Introduction to Beyond the Big Bang

Few of us can look into the starry sky on a clear night without experiencing a sense of awe. We can see that our world—the planet Earth with all its mountains, oceans, forests, cities, and villages—is part of a vastly larger universe. Gazing at the faraway stars, many barely visible, we think we can feel the immensity of the universe, but in fact, we have no real sense of its size. We cannot see that our sun and all the stars within our view are part of a giant pinwheel of more than 100 billion stars, a galaxy known as the Milky Way. And we have no reason even to suspect that other galaxies exist. Astronomers have found, however, that space is teeming with galaxies, about 100 billion of them within the reach of our most powerful telescopes.

How did this vast and magnificent universe come to be? That is a question that cosmology, the branch of science that studies the universe as a whole, seeks to answer. Cosmologists have made tremendous strides since the early 1900's. We have advanced from the belief that the Milky Way is an “island universe,” a solitary galaxy surrounded by a sea of emptiness, to the picture of a universe filled with galaxies. We have also progressed from the belief that the universe is motionless to the view that it has expanded from a dramatic explosion, the big bang, which astronomers think occurred 10 billion to 15 billion years ago. But despite these advances, the most fundamental questions about the universe remain unanswered.

For example, the traditional form of the big bang scenario does not attempt to address questions about the very beginning of the universe. Although it is called the big bang theory, it is really only a theory of what happened after the big bang explosion. The theory describes how the early universe expanded and cooled and how a nearly uniform cloud of gaseous matter condensed into clumps to form stars and galaxies. But the theory gives not even a clue about what the universe emerged from, or what caused it to explode into being.

Today many cosmologists are looking for ways to go beyond the big bang theory, to describe the very moment of creation in terms of the laws of nature. Although our understanding of these laws is still incomplete, there has been much progress in constructing a theory of how the universe may have come into existence. If the new ideas are correct, then the universe is far older than 15 billion years—and vastly larger than had been previously thought. Furthermore, the new ideas imply that the big bang occurred not just once, but countless times. Each big bang produced a huge expanse of space, often called a “bubble universe.” These bubble universes are similar to the observable universe (the region within the reach of our telescopes), but they are separate from it, and possibly much larger.

The Beginning of Modern Cosmology

The modern view of cosmology is based on the work of Edwin P. Hubble, an American astronomer. In 1929, he discovered that all the galaxies in the universe are rushing away from one another, which means that the universe as a whole is expanding. Since matter is known to cool as it expands, cosmologists concluded that the universe began in an immensely hot, dense state, often called the primordial fireball.

The theory of the primordial fireball, also called the big bang theory, has become widely accepted because a number of its predictions have been confirmed by observation. The first strong evidence that the universe had a fiery birth came in 1965. That year Arno A. Penzias and Robert W. Wilson, researchers working at Bell Telephone Laboratories in New Jersey, discovered that weak microwave radiation is arriving at Earth from all directions in space. Cosmologists interpreted these microwaves, now called the cosmic background radiation, as the faint glow that survives today from the blazing heat of the primordial fireball. Today we know, based on data obtained from the Cosmic Background Explorer (COBE) satellite in the early 1990's, that the properties of the background radiation agree precisely with what we would expect from the glow of hot matter in the early universe.

In addition, physicists have studied how the nuclei of atoms would have formed in a fireball as it cooled. They found that the nuclei in the universe would be mainly of the lightest known types: two isotopes (forms) of hydrogen, two isotopes of helium, and an isotope of lithium. The amounts of these elements that astronomers have measured in our universe agree well with what the theorists calculated. Cosmologists believe that heavier elements formed much later, in the interior of stars, so they do not provide a test for the big bang theory.

Rethinking the Big Bang Theory

Despite the success of the big bang theory in accounting for the present state of the universe, there is strong evidence that the theory is incomplete. Because the theory describes only the aftermath of the primordial explosion, and not the explosion itself, there are mysteries that the big bang theory leaves unresolved. Most importantly, the theory gives no explanation for two remarkable properties of the observable universe: its uniformity and its average density of mass.

