Companion to the Cosmos

INTRODUCTION

Where do we come from?

Everybody is intrigued by the story of our origins—how the Universe came into being, how it got to be the way it is, and why it is a suitable home for life forms like ourselves. The question ‘where do we come from?’ is the most profound question it is possible to ask, and the ability to provide a reasonably complete answer to that question arguably ranks as the greatest achievement of human thought.

Virtually all of our information about the Universe at large comes from studies of electromagnetic radiation—light, radio waves, X-rays and other variations on the theme—all of which travels at the speed of light, 30,000 million cm per second. Although this is a very large speed, the Universe itself is very large, so that light and other forms of electromagnetic radiation take a long time to reach us from other stars and galaxies. Even for a relatively nearby star, light spends years on its journey to Earth, so that we see the star as it was years ago, when the light left it. Farther away across the Universe, we can detect light from galaxies and quasars so remote that the light has been millions, hundreds of millions or even, in some cases, thousands of millions of years on its journey across space to us, and we see these objects as they were that long ago, when the Universe was correspondingly younger.

The disadvantage of this is that light from such distant objects is very faint by the time it reaches us, and can be analysed only with the aid of powerful telescopes and sensitive electronic detectors. But to a large extent this disadvantage is offset by the great advantage that we are literally looking farther back in time as we look farther out into space, seeing the Universe as it used to be and getting some idea of how it has evolved. The key discovery in all of cosmology is that the Universe we see around us is indeed evolving—that it is different now from the way it used to be, and that it had a definite origin at a certain moment in time. But, in fact, you do not need either large telescopes or sensitive electronic detectors to work that out—all you need is the evidence provided by your own two eyes.

The most fundamental astronomical observation is that night follows day. Although the importance of this observation was not appreciated until the 18th century, and the explanation was not forthcoming at all until the 19th century, and not widely understood until the 1980s, this single observation is enough to tell us that the Universe had an origin at a definite time in the past, and that it has not always been as we see it today.

The fundamental question concerns the darkness of the night sky—how can a dark Universe be full of bright stars? The question is now known as Olbers’ Paradox, after the German astronomer Heinrich Wilhelm Olbers, although he was not, in fact, the first person to puzzle over it. Put at its simplest, the puzzle is that if the Universe is infinite in extent, going on forever in all directions, and if every region of the Universe is similar, in an average sense, to the region we live in, then in every direction we look our line of sight should intersect the surface of a star. Every point on the night sky should be bright!

We now know that stars are grouped into galaxies, like our own Milky Way, islands in the Universe which may each contain hundreds of billions of stars; but the ‘paradox’ can easily be rephrased to take account of that. We also know that, even if the Universe is not infinite, it is certainly big enough for the puzzle to hold with full force—if galaxies like those we see today have existed everywhere in the Universe, forever.

The resolution of the puzzle is straightforward, but required a revolution in the way people thought about the Universe. It is simply that the stars and galaxies have not existed forever—that, if you like, there has not been enough time since the birth of the Universe to fill up all the dark spaces between the stars with light. The darkness of the night sky is alone enough to tell us that the Universe had a definite beginning.

The answer seems obvious, to modern generations reared on the idea of a Universe born in a Big Bang. But it is a sign of how revolutionary the idea was that proper discussions of Olbers’ Paradox only took place decades after the discovery that the Universe is expanding forced astronomers to reject the idea of an eternal, unchanging cosmos and to begin thinking about the evolution of the Universe itself.

The discovery of the expansion of the Universe came only in the late 1920s, when the American astronomer Edwin Hubble and his colleagues established that galaxies are moving apart from one another. Modern cosmology really began only with that discovery, and the time that has elapsed from the discovery of the expanding Universe to the publication of this book is almost exactly one human lifetime, the standard ‘three score years and ten’ referred to in the Bible. Just one lifetime ago, the idea that the Universe was eternal and unchanging was an ‘obvious’ scientific truth, so unquestioned that when Albert Einstein first developed his general theory of relativity, and discovered that the simplest version of the equations required the Universe to be expanding, he added an extra term to the equations to hold them still. He later described this as the ‘biggest blunder’ of his career.

The lesson to be drawn from all this is not that we are so much more clever and insightful than the astronomers of seventy years ago, but that if even Einstein could make such a cosmic blunder we should, perhaps, be wary of taking too much of what we think we know about the Universe at face value. What seems obvious and commonsense to us just may, in another 70 years or so, seem as laughably out of date as we find the notion of an eternal, unchanging Universe. But that is not to say that we should not take any of the present understanding of the Universe and its origins seriously. Stars and galaxies and the structure of the Universe today are very well understood. The question is how far out into the Universe (and correspondingly back in time) we can push our good understanding of the Universe, and where (and when) speculation begins to play a major part. The boundary lies farther back in time, and under much more extreme conditions, than you might guess.

If galaxies are moving apart today, that must mean that they used to be closer together than they are now. One important point about the expanding Universe is that the galaxies are not moving through space, like fragments of bomb from a great explosion, but space itself is stretching, and carrying the galaxies along for the ride (this was the prediction from the general theory of relativity that Einstein himself initially refused to accept). Long ago, there was no space between what are now the galaxies, which must have overlapped each other; before that, there was no space between what are now the stars, which must have touched; and before that, there must have been a time when there was no space between atoms, which merged into one another.

