Cosmic Coincidences

COSMIC COINCIDENCES

Dark Matter, Mankind, and Anthropic Cosmology

by

JOHN GRIBBIN AND MARTIN REES

Produced by ReAnimus Press

Other books by John Gribbin and Martin Rees:

In Search of the Big Bang

Ice Age

In Search of the Double Helix

Q is for Quantum

The Sad Happy Story of Aberystwyth the Bat

A delightful children's tale by the best-selling author John Gribbin's son Ben

© 2014, 1989 by John Gribbin and Martin Rees. All rights reserved.

http://ReAnimus.com/authors/johngribbin

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Table of Contents

INTRODUCTION

PART ONE

CHAPTER ONE

CHAPTER TWO

CHAPTER THREE

PART TWO

CHAPTER FOUR

CHAPTER FIVE

CHAPTER SIX

CHAPTER SEVEN

CHAPTER EIGHT

CHAPTER NINE

PART THREE

CHAPTER TEN

CHAPTER ELEVEN

FURTHER READING

ABOUT THE AUTHORS

INTRODUCTION

Why Are We Here?

There are three motives for studying the Universe. The first is discovery: to learn what’s out there, whether in our own Solar System or in the extragalactic realm. This vicarious exploration—of the surface of Mars, or the patterns of spiral galaxies—is something a wide public can share.

For the astrophysicist, this exploration is preliminary to a second goal: to understand and interpret what we see, in terms of the laws of physics established here on Earth, and to place our entire Solar System in an evolutionary context that can be traced back to the birth of the Milky Way Galaxy, and beyond—right back, indeed, to the initial instants of the so-called Big Bang with which our Universe began.

To the physicist, there is a third motive: The cosmos is a laboratory offering more extreme conditions than can be simulated on Earth. Known laws can be tested, perhaps to the breaking point, by applying them, for instance, to the amazing densities of neutron stars; and a better understanding of the astounding temperatures and energies of the Big Bang could reveal new laws. Essentially all that we know about gravity—one of the four fundamental forces, and the one that controls the motions of stars, galaxies, and the entire expanding Universe—comes from astronomy.

Astronomy is, of course, an old pursuit—perhaps it was the first science to become professionalised—but it has greatly enlarged its scope during the past two decades. Recent progress has been largely driven by experimental and observational advances. No armchair theorist, even equipped with current physical knowledge, could have envisaged the extraordinary phenomena and objects that have been discovered. This burgeoning is due partly to technical improvements in optical astronomy, but even more to the new windows on the Universe opened up by radio astronomy and by observations from space. Valuable data are also obtained in other ways—from underground neutrino detectors and gravitational-wave experiments. There are few branches of terrestrial physics, indeed, that do not find application somewhere in astronomy.

In this book, we have (especially in the middle section) described those recent developments that we have found (from our experience of lecturing and writing) that seem to fascinate nonspecialists most. We aim to answer the questions that we most often are asked. Few of these topics—quasar spectra, protogalaxies, gravitational lenses, gravitational waves, and cosmic strings—have yet been given due prominence in nontechnical publications. On the other hand, stories such as that of black holes are not emphasised here because such exotic objects have become so familiar from the many excellent books that already exist.

All these topics relate to a single overall conclusion—something that has as much right to be called a paradigm shift as anything in twentieth-century astronomy. This is the realisation that the dynamics of our Universe, and of all the galaxies in it, are controlled not by what we see but by dark matter. Only 10 percent (at most) of the Universe shines; what we see is a biased and incomplete sample of the Universe’s overall contents. Without the dark matter, our Universe would be a very different place: Dark matter controls the structure and eventual fate of the Universe. Discovering what the dark stuff is surely rates as the number-one problem confronting cosmologists today.

The search for a solution to this puzzle is a natural development from recent discoveries in cosmology that have been reported in earlier books. A fuller description of Big Bang cosmology and the expanding Universe can be found in In Search of the Big Bang; the ultimate fate of the Universe, and evidence that dark matter does indeed exist, are discussed in detail in The Omega Point. Here, moving on from such discoveries, we are more concerned with the exact nature of the dark matter, the stuff of the Universe, than with the detailed proof that there is some sort of dark matter around.

