The Origin Of The Universe

Other Books in the SCIENCE MASTER SERIES

The Periodic Kingdom

by P.W. Atkins

Nature’s Numbers

by Ian Stuart

The Origin of Humankind

by Richard Leakey

The Last Three Minutes

by Paul Davies

Kinds of Minds

by Daniel C. Dennett

How Brains Think

by William H. Calvin

River Out of Eden

by Richard Dawkins

Laboratory Earth

by Stephen H. Schneider

Beautiful are the things we see

More beautiful those we understand

Much the most beautiful those we do not comprehend.

—Niels Steensen (Steno) 1638–1686

PREFACE

We are living in the universe’s prime, long after most of the exciting things have happened. Gaze into the sky on a starry night and you will see a few thousand stars, most straddling the darkness in a great swath we call the Milky Way. This is all the ancients knew of the universe. Gradually, as telescopes of greater and greater size and resolution have been developed, a universe of unimagined vastness has swum into view. A multitude of stars gathered into the islands of light we call galaxies, and all around the galaxies is a cool sea of microwaves—the echo of the big bang some fifteen billion years ago. Time, space, and matter appear to have their origins in an explosive event from which the present-day universe has emerged in a state of overall expansion, slowly cooling and continuously rarifying.

In the beginning, the universe was an inferno of radiation, too hot for any atoms to survive. In the first few minutes, it cooled enough for the nuclei of the lightest elements to form. Only millions of years later would the cosmos be cool enough for whole atoms to appear, followed soon by simple molecules, and after billions of years by the complex sequence of events that saw the condensation of material into stars and galaxies. Then, with the appearance of stable planetary environments, the complicated products of biochemistry were nurtured, by processes we still do not understand. But how and why did this elaborate sequence of events begin? What do modern cosmologists have to tell us about the beginning of the universe?

The various creation stories of ancient times were not scientific theories in any modern sense. They did not attempt to reveal anything new about the structure of the world; they aimed simply to remove the specter of the unknown from human imaginings. By defining their place within the hierarchy of creation, the ancients could relate the world to themselves and avoid the terrible consideration of the unknown or the unknowable. Modern scientific accounts need to achieve much more than this. They must be deep enough to tell us more about the universe than what we have put into them. And they must be broad enough to make predictions, as a check on their credentials to explain the things we already know about the world. They should bring coherence and unity to collections of disconnected facts.

The methods employed by modern cosmologists are simple, but not necessarily obvious to the outsider. They begin by assuming that the laws governing the workings of the world locally, here on Earth, apply throughout the universe until one is forced to conclude otherwise. Typically one finds that there are some places in the universe, especially in the past, where extreme conditions of density and temperature are encountered which are outside our direct experience on Earth. Sometimes our theories are expected to continue to work in these domains—and, indeed, do. But on other occasions we are working with approximations to the true laws of nature—approximations that possess known limits of applicability. When we reach those limits, we must try to establish better approximations to cover the unusual new conditions we have found. Many theories make predictions that we cannot test by observation. Indeed, it is those sorts of predictions that often dictate the types of observatory or satellite to be developed in the future.

Cosmologists often talk about constructing “cosmological models.” By this they mean producing simplified mathematical descriptions of the structure and past history of the universe which capture its principal features. Just as a model airplane reproduces some, but not all, of the features of a real airplane, so a model universe cannot hope to incorporate every detail of the universe’s structure. Our cosmological models are very rough and ready. They begin by treating the universe as if it were a completely uniform sea of material. The clumping of material into stars and galaxies is ignored. Only if one is investigating more specific issues, like the origins of stars and galaxies, are the deviations from perfect uniformity considered. This strategy works remarkably well. One of the most striking features of our universe is the way in which the visible part of it is so well described by this simple idealization of it as a uniform distribution of material.

Another important feature of our cosmological models is that they involve properties—like density or temperature—whose numerical values can be found only by observation, and only particular combinations of observed values for a number of these quantities will be allowed by the model. In this way compatibility between the model and the real universe can be checked.

Our exploration of the universe has taken off in different directions. Besides satellites, spacecraft, and telescopes, we have employed microscopes, atom-smashers and accelerators, computers and human thinking to enlarge our understanding of the entire cosmic environment. Besides the world of outer space—the stars, galaxies, and great cosmic structures—we have come to appreciate the labyrinthine subtlety within the depths of inner space. There we find the subatomic world of the nucleus and its parts: the basic building blocks of matter—so few in number, so simple in structure, but in combination capable of being organized into the vast panoply of complexity we see around us and of which we are a peculiar part.

