To Alina
Table of Contents
Title Page
Prologue
PART I - GENESIS
1 - What Banged, How It Banged, and What Caused It to Bang
2 - The Rise and Fall of Repulsive Gravity
THE FABRIC OF SPACE AND TIME
THE GRAVITY OF EMPTY SPACE
3 - Creation and Its Discontents
FRIEDMANN’S UNIVERSES
THE MOMENT OF CREATION
ACCEPTING THE INEVITABLE
4 - The Modern Story of Genesis
TUNNELING THROUGH THE IRON CURTAIN
THE PRIMEVAL FIREBALL
GAMOW’S ALCHEMY
COSMIC MICROWAVES
THE SMOKING GUN
IMPERFECTIONS OF CREATION
THE MODERN STORY OF GENESIS
5 - The Inflationary Universe
COSMIC PUZZLES
FALSE VACUUM
COSMIC INFLATION
6 - Too Good to Be Wrong
THE GRACEFUL EXIT PROBLEM
SCALAR FIELD
SLOW DOES IT
THE NUFFIELD WORKSHOP
A RECIPE FOR OVERNIGHT SUCCESS
UNIVERSE AS A FREE LUNCH
PART II - ETERNAL INFLATION
7 - The Antigravity Stone
8 - Runaway Inflation
UNIVERSE BEYOND THE HORIZON
ETERNAL INFLATION
A GLIMPSE OF ETERNITY
LINDE’S CHAOTIC INFLATION
9 - The Sky Has Spoken
RETURN OF THE COSMOLOGICAL CONSTANT
IMAGES OF THE BLAZING PAST
10 - Infinite Islands
THE FUTURE OF CIVILIZATIONS
TIME IS OF THE ESSENCE
THE BIG PICTURE
11 - The King Lives!
CADAQUÉS
LIMITED OPTIONS
COUNTING HISTORIES
HISTORY REPEATS ITSELF
WHAT ELSE IS NEW?
THE MEANING OF THE WORD “IS”
SOME WAYS OUT
A FAREWELL TO UNIQUENESS
PART III - PRINCIPLE OF MEDIOCRITY
12 - The Cosmological Constant Problem
VACUUM ENERGY CRISIS
IN SEARCH OF A DEEP SYMMETRY
THE COINCIDENCE PROBLEM
13 - Anthropic Feuds
CONSTANTS OF NATURE
FINE-TUNING THE UNIVERSE
THE ANTHROPIC PRINCIPLE
WHAT DOES THE ANTHROPIC PRINCIPLE HAVE IN COMMON WITH PORNOGRAPHY?
THE COSMOLOGICAL CONSTANT
14 - Mediocrity Raised to a Principle
THE BELL CURVE
COUNTING OBSERVERS
CONVERGING ON THE COSMOLOGICAL CONSTANT
SUPERNOVAE TO THE RESCUE
EXPLAINING THE COINCIDENCE
PROS AND CONS
15 - A Theory of Everything
IN SEARCH OF THE FINAL THEORY
THE PROBLEM WITH GRAVITY
THE HARMONY OF STRINGS
THE LANDSCAPE
THE BUBBLING UNIVERSE
A PROGRAM FOR THE TWENTY-FIRST CENTURY
PART IV - BEFORE THE BEGINNING
16 - Did the Universe Have a Beginning?
A PROBLEM WITH THE COSMIC EGG
A CYCLIC UNIVERSE
DE SITTER SPACE
BEYOND UNREASONABLE DOUBT
A PROOF OF GOD?
17 - Creation of Universes from Nothing
INFLATION AT THE END OF THE TUNNEL
TUNNELING FROM NOTHING
THE UNIVERSE AS A QUANTUM FLUCTUATION
MANY WORLDS
THE HAWKING FACTOR
MUCH ADO ABOUT NOTHING
18 - The End of the World
GRIM OPTIONS
INFLATIONARY TWIST
GALACTIC SOLITUDE
THE FINAL VERDICT
19 - Fire in the Equations
ALFONSO’S ADVICE
GOD AS A MATHEMATICIAN
MATHEMATICAL DEMOCRACY
MANY WORLDS IN ONE
Epilogue
Notes
Acknowledgments
Index
Notes
Copyright Page
Prologue
The stunning success of the book took everybody by surprise. The author, a quiet, even demure physics professor named Alex Vilenkin, has become an instant celebrity. His talk show engagements have been booked solid six months in advance. He has hired four bodyguards and has moved to an undisclosed location to avoid paparazzi. His sensational bestseller, titled Many Worlds in One, describes a new cosmological theory that says that every possible chain of events, no matter how bizarre or improbable, has actually happened somewhere in the universe—and not only once, but an infinite number of times!
The consequences of the new theory are mind-boggling. If your favorite football team did not win the championship, don’t despair: it did win on an infinite number of other earths. In fact, there is an infinity of earths where your team wins every single year! If your discontent goes beyond football and you are completely fed up with how things are, again Vilenkin’s book has something to offer. According to the new theory, most places in the universe are nothing like our Earth and are even ruled by different laws of physics.