If the big bang had been an ordinary explosion, like a blast of dynamite or a nuclear bomb, we would not expect the universe to be uniform. Objects would be distributed randomly and unevenly. However, that is not the case—the universe appears to be remarkably uniform. If we could imagine stepping back and looking at the very big picture, we would find that the distribution of stars and galaxies is very much the same throughout the universe. The most striking evidence for the uniformity of the universe comes from the cosmic background radiation. Precise measurements have shown that the intensity of the radiation is almost exactly the same from every direction in the sky.

Cosmologists usually discuss the average density of mass in the universe in terms of its effect on the fate of the universe. Recall that the universe is expanding and that the expansion is being slowed by the gravitational attraction that exists between any two masses. If the average mass density is greater than a certain value, called the critical density, the force of gravity will eventually halt the expansion of the universe and cause it to collapse into what is sometimes called the “big crunch.” If the average density is less than or equal to the critical density, the universe will continue to expand forever.

Searching For the Exact Density of Mass

Unfortunately, it is very difficult to measure the density of mass in the universe. Cosmologists only know that the average density lies somewhere near the critical value. Nonetheless, they have calculated that the slightest variation from the critical density soon after the birth of the universe would have had a significant effect on its development. At one second after the big bang, for example, the average mass density must have been less than 1.000000000000001 times the critical value, or else the universe would have stopped expanding and collapsed in on itself before reaching its present age. But the density must have been more than 0.999999999999999 times the critical value, or else the matter in the universe would have flown apart so quickly that stars and galaxies would never have formed.

Because the traditional big bang theory could not explain why the universe is so uniform or why the density of the primordial fireball was so extraordinarily close to the critical value, cosmologists realized that the theory would have to be refined. In the 1980's, they developed a new theory that accounted for these two phenomena—the inflationary universe theory. The theory proposes that just an instant after it emerged into being, the infant universe was propelled by a force called inflation. In just an eyeblink of cosmic time, the universe increased in size by a factor of trillions of trillions. The theory was first suggested by me in a form which did not quite work. The first fully successful version of inflation was proposed by Andrei Linde, then at the Lebedev Institute in Moscow, and independently in a publication by Andreas Albrecht and Paul Steinhardt at the University of Pennsylvania.

The Inflationary Universe

The inflationary universe theory was developed by combining ideas in cosmology with findings from another area of study, particle physics. The connection between these two fields at first seems odd, since the goal of cosmology is to understand the largest objects that we know of, while the goal of particle physics is to understand the smallest. Particle physicists study the protons, neutrons, and electrons that combine to make atoms, as well as a host of other particles, such as the quarks that join to form protons and neutrons. The relationship between the two fields, however, arises naturally from the big bang theory, which implies that the early universe was far hotter than any furnace that can be produced on Earth. Our only hope to understand the behavior of matter at these extreme temperatures is to understand the particles from which the matter is made, and the forces by which the particles interact.

The crucial concept from particle physics that makes inflation possible is the predicted existence of a peculiar form of matter, called a false vacuum, that can turn gravity on its head. Gravity is normally an attractive force between any two objects, but a false vacuum produces a gravitational force that repels. A false vacuum was still just a theoretical idea in 1997, as the energy needed to produce it was much larger than anything available. Nonetheless, particle physicists were reasonably sure that this form of matter can exist.

The Expansion of the False Vacuum

The inflationary theory proposes that the infant universe, or some small patch of it, was filled with a false vacuum. The repulsive gravitational force of the false vacuum would then have set off an extremely rapid expansion. In a period of time much shorter than we could ever imagine—perhaps 10 to the minus 37th power second—the region of false vacuum doubled in size. (The time span 10 to the minus 37th power second is a fraction of a second—written out as a decimal point followed by 36 zeros and the numeral 1.) Then in the same amount of time, it doubled again, and then again and again at least 100 times. This is called exponential expansion. In a twinkling, the universe became at least 10 to the 30th power (1 followed by 30 zeros) times larger than it had been. While any ordinary form of matter would have been diluted to a negligible density by such a gargantuan increase in volume, the peculiar properties of the false vacuum imply that its density of energy was unaffected by the expansion. The total energy in the false vacuum, therefore, grew enormously during the period of inflation.

