 102 1.2 x 1015 End of electromagnetic and weak force unification
10-6 s 1.0 1.2 x 1013 Confinement: quarks and gluons become bound in the hadrons
1 s 1.0 x 10-3 1.2 x 1010 Weak interactions drop out of thermal equilibrium (weak freeze-out)
~ 3 min 1.0 x 10-4 1.2 x 109 End nucleosynthesis of light elements
103 y 1.0 x 10-8 1.2 x 105 End of radiation dominance; begin matter dominance
105 y 3.0 x 10-10 3.6 x 103 Electrons and protons combine to form atoms; photons and matter decouple
109 y 8.0 x 10-13 ~ 10 The first galaxies have formed by now
~ 1010 y 2.3 x 10-13 ~ 3 The present Universe
*Since big bang   **Conversion: 1 GeV = 109 electron-Volts (eV) = 1.2 x 1013 kelvins (K)

The temperatures quoted are for the photons. Until the time of decoupling, photons and matter had the same temperature, but after decoupling matter and radiation no longer are in equilibrium and don't have the same temperature.