Astronomers know a lot about stars and galaxies. Physicists know a lot about atoms. Astrophysicists have no trouble describing the behaviour of a soup of matter and radiation so thick that individual atoms merge into one another, with electrons from the outer parts of the atoms being dislodged, leaving the nuclei at the hearts of the atoms exposed. Such a soup of nuclei and electrons, together with radiation, is called a plasma. But even this is not the limit of our sound understanding of matter and radiation under extreme conditions. Indeed, experiments carried out at particle accelerator laboratories like the ones at CERN, in Geneva, or Fermilab, in Chicago, provide insight into the behaviour of atomic nuclei themselves, and the protons and neutrons of which they are made. Physicists tell us—and back their claims up with convincing evidence—that they even understand the behaviour of matter, space, time and energy under conditions so extreme that atomic nuclei themselves are packed together cheek by jowl, and are broken up into their constituent parts.

Physicists also make much more extravagant claims about understanding what goes on under much more extreme conditions than this, but those claims are not always so well backed up by a solid weight of evidence. It is at that point that speculation starts to play a part, initially modest, in their cosmological musings, a part which grows in importance as they consider ever more extreme conditions. We know about atomic nuclei, protons and neutrons because all those things exist in the Universe today and can be studied directly in experiments of different kinds. And it is, therefore, not hopelessly unrealistic to believe that physicists really can tell us what things were like when the entire Universe was as dense as the nucleus of an atom is today, and that they can also tell us how it evolved from this hot, dense state (the Big Bang itself) into the collection of galaxies, stars, planets and people that we see today. Indeed, many physicists would argue that we are being cautiously conservative, in setting our sights so low, and in claiming that our really good understanding of the Universe ‘only’ extends from the time when it had nuclear density up to the present day. But that’s OK; in this context, it is better to err on the side of conservatism. So—when was the entire Universe in such a dense, hot state? When was the Big Bang?

If we imagine ‘winding back’ the present expansion of the Universe, it would mean that everything in the Universe as we know it—space, time, matter and energy—emerged from a point of infinite density and zero volume (a singularity) about 15 billion years ago. The exact time is not known, because details of the expansion of the Universe are hard to measure and interpret, but this does not matter. What matters is that the expansion tells us that there was a superdense state, perhaps a bit more than 15 billion years ago, perhaps a bit less, and that taken to extremes the superdense state seems to have started from a singularity. All this is borne out by the equations of the general theory of relativity—but nobody believes that this is really exactly what happened; the effects of quantum physics would dominate the situation close to a singularity, and would ensure that the hypothetical mathematical point was in fact blurred out by a process known as quantum uncertainty.

The question of what exactly went on close to the singularity, and how the quantum processes gave rise to the Big Bang, is one of the most respectable of current cosmological speculations, and attempts to answer that question form the basis of a great deal of research in cosmology today. But there is no need to worry about this just yet. The conditions we are interested in now, as the earliest time and place where the completely solid, securely founded understanding of the physics of objects that exist in our everyday world could be applied, occurred a full tenth of one-thousandth of a second (0.0001 sec) after the time represented by the singularity—sometimes called the ‘moment of creation’ or the ‘birth of the Universe’. In the sense that all of the science involved is well understood, astrophysicists can talk with complete confidence about everything that has happened after the first tenth of one-thousandth of a second; the uncertainties that remain in describing the subsequent evolution of the Universe are simply a result of our imperfect observations of the Universe at large, and our imperfect ability at applying the known laws of physics to describe complicated systems. The interval before that, back to the moment of creation, is still partly a mystery, not only because of our imperfect ability to apply the laws of physics but also because we are not quite sure exactly what the laws of physics are that operate under such extreme conditions.

But by everyday standards, the conditions that existed 0.0001 sec after the moment of creation were extreme enough. At that time, the density of the Universe was 10¹⁴ grams per cubic centimetre—100,000 billion times the density of water. The temperature was 1,000 billion degrees above absolute zero (10¹² K, which for such large numbers is essentially the same as 10¹² Celsius), and the Universe consisted of a cosmic fireball of hot radiation.

Under such extreme conditions, individual particles (such as protons, neutrons and electrons) do not have much of an independent existence. Individual photons from the fireball radiation (‘particles of light’) carry so much energy at these temperatures that they are able to convert themselves into pairs of particles, swapping energy for mass in line with Einstein’s famous equation E = mc². The pairs of particles made in this way almost always consist of an everyday particle (such as a proton) and its so-called antimatter counterpart (in this case, an antiproton). When a particle meets up with an equivalent antiparticle, the pair annihilate, giving back in the form of radiation the energy they were made from. In the Big Bang, radiation was constantly being turned into matter, and matter was constantly being turned back into radiation, in a seething maelstrom of activity.

But as the cosmic fireball expanded and cooled, the individual photons in the fireball had less and less energy. Soon, they did not have enough energy to make any more protons and neutrons. If the conversion of radiant energy into matter-antimatter pairs had always been precise, that would have meant that the cooling Universe was left with an exactly equal number of protons and antiprotons, and an exactly equal number of neutrons and antineutrons. Before long, under those extreme conditions, every particle would have met an antiparticle partner and annihilated, leaving nothing but radiation in the cooling Universe. But because of a tiny imbalance in the laws of physics, whose significance was first appreciated in the 1960s by the Soviet physicist Andrei Sakharov, there was a tiny excess of the kind of matter we are made of left over at the end of this process—there was just one everyday particle left over for every billion photons of radiation left in the fireball. Everything that we can see in the Universe today is made out of the one-in-a-billion particles (protons+neutrons) manufactured in this way in the Big Bang fireball.

