What a Wonderful World (29 page)

If you spent your whole life sitting on a chair in the middle of a field, you would find it hard – if not impossible – to create a
mental
picture of the Earth. Astronomers are similarly handicapped. They spend their whole lives pinned to the surface of a tiny ball of rock in an anonymous cosmic backwater. But, despite their overwhelming handicap, they have had remarkable success in concocting a picture of the cosmos. Not only do they know the content and extent of the Universe but they also have a pretty good idea of how it all came into being in the first place.

Nature has been kind to us. We do not live on a planet such as Venus shrouded in impenetrable clouds. We do not live in a
star-choked
region of the Galaxy such as the heart of the Milky Way where night is unknown. We have not appeared on the cosmic scene so late in the day that most of the stars have exhausted their fuel and sputtered out. Instead, using our Earthbound telescopes, we can see all the way to the Universe ’s distant horizon.

Previous generations would have killed for the kind of picture we have of our Universe. The Earth, along with a handful of other planets and moons and assorted leftover rubble from the formation of the Solar System, orbits the Sun. The Sun, in turn, orbits the centre of the Milky Way, a great pinwheel of about 100 billion stars turning ponderously in the night. At the Sun’s
location
, about two-thirds of the way out towards the outer rim, it takes about 220 million years to complete a circuit, which means
the last time the Earth was where it is at this very moment the dinosaurs were just beginning their 150-million-year reign.

The Milky Way, however, is but one island of stars, or galaxy, among 100 billion others. To get some idea of the scale of things, imagine the Universe is a sphere a kilometre across. It would be filled with 100 billion galaxies, each roughly the size of an aspirin, with the nearest galaxy, Andromeda, just over 10 centimetres away from us. Andromeda and a handful of the closest galaxies are bound to us by gravity. But all the other galaxies are fleeing from each other like pieces of cosmic shrapnel in the aftermath of an explosion.

The recession of the galaxies was discovered by American astronomer Edwin Hubble in 1929. An unavoidable consequence of this recession is that the Universe must have been smaller in the past. In fact, if the expansion of the Universe is imagined running backwards like a movie in reverse, a time is reached – about 13.8 billion years ago – when everything in creation was crowded into the tiniest of tiny volumes. This was the moment of the Universe’s
birth
: the big bang.

One of the most profound discoveries in the history of science is undoubtedly that the Universe has not existed for ever, that it was born, that, in the words of the Belgian priest and
mathematician
George Lemaître, there was ‘a day without a yesterday’.

When the Universe was squeezed into a small volume, it must have been hot, for the same reason that air squeezed in a bicycle pump gets hot. The big bang was a
hot
big bang. The Universe was therefore born in a hot, dense state – a fireball. It has been
expanding and cooling ever since and, out of the cooling debris, have congealed the galaxies we see about us, including the Milky Way.
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The fireball of the big bang was like the fireball of a nuclear bomb. But whereas the heat of a nuclear fireball dissipates into the surrounding air in an hour, a day, a week, the heat of the
big-bang
fireball had nowhere to go. It was bottled up in the Universe, which, by definition, is all there is. Consequently, the heat of the big bang is still all around us today.

Although this cosmic background radiation was once
blindingly
bright, it has been greatly cooled by the expansion of the Universe since the big bang and no longer appears as visible light. Instead, it appears as microwaves, a type of light invisible to the naked eye but that can be picked up by your TV.
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Tune your television between the stations. One per cent of the static, or snow, on the screen is the leftover heat from the big bang. Before it was intercepted by your TV aerial, it had been travelling for 13.8 billion years across space – and the very last thing it touched was the fireball of the big bang.

The cosmic background radiation is the most striking feature of our Universe. A remarkable 99.9 per cent of the particles of
light, or photons, in the Universe are tied up in this afterglow of the big bang and a mere 0.1 per cent in the light of stars and galaxies. If we had eyes sensitive to microwaves rather than to visible light, we would see the whole of space glowing white like the inside of a giant light bulb.
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The afterglow of the big-bang fireball together with the expansion of the Universe are two powerful pieces of evidence that the Universe started out in a hot and dense state and has been expanding and cooling ever since.
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One other major piece of evidence is that about 25 per cent of the mass of the Universe is in the form of helium, the second heaviest element. Starlight is a by-product of the fusion of the lightest element, hydrogen, into helium.
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By estimating how much starlight there is in the
Universe
, astronomers can deduce that stars have converted only 1 or 2 per cent of the Universe ’s initial hydrogen into helium. So the helium had to be forged somewhere else.

The cores, or nuclei, of hydrogen atoms, being charged, repel each other ferociously. They can overcome their mutual aversion and stick together to make helium nuclei only if they slam into each other at high speed, which is synonymous with high
temperature
, and if they run into each other frequently, which is synonymous with high density. These twin conditions are believed to have been satisfied in the fireball of the big bang between about 1 and 10 minutes after the birth of the Universe. Calculations show that 25 per cent of the Universe ’s hydrogen should have been transformed into helium. And this is exactly the percentage of helium astronomers observe throughout the Universe.

The big-bang picture of the birth of the Universe provokes a host of questions. One of the most common is: where did the big 
bang happen? Here, the very term ‘big bang’, coined by English astronomer Sir Fred Hoyle on a BBC radio programme in 1949, sows seeds of confusion. After all, a bang, or explosion, happens at a particular location and the shrapnel flies outwards into
pre-existing
space. But the big bang did not happen at one location. It happened
everywhere
. And there was no pre-existing space. Space, along with matter, energy – and even time – were all created together in the big bang.

