At first, there was nothing. No matter, no energy, no time or space: in short, no universe. And then, suddenly, out of this nothing, an infinitesimally small point of energy was formed. This point began to expand, and as it did so, matter was created and so our universe was born.

So says the Big Bang theory. It's a fairly familiar theory by now but how do scientists know this? How can they possibly have any idea what happened at the beginning of the universe or even know for sure that the universe had a beginning? How did someone come up with this theory?

Well, it all started almost a century ago, with an astronomer doing some research into distant galaxies . . .

In 1913, Vesto Melvin Slipher was working at an observatory in Arizona, studying the light emitted by stars in remote galaxies. He knew that each element emits particular colours of light, so since stars all consist mainly of hydrogen, they should all give out the same specific colours of light. But when he analysed his results, he was surprised to find that the light from every star was redder than it should have been. Slipher realised that this was similar to the effect of hearing an ambulance siren moving away from you: it seems to make a different sound from when it is stationary, because the moving ambulance is stretching the sound waves. The same thing was happening with the light waves: they were being stretched, making them redder (red light waves are longer than others). This effect, which Slipher called 'redshift', clearly showed that the galaxies were moving away from each other. Without even meaning to, Slipher had answered the question of whether or not the universe was expanding (a problem that had given even Einstein difficulties!): if the galaxies were moving apart, it must be expanding.

Edwin Hubble then used Slipher's redshift measurements, together with his own data on the distances between other galaxies and our own, and found that the further away a galaxy was, the more its light was redshifted. In other words, more distant galaxies were moving away from us faster than nearer ones. A physicist named Gamow was one of the first to realise what Hubble's findings meant: at some point in the past, the whole universe had 'exploded' in a 'Big Bang'. Matter that was thrown out with greater speed had travelled further to begin with, so when this matter formed galaxies, they would be further away than those formed from matter moving more slowly, and would also still be moving faster. For most scientists, the idea that the universe had not always existed was hard enough to accept, but the most incredible thing about the Big Bang theory (the name was originally a derisive description coined by its opponents) was what Gamow said the universe must have started as: an infinitely small, infinitely dense point containing everything that is in the universe today. At first, the Big Bang theory had few supporters; it was just too hard to believe.

But there was another part to Gamow's theory. Energy can never be created or destroyed, and so, he reasoned, the enormous amount of energy that must have been generated by the Big Bang would still exist, spread throughout the universe. Because the universe was expanding, though, this energy would have been spread over a larger and larger area, and so would have cooled down. Its temperature, Gamow predicted, would by now be around -270C.

In the 1960s, two scientists in New Jersey, Penzias and Wilson, found some interference on their radio receiver they couldn't get rid of, no matter what they tried; even evicting the pigeons from the receiver didn't work! This mysterious noise came constantly from all directions in the sky, and was at a temperature of roughly -270C. Finally, they consulted an astrophysicist, who realised that what they had found was what he had been trying unsuccessfully to find: the predicted cosmic background radiation (CBR), the leftovers of the Big Bang. (After talking to Penzias and Wilson, he reportedly told his colleagues: "We've been scooped"). It was largely due to this discovery that the Big Bang theory began to be almost universally accepted, since no other theory could explain the existence of this radiation.

But where, you're probably wondering, does television come into it? The answer is that static on your TV is caused by electromagnetic radiation, of which light, microwaves, X-rays and CBR are all types. When your TV screen is only showing static, roughly one per cent of what you are seeing is actually cosmic background radiation. So next time someone complains that there's nothing on television tonight, you can tell them: there's always the birth of the universe.