By one-hundredth of a second after the moment of creation, things were calming down a little. The temperature had dropped to 100 billion K (10¹¹ K), and protons and neutrons were no longer being manufactured out of radiation, although they were still being buffeted by the dense sea of photons in which they swam. Initially, there was the same number of neutrons as there was protons. But neutrons, unlike protons, are unstable particles, and left to their own devices they will each spit out an electron (in a process known as radioactive decay) and convert themselves into protons. Today, this process is slow compared with the changes that were going on in the Universe when it was a fraction of a second old. On average, if you have an isolated neutron, it takes more than 10 minutes for it to decay in this way. But the buffeting that neutrons were getting in the cosmic fireball encouraged the change. So by the time the temperature of the Universe had fallen to 30 billion K, just over one-tenth of a second after the moment of creation, the proportion of neutrons to protons had dropped from 50:50 down to 38 per cent neutrons and 62 per cent protons. By the time the Universe had cooled to 10 billion K, 1.1 seconds after the moment of creation, the density was down to 380,000 times that of water and there were only 24 neutrons left for every 76 protons. But, like most of us, the Universe slows down and is less susceptible to change as it gets older; at last, the headlong pace of change in the early Universe had slowed to the point where change could be measured over seconds, rather than in fractions of a second.

After 13.8 seconds, the temperature had dropped to 3 million K and, with correspondingly less energy available to knock them about, the rate at which neutrons were being converted into protons had slowed dramatically. There were still 17 neutrons left for every 83 protons in the Universe, and occasionally nuclei of the isotope deuterium (heavy hydrogen) could form in the fireball as an individual proton and an individual neutron stuck together temporarily before being knocked apart in a collision. Just 3 minutes and 2 seconds after the moment of creation, the temperature of the entire Universe had cooled to the point where it was only 1 billion K, 70 times as great as the temperature at the heart of the Sun today, which is some 15 million K. There were still 14 neutrons around for every 86 protons, and by now the Universe was so old that the natural decay of the neutrons started to be important. Although the average lifetime of a free neutron is more than 10 minutes, as is the way with averages, some live longer and some decay sooner. In every 100 seconds from now on, one in ten of the remaining free neutrons would turn itself into a proton spontaneously. But the neutrons were saved from extinction because just at this time, a little over three minutes into the life of the Universe, conditions had eased to the point where neutrons began to combine with protons to form stable nuclei, first of deuterium and then of helium. The nuclei still collided with each other, and with other particles, but the temperature was now so low that there was not enough energy in these collisions to break the nuclei up. Almost immediately, the remaining neutrons (about 13 for every 87 protons) were locked up in nuclei of helium-4, each of which contains two neutrons and two protons. The proportion of the total mass of neutrons and protons converted into helium was just twice the proportional number of neutrons, which is 26 per cent, and the process was completed by 3 minutes and 46 seconds after the moment of creation.

The exact numbers that emerge from this description of the birth of the Universe are not just pulled out of the hat. They come from a combination of the general theory of relativity, which tells us how fast the Universe was expanding and cooling, and the known facts about the behaviour of particles such as neutrons and protons, and atomic nuclei, deduced from experiments here on Earth. This combination provides the so-called ‘standard model’ of cosmology, and one of its great triumphs is that it predicts that 26 per cent of the mass of each of the first stars formed in the Universe (which are therefore the oldest stars seen today) should be in the form of helium. This exactly matches the actual amount of helium observed in old stars, using the technique of spectroscopy. The other great prediction of the standard model is that the Universe should be filled with a sea of radiation left over from the fireball. This radiation, at a temperature of about 1 billion K when helium began to form less than 4 minutes after the moment of creation, should have cooled, according to the standard model, all the way down to below 3 K (just under -270°C) today, 15 billion years later. The cosmic microwave background radiation discovered in the 1960s, and predicted by the Big Bang theory, exactly fits the bill. It is hardly any wonder, then, that the standard Big Bang model of the Universe is regarded as one of the jewels in the crown of modern science.

Once helium had formed in the expanding Universe, the processes which led to the formation of galaxies, stars, planets and people seem to have proceeded inexorably on their way. Some of the details of these later processes remain obscure. This is partly because they represent increasingly complex phenomena—one of the strangest aspects to take on board about cosmology is that we know so much about the fireball of the Big Bang because it was a very simple place, running in accordance with detailed laws of physics that are very well understood, and involving a few simple kinds of particle, such as protons, neutrons and electrons. Once you get involved with atomic nuclei, you have more complexity to deal with; atoms themselves interact in accordance with the laws of chemistry, producing another layer of complexity; and life itself involves extremely complex chemistry and the way some complex arrangements of atoms and molecules (such as people) interact with their physical environment. We know less and less about more and more complex systems.

Another source of uncertainty in bringing the story of the Universe up to date concerns the amount of dark matter there is in the Universe, and its nature. Obviously, not everything in the Universe is in the form of a bright star that we can see with our telescopes. Indeed, there is sound evidence that tens of times as much dark matter came out of the cosmic fireball as there was matter in the form of protons and neutrons, the kind of nuclear matter which forms stars, galaxies, planets and people. The influence of this dark matter is seen by the way in which it tugs on the visible matter through gravity. Without the dark matter, nuclear matter would have been spread ever more thinly as the Universe expanded, never clumping together to form stars and galaxies at all; it is only thanks to the gravitational influence of the dark stuff that we exist.

