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You twiddle on.

Now you’re seeing X-rays, high-energy light radiated by gas heated to hundreds of thousands of degrees as it swirls down onto exotic objects like black holes. Once again, the most striking feature of the sky is that it is mostly black.

You twiddle back the other way, zipping back through ultraviolet light and visible light to infrared light, given out by objects much colder than the Sun. Now the sky is peppered by stellar embers—stars so recently born they are still swathed in shimmering placental gas and bloated red giants in their death throes. But despite the fact that the sky is lit by a new population of stars, its most striking thing remains the same. It is mostly black.

You twiddle on. Now you are seeing microwaves—the kind of light used for radar, mobile phones, and microwave ovens. But something odd is happening. The sky is getting brighter. Not just bits of it—all of it!

You take off the glasses, rub your eyes, and put them back on. But nothing has changed. Now the whole sky, from horizon to horizon, is glowing a uniform, pearly white. You twiddle further, but the sky just gets brighter and brighter. The whole of space seems to be glowing. It’s like being inside a giant lightbulb.

Are the glasses malfunctioning? No, they are working perfectly. What you are seeing is the cosmic background radiation, the relic of the fireball in which the Universe was born 13.7 billion years ago. Incredibly, it still permeates every pore of space, greatly cooled by the expansion of the Universe so that it now appears as low-energy microwaves rather than visible light. Believe it or not, the cosmic background radiation accounts for an astonishing 99 per cent of the light in today’s Universe. It is incontrovertible proof that the Universe began in a titanic explosion—the Big Bang.

The cosmic background radiation was discovered in 1965. But the realisation that there had been a Big Bang actually came earlier. In fact, the first step was taken by Einstein.

THE ULTIMATE SCIENCE

Einstein’s theory of gravity—the general theory of relativity—describes how every chunk of matter pulls on every other chunk of matter. The biggest collection of matter we know of is the Universe. Never one to shy away from the really big problems in science, Einstein in 1916 applied his theory of gravity to the whole of creation. In doing so he created cosmology—the ultimate science—which deals with the origin, evolution, and ultimate fate of the Universe.

Although the ideas behind Einstein’s theory of gravity are deceptively simple, the mathematical apparatus is not. Working out exactly how a particular distribution of matter warps space-time is very hard indeed. It was not until 1962, for instance—almost half a century after Einstein published his general theory of relativity—that New Zealand physicist Roy Kerr calculated the distortion of space-time caused by a realistic, spinning, black hole.

Figuring out how the whole Universe warps space-time would have been impossible without making some simplifying assumptions about how its matter is spread throughout space. Einstein assumed that it makes no difference where in the Universe an observer happens to be. In other words, he assumed that the Universe has the same gross properties wherever you are located and, from wherever you are located, it looks roughly the same in every direction.

Astronomical observations since 1916 have actually shown these assumptions to be well founded. The Universe’s building blocks—which Einstein and everyone else were unaware of at the time—are galaxies, great islands of stars like our own Milky Way. And modern telescopes do indeed show them to be scattered pretty evenly around the Universe, so the view from one galaxy is much the same as the view from any other.

Einstein’s conclusion, after applying his theory to the Universe as a whole, was that its overall space-time must be warped. Warped space-time, however, causes matter to move. This is the central mantra