The entire period of inflation lasted just a tiny fraction of a second because the false vacuum is unstable, just as the nuclei of radioactive elements are unstable. An unstable nucleus of uranium, for example, will decay by breaking up into a nucleus of thorium and a nucleus of helium. So, too, a false vacuum “decays” into other forms of matter. It converts its huge store of energy into a tremendously hot gas of essentially all the elementary particles. The gas is so hot that the particles cannot combine even to form protons or neutrons.

The False Vacuum Theory and the Big Bang

This expanding hot soup of particles, produced by the expansion and subsequent decay of the false vacuum, is exactly the form of matter that the traditional big bang theory had assumed made up the primordial fireball. Thus, the inflationary theory provides the detailed description of the initial explosion that was absent from the original form of the big bang theory. The description of the universe after the decay of the false vacuum is the same in both the theory of inflation and the older form of the big bang theory.

The inflationary universe theory can explain the nearly uniform distribution of matter throughout the universe, because the enormous burst of expansion makes it possible for the universe to have started out much smaller than had been previously thought. Just before inflation began, the region of space that we call the observable universe would have been more than a billion times smaller than a proton. Since the region was so small, both the temperature and the distribution of matter had a chance to become uniform before inflation kicked in. This uniformity would have been preserved as inflation expanded this tiny region to an immensely larger volume.

The inflationary theory can also explain why the average mass density in the universe is so close to the critical value. As the universe evolves, both the mass density and its critical value change, since the critical density is determined by the rate of expansion. (If the rate of expansion were high, for example, then a strong gravitational force would be needed to reverse it, and the critical density would be high.) During inflation, the expansion rate of the universe was controlled by the gravitational repulsion of the false vacuum, which according to calculations had exactly the needed strength. Whatever the conditions were before inflation—whether the average mass density was greater or less than the critical value–the inflationary expansion would have adjusted the balance between the rate of expansion and the mass density. When the period of inflation ended, after 100 or more doublings in the size of the universe, the mass density would have been extraordinarily close to the critical value.

Because inflation is the only known explanation for the uniformity of the universe and the fact that the universe's mass density is very close to the critical value, cosmologists are fairly certain that the universe did indeed go through a period of inflation. Nonetheless, the theory of inflation is not the final word in cosmology, for at least three reasons. First, although inflation describes the universe at an extraordinarily early moment in time, it does not explain the actual origin of the universe—time zero. Second, inflation is not a unique theory, but rather a class of theories. Most of the versions present the same basic scenario of the early universe, but they differ in their details. There was still much work to be done in 1997 to determine which version of the theory, if any, is correct. And third, though inflation is a persuasive theory, decisive tests of its predictions were still needed.

Eternal Creations

Nonetheless, the inflationary theory has led to new speculation about the nature of our universe. One of the most fascinating new ideas is the possibility that inflation never ends.

To understand how this could be so, recall that inflation is driven by a false vacuum and ends when the false vacuum decays. The false vacuum, however, does not decay all at once. In most versions of the inflationary theory, it decays with a fixed half-life, like a radioactive material. The half-life is the time it takes for half of a radioactive substance or a false vacuum to decay. Imagine that we could sprinkle a region of false vacuum uniformly with hundreds of tiny probes. After one half-life, half of the probes, on average, would be in regions that still contain false vacuum. The other half would be in regions in which the false vacuum has decayed into a hot soup of particles. The decay of the false vacuum would look just like the big bang in our own past, so each region in which the false vacuum decayed would become a new universe. Cosmologists call these universes “bubble universes,” and many think our own universe is such a bubble.

Although a radioactive material will eventually decay completely, a false vacuum behaves differently. While the false vacuum is decaying into bubble universes, it is also exponentially expanding. The time it takes for the false vacuum to double in size—perhaps 10 to the minus 37th power second—is much shorter than the false vacuum's half-life. Thus, despite continually losing regions of itself to decay, the remaining false vacuum grows ever larger, and inflation never stops. The result is the eternal creation of an infinite number of bubble universes as the false vacuum goes on inflating and decaying. Each bubble universe continues to expand at an ever-decreasing rate for billions of years as it evolves. The fate of each bubble universe is determined by how much mass it contains. Some bubble universes will eventually collapse by the force of gravity into a big crunch; others will expand forever, becoming cold and dark. According to the theory, however, the overall universe—the false vacuum and all the bubbles that it spawns—continues forever.