It is no exaggeration to say that we would not be here to wonder at the Universe if the dark stuff were not around. We can imagine ways in which the Universe might have emerged from the Big Bang without this background sea of stuff, so that stars, galaxies, and creatures like us would never have been produced. And yet we are here, and this relates to the second main theme of our book.

Science deals mostly with complex manifestations of laws that in essence are well known—the real scientific challenge lies in understanding the rich complexity inherent in these phenomena. Cosmology and particle physics are, however, the two frontier areas, where even the basic laws are still mysterious. Moreover, deep interconnections are becoming apparent between these two endeavours—the study of the cosmos and of the microworld. For example, the dark matter that dominates the Universe is probably in the form of myriads of tiny particles whose individual properties can be understood only in microphysical terms.

The study of the Universe, and our place in it, evolves in a piecemeal way. We can make progress only by tackling problems in bite-sized pieces, and specialists are perforce concerned with technical details. But the occupational risk of astrophysicists, and indeed of all scientists, is to forget that one is wearing blinkers—that there are broader questions at issue, and that a main goal of our piecemeal efforts is eventually to elucidate them.

Why is our Universe the way it is? What is our place in it? Could things have been otherwise, and could alternative universes exist? Why, above all, does the Universe have the symmetry and simplicity that have allowed us to make any progress in understanding it? These issues, where even the specialists are still groping for clues, are the ones that come up most frequently in general discussions. Their investigation sometimes goes by the name of anthropic cosmology—but giving the investigation a name doesn’t mean that we yet have all, or any, of the answers.

As the frontier of cosmological knowledge has advanced, its periphery has expanded, and issues that were once purely conjectural have come within the scope of serious investigation. Questions about how the Universe began and how it may end can now be addressed scientifically, and not just in our nonprofessional moments. Such debates have been the subjects of previous books; here, we have not shied away from more speculative issues, coming just within the periphery of respectable science, and we try to give the flavour of current debates that are on (but not beyond) the frontiers of the subject.

The unifying theme of this book can be stated in nontechnical terms—indeed, there is no other way to express it, since the specialists are as perplexed about the answer as anyone else: What features of the Universe were essential for the emergence of creatures such as ourselves, and is it through coincidence, or for some deeper reason, that our Universe has these features? We hope our discussion of these issues will answer some of the questions in your mind.

John Gribbin Martin Rees October 1988

PART ONE

Cosmic Coincidences

CHAPTER ONE

How Special Is the Universe?

Science is not simply the accumulation of more and more facts about the natural world. If that were the case, science would long since have ground to a halt, clogged up by the accumulation of vast amounts of data. Instead, science proceeds because of our ability to discern patterns and regularities in the natural world. As we come to see how previously unconnected facts hang together, we fit more data into laws of greater scope and generality, and we need to remember fewer independent basic facts, from which all the rest can be deduced. The astonishing triumph of modern science, especially physics and astronomy, is its ability to describe so many of the bewildering complexities of the natural world in terms of a few underlying principles. But this success seems to rest upon the fact that our Universe is constructed along very simple lines. The laws of physics are straightforward enough to be understood by human minds, and the laws we deduce from experiments here on Earth seem to apply across the Universe, at all places and at all times. Is such simplicity an inevitable feature of the Universe? Is it merely a coincidence that creatures intelligent enough to understand a few simple physical laws exist in a world where only those physical laws are needed to explain how everything works? Or is there some deeper plan that ensures that the Universe is tailor-made for humankind?

