Did the universe really begin in Big Bang Theory?
Our universe was born about 13.7 billion years ago in a massive expansion that blew space up like a gigantic balloon. This was a period of cosmic inflation that lasted mere fractions of a second — about 10^-32 of a second, according to physicist Alan Guth’s 1980 theory that changed the way we think about the Big Bang forever.
When cosmic inflation came to a sudden and still-mysterious end, the more classic descriptions of the Big Bang took hold. A flood of matter and radiation, known as “reheating,” began populating our universe with the stuff we know today: particles, atoms, the stuff that would become stars and galaxies, and so on.
Cosmologists suspect that the four forces that rule the universe — gravity, electromagnetism, and the weak and strong nuclear forces — were unified into a single force at the universe's birth, squashed together because of the extreme temperatures and densities involved.
But things changed as the universe expanded and cooled. Around the time of inflation, the strong force likely separated out. And by about 10 trillionths of a second after the Big Bang, the electromagnetic and weak forces became distinct, too.
Just after inflation, the universe was likely filled with a hot, dense plasma. But by around 1 microsecond (10 to the minus 6 seconds) or so, it had cooled enough to allow the first protons and neutrons to form, researchers think.
In the first three minutes after the Big Bang, these protons and neutrons began fusing together, forming deuterium (also known as heavy hydrogen). Deuterium atoms then joined up with each other, forming helium-4.