The theory of eternal inflation invites us to ask further questions about the origin of the universe. For example, if the universe includes an infinite number of bubble universes, are the laws of physics the same within all of them? If inflation can continue without end, is it possible that it is also without beginning? If not, then how did the universe begin? To describe what cosmologists are thinking about these questions, we need to summarize what physicists have learned about the fundamental laws of nature.

The Laws of Physics–searching For A Better Understanding

Although the world around us appears unimaginably complex, most physicists believe that it is governed by an underlying simplicity. They attribute the spectacular complexity to the huge numbers of atoms and molecules that make up every object that we see and touch.

The interactions among subatomic particles appear to be governed by four known forces of nature. In the order of their strength, the first of these forces is simply called the strong force. It is responsible for binding quarks together to form protons and neutrons. It also holds protons and neutrons together inside an atomic nucleus. The electromagnetic force holds electrons in orbit around the atomic nucleus. The weak force is responsible for some types of radioactive decay. These forces are transmitted by particles. For example, photons transmit the electromagnetic force and gluons transmit the strong force.

Gravity–the Big Weakling

Surprisingly, the weakest known force of all is gravity. The force of gravity between two elementary particles is so weak that it has never been detected. Gravity appears strong, however, because gravity is long-range and always attractive. The weight of a refrigerator, for example, is caused by the attraction between all its 10 to the 28th power particles and all 10 to the 52nd power particles of the Earth. For similar reasons, gravity is the dominant force controlling the evolution of the universe.

Cosmologists believe that in the first moments of the universe, there existed only one master force. As the universe expanded and cooled, the master force split into the four forces known today. A major goal of modern physics is to understand the mathematical relationships that unify all the forces. So far, we have been able to develop a theory that unifies the strong, electromagnetic, and weak forces. But we have been only partially successful with gravity.

Under most circumstances, gravity is accurately described by the law formulated by the English physicist and mathematician Isaac Newton in the 1600's. Newton said that gravity is a force by which any two masses attract each other. In 1916, however, the German-Swiss physicist Albert Einstein invented a new theory of gravity, the general theory of relativity, which was found to be more accurate, especially when very large masses are involved.

Einstein's Theory of Relativity

Einstein said gravity is not a force, but a distortion in the geometry of space and time. Previously, physicists had concerned themselves only with the motion of matter and treated space as merely a fixed backdrop. But in general relativity, Einstein saw space as an elastic material, with the ability to bend, twist, and stretch. General relativity is completely adequate for describing the revolutions of planets and other large-scale motions, but it is not a candidate for being part of the truly fundamental laws of nature. The problem is that general relativity is not consistent with the quantum theories that physicists have developed to explain the behavior of atoms and elementary particles.

The key difference between quantum theories and so-called classical theories of physics, such as Newton's and Einstein's, concerns the question of predictability. A classical theory can be used to make clear predictions for every property of a system at any time in the future. A quantum theory, in contrast, can be used only to calculate the probabilities of different outcomes. Physicists are now convinced that the underlying laws of nature are all properly phrased in terms of quantum theory, and that they have found a successful quantum description of the strong, electromagnetic, and weak forces. Now they want to explain the origin of the universe by developing a quantum theory of cosmology. A major step in that direction would be a successful quantum theory of gravity.

Superstring Theory

The need to develop a quantum theory of gravity has bedeviled theoretical physicists since the 1940's, and one approach after another has met with failure. However, after decades of frustration, many physicists now believe that we are well along the road to a solution, in the form of a quantum theory called superstrings. As its name suggests, the theory conceives of fundamental particles, such as electrons or quarks, not as pointlike objects but rather as tiny strings, with lengths of about 10 to the minus 33nd power centimeter. The particles that transmit forces between particles of matter are also viewed as strings. The theory holds that the force of gravity is conveyed by a looplike particle called a graviton, and all the various kinds of particles exist in a 10-dimensional realm–the 4 dimensions we are familiar with in our everyday world (height, width, depth, and time) and 6 extra dimensions.

