Once upon a beginning of time, there was a Big Bang. At the exact moment of the Bang, a “cosmic egg” was conceived. Over time, it would cool down, spread out, and grow into the Universe. It would never stop growing.
One-tenth of a second after the Bang, there was enough energy to create matter: neutrons, protons, and electrons, some stable, some unstable. The Universe had a temperature of 30 billion degrees Kelvin and a density 30 million times that of water.
One and one-tenth of a second after the Bang, the Universe had a temperature of 10 billion degrees Kelvin and a density 380,000 times that of water.
Just under fourteen seconds after the Bang, it had cooled to 3 billion degrees Kelvin. This made the neutrons and protons and electrons move more slowly; slowly enough, in fact, to stick together if they happened to collide. So, at this point in the egg’s development, one proton and one neutron and one electron could collide and form the first atom, deuterium.
Three minutes and 2 seconds after the Bang, the temperature dropped to below one billion degrees Kelvin. It was then cool enough for two deuterium atoms to collide and form another kind of atom, helium.
Thirty-four minutes after the Bang, the Universe was 300 million degrees Kelvin. It was only 10 percent as dense as water. The deuterium and helium atoms were still bouncing around, usually too much to form stable entities for significant periods of time.
Seven hundred thousand years after the Bang, the Universe was the same temperature as today’s Sun—about 4,000 degrees Kelvin. This was finally cool enough for all of the atoms to be stable. For the next few billion years, they morphed into stars and galaxies.
Fifteen Billion years after the Bang, the Universe exists as it does now. Now.