The most exciting and profound new physics, the first glimpses of twenty-first-century physics, are now coming from astronomy. Stunning new, supersensitive instruments and dazzling theoretical models have combined to squeeze revolutionary data from the faintest observations. From satellites in outer space and camps 800 metres from the South Pole, astronomers are mapping the shape of space and reaching back to the birth of time.

Despite its many successes, the Big Bang model led to some new, deeply perplexing puzzles. Suppose our telescopes look at very distant objects in opposite directions. They might be so remote that nothing could travel from one to the other. Even at the speed of light, the journey would take longer than the 15 billion years since the Big Bang. Yet the universe in opposite directions looks pretty much the same; in fact, it is exceedingly uniform. The cosmic microwave background, for example, comes to us from the farthest corners of the universe, but is the same whichever way we look, to within one part in 10,000 or more. This is suspicious. What could have coordinated or matched conditions in regions so far from each other? Since this coordination seems to have extended beyond the horizon that light could reach, it was called the horizon problem.

There are other big problems. Imagine throwing a stone straight up into the sky. Three things might happen. Ordinarily, the stone will rise upwards, slow down and fall back to Earth.