BEFORE THE BIG BANG

By Tom Yulsman

Note: This is the first part of an article that originally appeared in the September 1999 issue of Astronomy magazine, pp. 38-46

Part 1

In the Beginning, there was the singularity — that mind bogglingly small point of Brobdingnagian density that put the BANG in the big bang.
 
Or was there?
 
The singularity is what you’d see if the classic movie of the big bang and subsequent cosmic evolution were run in reverse to the opening frame. All the planets now extant, all the stars, all the galaxies, all the intergalactic gas and dust,  in short, all the matter and energy in the universe, would zoom backward in time and converge into an infinitely tiny, infinitely dense — and singularly unexplainable — spec. Here, all the laws by which scientists explain nature would break down.
 
As a theory of early cosmic evolution, the big bang is fantastically successful. But it rests on the deep enigma of the singularity, and for scientists, an explanation for creation built on the unexplainable is just not satisfying.
 
“The problem we have is that every particle in the universe originated in the singularity,” says Neil Turok, a mathematical physicist at Cambridge University in England. “That's unacceptable because there are no laws of physics that tell you how they came out of it.”
 
And so scientists are seeking to refine their explanation of the ultimate origin of the universe. “Many cosmologists are now asking the deeper questions of how the universe really began,” says Alan Guth, a cosmologist at the Massachusetts Institute of Technology. “What started the big bang?”
 
The hunt for an answer grew more interesting in 1998, when Turok and his Cambridge colleague, Stephen Hawking, announced they had found a way to sidestep the singularity, if not eliminate it entirely. In the process, they said they had arrived at a compellingly simple and complete explanation for how the universe began.
 
Well, not precisely. In Turok and Hawking's mind-bending creation model, the universe has no discrete beginning. Instead, the Cambridge scientists propose a quantum theory of the origin of the universe in which there is no distinction between time and space, and no distinct point at which either can be said to begin.
 
If all that seems a bit much to wrap your neurons around, consider the microscopic primordial object within which Turok and Hawking say this blending of space and time occurred. They dubbed it a “pea instanton.” And what an extraordinary pea it is, for Turok and Hawking insist that it's the seed from which the universe sprouted. They boldly argue that not only do all the laws of physics work just fine within their instanton, but those laws actually imply its existence.
 
Turok and Hawking’s pea instanton is attractive as a scientific theory because it’s testable. The theory makes specific predictions about the nature of the universe that sprouts from the instanton — predictions that other scientists can check by seeing how accurately they reflect the universe we actually live in.  “One can predict everything about the universe from this initial instanton,” Tuork comments. “The calculations are very precise.”
 
But of course, precision doesn’t guarantee truth. And some cosmologists question whether the pea instanton really is getting at some universal truth. “This proposal has led to controversy,” Guth notes. “So it is too early to tell if some form of it might eventually become the accepted theory of how the universe began. In any case, it does seem rather amazing that such a far-reaching question can even be debated in scientific terms.”
 
The fact that scientists can have the debate at all is due in no small part to Guth’s own proposal in 1981 that the infant universe experienced a bout of gargantuan inflationary growth that led to the big bang. The physics of this inflationary universe theory, as it came to be called, is so far reaching that it explains in one stroke why the big bang happened, how matter was created, and why the universe looks the way it does in several fundamental ways.
 
Inflation, in turn, has spawned other theories. One in particular stands in contrast to the approach taken by Turok and Hawking in that it describes cosmic evolution as a much more complex process — and one that may have no discernible beginning at all. This theory holds that our universe was born as a tiny bubble of spacetime that inflated out of a pre-existing region and then experienced a big bang. That region, in turn, inflated out of a previous one, and so on down the line such that the original birth of spacetime — the ultimate origin of the universe — happened so far back in the past that it may well be pointless to ask how it happened.
 
In this view, our universe is but one component of a gigantic, fractal, multi-branching multiverse. It has been growing through a series of many big bangs for much longer than our little region of the multiversal whole. And it will continue to grow eternally into the future.
 
Although fundamentally different in many respects, the Turok-Hawking proposal is not necessarily incompatible with this idea, Guth points out. Although the ultimate origin of the multiverse would have occurred in the deepest mists of cosmic time, it is at least possible (if not testable) that the pea instanton was how it all got started from nothing, he says..

But the theory does face an uphill battle for acceptance by skeptical cosmologists. For example, Stanford University cosmologist Andrei Linde, a chief proponent of the multiverse theory, dismisses the pea instanton as a mathematical abstraction with no clear basis in reality. Moreover, he questions whether it's even possible to explain how the universe could have been created from nothing.

“Stephen is an extremely talented person,” Linde is careful to say of Hawking. “Sometimes, however — this is my interpretation — he trust mathematics so much that he makes calculations first and then interprets them later.” Linde finds this trust in the primacy of the math on this subject to be a bit like religious faith. “You know, it is such an esoteric science, creation from nothing, you can come away with your interpretations and there is no way to check it. It is like a religion.”

