Amid all the media hubbub, it was difficult to tell just what COBE had found and impossible to tell what it all meant. Many people who watched TV the night of the ‘cosmic ripples’ announcement or read the newspapers were rendered dizzy by the convoluted cosmological explanations. They wondered whether it was mere hype or whether the COBE satellite really had discovered something of great importance.
One thing was for sure: COBE did not unravel the mystery of the Universe, as some newspapers claimed. But the satellite did supply important information that provides a crucial missing link in modern astronomical theory. COBE found that the temperature of the cosmic background radiation differed ever so slightly in different directions. The sky contained hot spots and cold spots, often referred to as ripples. The hot spots were just 30 millionths of a degree hotter than the average temperature of the sky, so it was no wonder it had taken more than a quarter of a century to find them. The motion of the Earth through space created an effect 100 times bigger.
The hot spots marked regions of the early Universe that were marginally less dense than average, while the cold spots marked the denser regions, or lumps. The lumps were on an enormous scale – between 100 million and 2,500 million light years across. They were the oldest and largest structures in the Universe – the ‘seeds’ of giant clusters of galaxies in today’s Universe.
Now at least we knew we existed.
Of course, in interpreting the COBE result, astronomers were making the tacit assumption that the last time the photons of the cosmic background radiation were in contact with matter was when atoms formed 380,000 years after the Big Bang. But what if the photons of the background radiation had interacted with particles of matter during their long journey to the Earth? They might be telling us nothing at all about the lumpiness of matter at the beginning of the Universe.
One way this could have happened was if the Universe had been reheated to thousands of degrees at some time during the past 13.7 billion years. Electrons would have been freed from atoms so that they could scatter the background photons. The reheating could have been caused by an early generation of stars which blazed brightly at the dawn of time, before any galaxies formed. If there had been such a generation of stars, then the cosmic background radiation, instead of carrying a snapshot of the Universe 13.7 billion years ago, might be carrying an imprint of this later era.
But as the COBE team continued to measure the spectrum of the fireball radiation with ever greater precision, this possibility began to look increasingly unlikely. If the Universe had been reheated in the past 13.7 billion years, then this should also show up as a distortion of the fireball spectrum. Instead, the spectrum showed no discernible deviations from a perfect black body, strong evidence that it did indeed come directly from the Big Bang.
Some theorists suggested that on their way to the Earth the photons of the cosmic background radiation might have instead been influenced by the gravity of so-called cosmic strings. These bizarre objects, conjured up by some theorists, were likened to the cracks that form in ice as it freezes, except that these ‘cracks’ formed in the fabric of space as it cooled after the Big Bang. Cosmic strings were bits of space that got left behind in a hot dense state as the Universe cooled. Preserved along their length were the conditions of enormous density that prevailed in the first moments of creation. If cosmic strings were scattered about the Universe, then any cosmic background photons passing near one would lose energy pulling themselves free of its intense gravity.
But the idea that cosmic strings had caused the cold spots COBE had seen in the sky had its problems. If such bizarre objects really exist in the Universe, then they ought to distort the images of distant galaxies. So far, astronomers have not seen such an effect.
However, even if the cosmic background radiation did come straight from the Big Bang, there were things other than the lumpiness of matter that could have left their mark on it. For instance, the hot and cold spots could have been caused by gravitational waves – ripples in the very fabric of space – created by violent events in the first split second after the Big Bang. The American physicist Craig Hogan has even suggested that the variation in the temperature of the sky might be caused by astronomical objects at very large distances. If there were a lot of them, their light could add up and produce the lumpy signal seen by COBE. But Charles Bennett believes this idea can be ruled out. ‘We’ve done correlations with databases of distant extragalactic objects and we don’t find you can explain most of the signal that way,’ he says.
Bennett admits there is no unique way to explain the COBE result. But he thinks the alternative ideas are unlikely. ‘The COBE team feels that the simplest explanation for what we are seeing is lumps of matter in the early Universe,’ he says.
The implications of the COBE result go far beyond galaxy formation. For one thing, the result bolsters the theory that most of the Universe is made of invisible – ‘dark’ – matter. The reason for this is that the lumps of matter COBE had found in the early Universe were simply not big enough for their gravity to pull in the matter to make galaxies or clusters of galaxies in the 13.7 billion years available since the Big Bang. They needed help – from a lot of dark matter.