In broad outline, the story of how we got to be here can be picked up about 300,000 years after the Big Bang. At that point, the Universe was still a hot soup, at a temperature of about 5,000 K, a little cooler than the surface of the Sun is today. The nuclear stuff of the Universe was largely in the form of individual protons (nuclei of hydrogen atoms) and helium nuclei, moving in a sea of electrons and dark matter. Until that time, any nucleus that tried latching on to an electron or two and forming an atom would quickly be involved in a collision with an energetic photon, which would rip the electron away from it. Because nuclei each have a positive electric charge and electrons each have a negative charge, and photons like to interact with charged particles, this meant that the Universe was full of charged particles, interacting with photons and making the Universe opaque. No photon could travel far without bouncing off a charged particle and continuing on its journey in a crazy zig-zag path, like the ball in some demented cosmic pinball machine.

Then, quite suddenly by cosmological standards, as the temperature fell the photons no longer had enough energy to disrupt atoms as they tried to form. Each proton captured an electron, each helium nucleus captured two electrons, and all the charged particles were locked up as electrically neutral atoms. There were no longer any charged particles for photons to bounce off, and they streamed through space, around the atoms, essentially unhindered. Overnight, as it were, the Universe became transparent. It is the radiation from that time, a few hundred thousand years after the Big Bang, which has been streaming silently through transparent space ever since, and which we now detect as the background radiation.

When the atoms formed, they were already clumped together in large streamers and sheets of higher than average density, pulled together by the gravitational influence of dark matter in the Universe. Within the great sheets of atomic material formed in this way, even though the Universe as a whole continued to expand, large quantities of gas were pulled together by gravity into sheets surrounding voids of atomic material. As gravity pulled the gas into thinner sheets, clumps formed within the sheets and shrank in their turn, breaking up into smaller fragments, which shrank and fragmented in their turn (actually, not literally ‘in their turn’ in the sense of one after another; the fragmentation and collapse was going on at all levels simultaneously). The smallest fragments formed in this way became stars, nested inside galaxies, which are themselves nested inside clusters of galaxies within superclusters that form chains, filaments and sheets of bright stuff, forming a frothy distribution of visible material throughout the dark void of the Universe at large.

It was only after the first generations of stars formed in this way and ran through their life cycles that stars like the Sun and planets like the Earth could form. The first stars contained only hydrogen and helium. Heavier elements, including such atoms as those of carbon, oxygen and nitrogen that are essential for life as we know it, were manufactured inside stars by nuclear fusion, and spread through large regions of the young galaxies when the more massive and short-lived of those first-generation stars exploded at the ends of their lives.

The Sun formed much later, only about 5 billion years ago, from the debris of such stellar explosions. The specific collapsing cloud of gas from which the Sun was born probably contained enough matter to make several hundred stars, which formed together as the fragmenting cloud collapsed, but have since gone their separate ways. One glob of matter in that cloud contained a little more mass than the Sun has today, and as the glob collapsed under its own weight, most of the matter formed a hot ball of gas while some formed a ring of material around the embryonic star. The heat of the young star blew away many of the lighter atoms from the ring, leaving a system made up of tiny particles of dust which gradually stuck together and aggregated to form the planets. After that, the story involves geophysics and biology, not cosmology and cosmogony, as life emerged and evolved on the surface of at least one of those planets.

One of the most important features of this story, which still puzzles many astronomers, is that, although it all seems inevitable given what we know about the way the laws of physics work, only very small changes in the way those laws work could have prevented all this happening. Could the laws have been different? For example, if the Universe had expanded slightly more slowly, by the time it had cooled to the point where helium nuclei could form there would have been no neutrons left to make helium nuclei with. If the expansion had proceeded only a little more quickly, there would have been so many neutrons left that virtually all of the nuclear matter would have emerged from the Big Bang in the form of helium, with no free protons left to make hydrogen at all. Either way, the Universe would have been a very different place. Stars made entirely of helium, for example, would rapidly run through their life cycles and quickly fade away, perhaps not allowing time enough for life to evolve on any planets that orbited them.

The fact that the Universe contains some helium, but not 100 per cent, depends on a balance between gravity (which determines how fast the Universe expands) and the nuclear forces involved in the formation of helium nuclei, which determine how quickly protons and neutrons combine to make helium. If the balance had been slightly different, we would not exist; so the fact that we exist can be used to work out what some of the properties of the Universe, and the laws of physics, must be. This is an example of what is known as anthropic reasoning, or anthropic cosmology; there is a lively debate about whether this is a mere tautology or whether it can be used to tell us something deeply significant about the way the Universe works.

One exciting possibility is that there may be other universes, in which the laws of physics operate in different ways to the way they operate in our Universe, and life forms like us cannot exist. This brings us right back to the question of what went on before the Big Bang, during the first split-second of the existence of the Universe, and at the moment of creation itself. This is the realm of inflationary cosmology, the most important and dramatic development in cosmology today.

The search for the Big Bang ended in the spring of 1992, when NASA astronomers announced the results of observations of the cosmic background radiation made by the COBE satellite. Those observations left no room to doubt that the Universe as we know it has evolved from a very hot, very dense state—the Big Bang. But they went further. Tiny variations in the strength of the background radiation from place to place on the sky, which soon became known as ‘ripples’, exactly matched the predictions of the theory of inflation, developed during the 1980s. The combination of inflation and the hot Big Bang was established beyond reasonable doubt as the only good description of the birth of the Universe.

But that does not mean that there is no more work for the cosmologists to do, or that there will be no more exciting stories about the birth of the Universe. With the success of the Big Bang theory, attention has now turned to how and why the Universe got to be in a hot, dense state in the first place—the details of the theory of inflation. There are now many variations on this particular theme, opening up for truly scientific debate one of the last provinces of the philosophers and metaphysicists, and addressing the question ‘how did time itself begin?’

