Tufts University is understandably a little ambivalent about its Antigravity Rock. On the plus side, it’s not an unattractive boulder, as boulders go; it has a catchy name and bears a handsome inscription; and it came with a modest research grant. Unfortunately, it doesn’t float, the inscription on its plaque posits an unlikely relationship between antigravity and safe airplane flight, and the money is available only for research on antigravity.
Alexander Vilenkin is one Tufts faculty member who believes the pros outweigh the cons. In fact, he rarely fails to offer visitors the opportunity to stroll across campus to check the boulder out. Many accept, the alternative being to stay in Vilenkin’s modest, slightly cheesy office and sit in his Metastable Chair, a rickety piece of furniture that affords a fair chance of experiencing the rush of gravity firsthand.
Vilenkin seems to enjoy the air of wry surrealism that these gravity-related attractions lend his existence. And why shouldn’t he lighten things up a bit? Giggles don’t come easily in cosmology, his line of work. Vilenkin is the author, for example, of a widely accepted explanation of how our universe leapt into existence from nothing. (Antigravity happens to play a cameo role in this explanation, providing a loophole for using the grant.) More recently he has come up with a solution to the long-standing problem of why our universe hasn’t dissipated into cold dust or collapsed into a fantastically hot nugget. According to physics as currently understood, one of these events should have happened long ago.
This later work rests on the reasonable but deceptively simple proposition that, of all the civilizations that might exist in this or any other universe, ours--meaning our planet’s--is probably not extraordinary. Vilenkin’s accomplishment has been to deduce from this general statement specific predictions of some of the physical properties we should expect to observe in nature. Physicists have long understood what these properties are and how they affect the universe, but they have gnashed their teeth over exactly why nature settled on these particular ones. According to Vilenkin, the explanation has been staring us in the face: it all has to do with our civilization’s mediocrity.
Mediocrity may seem a modest hook upon which to hang all the mysteries of the universe, but it suits Vilenkin. He doesn’t go in for the bombast that characterizes many of cosmology’s brethren. The 46-year-old physicist is soft-spoken, even demure. With his tamed Russian accent, slight build, and soft features, he exudes the grace of a poet-in- residence.
Then there’s Tufts. Stuck in the middle-class normalcy of Medford, Massachusetts, Tufts is almost literally, and very much metaphorically, in the shadows of those towers of higher education, MIT and Harvard, just 15 minutes down the road in urbane Cambridge. Blinded by the light of those two academic giants, some people mistake Tufts for a mediocre institution.
MIT and Harvard, needless to say, have their own takes on how the universe originated, incorporated in the persons of Alan Guth and Sidney Coleman, respectively. Guth took the field by storm in 1980 when he suggested that the universe, after its birth in the Big Bang, did not merely expand but also inflated, its borders rushing out at faster-than- light speed to create its vast scope in mere instants. Radical as that notion seemed at the time, it is now the most popular choice among cosmologists to explain most of the characteristics of the universe we observe today. A few years ago at Harvard, meanwhile, particle physicist Sidney Coleman dabbled in cosmology long enough to shake everyone up with his theory that wormholes--submicroscopic tunnels to other universes--set the physics of our own universe. Vilenkin’s work builds on Guth’s idea and offers an alternative to Coleman’s.
Vilenkin’s fascination with cosmology dates back to high school in the Ukraine, where he split his passions between the writings of Karl Marx and those of Albert Einstein. Although his interest in Marx faded under the harsher illumination of study at the University of Kharkov, he says, Einstein’s work seemed beautiful to me. Unfortunately, few professors at the university could do anything to satisfy Vilenkin’s curiosity about cosmology; his frustration grew worse when he was rejected by Soviet graduate schools. He attributes this snubbing in part to anti- Semitism. Having good contacts can help you get around being Jewish, he explains, but my father was a professor and not a very practical man, and never made those contacts.
Unable to get work as a physicist, Vilenkin took a job as a night watchman in a zoo and set about doing cosmology on his own. After being allowed to emigrate in 1976, he came across an advertisement for the graduate program in physics at the State University of New York at Buffalo. He had better luck getting accepted in Buffalo than in the Ukraine, and he whipped through the Ph.D. program in just one year. Eventually he landed a job at Tufts, in condensed-matter physics. No one complained when, after a while, he quietly took up cosmology again.
