Quantum Theory Cannot Hurt You (16 page)

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.

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 to reach us. So the only stars and galaxies we see are those that are near enough that their light has taken less than 13.7 billion years to get to us. Most of the stars and galaxies in the Universe are so far away that their light will take more than 13.7 billion years to reach us. The light of these objects is still on its way to Earth.

Therefore, the main reason the sky at night is dark is that the light from most of the objects in the Universe has yet to reach us. Ever since the dawn of human history, the fact that the Universe had a beginning has been staring us in the face in the darkness of the night sky. We have simply been too stupid to realise it.

Of course, if we could wait another billion years, we would see stars and galaxies so far away that their light has taken 14.7 billion years to get here. The question therefore arises of whether, if we lived many trillions of years in the future when the light from many more stars and galaxies had time to reach us, the sky at night would be red. The answer turns out to be no. The reasoning of Kepler and Olbers is based on an incorrect assumption—that stars live forever. In fact, even the longest-lived stars will use up all their fuel and burn out after about 100 billion years. This is long before enough light has arrived at Earth to make the sky red.

The Big Bang has enormous explanatory power. Nevertheless, it has serious problems. For one it is difficult to understand where galaxies like our Milky Way came from.

The fireball of the Big Bang was a mix of particles of matter and light. The matter would have affected the light. For instance, if the matter had curdled into clumps, this would be reflected in the afterglow of the Big Bang—it would not be uniform all over the sky today but would be brighter in some places than others. The fact that the afterglow is even all around the sky means that matter in the fireball
of the Big Bang must have been spread about extremely smoothly. But we know that it could not be spread completely smoothly. After all, today’s Universe is clumpy, with galaxies of stars and clusters of galaxies and great voids of empty space in between. At some point, therefore, the matter in the Universe must have gone from being smoothly distributed throughout space to being clumpy. And the start of this process should be visible in the cosmic background radiation.

Sure enough, in 1992, very slight variations in the brightness of the afterglow of the Big Bang were discovered by NASA’s Cosmic Background Explorer Satellite, COBE. These cosmic ripples—one of the scientists involved was more picturesque in likening them to “the face of God”—showed that, about 450,000 years after the Big Bang, some parts of the Universe were a few thousandths of a per cent denser than others. Somehow, these barely noticeable clumps of matter—the “seeds” of structure—had to grow to form the great clusters of galaxies we see in today’s Universe. But there is a problem.

Clumps of matter grow to become bigger clumps because of gravity. Basically, if a region has slightly more matter than a neighbouring region, its stronger gravity will ensure that it will steal yet more matter from its neighbour. Just as the richer get richer and the poor get poorer, the denser regions of the Universe will get ever denser until, eventually, they become the galaxies we see around us today. The problem the theorists noticed was that 13.7 billion years was not enough time for gravity to make today’s galaxies out of the tiny clumps of matter seen by the COBE satellite. The only way they could do it was if there was much more matter in the Universe than was tied up in visible stars.

Actually, there was strong evidence for missing matter close to home. Spiral galaxies like our own Milky Way are like giant whirlpools of stars, only their stars turn out to be whirling about their centres far too fast. By rights, they should fly off into intergalactic space just as you would be flung off a merry-go-round that someone had spun too fast. The extraordinary explanation that the world’s astronomers have come up with is that galaxies like our Milky Way
actually contain about 10 times as much matter as is visible in stars. They call the invisible matter dark matter. Nobody knows what it is. However, the extra gravity of the dark matter holds the stars in their orbits and stops them from flying off into intergalactic space.

If the Universe as a whole contains 10 times as much dark matter as ordinary matter, the extra gravity is just enough to turn the clumps of matter seen by COBE into today’s galaxy clusters in the 13.7 billion years since the Universe was born. The Big Bang picture is saved.
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The price is the addition of a lot of dark matter, whose identity nobody knows—well, almost, nobody. In the words of Douglas Adams in
Mostly Harmless
: “For a long period of time there was much speculation and controversy about where the so-called ‘missing matter’ of the Universe had gotten to. All over the Galaxy the science departments of all the major universities were acquiring more and elaborate equipment to probe and search the hearts of distant galaxies, and then the very centre and the very edges of the whole Universe, but when eventually it was tracked down it turned out in fact to be all the stuff which the equipment had been packed in!”

The fact that the standard Big Bang picture does not provide enough time for matter to clump into galaxies is not the only problem with the scenario. There is another, arguably more serious, one. It concerns the smoothness of the cosmic background radiation.