At the same time, with the basics of the Big Bang theory firmly in place, cosmologists are increasingly interested in the details of how the Universe got to be the way we see it today. One of the predictions of inflation is that the Universe should contain a great deal of dark matter, much more than we can see in the form of bright stars and galaxies. The presence of at least some of this dark stuff is also revealed, as we have mentioned, by the way that galaxies move. There is a lively debate among the experts about exactly what this dark matter might be, and experimenters on Earth are trying to capture particles of the dark matter in their laboratories.

Another enduring puzzle is the exact value of the Hubble constant, the number which measures how rapidly the Universe is expanding. It is still extremely difficult to measure this parameter, which also indicates the age of the Universe, and current estimates range from about 50 km per second per Megaparsec to about 80 km per second per Megaparsec.

All of these debates about the details of the Big Bang are sometimes presented in news stories as if the uncertainties threaten the Big Bang theory itself. For example, if the Hubble constant is indeed as large as 80, in the usual units, then the simplest version of the cosmological models would tell us that the time that has elapsed since the Big Bang is less than 10 billion years. That would be embarrassing, since many stars are known to be older than that. Obviously, the Universe cannot be younger than the stars it contains! But even if this measurement of the Hubble constant turned out to be correct, that would not spell the death of the Big Bang theory. More subtle versions of the cosmological models can quite easily accommodate such a large value of the constant and keep the age of the Universe greater than that of its oldest stars. It is always nice if the simplest version of a theory turns out to be a good description of reality—but as we all know from everyday experience, life just is not like that, so why should the Universe be so simple? As Richard Feynman has pointed out, the simplest thing would be nothing at all, and nature is far more inventive than that (see Brian Hatfield, ed., Feynman Lectures on Gravitation, Addison-Wesley, 1995).

One possibility is that we are being too parochial, and that our corner of the Universe, even though it may be billions of light years across, is not big enough to provide a reliable guide to the Universe at large. Cosmologists baffled by the apparent evidence that the Universe is younger than the stars it contains may simply have been guilty of reading too much into our immediate surroundings in the Universe. According to a group of Chinese researchers, the problem is that we live in a low-density bubble which is not typical of the Universe at large. When the appropriate measurements are made on large enough scales, everything slots into place.

The kinds of scale that cosmologists deal with are much greater than the distances between stars. They are interested in the distances between clusters of galaxies, and regard a whole galaxy of several hundred billion stars, like our Milky Way, merely as a ‘test particle’ in the Universe at large. Their efforts to measure the scale of the Universe are rather like trying to measure the distribution of island archipelagos across the Pacific Ocean from a base on one of those islands—with the added complication that each archipelago is moving apart from every other archipelago as the Universe expands.

The key question, which has not really been considered much by cosmologists until now, is how typical the region of the Universe over which we can make these measurements is. Just as the hypothetical Pacific islander mapping the known ‘universe’ may be unaware of the existence of the continents on either side of the ocean, so our local bubble of space may not give us enough information to predict the behaviour of the entire Universe. Xiang-Ping Wu, of the Beijing Astronomical Observatory, and several colleagues, suggested in 1995 that this is indeed the case.

They pointed out that, although this kind of study of the Universe extends out to distances of a few hundred million light years, if the measurements made for clusters at different distances are analysed separately, instead of all being lumped together to give one average figure, they show that the density of matter in the Universe increases the further out we look. On a scale of about 30 million light years, the density is only 10 per cent of the critical value, while on a scale of 300 million light years it may be as much as 90 per cent of the critical value.

The direct implication of this is that, on the scale over which recent measurements of the expansion of the Universe have been made by the Hubble Space Telescope, the expansion rate (given by the Hubble constant) is bigger than the overall average expansion rate by as much as 40 per cent. This means that the age of the Universe has been underestimated by 40 per cent, which is almost exactly the correction needed to boost the age from about 8 billion years to about 12 billion years, matching the ages of the oldest stars. In cosmological terms, it may be that our Pacific islanders have just discovered America.

But whether or not the volume of space we live in is typical of the Universe at large, as we look towards the way cosmology will develop as we enter the 21st century, inflation seems certain to hold centre stage. The marriage between quantum physics and cosmology could be said to have been consummated by the discoveries made by COBE, and since confirmed and refined by several other observations, both from space and from ground-based experiments. The key question that the discovery of the cosmic ripples answered was how irregularities as large as galaxies and clusters of galaxies could ever have grown up in the Universe as it emerged from the Big Bang fireball. The ripples correspond to fluctuations in the temperature of the background radiation from different parts of the sky of only 30 millionths of a degree Kelvin, representing an incredible achievement in measuring them at all. But fluctuations of this size, corresponding to differences in the density of the Universe from place to place at the time when matter and radiation decoupled and went their separate ways, are exactly right to have allowed the growth of the kind of irregularities we see in the Universe today over many billions of years.

The icing on the cake is that inflation can tell us where these irregularities seen at the era of decoupling came from. During the split-second of rapid expansion that we call inflation, quantum processes should, according to the theory, have created tiny distortions in the structure of the Universe. Inflation took these quantum irregularities, and blew them up to the size of clusters of galaxies. The nature of the cosmic ripples measured by COBE exactly matches the kinds of distortion that would have been produced in this way.