Why, then, do we not see these extra dimensions? The answer can be understood by thinking about an ordinary soda straw. By looking closely at the straw, you can see that it is a curved two-dimensional surface. However, if you look from a distance, you cannot see the thickness, and the straw looks like a one-dimensional line. Superstring theory proposes that the 6 extra dimensions are curled in this way, with a circumference of about 10 to the minus 32nd power centimeter. Unfortunately, the mechanism that causes this curling is not well understood.

While superstring theory was invented as a quantum theory of gravity, it appears to be much more far-reaching than anyone had anticipated. If the superstring theory is right, it could very well be the conclusion of our search for the fundamental laws of nature.

New Speculation About Our Universe

We can now return to the question of whether the laws of physics within all the bubble universes can be expected to be the same. Observations of light from distant galaxies show that the laws of physics appear to be the same throughout the visible universe, but this of course says nothing about other bubble universes. In addressing this question, cosmologists usually assume that the truly underlying laws of nature are the same everywhere, because otherwise we would have no clue about how to proceed. Nonetheless, the apparent laws of physics—those that we actually observe—may not be uniquely determined by the underlying laws. In string theory, for example, there may be more than one way that the six hidden dimensions can curl up, so the apparent laws of physics may depend on how the curling happens to have occurred in our bubble universe. Indeed, it is even conceivable that the space-time of other bubble universes may not seem four-dimensional.

If inflation can continue without end, we might also ask if it could occur without a beginning? If so, there would be no need to seek a theory of the actual origin of the universe. The issue is not definitively decided, but calculations indicate that even eternal inflation cannot remove the need for a beginning.

Starting From Nothing

In considering a scientific theory of creation, one crucial issue is the starting point. Any account of the creation of the universe must begin with an initial state in which there is no universe—but what does that mean? One might consider starting with empty space, but in the context of general relativity, empty space is essentially a material, capable of twisting and stretching like a piece of rubber. To most cosmologists, therefore, empty space is a kind of universe. To discuss a starting point with no universe, cosmologists speak of a state they call absolute nothingness, in which neither matter, space, nor time exist.

If the universe originated from nothing, an issue that must be understood is the conservation of energy. The laws of nature, physicists believe, imply that energy is never created or destroyed, so the total amount of energy can never change. But the total energy of the universe seems to be huge, so how could the universe have begun from nothing? The answer hinges on the fact that the energy of a gravitational field is negative. If the average gravitational field of the universe is strong enough, then the negative energy of gravity can cancel the positive energy of everything else, so that the total energy is zero. The inflationary theory, and especially the theory of eternal inflation, depend on the fact that whole bubble universes can arise with no input of energy.

Fine Tuning the Theories

Although an accepted theory of the creation of the universe from nothing does not exist, serious speculations have been proposed by four noted cosmologists: Alexander Vilenkin of Tufts University in Massachusetts, Andrei Linde of Stanford University in California, and working collaboratively, Stephen Hawking of Cambridge University in Great Britain and James Hartle of the University of California at Santa Barbara. These proposals use the unpredictability of quantum theory to explain the origin of the universe as the random creation of space and time out of a state of absolute nothingness. The ideas are only approximate, based on our imperfect understanding of the quantum version of general relativity. While one hopes that someday the origin of the universe will be addressed by superstring theory, at present the theory is not understood well enough to make this possible.

Likewise, the theory of inflation itself, though it seems valid, must be put on a more solid footing before it can confidently be called the correct description of the very early moments of our universe. More work is needed to test the predictions of inflation, and to determine which version of the theory, if any, is correct. For example, most versions of the theory predict the creation of slight variations in the density of the universe, which can be explored by measuring the very faint nonuniformities in the cosmic background radiation. Cosmologists had high hopes in 1997 that some of these questions would be answered by the Microwave Anisotropy Probe, a highly sensitive satellite that was to make further measurements of the cosmic background radiation. The satellite was launched by the National Aeronautics and Space Administration in 2001. The European Space Agency was planning to launch an even more sensitive probe, called Planck, in 2004. The measurements provided by these probes might leave us totally baffled, or they could help settle the inflation issue.

It is an exciting time in cosmology, and the stakes are high, because if the creation of our universe can someday be described by the laws of physics, we would be left with just one great mystery of existence: What was it that determined the laws of physics?