It helped that Vilenkin made his mark quickly. In 1982 he produced an explanation of how galaxies might have formed even if, as theory had it, matter had been distributed too uniformly throughout the universe to clump together. The answer, he said, lay in vast concentrations of energy called cosmic strings, which snake their way through the universe, drawing matter together with their gravity. This work impressed even cosmology guru Stephen Hawking, who invited Vilenkin to an elite gathering of cosmologists at Cambridge University. A year later Vilenkin produced another theory to justify his growing reputation. At the time, almost all variations of the Big Bang theory started with the bang itself. Vilenkin, however, reached back further. If the Big Bang created all matter and energy and time and space from a tiny speck, where did the speck come from? How did it burst into being? These were not questions that cosmologists considered answerable. Physics had given them no tools for picking apart the nature of creation itself, only its results.
Or had it? Vilenkin notes that Saint Augustine was warned by his colleagues that God prepared hell for people who asked about pre-creation, but Saint Augustine refused to believe God punished curiosity about the ultimate miracle. Vilenkin decided to jump in, too. He started by pondering a curious symmetry: physicists used roughly the same theory to describe the Big Bang as they did to describe the tiniest subatomic particles. This theory is quantum mechanics, and it imbues particles with a fuzzy, wavelike nature that allows them to be sort of smeared out in space and time. Without this smearing, what happened at the moment of the Big Bang would be inconceivable. Classical physics, including Einstein’s theory of relativity, does not allow for packing an entire universe of matter or energy into a pointlike speck, because gravity would become infinitely large, and the equations of classical physics break down in the face of infinity. But smear that matter out, quantum mechanically, and gravity is no longer infinite. It is only unthinkably immense.
Since cosmologists were already using some of the tools of quantum mechanics, Vilenkin reasoned, why not borrow another of its tricks? According to quantum mechanics, the emptiest possible void of outer space is never really completely empty. It is filled with tiny particles of matter that are always popping into existence and then, an instant later, popping out again. These are called virtual particles, and they exist because of a quirk of quantum mechanics. The equations of that theory allow empty space to be described as an energy field that has an average value of zero. An average value of zero, however, means that at any particular spot the energy level of empty space can fluctuate--it can assume a positive value at one moment in one particular spot and a negative value the next moment somewhere else. Every once in a while, one of these random fluctuations will be large enough so that a particle will spring into being, only to snuff itself out a moment later. These virtual particles blink in and out of existence throughout the universe all the time, and their reality is widely accepted by physicists.
If a particle can pop into existence from nothing, why not a whole universe? Vilenkin wondered. If space can be thought of as an energy field with an average value of zero, why not think of pre-creation nothingness as a sort of space-time field whose average value is zero? Rather than a virtual particle popping into existence, a whole universe, along with matter and energy and space and time and everything else, pops into existence from nothing. Once he started to think about the universe in this way, he raised the possibility of not just one universe but many. Proto-universes could be popping into existence all the time. Of course, most of these universes would instantly snuff themselves out, just as virtual particles do. They would amount to nothing but fluctuations, random hiccups in nothingness. Eventually, though, one of these hiccups would have enough energy to escape instant annihilation. It would instantly expand. A universe would thus be born in a big bang.
Despite some doubts--it began to look like a very crazy idea to me after a while, Vilenkin says--he plowed ahead. Using the accepted mathematics of quantum mechanics, he produced a reasonably rigorous description of the instant of the birth of the universe. The preuniversal nothingness he described was the purest form of nothingness imaginable. Since matter and energy create time and space, Vilenkin’s nothingness had neither. There was no countdown to the Big Bang, because time did not yet exist. In a stroke, he reduced creation from a metaphysical event to a physical one. What had seemed unknowable was suddenly reduced to a set of equations.
The man who only a few years before had guarded the sleep of zebras was now inundated with invitations to speak to the cosmological cognoscenti. After a talk he gave at Harvard, Guth and Coleman enthusiastically discussed his theory with him. But these talks did not leave Vilenkin feeling satisfied. One question kept popping up like some annoying virtual pebble in his shoe. When would he produce a testable prediction? It was a valid question, and one Vilenkin often asked of himself. After all, testability is what separates physics from mere philosophy. Anybody can make up a plausible story about what causes eclipses. But predicting an eclipse--that’s science.
With this shortcoming in mind, Vilenkin turned his attention to the hottest problem in cosmology: the theory of inflation. Guth had put forth this theory, which explains how the universe expanded after the Big Bang, to address some puzzling observations--not least that the universe appears to be very flat.