This success is all the more impressive because inflation theory was not invented in order to explain where galaxies came from. The motivation for the development of the theory came from two puzzles that astronomers began to focus on in the 1970s. The first is called the horizon problem, and is simply that the Universe looks the same in all directions—in particular, the temperature of the background radiation is the same on opposite sides of the sky. But how do regions on opposite sides of the sky know how to keep in step with each other? After all, there has not been sufficient time since the Big Bang for light (or anything else) to travel across the Universe and back. The second puzzle is related to the presence of dark matter. It is that the Universe is very nearly flat, in the sense that it sits just on the dividing line between expanding forever and one day recollapsing.

The flatness problem can be understood in terms of the density of the Universe. The density parameter is a measure of the amount of gravitating material in the Universe, usually denoted by the Greek letter omega (Ω), and also known as the flatness parameter. It is defined in such a way that if spacetime is exactly flat then Ω = 1. Before the development of the idea of inflation, one of the great puzzles in cosmology was the fact that the actual density of the Universe today is very close to this critical value—certainly within a factor of ten. This is curious because, as the Universe expands away from the Big Bang, the expansion will push the density parameter away from the critical value. If the Universe starts out with the parameter less than 1, Ω gets smaller as the Universe ages, while if it starts out bigger than 1 Ω gets bigger as the Universe ages. The fact that Ω is between 0.1 and 1 today means that in the first second of the Big Bang it was precisely 1 to within one part in 10⁶⁰. This makes the value of the density parameter in the beginning one of the most precisely determined numbers in all of science, and the natural inference is that the value is, and always has been, exactly 1. One important implication of this is that there must be a large amount of dark matter in the Universe. Another is that the Universe was made flat by inflation.

It works like this. During the first split-second of the existence of the Universe, everything we can see now was squashed together in a region smaller than the nucleus of an atom today. The hot fireball was so small that there was no problem about photons criss-crossing it at the speed of light and smoothing everything into a homogeneous state. But under these conditions, quantum processes associated with the development of the separate forces of nature that we know about today (gravity, the electromagnetic force and the two forces that operate inside nuclei) acted as a kind of antigravity, forcing the embryonic Universe to expand extremely rapidly. This expansion blew up the tiny fireball to at least the size of a basketball, and perhaps much bigger, in a tiny fraction of a second, doubling the size of the Universe hundreds of times in an interval too short to be measured by any conventional clock. Then, inflation switched off, and the more sedate expansion described by the standard Big Bang model and outlined earlier took over.

The key point is that during inflation opposite regions of the Universe were blasted apart, in a sense, ‘faster than light’. Nothing can travel through space faster than light, but it was space itself that expanded in this way, carrying regions that used to be literally in touch with one another out of the range of light communication. That is why opposite sides of the Universe still have the same temperature today—they started out the same, even though they have not ‘communicated’ with one another since the first split-second after the birth of the Universe. This awesome expansion also ensures that spacetime gets highly flattened, in much the same way that the wrinkly surface of a prune becomes a smooth, flat surface when the prune is placed in water and swells up.

The reason why inflation has become established as the most exciting area of cosmological research today is that the ideas from quantum theory that led to this dramatic resolution of the cosmological puzzles were entirely derived from studies of particle physics, the so-called grand unified theories, or GUTs. The physicists who developed GUTs had no idea that they could be applied to describe the birth of the Universe; the fact that these theories work so well in solving problems they were never designed for is taken as strong evidence that they are telling us something fundamental about the way the Universe works.

Quantum theory can even explain the origins of the tiny primordial seed from which the Universe as we know it inflated—so-called ‘quantum fluctuations’ allow such minuscule regions of intense energy to appear literally out of nothing at all, making the Universe, in a memorable phrase, ‘the ultimate free lunch’. An alternative possibility suggests that new baby universes may be created by inflation where matter collapses into a black hole, forming a singularity like the one in which the Universe was born, from which inflation creates a new universe. On that picture, our own Universe may have formed from a black hole in another universe, and so on, in an interconnected web which had no beginning and will have no end.

The clean simplicity of the original simple picture of inflation has now, inevitably, begun to be obscured by refinements, as inflationary cosmologists add bells and whistles to their models to make them match more closely the Universe we see about us. Some of the bells and whistles, it has to be said, are studied just for fun. Andrei Linde, who now works at Stanford University and has been the chief proponent of inflation in the 1990s, has taken great delight in pushing inflation to extremes, and offering entertaining new insights into how the Universe might be constructed. For example, could our Universe exist on the inside of a single magnetic monopole produced by cosmic inflation? According to Linde, it is at least possible, and may be likely. And in a delicious touch of irony, Linde made this outrageous claim in a lecture at a workshop on the birth of the Universe held in Rome, where the view of creation is usually rather different.

One of the additional reasons why theorists came up with the idea of inflation in the first place (alongside the horizon problem and the flatness problem) was precisely to get rid of magnetic monopoles—strange particles carrying isolated north or south magnetic fields, predicted by many grand unified theories of physics, but never found in nature. Standard models of inflation solve the ‘monopole problem’ by arguing that the seed from which our entire visible Universe grew was a quantum fluctuation so small that it contained only one monopole. That monopole is still out there, somewhere in the Universe, but it is highly unlikely that it will ever pass our way.

But Linde has discovered that, according to theory, the conditions that create inflation persist inside a magnetic monopole even after inflation has halted in the Universe at large. Such a monopole would look like a magnetically charged black hole, connecting our Universe through a wormhole in spacetime to another region of inflating spacetime. Within this region of inflation, quantum processes can produce monopole-antimonopole pairs, which then separate exponentially rapidly as a result of the inflation. Inflation then stops, leaving an expanding Universe rather like our own, which may contain one or two monopoles, within each of which there are more regions of inflating spacetime.

