General Astronomy/The First Three Minutes
The first three minutes after the Big Bang is a time period worth of a whole book (indeed, such has been written). It is important because it was during this very short time period that all the hydrogen and most of the helium in the Universe today was formed. These are the two most abundant elements in the Universe, and are the basic materials from which the first galaxies and stars formed.
At the very instant of the Big Bang, the Universe formed as a rapidly expanding ball of energy at extremely high temperatures (trillions of degrees). As soon as it began to expand, it began to cool, and as it cooled some of the energy "froze" out in what physicists call a "symmetry breaking." This is similar to water freezing into ice, except in this case it was pure energy solidifying into matter. The conditions were right for about 75 percent of the "frozen" energy to become proton (hydrogen nuclei) and the rest to form combinations of protons and neutrons in the helium nuclei. This continued for only about 3 minutes, after which the Universe had expanded and cooled to the point that the process was no longer possible.
Thus, just some 3 minutes after the Big Bang began, the Universe was composed of hydrogen and helium nuclei with a great deal of leftover energy.
This seminal 3 minute period is populated with very short periods known as the Planck Era, the GUT Era, the Electroweak Era, the Particle Era, and the Era of Nucleosynthesis. However, the important thing to know is that it was during this very short time that the major building blocks for today's Universe were formed -- and have persisted to this day.
On final note may be in order. Most of the things around us in everyday life are composed of materials other than hydrogen and helium, although obviously there is much hydrogen in ordinary water, and protons (hydrogen nuclei) are part of all matter. But what about the heavier elements such as carbon, oxygen, silicon, iron and others? They were not formed in the Big Bang, so where did they come from?
After the first 3 minutes of the Big Bang, no further elements were created for millions of years, so the Universe was entirely hydrogen, helium and energy. However, eventually clouds of hydrogen and helium collected and contracted into stars under the force of gravity. In the cores of these stars nuclear fusion began and hydrogen fused into more helium, helium fused into carbon, and various other elements up to iron. For smaller stars, this is where it stopped. However, some of the most massive stars underwent enormous supernova explosions. In the supernovas conditions were right to form even heavier elements, all the way up to Uranium.
It is a sobering thought that every atom in your body can be traced back (in theory anyway) to the Big Bang, and the nitrogen and oxygen you breathe, the carbon your body is composed of all came from some ancient star. And the heavier elements, of which there are traces in your body, formed in the fiery furnaces of supernovas! No wonder the late Carl Sagan was fond of saying that we are all "star stuff."