The Big Bang

Everything you can see, every star, planet, atom and ray of light, was once packed into a space far smaller than a grain of sand. About 13.8 billion years ago that space began to stretch. It has not stopped. That stretching is the Big Bang, and it is still happening around you right now.

Not an explosion in space, but an expansion of space

The name is misleading. The Big Bang was not a bomb going off in an empty room, with debris flying into a void. There was no room and no void. Space itself was tiny, and space itself grew. Galaxies today are not flying through space away from some central point. The space between them is getting bigger, the way the distance between two raisins grows as a loaf of bread rises in the oven. Every raisin sees every other raisin moving away, and none of them is at the centre.

This is why astronomers can say the universe has no edge and no middle. Wherever you stand, the view is roughly the same: galaxies in every direction, all receding, with the most distant ones receding fastest.

The first second

In the first fraction of a second the universe was unimaginably hot and dense. The four forces of nature, gravity, electromagnetism and the two nuclear forces, had not yet separated into their distinct forms. Within a trillionth of a trillionth of a trillionth of a second, a brief burst called inflation blew the universe up by a factor of at least a hundred trillion trillion, smoothing it out and stretching tiny quantum ripples into the seeds of future galaxies.

By the time one second had passed, the temperature had fallen to about 10 billion degrees. That is cool enough for the basic building blocks of matter, protons and neutrons, to exist without being torn apart. Matter and antimatter had been created in almost equal amounts and had mostly annihilated each other in flashes of light. A tiny surplus of matter, roughly one part in a billion, survived. Everything made of atoms, including you, is that leftover.

The first three minutes

Between about one second and three minutes, the universe became a nuclear furnace. Protons and neutrons collided and stuck, forming the nuclei of helium along with traces of lithium and a hydrogen isotope called deuterium. Then expansion cooled things too much for fusion to continue, and it stopped. The recipe it left behind, about 75 percent hydrogen and 25 percent helium by mass, matches what we measure in the oldest gas clouds today almost perfectly. That match is one of the strongest pieces of evidence that the Big Bang really happened.

The universe becomes transparent

For the next 380,000 years the universe was a glowing fog. It was full of free electrons, and light cannot travel far through a sea of electrons without scattering. Then the temperature dropped to about 3,000 degrees, cool enough for electrons to settle onto nuclei and form the first complete atoms. Suddenly the fog cleared, and light could stream freely for the first time.

That light is still travelling. The expansion of space has stretched it from a visible orange glow into faint microwaves, a few degrees above absolute zero. We detect it coming from every direction in the sky and call it the cosmic microwave background. It is, quite literally, a baby photograph of the universe. The speckles in it, differences of only a few hundred-thousandths of a degree, are the places where matter was slightly denser. Gravity pulled on those dense patches for hundreds of millions of years until they collapsed into the first stars and galaxies.

How we know

Three independent lines of evidence point the same way. First, galaxies are moving apart, and the further away they are, the faster they recede, exactly as an expanding space predicts. Run the film backwards and everything converges on a single moment. Second, the amounts of hydrogen, helium and deuterium in old gas match what a hot early universe would cook up. Third, the cosmic microwave background exists, and has precisely the temperature and texture the theory requires. It was predicted in 1948 and found by accident in 1964, when two radio engineers could not get rid of a faint hiss in their antenna.

What we still don't know

The Big Bang theory describes how the universe evolved from a hot dense state. It does not say what, if anything, came before, or why there was something rather than nothing. The physics we have breaks down at the very first instant, where quantum mechanics and gravity would need to be combined into a single theory that nobody has yet written. Whether time itself began at the Big Bang, or whether our universe is one of many, are questions still open. We know an astonishing amount about the first three minutes. The first zero seconds remain a mystery.