The result is a never-ending fractal structure, with inflating universes embedded inside each other and connected through the magnetic monopole wormholes. Our Universe may be inside a monopole which is inside another universe which is inside another monopole, and so on indefinitely. What Linde calls ‘the continuous creation of exponentially expanding space’ means that ‘monopoles by themselves can solve the monopole problem’. Although it seems bizarre, the idea is, he stresses, ‘so simple that it certainly deserves further investigation’.

That variation on the theme really is just for fun, and it is hard to see how it could ever be compared with observations of the real Universe. But most of the modifications to inflation now being made are in response to new observations, and in particular to the suggestion that spacetime may not be quite ‘flat’ after all. In the mid-1990s, many studies (including observations made by the refurbished Hubble Space Telescope) began to suggest that there might not be quite enough matter in the Universe to make it perfectly flat—most of the observations suggest that there is only 20 per cent or 30 per cent as much matter around as the simplest versions of inflation require. At first sight, the shortfall is embarrassing, because one of the most widely publicized predictions of simple inflation was the firm requirement of exactly 100 per cent of this critical density of matter. But there are ways around the difficulty; and here are two of them to be going on with.

The first suggestion is almost heretical, in the light of the way astronomy has developed since the time of Copernicus. Is it possible that we are living near the centre of the Universe? For centuries, the history of astronomy has seen humankind displaced from any special position. First the Earth was seen to revolve around the Sun, then the Sun was seen to be an insignificant member of the Milky Way Galaxy, then the Galaxy was seen to be an ordinary member of the Universe. But now comes the suggestion that the ‘ordinary’ place to find observers like us may be in the middle of a bubble in a much greater volume of expanding space.

The conventional version of inflation says that our entire visible Universe is just one of many bubbles of inflation, each doing their own thing somewhere out in an eternal sea of chaotic inflation, but that the process of rapid expansion forces spacetime in all the bubbles to be flat. A useful analogy is with the bubbles that form in a bottle of fizzy cola when the top is opened. But that suggestion, along with other cherished cosmological beliefs, has now been challenged by Linde, working with his son Dmitri Linde (of CalTech) and Arthur Mezhlumian (also of Stanford).

Linde and his colleagues point out that the Universe we live in is like a hole in a sea of superdense, exponentially expanding inflationary cosmic material, within which there are other holes. All kinds of bubble universes will exist, and it is possible to work out the statistical nature of their properties. In particular, the two Lindes and Mezhlumian have calculated the probability of finding yourself in a region of this super-Universe with a particular density—for example, the density of ‘our’ Universe.

Because very dense regions blow up exponentially quickly (doubling in size every fraction of a second), it turns out that the volume of all regions of the super-Universe with twice any chosen density is 10 to the power of 10 million times greater than the volume of the super-Universe with the chosen density. For any chosen density, most of the matter at that density is near the middle of an expanding bubble, with a concentration of more dense material round the edge of the bubble. But even though some of the higher-density material is round the edges of low-density bubbles, there is even more (vastly more!) higher-density material in the middle of higher-density bubbles, and so on forever.

The discovery of this variation on the theme of fractal structure surprised the researchers so much that they confirmed it by four independent methods before venturing to announce it to their colleagues. Because the density distribution is nonuniform on the appropriate distance scales, it means that not only may we be living near the middle of a bubble universe, but the density of the region of space we can see may be less than the critical density, compensated for by extra density beyond our field of view.

This is convenient, since those observations by the Hubble Space Telescope have suggested that cosmological models which require exactly the critical density of matter may be in trouble. But there is more. Those Hubble observations assume that the parameter which measures the rate at which the Universe is expanding, the Hubble constant, really is a constant, the same everywhere in the observable Universe. If Linde’s team is right, however, the measured value of the ‘constant’ may be different for galaxies at different distances from us, truly throwing the cat among the cosmological pigeons. We may seem to live in a low-density universe in which both the measured density and the value of the Hubble constant will depend on which volume of the Universe these properties are measured over. It might sound like the fevered imaginings of an over-enthusiastic theorist—if it were not for those intriguing Chinese observations which suggest that the Universe around us really is like that!

That would mean abandoning many cherished ideas about the Universe, and may, in spite of the observational evidence, still be too much for many cosmologists to swallow. But there is a simpler solution to the density puzzle, one which involves tinkering only with the models of inflation, not with long-held and cherished cosmological beliefs. That may make it more acceptable to most cosmologists—and it is so simple that it falls into the ‘why did I not think of that?’ category of great ideas.

A double dose of inflation may be something to make the government’s hair turn grey—but it could be just what cosmologists need to rescue their favourite theory of the origin of the Universe. By turning inflation on twice, they have found a way to have all the benefits of the inflationary scenario, while still leaving the Universe in an ‘open’ state, so that it will expand forever.

In the simplest inflation models, the big snag is that after inflation even the observable Universe is left like a mass of bubbles, each expanding in its own way. We see no sign of this structure, which has led to refinements of the basic model to ensure homogeneity. Now, however, Martin Bucher and Neil Turok, of Princeton University, working with Alfred Goldhaber, of the State University of New York, have turned this difficulty to advantage.

They suggest that, after the Universe had been flattened by the original bout of inflation, a second burst of inflation could have occurred within one of the bubbles. As inflation begins (essentially at a point), the density is effectively ‘reset’ to zero, and rises towards the critical density as inflation proceeds and energy from the inflation process is turned into mass. But because the bubble from which the Universe is expanding has already been homogenized, there is no need to require this bout of inflation to last until the density reaches the critical value. It can stop a little sooner, leaving an open bubble (what we see as our entire visible Universe) to carry on expanding at a more sedate rate.