Einstein showed that matter and energy determine the shape of space; one of the consequences of this is the phenomenon we perceive as gravity--just as a bowling ball placed on a bed creates a small valley in the mattress, a massive object (or a large energy level) warps space-time in such a way as to create a valley into which other objects naturally fall. If our universe had been created with lots of matter and energy, then the result should be a tremendous warping inward of space; eventually the universe’s expansion would be reversed, and it would collapse in a Big Crunch. If, on the other hand, the universe began with only a little bit of matter and energy in it, then it would warp in the opposite direction and go on expanding forever.
Scientists have taken great pains to measure the extent of this warping, comparing their observations of distant objects such as quasars with where they would expect them to be. As far as they can tell, the universe is not warped--it is perfectly flat. When you consider that the expansion of the universe would have exaggerated any warping that existed immediately after the Big Bang, this present flatness is all the more surprising. The universe, it seems, has achieved a perfect balance between the Big Crunch and eternal expansion. But why, scientists asked, should our universe happen to have achieved this miraculous poise? There was no reason at all that anybody could think of.
Guth posed his inflation theory to remove the need for a miracle. In his scenario, the universe at the time of the Big Bang was so strange that gravity was actually repulsive. This antigravity caused space to expand so quickly as to dwarf the speed of light. (Einstein’s theories may not allow matter or energy to travel through space faster than light, but they place no such restriction on space itself.) In much less than a billionth of a second, a newborn universe immeasurably smaller than an atom mushroomed into a vast cosmos stretching far beyond the reaches of our observation. Even if such a universe were as bent as a balloon giraffe, it would be so unthinkably vast that the best telescopes would not be able to see far enough to detect any curvature at all. Its huge size would hide its curvature from us, much as Earth’s curvature is invisible from the ground. If our universe did indeed inflate itself in this way, it would appear flat without actually being flat--and physicists would no longer have to invent a reason to explain its apparent flatness.
Guth’s inflation theory was less helpful, however, in explaining the nagging mystery of the cosmological constant, one of the most troublesome aspects of cosmological theory. Loosely put, the constant is a measure of how much energy is tied up in empty space, and it is expressed in units of energy per volume. Remember, the quirks of quantum mechanical math allow space to be described as an energy field with an average value of zero--but they don’t demand that that average value actually be zero. In fact, there is no known reason that it should be, other than the aesthetic preferences of physicists. Energy levels throughout the vastness of the universe could, in theory, fluctuate around some other average, positive or negative.
Some physicists have calculated that the cosmological constant should in fact have a monstrously large positive value. The problem is that a large positive constant would act like antigravity, causing the universe to expand so fast that it would tear itself apart. A large negative constant, on the other hand, would have turned our universe into a fun- house mirror, in which you could look out a window and see into the room behind you. As Guth and Vilenkin knew only too well, neither case is true-- space is pretty darned straight in the observable universe, as far as anyone can tell, and the universe has obviously not torn itself apart-- which suggests that the cosmological constant is indeed either zero or very close to it.
Physicists have long been convinced that this jarring disparity between expectation and observation is far beyond the reach of coincidence and that some unknown law or phenomenon is driving the constant down. What’s more, they reason that if something is pushing the constant down from its natural, mind-bogglingly high value to very close to zero, then it must be pushing it to exactly zero. That’s because, to a physicist’s way of thinking, zero is far more natural than, say, .000236. Coleman’s wormhole theory provides an Alice-in-Wonderland explanation for a zero cosmological constant. Immediately after our universe’s birth, tiny wormholes connecting it to older universes with cosmological constants of zero supposedly allowed it to peer into those universes and adopt the same constant for itself. Coleman’s theory, however, relies too much on conjecture for most tastes, as do most other theories that attempt to rectify this problem.
In this case, though, some physicists have gotten so desperate to settle the matter of unnaturalness that they have resorted to a very odd (and somewhat circular) argument known as the anthropic principle. The very existence of human beings, the argument goes, can explain certain characteristics of the universe that are otherwise mysterious. In a nutshell: If a constant must have a certain value in order to support the eventual development of life as we know it, then it cannot have had any other value, or we wouldn’t be here to theorize about it.
This reasoning may sound like a cosmic cop-out, but it isn’t. Even if the odds are long--say, one in a trillion--that a universe would get precisely those constants needed to support life, beating those odds doesn’t require a special scientific explanation. Lottery winners might attribute their fortunes to divine intervention, but to the rest of us it’s just dumb luck--after all, someone has to win. Our universe just happens to be a lucky lottery winner.