The peculiar idea that most of the matter of the Universe is invisible had its origin back in the 1930s. It was then that the Swiss-American astronomer Fritz Zwicky discovered a peculiar thing when measuring how fast galaxies were flying about inside clusters of galaxies. Zwicky found that most galaxies were moving faster than they should. They ought to have broken free of the gravitational clutches of their parent clusters long ago and sailed off into the wider Universe.
The only explanation that Zwicky could offer for why they had not sailed off was that the parent clusters contained more matter than he could see with his telescope. It was the combined gravity of this hidden, or dark, matter, said Zwicky, that was keeping the visible galaxies prisoners within their clusters.
Zwicky was a little ahead of his time in coming to this conclusion, and it took the rest of the astronomical community several decades to catch up. But, by the 1980s, it was abundantly clear to everyone that Zwicky’s anomaly could not be swept under the carpet.
The evidence for dark matter was incontrovertible. Everywhere astronomers looked in the Universe they found evidence of its ghostly presence. Even our own Milky Way was found to be embedded in a massive spherical cloud of dark matter, which greatly outweighed all of its visible stars. Astronomers now believe that about 85 per cent of the matter of the Universe is in the form of ‘non-luminous’ dark matter, detectable only because its gravity bends the trajectories of the visible stars and galaxies.
This disconcerting discovery has put astronomers in a hugely embarrassing position. Everything they have been studying with their telescopes these past 400 years turns out to be only a tiny fraction of all there is. Ordinary matter, which scientists have dedicated themselves to understanding – the stuff of planets and stars and the atoms of our own bodies – is no more than a minor contaminant in the Universe.
And what is even more embarrassing to the astronomers is that they have no good idea what the dark matter is made of. There has been no shortage of suggestions. For instance, it could be made of collapsed stars like black holes or even of brown dwarfs, failed stars that are so faint we could easily miss them with our telescopes.1 Then again, the dark matter could be made of hitherto undiscovered subatomic particles. Physicists have given these hypothetical particles names like neutralinos, axions and gravitinos, but nobody is hugely confident that any of them really exists.2
But whatever the true identity of the dark matter, COBE’s discovery of lumps in the early Universe only emphasised that there had to be an awful lot of it around. Without it, clusters of galaxies simply could not form.
According to the accepted theory of galaxy formation, regions of the early Universe where the matter was slightly denser than elsewhere naturally grew at the expense of other regions. They pulled in more and more matter because their gravity was stronger than that of their surroundings. But the trouble with the lumps which COBE found was that they were only marginally denser than their surroundings. It would take the gravity of such lumps longer than the 13.7-billion-year history of the Universe to pull in enough matter to make a cluster of galaxies.
But if the Universe contains a lot of dark matter, the dark matter would have speeded things up because it would have curdled into clumps much sooner after the Big Bang. The reason for this is that it was unaffected by radiation. It neither emitted light nor absorbed it, nor interacted with light in any other way. This was in marked contrast to ordinary matter, which was constantly being blasted apart by the photons of the fireball radiation.
Each clump of dark matter that formed would have exerted a strong gravitational pull on its surroundings. However, ordinary matter would not have fallen into its clutches immediately; the pressure of fireball radiation would have kept it spread out very smoothly. But though it would have been smooth, it would not have been dead smooth. Around the lumps of invisible dark matter, ordinary matter would have been concentrated ever so slightly.
Finally, when atoms formed 380,000 years after the Big Bang, ordinary matter was freed from the tyranny of radiation so that it could begin to clump. At this time, according to the theory, ordinary matter was denser by about ten parts per million in the vicinity of each clump of dark matter than it was on average in the Universe. This is very close to the density difference measured by COBE for lumps of matter in the early Universe.
Once atoms formed and the Universe became transparent to radiation, there was nothing to keep ordinary matter out of the gravitational clutches of the dark matter. It quickly clumped to form stars and galaxies. With dark matter helping it along, this process of galaxy formation was greatly accelerated. In fact, it could be completed in the time available since the Big Bang.