According to Bucher and his colleagues, an end product looking very much like the Universe we live in can arise naturally in this way, with no ‘fine tuning’ of the inflationary parameters. All they have done is to use the very simplest possible version of inflation, but to apply it twice. And you do not have to stop there. Once any portion of expanding spacetime has been smoothed out by inflation, new inflationary bubbles arising inside that volume of spacetime will all be pre-smoothed and can end up with any amount of matter from zero to the critical density (but no more). This should be enough to make everybody happy. Indeed, the biggest problem now is that the vocabulary of cosmology does not quite seem adequate to the task of describing all this activity.

The term ‘Universe’, with the capital ‘U’, is usually used for everything that we can ever have knowledge of, the entire span of space and time accessible to our instruments, now and in the future. This may seem like a fairly comprehensive definition, and in the past it has traditionally been regarded as synonymous with the entirety of everything that exists. But the development of ideas such as inflation suggests that there may be something else beyond the boundaries of the observable Universe—regions of space and time that are unobservable in principle, not just because light from them has not yet had time to reach us, or because our telescopes are not sensitive enough to detect their light.

This has led to some ambiguity in the use of the term ‘Universe’. Some people restrict it to the observable Universe, while others argue that it should be used to refer to all of space and time. If we use ‘Universe’ as the name for our own expanding bubble of spacetime, everything that is in principle visible to our telescopes, then maybe the term ‘Cosmos’ can be used to refer to the entirety of space and time, within which (if the inflationary scenario is correct) there may be an indefinitely large number of other expanding bubbles of spacetime, other universes with which we can never communicate. Cosmologists ought to be happy with the suggestion, since it makes their subject infinitely bigger and therefore infinitely more important; and it is in this spirit that we have offered you not merely a guide to the Universe, but a Companion to the Cosmos.

Note to the Reader

Cross-references to another entry are printed in italic bold type. Cross-references are selective and are used only when the other entries are directly relevant to the entry that is being read.

A-Z DICTIONARY

A

AAT = Anglo-Australian Telescope.

aberration An apparent shift in the position of a star caused by the finite speed of light and the motion of the Earth in its orbit around the Sun. Over a year, the star seems to move in a small ellipse around its average position. The effect was discovered by James Bradley in 1729 and used by him to measure the speed of light.

absolute magnitude The apparent magnitude that a star or other bright object would have if it were at a distance of exactly 10 parsecs from the observer (see magnitude scale).

absolute zero The lowest temperature that could ever be attained. At absolute zero, atoms and molecules would have the minimum amount of energy allowed by quantum theory. This is defined as 0 on the Kelvin (K) scale of temperature; 0K is -273.15°C, and each unit on the Kelvin scale is the same size as 1 degree Celsius.

absorption line A narrow feature in a spectrum, corresponding to absorption of electromagnetic radiation at a well-defined wavelength. The pattern of absorption lines in a spectrum is like a fingerprint, identifying the elements that are absorbing the radiation.

absorption nebula A cold cloud of gas and dust in space that is visible only because it blocks out the light from more distant stars. See nebulae.

abundance of the elements See cosmic abundances.

accretion Two kinds of accretion are important in the Universe. The first is the process where small particles collide with one another and stick together to make larger objects. Collisions have to be ‘just right’ for this to occur—if the impacts are too hard, they will break up the objects (fragmentation) instead of allowing them to stick together. When the Solar System formed from a cloud of gas and dust in space, collapsing under its own gravitational pull, the young Sun was left surrounded by a disc of material which settled into the equatorial plane. This must have been rather like a grander version of the rings seen around Saturn today. The planets and other objects in the Solar System formed by accretion in this swirling disc of material, starting with tiny grains less than 1 mm across.

The second kind of accretion occurs when a massive object gathers in material from its surroundings by the pull of its gravitational field. This happens in a modest way for an ordinary star like our Sun, which is constantly accreting material from interstellar space. Much more dramatic accretion can occur with objects that have stronger gravitational fields, such as neutron stars and black holes. Then, material being sucked down on to the object (perhaps from a nearby companion star in a binary system) forms an accretion disc. As material gains energy by falling in the gravitational field, and the atoms collide with one another in the disc, they can become so hot that they radiate X-rays. Processes like this, operating on a large scale at the centres of some galaxies, involving black holes many millions of times more massive than our Sun, may provide the power source in quasars.

accretion disc A ring of material surrounding a star or other object from which matter spirals inward to fall on the object inside the disc. See accretion.

active galaxy A galaxy that is emitting a large amount of energy from its central region, known as the nucleus. This gives such objects an alternative name, active galactic nucleus, usually shortened to AGN. The term covers a variety of energetic galaxy types discovered at different times and given different names, including Seyfert galaxies, N galaxies, BL Lac objects and quasars. It is now thought that these are all powered by essentially the same process, involving the accretion of matter on to a supermassive black hole at the centre of the active galaxy.

Active galaxy. The active galaxy Cygnus A, ‘photographed’ at radio wavelengths. The galaxy itself is a small dot at the centre of the image; activity associated with a black hole at the heart of the galaxy emits two jets of material in opposite directions.

As material from the galaxy falls into the black hole, gravitational energy associated with its mass is released and converted into electromagnetic radiation, including light, X-rays and radio waves. This process is so efficient that 10 per cent or more of the mass of the