These two frontiers of our understanding—the small world of the elementary parts of matter and the astronomical world of the stars and galaxies—have come together in unexpected ways in recent times. Where once they were the domains of different groups of scientists attempting to answer quite different questions by separate means, now their interests and methods are intimately entwined. The secret of how galaxies came into being may well be fathomed by the study of the most elementary particles of matter in particle detectors buried deep underground; the identity of those elementary particles may be revealed by observations of distant starlight. And as we try to reconstruct the history of the universe, searching for the fossil remnants of its youth and adolescence, we find that by the coming together of the largest and the smallest aspects of the physical world our appreciation of the unity of the universe becomes more impressive and complete.

This little book aims to provide a short account of the Beginning for beginners. What evidence do we have about the early history of the universe? What are the latest theories about how the universe could have begun? Can we test them by observation, and how does our own existence relate to them? These are some of the questions that will arise on our journey to the origins of time. I shall present some of the latest speculative theories about the nature of time, the “inflationary universe,” and “wormholes,” and along the way explain the significance of the COBE satellite observations that were greeted with such euphoria in the spring of 1992.

I would like to thank my cosmological colleagues and collaborators for their discussions and discoveries, which have made possible a modern story of the origin of the universe. Anthony Cheetham and John Brockman deserve credit for their conception of this project. It remains to be seen whether they were as wise to invite me to participate in it. I would also like to thank Gerry Lyons and Sara Lippincott for their editorial guidance. My wife, Elizabeth, has provided a vast amount of assistance, which helped bring things quickly to completion without pushing everything else off to infinity. As always, I am indebted to her for everything. Junior members of the family—David, Roger, and Louise—have seemed singularly unimpressed by the project. But they do like Sherlock Holmes.

Brighton

March 1994

CHAPTER 1

THE UNIVERSE IN A NUTSHELL

“I must thank you,” said Sherlock Holmes, “for calling my attention to a case which certainly presents some features of interest.”

—The Hound of the Baskervilles

How, why, and when did the universe begin? How big is it? What shape is it? What’s it made of? These are questions that any curious child might ask, but they are also questions that modern cosmologists have wrestled with for many decades. One of the attractions of cosmology for popular writers and journalists is that so many of the questions at the frontiers of the subject are easy to state. Look at the frontiers of quantum electronics, DNA sequencing, neurophysiology, or pure mathematics and you will not find that the problems of the expert translate so readily into the vernacular.

Until the early years of the twentieth century, neither philosophers nor astronomers had questioned the notion that space was absolutely fixed—an arena in which the stars, the planets, and all the other heavenly bodies played out their motions. But during the 1920s this simple picture was transformed: first by the suggestions of physicists exploring the consequences of Einstein’s account of gravity, and then by the results of observations of light from stars in distant galaxies by the American astronomer Edwin Hubble.

Hubble made use of a simple property of waves. If their source moves away from the receiver, the frequency with which waves are received falls. To see this, wiggle your finger up and down in some still water and watch the wave crests moving off to some other point on the water’s surface. Now move your finger away from that point as you make waves, and they will be received less frequently than they were emitted. Now move your finger toward the reception point, and the reception frequency goes up. This property is shared by all waves. In the case of sound waves, it is responsible for the change in pitch of a train whistle or a police siren as it passes you. Light is also a wave, and when its source is moving away from the observer the decrease in the frequency of the light waves means that visible light is observed to be slightly redder. Hence, this effect is called a “redshift.” When the light source is approaching the observer, the reception frequency increases, visible light gets bluer, and it is called a “blueshift.”

Hubble discovered that the light from the galaxies he was seeing displayed a systematic redshifting. By measuring the extent of the shift, he could determine how fast the sources of light were receding; and by comparing the apparent brightnesses of stars of the same sort (stars whose intrinsic brightnesses would be the same) he could deduce their relative distances away from us. What he discovered was that the farther away the source of light, the faster it was moving away from us. This trend is known as Hubble’s Law, and its illustration with modern data is shown in figure 1.1. In figure 1.2 is shown an example of the light signal received from a distant galaxy, displaying the shift of the spectrum of various atoms toward the red, as compared with that emitted from the same atoms in the laboratory.