These questions, which relate to our place in the Universe, and are concerned with the issues of what has been dubbed anthropic cosmology, are addressed in this book. The success of science in explaining complex patterns of behaviour by simple laws can be seen by a few examples. The regular courses of the Moon and planets across the sky had been known since ancient times, but were explained only when Newton realised that they were governed by the same gravitational force that holds us down on the Earth. And the complexity of chemistry, which so baffled the alchemists, began to be understood when Mendeleev, in the nineteenth century, found regularities in the way the properties of elements related to one another; these regularities are now attributed to the fact that atoms are made from just three basic types of component, the protons and neutrons (together making up the nucleus) and the electrons (which are distributed outside the nucleus in accordance with the laws of quantum mechanics).

Physicists have now reduced nature still further. They believe that the basic structure of the entire physical world—not just atoms but stars and people as well—is in principle determined by a few basic constants. These are the masses of a few so-called elementary particles, and the strengths of the forces—electric, nuclear, and gravitational—that bind those particles together and govern their motions.

In terms of these simple rules, some natural phenomena are more easily explained than others. Biological processes, for example, are much harder to understand than the fall of an apple from a tree or the orbit of a planet around the Sun. But it is complexity, not sheer size, that makes a process hard to comprehend. We already understand the inside of the Sun better than we do the interior of the Earth. The Earth is harder to understand because the temperatures and pressures inside it are less extreme, and therefore more subtle, than those inside the Sun. Complex structures—chemical compounds containing many atoms joined together—exist inside the Earth; inside the Sun, however, everything is reduced by the heat and pressure to the constituent atomic nuclei and electrons, and their behaviour is governed by the basic rules.

Our Universe contains thousands of millions of galaxies, and each of those galaxies may, like our Milky Way, contain thousands of millions of stars, more or less like our Sun. Observations show that the Universe is expanding, with groups of galaxies moving apart from one another as time goes by. Cosmologists infer that there was a time, roughly 15 billion years ago, when all of the matter and energy of the Universe, and space and time as well, were concentrated in a superhot region, a fireball known as the Big Bang. In the earliest stages of the primordial fireball, matter would surely have been reduced, or broken down, into its most primitive constituents—a thermal soup at a temperature of 10 billion degrees Celsius, initially expanding at such a rate that it doubled in size every second. In this sense, conditions in the Big Bang were even simpler than those inside the Sun today. So we can realistically hope to explain why the Universe is expanding the way it is—it isn’t presumptuous to try to understand the physics involved. Perhaps we can also understand how stars and galaxies came into existence in the expanding Universe, and therefore begin to appreciate the nature of our own origins. But as soon as we begin to gain an understanding of these processes, we immediately run into the puzzle of the cosmic coincidences.

The Anthropic Universe

The Universe is a simple place, but we are complex creatures. One reason for this is that we do not inhabit a typical place in the Universe. Most of the Universe is empty space, filled with a weak background sea of electromagnetic radiation, with a temperature only 3 degrees above the absolute zero of temperature, which lies at -273 degrees C. But we live on a planet, which orbits around a simple, stable star. Conditions inside that star—our Sun—provide the energy that life, including human life, needs; conditions on the surface of that planet—the Earth—allow for the complexity that seems essential to life as we know it. Clearly, our home represents a special place in the Universe (although not necessarily a unique place). Slightly more subtly, we can see that we also exist at a special time in the Universe. In the Big Bang itself, conditions were too extreme for the complexity that represents human life to exist; today, they are just right (at least on one planet, orbiting one star in one galaxy). In the future, perhaps conditions will once again be unsuitable for life as we know it. We exist here and now because of the exact relationships between the basic forces and particles. And this raises many questions.

Why, for example, are stars so big? The strength of the electrical force between two protons (in, say, a hydrogen molecule) can be compared with the gravitational force between the same two particles. Electrical forces are 10³⁶ (a 1 followed by 36 zeroes) times stronger than gravitational forces, and on the scale of an atom gravity can be completely ignored. But when large numbers of atoms are grouped together, the force of gravity increases as the total mass increases. Each atom has zero net electrical charge, because the positive charge on each proton is exactly balanced by the negative charge on an electron in the atom (some people, incidentally, see this exact balance between the charge on an electron and the charge on a proton as a remarkable coincidence in its own right). So, a large mass carries no net electrical charge and exerts no net electrical force. When an apple falls from a tree, it does so not because of the electrical forces pulling it towards the Earth, but because of the accumulated gravitational force of the enormous numbers of atoms that together make up the Earth. In fact, the apple is held together by electrical forces, acting between its constituent atoms and molecules. The same forces hold together the atoms and molecules of the stem that attaches the apple to the tree. The apple falls if, and when, the gravity of the whole Earth is strong enough to overcome the electrical forces in the stem and break the apple free from its parent tree. The gravity of the whole Earth is needed to break the electrical forces involving the relatively few atoms in the stem of the apple.