The most controversial aspect of the book is the claim that each of us has an infinite number of identical clones living on countless earths scattered throughout the universe. Much sleep has been lost over this issue. People feel their unique identities have been stolen. So attendance at psychoanalysts’ offices has doubled, and sales of the book have soared. Using his theory, Vilenkin also predicted that on some earths his book would be a phenomenal success. But to be fair, he had to admit there were infinite others where it would be a complete flop …
We live in the aftermath of a great explosion. This awesome event, called somewhat frivolously “the big bang,” occurred some 14 billion years ago. The whole of space erupted in a hot, rapidly expanding fireball of matter and radiation. As it expanded, the fireball cooled down, its glow steadily subsided, and the universe slowly descended into darkness. A billion years passed uneventfully. But gradually, galaxies were pulled together by gravity, and the universe lit up with myriads of stars. Planets revolving about some of the stars became home to intelligent creatures. Some of the creatures became cosmologists and figured out that the universe originated in the big bang.
Compared with historians and detectives, the great advantage cosmologists have is that they can actually see the past. Light from remote galaxies takes billions of years to reach our telescopes on Earth, so we observe the galaxies as they were in their youth, when their light was emitted. Microwave detectors pick up the faint afterglow of the fireball, yielding an image of the universe at a still earlier epoch, prior to the formation of galaxies. We thus see the history of the universe unfolding before us.
This wonderful vision, however, has its bounds. Even though we can trace the history of the cosmos to less than a second after the big bang, the bang itself is still shrouded in mystery. What triggered this enigmatic event? Was it the true beginning of the universe? If not, then what came before? There is also a fundamental limit to how far we can see into space. Our horizon is defined by the maximum distance light could have traveled since the big bang. Sources more distant than the horizon cannot be observed, simply because their light has not yet had time to reach Earth. This leaves us wondering what the rest of the universe is like. Is it more of the same, or could it be that distant parts of the universe differ dramatically from our cosmic neighborhood? Does the universe extend to infinity, or does it close in on itself, like the surface of the Earth?
These are the most basic questions about the universe. But can we ever hope to answer them? If I claim that the universe ends abruptly beyond the horizon, or that it is filled with water and inhabited by intelligent goldfish, how can anyone prove me wrong? Cosmologists, therefore, focus mostly on the observable part of the universe, leaving it to philosophers and theologians to argue about what lies beyond.
But if indeed our quest must end at the horizon, wouldn’t that be a great disappointment? We may discover scores of new galaxies and map the entire visible universe, just as we mapped the surface of the Earth. But to what end? Mapping our own galaxy could serve a practical purpose, since we may want to colonize it some time in the future. But galaxies billions of light-years away are not likely prospects for colonization. At least not in the next few billion years. Of course, the appeal of cosmology is not in its practical utility. Our fascination with the cosmos is of the same nature as the feeling that inspired ancient creation myths. It is rooted in the desire to understand the origin and the destiny of the universe, its overall design, and how we humans fit into the general scheme of things.
Cosmologists who do rise to the challenge of the ultimate cosmic questions lose all their advantage over detectives. They can rely only on indirect, circumstantial evidence, using measurements made in the accessible part of the universe to make inferences about the times and places that cannot be observed. This limitation makes it much harder to prove one’s case “beyond a reasonable doubt.” But because of remarkable recent developments in cosmology, we now have answers to the ultimate cosmic questions that we have some reason to believe.
The worldview that has emerged from the new developments is nothing short of astonishing. To paraphrase Niels Bohr, it may even be crazy enough to be true. That worldview combines, in surprising ways, some seemingly contradictory features: the universe is both infinite and finite, evolving and stationary, eternal and yet with a beginning. The theory also predicts that some remote regions have planets exactly like our Earth, with continents of the same outline and terrain, inhabited by identical creatures, including our clones, some of them holding copies of this book in their hands. This book is about the new worldview, its origins, and its fascinating, bizarre, and at times disturbing implications.
PART I
GENESIS
1
What Banged, How It Banged, and What Caused It to Bang
In the context of inflationary cosmology, it is fair to say that the universe is the ultimate free lunch.
—ALAN GUTH
On a Wednesday afternoon, in the winter of 1980, I was sitting in a fully packed Harvard auditorium, listening to the most fascinating talk I had heard in many years. The speaker was Alan Guth, a young physicist from Stanford, and the topic was a new theory for the origin of the universe. I had not met Guth before, but I had heard of his spectacular rise from obscurity to stardom. Only a month before, he belonged to the nomadic tribe of postdocs—young researchers traveling from one temporary contract to another, in the hope of distinguishing themselves and landing a permanent job at some university. Things were looking bleak for Guth: at age thirty-two he was getting a bit old for the youthful tribe, and the contract offers were beginning to dry out. But then he was blessed with a happy thought that changed everything.
Guth turned out to be a short, bouncy fellow, full of boyish enthusiasm, apparently untarnished by his long wanderings as a postdoc. From the outset, he made it clear that he was not trying to overthrow the big bang theory. There was no need to. The evidence for the big bang was very persuasive, and the theory was in good shape.
