How we know there was a Big Bang

The Big Bang theory says that 13.8 billion years ago everything we can see was packed together, very hot and very dense, and has been spreading out and cooling ever since. Nobody was there to watch. So why do astronomers believe it? Because it made four predictions that could have turned out wrong, and none did.

The first is the one that started it. Nearly every galaxy is moving away from us, and the further off it is, the faster it goes: twice as far, twice as fast. That is exactly what you would see from any galaxy if space itself were stretching. Run the stretching backwards and everything comes together.

The second is the oldest light. If the universe began hot, the glow of that heat should still be arriving, stretched and cooled, from every direction. It was predicted in 1948 and found by accident in 1964. This is the real map of it. No other idea about the universe had predicted any such thing.

The third is what the universe is made of. For a few minutes at the start, the whole of space was as hot as the middle of a star, and should have cooked its hydrogen into a mixture of three parts hydrogen to one part helium. The oldest, least-touched gas ever measured is 24.5 per cent helium.

The fourth is that the past looks different. Light from far away is old, so looking far is looking back. This real photograph reaches back about 13 billion years. The most distant galaxies in it are small, ragged and young. If the universe had always been here, far-off galaxies would look just like the ones nearby.

Any one of these might be explained some other way. Nobody has found another idea that explains all four at once. That is what it means to say the Big Bang is well established: not that it has been seen, but that it keeps passing tests it could have failed.