Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [68]
Penzias and Wilson did not accept the Big Bang origin of their mysterious static for at least two years. Nevertheless, for the discovery of the afterglow of creation, they carried off the 1978 Nobel Prize for Physics.
The cosmic background radiation is the oldest “fossil” in creation. It comes to us directly from the Big Bang, carrying with it precious information about the state of the Universe in its infancy, almost 13.7 billion years ago. The cosmic background is also the coldest thing in nature—only 2.7 degrees above absolute zero, the lowest possible temperature (–270 degrees Celsius).
The cosmic background radiation is actually one of the most striking features of our Universe. When we look up at the night sky, its most obvious feature is that it is mostly black. However, if our eyes were sensitive to microwave light rather than visible light, we would see something very different. Far from being black, the entire sky, from horizon to horizon, would be white, like the inside of a lightbulb. Even billions of years after the event, all of space is still glowing softly with relic heat of the Big Bang fireball.
In fact, every sugarcube-sized region of empty space contains 300 photons of the cosmic background radiation. Ninety-nine per cent of all the photons in the Universe are tied up in it, with a mere 1 per cent in starlight. The cosmic background radiation is truly ubiquitous. If you tune your TV between stations, 1 per cent of the “snow” on the screen is the relic static of the Big Bang.
DARKNESS AT NIGHT
The fact that the Universe began in a Big Bang explains another great mystery—why the night sky is dark. The German astronomer Johannes Kepler, in 1610, was the first to realise this was a puzzle.
Think of a forest of regularly spaced pine trees going on forever. If you ran into the forest in a straight line, sooner or later you would bump into a tree. Similarly, if the Universe is filled with regularly spaced stars and goes on forever, your gaze will alight on a star no matter which direction you look out from Earth. Some of those stars will be distant and faint. However, there will be more distant stars than nearby ones. In fact—and this is the crucial point—the number of stars will increase in such a way that it exactly compensates for their faintness. In other words, the stars at a certain distance from Earth will contribute just as much light in total as the ones twice as far away, three times away, four times away, and so on. When all the light arriving at Earth is added up, the result will therefore be an infinite amount of light!
This is clearly nonsensical. Stars are not pointlike; they are tiny discs. So nearby stars blot out some of the light of more distant stars just as nearby pine trees block out more distant pine trees. But even taking this effect into account, the conclusion seems inescapable that the entire sky should be “papered” with stars, with no gaps in between. Far from being dark at night, the night sky should be as bright as the surface of a typical star. A typical star is a red dwarf, a star glowing like a dying ember. Consequently, the sky at midnight should be glowing blood red. The puzzle of why it isn’t was popularised in the early 19th century by the German astronomer Heinrich Olbers and is known as Olbers’ paradox in his honour.
The way out of Olbers’ paradox is the realisation that the Universe has not in fact existed forever but was born in a Big Bang. Since the moment of creation, there has been only 13.7 billion years for the light of distant stars