Big Bang Overview

What we are able to discuss is what happened very soon after the Big Bang. And we do mean very soon. From about H00 of a second after the Big Bang onward, we can outline the major steps in the universe's evolution. It is basically a story of cooling and expanding. For a thorough review of modern cosmology, see Timothy Ferris's excellent and humorous The Whole Shebang (Touchstone, 1998).

At the earliest times that we can track, the universe was incredibly hot (1011 K) and filled with elementary particles, the building blocks of atoms: electrons, positrons, neutrinos, and photons of light. And for every billion or so electrons present at this time, there was one heavy particle (a neutron or proton). In this soup—an entire universe hotter than the core of a star—energy and matter were going back and forth as electrons and positrons annihilated to produce energy, and more were born from the energetic photons that filled the universe.

As the universe cooled, there was soon insufficient energy to create the electrons and positrons, so most of them annihilated, without new ones taking their place. After about the first three minutes of its existence, the universe had cooled to about 1 billion degrees, sufficiently cool for the nuclei of atoms to hold together, and later hydrogen, helium, and small amounts of lithium and beryllium were formed.

The universe then consisted of photons of light, neutrinos and antineutrinos, and a relatively small amount of nuclear material. Of course, the universe was still far too hot for electrons to come together with nuclei to form stable atoms. Not until about 300,000 years later (when the entire universe was at the temperature of a stellar photosphere) would nuclei be able to hold on to electrons.

It wasn't until after this first 300,000 years that the universe became transparent to its own radiation. What does that mean? Before the electrons settled out into atoms, they got in the way of all the photons in the universe and kept them bumping around. With

Star Words

Particles and antiparticles annihilate when they meet, converting their mass into pure energy. Electrons and anti-electrons (positrons) were continually created and annihilated in the early universe.

Star Words

Particles and antiparticles annihilate when they meet, converting their mass into pure energy. Electrons and anti-electrons (positrons) were continually created and annihilated in the early universe.

the electrons out of the picture, the universe went from a radiation-dominated state to a matter-dominated state.

From this point on, the universe continued simply to cool and coalesce, eventually forming the stars and galaxies of the observable universe.

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