Applied indiscriminately, the anthropic principle amounts to little more than rearranging the laws of nature to suit our need to exist. For this reason, Vilenkin had never been a particular fan of it. Most physicists try to stay away from it as much as possible--and so do I, he says. However, the principle makes more sense, and avoids this dubious circularity, when framed in terms of multiple universes. To a cosmologist, predicting the characteristics of our universe is equivalent to predicting the characteristics of any randomly selected universe and then asking how likely it is, according to the theory, that a universe with the same characteristics as ours would occur. (By characteristics, a cosmologist usually means nature’s fundamental constants--the speed of light, the electric charge of an electron, the mass of a quark--which form the bedrock of physics and serve to define our universe.) The cosmologist crunches equations that form the stuff of the new theory and comes up with a range of possible values for each constant, along with a bell curve indicating which values are likely and which are far-fetched.
For the new theory to pass muster, it must overcome physicists’ abhorrence of the unnatural--it must predict that our universe is not only possible but likely. In other words, each known constant should fall in the middle of the range of values produced by the equations, under the fat part of the curve. If not--if known constants fall under the extreme edge of the bell curve--then physicists worry that the theory leaves out something fundamental and try to think up a better one.
The anthropic principle helps narrow the very long odds of a universe’s having a cosmological constant close to zero. Among the multitude of all possible universes, a few are bound by sheer chance to have the constants that allow for the rise of life. Most of them wouldn’t: some would have too much gravity and crush themselves out of existence, others would have too little for planets to form; some universes would have such a tiny electric force that atoms wouldn’t stick together to form molecules, while others would have such a strong electric force that matter would clump together too tightly to allow stars to burn; and so on. The anthropic principle, however, says that we can rule out any universe that doesn’t support life. We don’t even need to consider it. As soon as you pose the question in terms of what constants you are likely to observe in the universe, you are already raising the issue of an observer, says Vilenkin. In this context, consideration of the anthropic principle is unavoidable.
Of course, physicists would prefer to discover the mechanism that caused the fundamental constants to take on their precise value, but the anthropic principle at least helps explain why some constants have such seemingly unnatural values. Physicist Steven Weinberg at the University of Texas at Austin used the anthropic principle in trying to explain why the cosmological constant was small. He showed that a huge cosmological constant, whether positive or negative, would have precluded our existence. A huge positive constant would produce overwhelming antigravity, causing matter to dissipate rather than clump into stars and galaxies. If the constant were negative, gravity would halt the universe’s expansion and cause it to collapse. Because we are here, neither of these events could have happened, and the cosmological constant must have limits that make it many trillions of times smaller than the huge value physicists would otherwise expect. Of course, that still leaves the constant much, much larger than observation shows it to be, but it’s a big step in the right direction.
The next big step was Vilenkin’s. In 1994 he was reading Richard Dawkins’s The Selfish Gene, which argues that successful genes--those that persist from generation to generation--are those that, by sheer luck, happen to improve the chances of their own organism’s survival and thus of the gene’s own perpetuation.
I was impressed with that idea, says Vilenkin. And at the same time, it started me thinking about the anthropic principle. Though from entirely different fields, the two ideas have a similar ring: Dawkins says, in effect, that a gene’s existence is all the justification needed for whatever odd-seeming function the gene embodies; similarly, the anthropic principle says that our existence justifies our odd-seeming constants of nature.
In mulling this over, Vilenkin added a twist. Weinberg, in pondering the likelihood of our universe’s springing into being from among a multitude of possible universes, had already used the anthropic principle to narrow the odds. Rather than calculating the odds from among all possible universes, one need only consider those that could possibly support life. But he hadn’t considered that some universes may spawn far more civilizations than others--one universe might be harsh and allow life merely to squeak by, while another, more fecund universe might be teeming with millions of inhabited solar systems.
If so, Vilenkin reasoned, a small percentage of universes would account for the lion’s share of civilizations. Any randomly chosen civilization--say, for example, ours--would be far more likely to come from one of these high-civilization-producing universes than from a stingy universe harboring only one or two civilizations. After all, if you randomly picked a person from among the billions of people on Earth, he or she would more likely hail from populous China or India than from tiny Liechtenstein or Luxembourg. This reasoning applies as well to civilizations and universes.
One conclusion that Vilenkin draws from this insight is that our civilization most likely exists in a fecund universe. Why should we assume there is anything special about us? he says, in his precise, slightly gloomy tone. If we are like most civilizations, then we are probably in a universe with many civilizations. In other words, our civilization is nothing special. For this reason, he dubbed this new criterion the principle of mediocrity.
The principle of mediocrity narrows down the target. Suddenly, we are looking for properties of the universe that will lead not merely to life but to a plethora of life. What are these more restrictive properties? The answers would become predictions of what we are likely to see in our universe.