The dark matter we have been talking about so far is known by the theorists as ‘cold’ dark matter. ‘Cold’ just means it consists of some kind of particles that are moving sluggishly. Such particles can be easily tamed by gravity and tend to clump rather like ordinary matter. ‘The cold dark matter model for making galaxies is a beautiful idea,’ says Jim Peebles. ‘I can say that because I was one of the people who invented it!’
Although cold dark matter could have helped galaxies to form more quickly, it has a problem. When astronomers simulate the whole process of galaxy formation on a computer, they find that they end up with clusters of galaxies which are subtly different from those they observe with their telescopes.
In recent years, astronomers have found that the Universe contains structures on scales bigger than they ever expected – great chains and walls of galaxies. Although cold dark matter is good at explaining some of the relatively small structures of the Universe, such as galaxies and clusters of galaxies, it is not good at making these large ones.
It is not absolutely clear that cold dark matter cannot explain these, because there are uncertainties in the observations and the theory. But even some of the proponents of cold dark matter – including Peebles – have begun to worry just a little. ‘The cold dark matter model of galaxy formation is in deep trouble,’ he says.
But there is another type of dark matter that theorists can envisage, and some have invoked it to help explain the way galaxies cluster. It is known as ‘hot’ dark matter. The particles that make this up would have come out of the Big Bang moving very fast – close to the speed of light, in fact. It is difficult for gravity to tame such particles, so they would be spread far more evenly throughout the Universe than particles of cold dark matter.
The gravity of hot dark matter would therefore tend to keep ordinary matter spread out. In contrast with cold dark matter, which is good at making the small-scale structures, hot dark matter would make the large-scale structures.
Some theorists have begun to claim that both types of dark matter are needed in the Universe. Of course, nobody said the Universe had to be simple and that there had to be just one type of dark matter.
Apart from bolstering the theory of dark matter, the hot spots found by COBE were widely claimed to prove another esoteric theory of the early Universe known as ‘inflation’. The theory predicts that hot spots should range over all sizes and that they should have the same temperature no matter what their size – precisely what COBE found. In fact, people went a little overboard in their claims for the theory because inflation is not the only theory to predict this. But the reason they went overboard is understandable: they desperately wanted inflation to be true. In the words of Jim Peebles: ‘If inflation is wrong, God missed a good trick!’
The reason the theory is so attractive to theorists is that it seems to solve at least one major cosmological puzzle, and at the same time explain just what the Big Bang was. ‘Inflation is a beautiful idea,’ says Peebles. ‘However, there are many other beautiful ideas that nature has decided not to use, so we shouldn’t complain too much if it’s wrong.’
According to the theory of inflation, proposed in 1980 by Alan Guth of MIT, there was an era before the Big Bang. Although this era lasted only a split second, the Universe managed to undergo an extraordinarily violent expansion, or ‘inflation’.
It is almost impossible to convey just how violent this expansion was. Some have likened it to a nuclear explosion compared with the hand grenade of the Big Bang. Others have simply pointed out that during inflation, space blew up from a volume smaller than a proton to a volume bigger than the Universe we see today. In numerical terms, inflation made the diameter of the Universe 1050 times bigger, where 1050 is mathematical shorthand for 1 followed by 50 zeroes.
Inflation was over and done by the time the Universe was a million-million-million-million-millionth of a second old. Thereafter, the Universe expanded at a much more sedate pace. This sedate expansion was the Big Bang, which until Guth had come along everyone had considered the most violent explosion imaginable.
The energy to inflate the Universe came from the vacuum. In fact, in the inflationary picture, in the beginning there is only the vacuum. Locked in a weird state with repulsive gravity known technically as the ‘false vacuum’, it expands, creating more vacuum whose repulsive gravity causes the vacuum to expand faster. Here and there, totally randomly, the false vacuum decays into ordinary vacuum. Think of bubbles forming throughout a liquid and you will get the picture. Our Universe was one such bubble among countless others. Inside, the tremendous energy of the vacuum was transformed into other forms, creating matter and heating it to an extraordinarily high temperature. In short, it created the hot Big Bang.