FIGURE 1.1

A modern illustration of Hubble’s Law, displaying the increase of recession speed of galaxies growing in direct proportion to their distance.

What Hubble had discovered was the expansion of the universe. Instead of a changeless arena in which we could follow the local perambulations of planets and stars, he found that the universe was in a dynamic state. This was the greatest discovery of twentieth-century science, and it confirmed what Einstein’s general theory of relativity had predicted about the universe: that it cannot be static. The gravitational attraction between the galaxies would bring them all together if they were not rushing away from each other. The universe can’t stand still.

FIGURE 1.2

The spectrum of a distant galaxy (known as Markarian 609), showing how three spectral lines (marked H_β, O, and O) near 5000 angstroms and two (marked H_α and N) near 6500 angstroms are systematically shifted toward higher wavelengths than they have when measured in the laboratory. The positions of the lines in the laboratory are indicated by the arrows marked LAB; the measured positions are the labeled peaks on the graph of the light spectrum. The shift toward the red (optical red light lies at about 8000 angstroms) enables the recession speed to be calculated.

If the universe is expanding, then when we reverse the direction of history and look into the past we should find evidence that it emerged from a smaller, denser state—a state that appears to have once had zero size. It is this apparent beginning that has become known as the big bang.

But we are going a little too fast. There are important things to appreciate about the present expansion of the universe before we start delving into the past. First of all, what exactly is expanding? In the movie Annie Hall, Woody Allen is found on his analyst’s couch telling of his anxiety about the expansion of the universe: “Surely this means that Brooklyn is expanding, I’m expanding, you’re expanding, we’re all expanding.” Thankfully, he was wrong. We are not expanding. Nor is Brooklyn. Nor is the Earth. Nor is the solar system. Nor, in fact, is the Milky Way galaxy. Nor even those aggregates of thousands of galaxies that we call “galaxy clusters.” These collections of matter are all bound together by chemical and gravitational forces between their constituents—forces that are stronger than the force of the expansion.

It is only when we get beyond the scale of great clusters of hundreds and thousands of galaxies that we see the expansion winning out over the local pull of gravity. For example, our near neighbor the Andromeda galaxy is moving toward us, because the gravitational attraction between Andromeda and the Milky Way is larger than the effect of the universal expansion. It is the galaxy clusters, not the galaxies themselves, that act as the markers of the cosmic expansion. A simple picture might be to think of specks of dust on the surface of an inflating balloon. The balloon will expand and the dust specks will move apart, but the individual dust specks will not themselves expand in the same way. They act like markers of the amount of stretching of the rubber that has occurred. Similarly, it is best to think of the expansion of the universe as the expansion of the space between clusters of galaxies, as illustrated in figure 1.3.

FIGURE 1.3

The expansion of the universe viewed as the expansion of space. Mark points on the surface of a balloon to represent galaxy clusters and inflate it. The space between the clusters increases, but the size of the clusters does not. This is analogous to a universe with two dimensions of space, represented by the surface of the balloon. Any cluster on the inflating surface sees all the other clusters receding from it. Notice that the center of the expansion does not lie on the surface of the balloon.

Next, we might worry about the implications of the fact that all the clusters are moving away from us. Why us? If we know anything about the history of science, it is that Copernicus demonstrated that the Earth is not at the center of the universe. Surely if we think that everything is moving away from us then we have reinstated ourselves back in the center of immensities. But this is not the case. The expanding universe is not like an explosion that has some origin at a point in space. There is no fixed background space into which the universe is expanding. The universe contains all the space there is!

Think of space as an elastic sheet. The presence and movement of material on this malleable space will produce indentations and curvature. The curved space of our universe is like the three-dimensional surface of a four-dimensional ball—something we cannot envisage. But imagine the universe as a flatland, with only two dimensions of space. It is then like the surface of a three-dimensional ball, which is easy to picture. Now imagine that this three-dimensional ball can get bigger—like our inflating balloon in figure 1.3. The surface of the balloon is an expanding two-dimensional universe. If we mark two points on it, those points will recede from each other as the balloon is inflated. Now put many marks all over the surface of the balloon and inflate it again. What you find is that at whatever mark you locate yourself, all the other marks will appear to expand away from you