The most important of these properties turns out also to be the simplest: size. The larger the universe, the more room there would be for life. Simple statistics, then, would seem to dictate that our universe is one of the largest of possible universes. The properties in our universe must also be those that would produce the greatest amount of inflation while still supporting abundant life.
Armed with these new considerations, Vilenkin began drawing conclusions about the conditions in our newborn universe, relating mostly to the way in which energy was distributed. These conditions, in turn, led to a somewhat different picture of the universe during the brief instant of inflation. In particular, the theory’s condition that the universe be very large implies that it inflated more slowly and for a longer period of time (though still a tiny fraction of a second) than most physicists had thought. What’s more, the point at which matter as we know it began to form occurred later after the Big Bang than expected, and cosmic strings probably played a larger role in helping matter to first gather into clumps. Unfortunately, none of these characteristics had unique consequences that, when verified by observation, could confirm his theory. Vilenkin, in other words, hadn’t come up with a testable prediction. Until, that is, he decided to consider the cosmological constant.
Vilenkin recognized that to end up with a universe that supported abundant life, the cosmological constant should lie in a narrow range--it would need to both allow life and lead to high inflation. Such a universe would be quite sensitive to the effects of a too-large constant on its finely tuned expansion. Even a fairly small constant could add enough repulsive force to cause the universe to expand so much that matter ends up being too dispersed to clump. And a fairly small negative constant could put the brakes on the universe’s expansion and cause it to collapse before life had a chance to evolve. In this respect, the cosmological constant is like the weather at a bubble-gum-chewing contest. You might be able to blow ordinary-size bubbles all day long in a hurricane, but when you’re shooting for the largest possible bubble, you’d better make sure the winds are still. To a giant, life-supporting universe struggling to inflate itself, even a small cosmological constant acts like a giant gust of wind.
Vilenkin concluded that the cosmological constant would have to be nearly zero--actually, some value between zero and .9. That is so close to zero that its effects would be subtle enough to have evaded detection by astronomers so far. Although Coleman and others had already come up with ways to explain a zero cosmological constant, Vilenkin’s conclusion had the subtle but all-important distinction of demanding not that the constant be precisely zero, only that it be very small. In fact, according to Vilenkin’s theory, it would be absurdly unlikely for the constant to turn out to be exactly zero.
A small, nonzero cosmological constant is the sort of prediction physicists dream of. It agrees for the most part with current observation. It differs from the standard prediction, which means it can be used to prove which of the theories is more likely correct. And most important, it holds the promise of an observational showdown with the standard prediction in the near future. Physicists are continually increasing the precision with which they can measure the cosmological constant. Within several years, they expect to measure it precisely enough to prove whether Vilenkin is right. Says Vilenkin: We’ll see.
Alan Guth, the originator of the inflation theory, is among those cosmologists who are counting on it. Things have been moving especially fast in this field in the past five years, he says, and it’s not unlikely we’ll be able to settle this question within the next five. Guth himself isn’t taking any bets on the outcome. He points out several potential problems with Vilenkin’s assumptions. For one, the anthropic principle depends on there being many possible universes whose constants vary randomly from one to another. Guth still holds out the possibility that there is only one universe and only one possible set of constants, and that our existence holds no sway over them. Another possible kink is that, in one increasingly popular model of inflation, inflation never stops--it just keeps pushing the universe out farther and farther. Guth favors this hypothesis, even though it might make the principle of mediocrity irrelevant. (Vilenkin claims he can make the principle work in an eternally inflating universe.) To be sure, Guth concedes that neither the one- universe nor the eternally inflating universe scenario is firmly established yet. If the laws of physics allow for many types of universes, and they’re not eternally inflating, the principle of mediocrity seems essential, he says.
Vilenkin himself claims to be easygoing about the possibility that he could be wrong. When you’re on uncertain ground, your end product is just a proposal, he says, shrugging. Just because one is trying to figure out the origins of the universe doesn’t mean one should take oneself too seriously.
If Vilenkin is correct about the cosmological constant, then cosmologists may look more favorably on other hard-to-test predictions that are corollaries of the principle of mediocrity, such as the existence of cosmic strings. And in time, and perhaps with encouragement from verifying measurements, Vilenkin and others will probably be able to wring more predictions from the principle. Yet even if all this were to happen, Vilenkin insists he would still feel as if he hadn’t quite lived up to certain expectations. I’m just not sure any of this will make airplanes any safer, he explains.