It was the British astrophysicist, Sir Fred Hoyle who first coined the phrase, "the Big Bang".
He did not believe that the universe began in a massive explosion. He mockingly used the Big Bang phrase as a metaphor for orgasm.
This phrase would haunt him for the rest of his life.
Hoyle instead believed that the universe had no specific beginning point in time, instead that it was evolving over infinite time in a "steady state".
The Big Bang was the beginning of EVERYTHING. There was nothing before it. No time and, no space.
The Big Bang occurred some 15 billion years ago or so.
According to general relativity (see "Relativity" topic page), the universe began with an infinite temperature and density.
As the universe expanded, its temperature decreased.
At about a hundredth of second after the Big Bang, the temperature would've been around 100 billion degrees celsius. At this time, the universe would have contained mostly photons, electrons and neutrinos, and their antiparticles, plus some protons and neutrons.
For the next three minutes, the universe cooled down to about a billion degrees celsius, protons and neutrons would've begun combining to form nuclei of hydrogen, helium and other light elements.
Hundreds of thousands of years later, when the temperature had dropped to a few thousand degrees celsius, the electrons would have slowed right down to the point where the light nuclei could capture them to form atoms.
Heavier elements such as carbon and oxygen would not form until billions of years later where they would form from the burning of helium in the centre of stars.
The reason that we know there was a Big Bang is due to the fact that the Universe is expanding. Galaxies are moving further away from each other. If we reversed all this, then all the galaxies must have originated from a single point. This was the origin of the Big Bang.
It was the physicists, Arno Penzias and Robert Wilson who discovered the echo of the Big Bang. In 1965 they detected a weak radio signal coming from every direction in the sky. This signal was equivalent to that emitted by an object at -270 degrees celsius (3 degrees above absolute zero). The only possible source for this radiation was the dying heat of the Big Bang., cooled by the expansion of the Universe.