Things reach the same temperature when heat travels from a hot body to a cold body. For instance, if you put your cold hand on a hot water bottle, heat will flow from the bottle until your hand
reaches the same temperature. The cosmic background radiation is basically all at the same temperature. This means that, as the early Universe grew in size, and some bits lagged behind others in temperature, heat always flowed into them from a warmer bit, equalising the temperature.

The problem arises if you imagine the expansion of the Universe running backwards like a movie in reverse. At the time that the cosmic background radiation last had any contact with matter—about 450,000 years after the Big Bang—bits of the Universe that today are on opposite sides of the sky were too far apart for heat to flow from one to the other. The maximum speed it could flow is the speed of light, and the 450,000 years the Universe had been in-existence was simply not long enough. So how is it that the cosmic background radiation is the same temperature everywhere today?

Physicists have come up with an extraordinary answer. Heat could have flowed back and forth throughout the Universe, equalising the temperature, only if the early Universe was much smaller than our backward-running movie would imply. If regions were much closer together than expected, there would have been plenty of time for heat to flow from hot to cold regions and equalise the temperature. But if the Universe was much smaller earlier on, it must have put on a big spurt of growth to get to its present size.

According to the theory of inflation, the Universe “inflated” during its first split-second of existence, undergoing a phenomenally violent expansion. What drove the expansion was a peculiar property of the vacuum of empty space, although that’s still hazy to physicists. The point is that there was this enormously fast expansion, which very quickly ran out of steam, and then the more sedate expansion that we see today took over. If the normal Big Bang expansion is likened to the explosion of a stick of dynamite, inflation can be likened to a nuclear explosion. “The standard Big Bang theory says nothing about what banged, why it banged or what happened before it banged,” says inflation pioneer Alan Guth. Inflation is at least an at-tempt to address such questions.

With inflation plus dark matter tagged on, the Big Bang scenario can be rescued. In fact, when astronomers talk of the Big Bang these days, they often mean the Big Bang plus inflation plus dark matter. However, inflation and dark matter are not as well-founded ideas as the Big Bang. Beyond any doubt, we know that the Universe began in a hot dense state and has been expanding and cooling ever since—the Big Bang scenario. That inflation happened is still not certain, and nobody has yet discovered the identity of dark matter.

One of the pluses of inflation is that it provides a possible explanation of the origins of structures such as galaxies in today’s Universe. For such structures to have formed, there must have been some kind of unevenness in the Universe at a very early stage. That primordial roughness could have been caused by so-called quantum fluctuations. Basically, the laws of microscopic physics cause extremely small regions of space and matter to jiggle about restlessly like water in a boiling saucepan. Such fluctuations in the density of matter were minuscule—smaller even than present-day atoms. However, the phenomenal expansion of space caused by inflation would actually have enhanced them, blowing them up to noticeable size. Bizarrely, the largest structures in today’s Universe—great clusters of galaxies—may have been spawned by “seeds” smaller than atoms!

Inflation, however, predicts something about our Universe that does not seem to accord with the facts. Currently, the Universe is expanding. However, the gravity of all the matter in the Universe is acting to brake the expansion. There are two main possibilities. One is that the Universe contains sufficient matter to eventually slow and reverse its expansion, causing the Universe to collapse back down to a Big Crunch, a sort of mirror image of the Big Bang in which the Universe was born. The other is that it contains insufficient matter and goes on expanding forever. Inflation predicts that the Universe should be balanced on the knife edge between these two possibilities. It will continue expanding, but slowing down all the time, and finally running out of steam only in the infinite future. For this to happen, the Universe must have what is known as the critical mass. The problem
is that, even when all the matter in the Universe—visible matter and dark matter—is added up, it amounts to only about a third of the critical mass. Inflation, it would seem, is a nonstarter. Well, that’s how it seemed—until a sensational discovery was made in 1998.

Two teams were observing “supernovas”—exploding stars—in distant galaxies. One team was led by American Saul Perlmutter and the other by Australians Nick Suntzeff and Brian Schmidt. Supernovas are exploding stars that often outshine their parent galaxy and so can be seen at great distances out into the Universe. The kind the two teams of astronomers were looking at were known as Type Ia supernovas. They have the property that, when they detonate, they always shine with the same peak luminosity. So if you see one that is fainter than another, you know it is farther away.

What the astronomers saw, however, was that the ones that were farther away were fainter than they ought to be, taking into account their distance from Earth. The only way to explain what they were seeing was that the Universe’s expansion had speeded up since the stars exploded, pushing them farther away than expected and making them appear fainter.

It was a bombshell dropped into the world of science. The sole force affecting the galaxies ought to be their mutual gravitational pull. That should be braking the expansion, not speeding it up.