The First Three Minutes

The big bang occurred a very long time ago. However, the formation of the light elements that constituted the visible material of the initial Universe occurred in a very short period of a little over three minutes at its beginning. It has become common to refer to this remarkable era as the "first three minutes". We now sketch the important things that happened in this period.

Confinement of the Quarks and Gluons

Until about 1 millionth of a second after the bang, matter was in the form of a hot soup of fundamental particles such as photons, quarks and gluons, neutrinos, electrons and antielectrons, and so on. There were no protons or neutrons yet, because the quarks and gluons that make them up were moving about freely (they are said to be deconfined). But at about 1 millionth of a second into the formation of the new Universe, the temperature had dropped enough that the quarks and gluons became bound together in neutrons and protons. This is called the confinement transition, and it occurs at a temperature of about 1 billion electron-Volts (1 GeV), which is about 10 trillion K; see the box below concerning temperature units. It greatly reduced the number of players in the drama, as illustrated in the above figure and this animation, which plots the number of different kinds of particles participating in the thermal equilibrium as a function of temperature (what we call "players in the drama", physicists call degrees of freedom).

Time ~ 1/100 Second

At this stage the temperature is about 100 billion kelvins and the density is more than a billion times that of water. The Universe is expanding rapidly and is still very hot; it consists of an undifferentiated soup of matter and radiation in thermal equilibrium. This temperature corresponds to an average energy for each particle in the Universe of about 8-9 MeV (million electron-Volts).

Temperature Units

We may express temperatures in kelvins as we have been, but astronomers who study the early Universe often use a special set of units called natural units. In this case temperature has the same units as energy. Because the temperatures of the early Universe are very large, the most common energy units used are the MeV (million electron-Volts) and GeV (billion electron-Volts). The conversion factor between these units and the kelvin scale is

1 GeV = 1.2 x 10¹³ K

with 1 GeV = 1000 MeV = 10⁹ eV. We shall, like many astronomers, use both sets of units in our discussion.

The electrons and positrons are in equilibrium with the photons, the neutrinos and antineutrinos are in equilibrium with the photons, antineutrinos are combining with protons to form positrons and neutrons, and neutrinos are combining with neutrons to form electrons and protons. At this point the number of protons is about equal to the number of neutrons.

Time ~ 1/10 Second

Now the temperature has dropped to around 50 billion kelvins and the density is a little over 10 million times that of water as the Universe continues to expand. Because a free neutron is slightly less stable than a free proton, neutrons are converted to protons by weak interactions, which are still in thermal equilibrium with the particles (we shall explain this in more detail shortly). Thus, the initial approximately equal balance between neutrons and protons begins to be tipped in favor of protons. By this time about 60 percent of the nucleons are protons and 40 percent are neutrons.