Theoretical studies of stars and their life cycles were stimulated by the challenge of observations—people saw the stars and wondered what they were made of. It is interesting, though, that the properties of stars could have been deduced by a physicist who lived on a perpetually cloud-bound planet. Such a physicist could have posed the question: Can one have a gravitationally bound fusion reactor, and what would it be like? He or she might then have reasoned like this: Because gravity is never cancelled out in the way electrical charges cancel, it must win out over electrical forces on a sufficiently large scale. But how large?

Imagine that we assemble a set of objects containing successively 10, 100, 1,000 atoms, and so on. The 24th object would be the size of a sugar lump—about 1 cubic centimetre. The 39th would be like a rock 1 kilometre across. Gravity starts off with a handicap of 10³⁶, but it gains on electrical forces as a two-thirds power.¹ So when we get to our 54th object, because 36 is two-thirds of 54, it will have caught up. Our 54th object will have the mass of Jupiter; anything bigger than Jupiter will start to get crushed by gravity. So, to be squeezed by gravity and heated to the point where nuclear fusion could ignite, an object must contain well over 10⁵⁴ atoms.

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¹The reason is simple: the force involved depends on mass M and radius R and is proportional to M/R; for uniform density, mass is proportional to the cube of radius, that is, radius is the one-third power of the mass, and M/R goes as the two-thirds power of mass.

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Gravitationally bound fusion reactors—stars—must be massive because gravity is so weak. Having inferred this, our hypothetical physicist could in principle calculate the entire life cycle of a star. Sir Arthur Eddington was the first person to express this line of argument clearly, in the 1920s; he went on to conclude that when we draw aside the veil of clouds beneath which our physicist is working and let him look up at the sky, there he will find a thousand million globes of gas, nearly all with masses [in this calculated range].

Gravity dominates the electrical forces, and crushes atoms out of existence, when the total mass of a collection of atoms approaches 10⁵⁷ times the mass of a proton. Even the interior of the Earth can resist the inward pressure of gravity and maintain atoms as distinct entities. But when the total mass nears that critical value, the structure of atoms is destroyed. What remains is a sea of freely mingling nuclei and electrons. Stars do indeed have masses around 10⁵⁷ times the mass of a proton. They are held together by gravity, and gravity initiates the process of nuclear fusion, when atomic nuclei are squeezed together to make new nuclei, which provides the energy that keeps stars hot. If gravity were even weaker, stars would be bigger still; if gravity were stronger, stars would be smaller and would run through their life cycles more quickly—perhaps so quickly that there would be no time for intelligent life to evolve on any planets orbiting those stars.

The basic forces also determine how big a human being can be. Our bodies, like all chemical structures, are held together by electrical forces. These forces are fixed by the basic laws of nature. But because the gravitational force acting on our bodies—our weight—depends on how many atoms the bodies contain, the force is bigger if people (or other creatures) are bigger. The bigger they come, the harder they fall. A simple calculation shows that any creatures much bigger than a human being, inhabiting the surface of a planet the size of the Earth, will simply break apart when they fall over. We are as big as we can be, given our lifestyle—or rather, the lifestyle of our recent ancestors. Whales can be big, because their mass is supported by the sea; but our ancestors, who were tree-dwelling primates, couldn’t be so big that an occasional fall would inevitably prove fatal.

