The Big Bang and the Origin of the Universe
Trace the universe's expansion backward in time and you arrive at a single conclusion: 13.8 billion years ago, everything began in a hot, dense point. Here's the key evidence behind the Big Bang theory.
Running the expansion in reverse
As Edwin Hubble discovered, the farther away a galaxy is, the faster it's receding from us. Run that fact backward in time and a natural conclusion follows: if everything is moving apart now, then in the distant past, all galaxies and matter must have been much closer together — eventually converging toward a single point.
Backing up that reasoning with calculation leads to the conclusion that the universe began about 13.8 billion years ago in an extremely hot, extremely dense state. That beginning is called the "Big Bang."
What the Big Bang actually was — expansion, not explosion
The name "Big Bang" often leads people to picture a giant explosion that happened somewhere in space, but the actual meaning is quite different. The Big Bang wasn't matter bursting outward into pre-existing empty space — it was space itself beginning to expand everywhere at once from an extremely hot state. Amusingly, the name comes from astronomer Fred Hoyle, a skeptic of the theory, who coined the phrase half-mockingly on a radio broadcast — and it stuck.
Immediately after the Big Bang, the universe was unimaginably hot and so dense that even atoms couldn't yet form. As the universe expanded and cooled, the lightest elements — hydrogen and helium — formed first, and stars and galaxies were born one after another over the following hundreds of millions of years.
The universe's first light — the cosmic microwave background
The strongest evidence for the Big Bang was discovered entirely by accident in 1965. American radio engineers Arno Penzias and Robert Wilson found a faint, uniform hiss coming from every direction of the sky through their communications antenna — and no amount of cleaning the equipment made it go away.
That hiss turned out to be the universe's "first light" — radiation released when the universe first became transparent shortly after the Big Bang, then spread in all directions. Once extremely hot, this light has cooled as the universe expanded over 13.8 billion years, and today it's observed uniformly across the sky as microwave radiation just 2.7 degrees above absolute zero. This is the "cosmic microwave background," and Penzias and Wilson won a Nobel Prize for finding it.
So what came before the Big Bang?
No one can yet say for certain what existed before the Big Bang. Known physics works well from a moment very shortly after the Big Bang onward, when the universe was extremely small and hot — but push back further, toward the very "beginning" of time and space itself, and you reach territory that known physics cannot yet explain.
So the question of "before the Big Bang" remains at the frontier of science. What is certain is that every star and galaxy we see — and every element making up our own bodies — trace back to that same hot, tiny beginning, 13.8 billion years ago.