When we look out at the Universe and see all the galaxies
fleeing
from us, it does not mean that the big bang happened here on Earth. When astronomers say the Universe is expanding, all they mean is that
every
galaxy is receding from
every other
galaxy. In other words, if we could magically transport ourselves to another galaxy far across the Universe, we would also see all the galaxies fleeing from us. Everyone is at the centre and no one is at the centre because
there is no centre.

An image often used to convey the idea is that of a cake with raisins baking in an oven. As the cake rises, every raisin recedes from every other raisin. No raisin is at the centre of the
expansion
. Of course, it is necessary to overlook the fact that a real cake has an edge – and imagine an infinite cake. But, then, all visual analogies of the big-bang expansion of the Universe
provide
at best a partial picture because it is
fundamentally
unvisualisable
.
The big bang, after all, happened in four dimensions of space–time – one dimension beyond what we, as lowly
three-dimensional
beings, can comprehend directly.

Many other questions provoked are by the big-bang picture of the birth of the Universe. What was the big bang? What drove the big bang? And, of course, what happened before the big bang? It is possible to answer all these questions. But only within
the context of a major – and it must be stressed, speculative – extension of the basic big-bang model.

The basic big-bang idea, for all its successes, contradicts our
observations
of the Universe in several serious ways. For one thing, the cosmic background radiation comes to us more or less equally from all directions in the sky. Or, to put it another way,
everywhere
in the sky has almost exactly the same temperature – 2.725° above absolute zero.
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This a problem because, if we imagine the expansion of the Universe running backwards like a movie in reverse to the time of the origin of the big-bang radiation, we find that regions of the Universe that are today more than 1° apart on the sky – twice the apparent width of the Moon – were not in contact with each other.
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Or, to be precise, there had been insufficient time since the beginning of the Universe for any influence – travelling even at the cosmic speed limit set by light – to pass between them. Consequently, if one bit of the fireball cooled down a bit faster than another, heat could not have travelled to it from its surroundings to equalise the temperature. The cosmic background radiation should therefore have an
uneven
temperature across the sky. It should not, as is the case, have the same temperature
everywhere in the sky.

The bizarre explanation, which has been embraced by many physicists, is that, in the Universe’s first split second of existence, it expanded far faster than at any time since – faster even than the speed of light.
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This period of inflation was so incredibly, mind-bogglingly fast that the Universe doubled in size, and doubled again,
more than 60 times over
. Inflation has been likened
to the explosion of an H-bomb compared to the puny stick of dynamite of the big-bang expansion that followed in its wake.

Inflation neatly explains why the temperature of the cosmic background radiation is the same everywhere we look in the sky. After all, if the Universe expanded far faster than we thought, it could have been smaller than we thought early on and yet still have reached its current size in 13.8 billion years. And, if it was smaller than we thought, then all bits could have been close enough to have exchanged heat, keeping the temperature of the Universe the same as it expanded.

Inflation, an idea from particle physics, was proposed in 1979 by the Russian physicist Alexei Starobinsky and independently in 1981 by the American physicist Alan Guth. Although the detailed physical mechanism underpinning the theory remains, frustratingly, obscure, inflation provides a majestic picture of the birth of our Universe and, most importantly, an explanation of what the big bang
was
.

This is the bizarre story now accepted by the majority of cosmologists. In the beginning was the inflationary, or false,
vacuum
. This was a weird, high-energy version of the true vacuum around us today.
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For a start, it had repulsive gravity.
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This caused the vacuum to expand, creating more vacuum, with more repulsive gravity, which caused the vacuum to expand even faster. Imagine you are holding a stack of banknotes between your hands and you pull your hands apart and more and more banknotes pop into existence. This is the way it was for the inflationary vacuum. Not surprisingly, physicists have dubbed inflation the ‘ultimate free lunch’.

But the inflationary vacuum was intrinsically unstable.
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Here and there, and totally at random, small patches disintegrated, or
decayed, into normal, lower-energy vacuum. And, when this happened, the tremendous energy of the inflationary vacuum had to go somewhere. It went into creating matter and simultaneously heating it to a tremendously high temperature.
It made hot big bangs.

Imagine a never-ending sea in which bubbles are appearing at random times and at random locations. Inside each bubble is a big-bang universe. One of those big-bang universes was
our Universe.

Now it is possible to answer some of those nagging questions about the big bang. The big bang was not a one-off. It was merely a local event in an ever-expanding ocean of inflationary vacuum. It was driven by the energy of that decaying vacuum. And the big bang was not the beginning. Other big bangs have been going off like stuttering firecrackers across the length and breadth of the inflationary vacuum ever since the inflationary vacuum began, well, inflating.

As fast as bubble universes are created, they are driven apart. In fact, new vacuum is created far faster than it is eaten away, so inflation, once started, is unstoppable. It is
eternal
. But, even though inflation will continue into the infinite future, surprisingly this does not mean that inflation started in the infinite past. It must have had a beginning. The question of ‘What happened before?’ is therefore simply pushed back from the big bang to an earlier time. Quantum theory might come to the rescue, however, since quantum theory allows stuff literally to pop out of nothing. All that would have been necessary was for a tiny patch of false
vacuum
to pop into existence and begin inflating. Since a prerequisite of this happening is the existence of quantum theory, the
question
now becomes: ‘Where did the laws of physics come from?’