One reason scientists were over-eager to say that the COBE result proved inflation was right was that the theory provides a natural way both to create tiny variations in the density of the Universe during the first split second of creation and then to magnify them to the size seen by the satellite.
It works this way. During the inflationary era, the vacuum was convulsed with so-called quantum fluctuations. Think of it as like the surface of a stormy sea. The places where the sea is high have more energy. And, as it is with a stormy sea, so it was with the inflationary vacuum. The high-energy patches of vacuum were magnified tremendously in size by the enormous inflation of the Universe. When, eventually, they decayed into normal vacuum and created matter, they created slightly more matter than neighbouring patches of decaying vacuum. In this way, they could have spawned the lumps of matter which were seen by COBE.
The implication of this is as startling as the idea of inflation itself. If the theory is right, then the huge chains and walls of galaxies seen by COBE, which are more than 100 million light years across, started out in the newborn Universe as tiny quantum fluctuations smaller than the size of an atomic nucleus. There could be no more dramatic connection between the physics of the very small and the very large.
Inflation is not unique in predicting the properties of the lumps of matter seen by COBE. But what is unique about the theory is that it explains in a very natural way one of the deepest puzzles of the cosmic background radiation: why its temperature is so nearly the same in all directions.
The problem is that the fireball radiation coming from opposite directions in the sky was emitted from regions of the early Universe that could not have been in contact with each other 380,000 years after the Big Bang. However, their temperatures could have kept in step as they cooled only if they were in contact.
To see why, imagine two mugs of hot coffee brought into contact. If the first one begins cooling marginally faster than the second, then heat will flow into the first mug from the second and the pair will promptly be brought back to the same temperature.3 A similar thing will happen if the second mug gets ahead of the first one. The two mugs will cool at the same rate so that at all times they share the same temperature. On the other hand, if the two mugs are not in direct contact – for instance, if they are in different parts of a room – there will be nothing to stop them cooling at different rates. If one is in a draught, for example, its temperature could easily drop more quickly than the other’s.
In the same way, if two regions of the early Universe were to have shared the same temperature as they cooled, heat must have flowed between them. But there is a limit to how fast this could have happened – the speed of light. So two regions could have stayed at the same temperature only if they were close enough for light to have travelled between them in the time since the beginning of the Universe.
And herein lies the problem with the cosmic background radiation. When astronomers look at the fireball radiation coming from opposite sides of the sky, what they are seeing is light emitted by regions that were much further apart than any influence could have travelled in the 380,000 years since the beginning of the Universe. In fact, only regions separated in the sky by less than about two degrees – four times the diameter of the Moon – could possibly have been in touch, and so have any right to share the same temperature.
But if the Universe did indeed go through an inflationary era before the Big Bang, this problem – known as the ‘horizon problem’ – has a very natural solution. Namely, that our Universe inflated from a region smaller than a proton in an atomic nucleus. The region was so small at the time inflation began that light had had plenty of time to cross it since the beginning of the Universe. So regions of the early Universe today seen on opposite sides of the sky were in very close contact before the inflationary era began. They had plenty of time to reach a common temperature.
The hot spots seen by COBE were so large that light could not have crossed them since the beginning of the Universe. This is the strongest evidence that they were imprinted on the Universe well before the time matter and radiation went their separate ways 380,000 years after the Big Bang. But this is not proof that they were imprinted in the first split second of the Universe, as inflation requires.
If inflation is right – and, in truth, the COBE result is simply compatible with the idea – then the implications for the Universe we live in are considerable. The region of space we see with our telescopes may be only a vanishingly small portion of the entire Universe. We are no more than an expanding bubble of space which grew from a region smaller than a proton in one corner of the Universe. Elsewhere, forever inaccessible to us, may be an infinity of other expanding bubble-universes spread throughout space like the froth on a great sea.
1. A black hole is left behind when a very massive star shrinks under its own gravity. In the process, its gravity becomes so strong that even light cannot escape – hence a black hole’s blackness.
2. At the moment, many physicists are carrying out experiments to look for such particles at the bottom of old mines or in mountain tunnels.
3. Heat always flows from a hot body to a cold body – something physicists have enshrined in the second law of thermodynamics.