The most convincing evidence is the expansion of the universe, discovered by Edwin Hubble in 1929. Hubble found that distant galaxies are moving away from us at very high speeds. If the motion of the galaxies is traced backward in time, they all merge together at some moment in the past, pointing to an explosive beginning of the universe.
Another major piece of evidence in favor of the big bang is the cosmic background radiation. Space is filled with microwaves of about the same frequency as we use in microwave ovens. The intensity of this radiation dwindles as the universe expands; hence what we now observe is the faint afterglow of the hot primeval fireball.
Cosmologists used the big bang theory to study how the fireball expanded and cooled, how atomic nuclei formed, and how the grand spirals of galaxies emerged from featureless gas clouds. The results of these studies were in excellent agreement with astronomical observations, so there was little doubt that the theory was on the right track. What it described, however, was only the aftermath of the big bang; the theory said nothing about the bang itself. In Guth’s own words, it did not say “what ‘banged,’ how it ‘banged,’ or what caused it to ‘bang.’”1
To compound the mystery, on closer examination the big bang appeared to be a very peculiar kind of explosion. Just imagine a pin balancing on its point. Nudge it slightly in any direction and it will fall. So it is with the big bang. A large universe sprinkled with galaxies, like the one we see around us, is produced only if the power of the primordial blast is fine-tuned with an incredible precision. A tiny deviation from the required power results in a cosmological disaster, such as the fireball collapsing under its own weight or the universe being nearly empty.
The big bang cosmology simply postulated that the fireball had the required properties. The prevailing attitude among physicists was that physics can describe how the universe evolved from a given initial state, but it is beyond physics to explain why the universe happened to start in that particular configuration. Asking questions about the initial state was regarded as “philosophy,” which, coming from a physicist, translates as a waste of time. This attitude, however, did not make the big bang any less enigmatic.
Now Guth was telling us that the veil of mystery surrounding the big bang could be lifted. His new theory would uncover the nature of the bang and explain why the initial fireball was so contrived. The seminar room fell suddenly silent. Everybody was intrigued.
The explanation the new theory gave for the big bang was remarkably simple: the universe was blown up by repulsive gravity! The leading role in this theory is played by a hypothetical, superdense material with some highly unusual properties. Its most important characteristic is that it produces a strong repulsive gravitational force. Guth assumed that there was some amount of this material in the early universe. He did not need much: a tiny chunk would be sufficient.
The internal gravitational repulsion would cause the chunk to expand very rapidly. If it were made of normal matter, its density would be diluted as it expanded, but this antigravity stuff behaves completely differently: the second key feature of the strange material is that its density always remains the same, so its total mass is proportional to the volume it occupies. As the chunk grows in size, it also grows in mass, so its repulsive gravity becomes stronger and it expands even faster. A brief period of such accelerated expansion, which Guth called inflation, can enlarge a minuscule initial chunk to enormous dimensions, far exceeding the size of the presently observable universe.
Figure 1.1. A chunk of gravitationally repulsive material.
The dramatic increase in mass during inflation may at first appear to contradict one of the most fundamental laws of physics, the law of energy conservation. By Einstein’s famous relation, E = mc2, energy is proportional to mass. (Here, E is energy, m is mass, and c is the speed of light.) So the energy of the inflating chunk must also have grown by a colossal factor, while energy conservation requires that it should remain constant. The paradox disappears if one remembers to include the contribution to the energy due to gravity. It has long been known that gravitational energy is always negative. This fact did not appear very important, but now it suddenly acquired a cosmic significance. As the positive energy of matter grows, it is balanced by the growing negative gravitational energy. The total energy remains constant, as demanded by the conservation law.
In order to provide an ending fo
r the period of inflation, Guth required that the repulsive gravity stuff should be unstable. As it decays, its energy is released to produce a hot fireball of elementary particles. The fireball then continues to expand by inertia, but now it consists of normal matter, its gravity is attractive, and the expansion gradually slows down. The decay of the antigravity material marks the end of inflation and plays the role of the big bang in this theory.
The beauty of the idea was that in a single shot inflation explained why the universe is so big, why it is expanding, and why it was so hot at the beginning. A huge expanding universe was produced from almost nothing. All that was needed was a microscopic chunk of repulsive gravity material. Guth admitted he did not know where the initial chunk came from, but that detail could be worked out later. “It’s often said that you cannot get something for nothing,” he said, “but the universe may be the ultimate free lunch.”
All this assumes, of course, that the repulsive gravity stuff really existed. There was no shortage of it in science fiction novels, where it had been used in all sorts of flying machines, from combat vehicles to antigravity shoes. But could professional physicists seriously consider the possibility that gravity might be repulsive?
They sure could. And the first to do that was none other than Albert Einstein.
2
The Rise and Fall of Repulsive Gravity
“We have conquered gravity!” the Professor shouted, and crashed to the floor.
—J. WILLIAMS and R. ABRASHKIN,
Danny Dunn and the Anti-Gravity Paint
Many Worlds in One: The Search for Other Universes Page 1