We shall look in more detail at these, and other cosmic coincidences in part 3 of the book. But it is worth spelling out now just how delicate the balances between the basic forces that permit our existence really are. For example, if the nuclear forces, which control the behaviour of protons and neutrons within the nucleus of an atom, were slightly stronger than they actually are, compared with electrical forces, then the di-proton (an atomic nucleus composed of two protons) would be stable. In our Universe, the electrical force of repulsion between two positively charged protons overwhelms the nuclear force of attraction between them, and di-protons do not exist. Two protons can be held in a stable atomic nucleus only if there is a neutron or two there as well; these uncharged particles add to the attractive force but do not affect the repulsive force. Now, stars gain their energy by fusing protons and neutrons together into such nuclei; if, instead, they could fuse pairs of protons together into di-protons, stars would evolve quite differently and the Universe would be a very different place. If, on the other hand, nuclear forces were slightly weaker than they are in our Universe, no complex nuclei could form at all. The entire Universe would be composed of hydrogen, the simplest element, whose atoms consist of a single proton and a single electron.

All the familiar chemical elements except hydrogen and primordial helium were, in fact, built up by nuclear transmutations inside stars that exploded long before our Solar System formed. Iron, carbon, oxygen, and the rest are all products of stellar nucleosynthesis, a process that is sensitive to several apparent accidents of physics, as Fred Hoyle pointed out in the 1950s. We shall look in detail at those coincidences later; what matters here is that the Universe seems to have been set up in such a way that interesting things can happen in it. It is very easy to imagine other kinds of universes, which would have been stillborn because the laws of physics in them would not have allowed anything interesting to evolve.

Imagine, for instance, tinkering with the Universe by varying the strength of gravity. Suppose that it were only 10²⁶, rather than about 10³⁶, times weaker than the electrical force. We would then have a smaller Universe, in which stellar processes would occur more rapidly. Stars, which are fusion reactors bound together by gravity, would each have only about 10-15 (one-millionth of a billionth) of the Sun’s mass. Although each one would have a mass of a trillion tons, it would take 10 million of them to add up to the mass of our Moon; and each would last for just about a year before burning out. Very probably, this would not provide time for life forms as complex as ourselves to evolve; in any case, complex structures could not grow very large before being crushed by gravity.

So the fact that we exist tells us, in a sense, what conditions are like inside stars and in the Universe at large. This is the mildest form of what is now known as anthropic reasoning, or anthropic cosmology. Given the brute fact that we are a carbon-based form of life, which evolved slowly on a planet orbiting around a star like our Sun (a so-called G-type star), there are some features of the Universe, some constraints on the possible values of physical constants, which can be inferred quite straightforwardly. This line of reasoning even helps us to understand the sheer size of the Universe.

A Universe Big Enough for Life

At first sight, it might seem that one planet like the Earth, circling one star like our Sun, would be sufficient to provide a home for life and the opportunity for intelligence to evolve. There is no way to set a precise figure on the extent of our Universe and the number of stars and planets it contains, but at the very least it contains a billion billion (10¹⁸) stars, and at least 1 percent of that number—some 10 million billion stars—are likely to be reasonably similar to our Sun. If we guessed that just 1 percent of those Sun-like stars actually possessed a retinue of planets that included a planet like the Earth, that would still provide a hundred thousand billion homes for life as we know it. This is a number so extravagantly large that it makes our place in the Universe seem utterly insignificant. And yet it may be necessary that all those billions of potential homes for life exist, simply because one home for life, our Earth, exists.

Consider the implications in terms of the linear size of the Universe, rather than the number of stars it contains. Cosmologists estimate, for good, sound reasons,² that the observable Universe is about 15 billion light-years across. A light-year is simply the distance that light can travel in one year, so it is no coincidence that this size is linked to the estimated age of the Universe—15 billion years. We can, in principle, see as far as light has had time to travel since the Universe began.

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²The details can be found in In Search of the Big Bang, by John Gribbin (see bibliography).

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The fireball of the Big Bang was a simple place, in the sense that matter was broken down into its component parts there. As the Universe expanded and cooled, those basic building blocks of matter formed into the simplest elements,