But mathematical abstractions have been Hawking’s stock in trade. And this certainly wouldn’t be the first time that one of the ideas he has championed has been met with initial incredulity only to gain support over time. (Of course it’s also true that some of Hawking’s mathematical abstractions have not checked out.)

WHAT PUT THE BANG IN THE BIG BANG?
Despite the enigma of the singularity, the big bang theory is unquestionably one of the most successful ideas in the history of science. It precisely predicted, for example, the relative amounts of the two most abundant elements in the universe, hydrogen and helium. It also successfully predicted that the universe would be filled with microwave energy, dubbed the cosmic microwave background  — a kind of echo of the big bang explosion. And the theory neatly explained why today's universe is expanding: The motion is a relic of the big bang's explosive beginnings.

But where did all the matter and energy in the universe come from, and what really put the BANG in the big bang? The theory actually does not say. In fact, contrary to common belief, the big bang does not explain the origin of the universe. It doesn't even explain a bang. Something had to happen before the big bang to get it going.

“Although the classical cosmological theory is called the big bang, the theory in fact contains no description whatever of the ‘bang,’” Guth is fond of saying in speeches about the early universe. “It proposes no answer at all to the question of what banged, how it banged, or what caused it to bang. It is really a theory of the aftermath of a bang.” The big bang description of creation, Guth says, simply begins with a cosmic fireworks display already in progress — a fireball of outreaching radiation and particles.

In his inflationary universe theory, Guth offered the first compelling explanation for what ignited the fireball. “Inflation is an attempt not to explain the origin of the universe,” he says, “but to provide a theory of the bang — a theory of what it was that set the universe into expansion and what created the energy and matter we see today.”

As a particle physicist at the Stanford Linear Accelerator back in the late 1970s, Guth never imagined that he would wind up making a profound contribution to the field of cosmology. But while at Stanford, Guth found himself working on a problem that lay at the intersection between the two scientific fields. In working on the problem, Guth focused his attention on the incomprehensibly tiny and hot patch of spacetime that was the early universe. There would have been no matter in this patch — spacetime would have consisted of a vacuum. Why study a patch of empty space?, you might ask. Well, a vacuum is not really empty. Like a sail billowing in the wind, the fabric of spacetime in a vacuum actually is roiled by quantum fields.

As Guth knew, the grand unified theories, or GUTS, of particle theory predicted that the vacuum of the early universe would have been dominated by particularly energetic fields.  As these primordial “scalar” fields fluctuated, their potential energy would have risen and fallen. Guth realized that it was theoretically possible for the potential energy of the fields to have gotten temporarily “stuck” at a high value. Admittedly, this was an assumption. But when Guth worked through its implications, he stumbled onto inflation.

His calculations showed that stuck scalar fields would have caused a tiny bubble of “false vacuum” to nucleate from the primordial patch of spacetime. The amazing thing about the bubble, Guth saw, was that it would have contained a huge amount of antigravitational energy.

Although this idea may seem unbelievable, it’s actually in accord with standard particle theories. Here’s how it works:
Since an ordinary vacuum contains energy in the form of quantum fields, gravitational energy must be present as well. To understand why, it helps to remember Einstein’s famous equation, E = mc2. According to the equation, energy (“E”) and the mass of matter (“m”) are really two forms of the same stuff. So if matter, such as planet Earth, exerts a gravitational pull, so must energy. In other words, the energy-containing quantum fields of an ordinary vacuum exert a gravitational pull, albeit a weak one.

But when Guth used Einstein's relativity equations to see what happen inside a bubble of false vacuum, he found that the gravitational energy would have the opposite effect of ordinary gravity. In other words, this gravity would push, not pull — it would, in essence, be a powerful of antigravity.

Starting out as small as one billionth the size of a proton, the initial bubble of false vacuum would have doubled in size many times in an incomprehensibly short interval, propelled in this exponential growth by the antigravity. According to one inflation model, in just 10-35 seconds, the bubble would have grown to the size of a basketball. And some inflation models predict far larger growth than that, perhaps as large as 101012 centimeters in diameter. That exponential number is a one followed by a trillion zeroes. (To print that many zeroes would require more than a million average size books.) Amazingly, if this picture of inflation is correct, it means that the portion of the universe we observe today is just an infinitesimally tiny mote compared to the whole.

The bubble of false vacuum would have had another peculiar property as well. According to particle theory, as the bubble expanded, the density of the energy within it actually would have remained constant. To picture this, imagine an inflating balloon. If the density of air inside is to be maintained, the total amount of air must be increased. Similarly, to maintain the same density of energy within the expanding bubble, the total energy must be increased — and by a huge amount, because the inflating bubble is growing exponentially. This theory seems to be saying that energy was created from nothing. And, in fact, Guth calls inflation the “ultimate free lunch.”

“It may sound as if I wasn’t there in my physics class when they talked about the conservation of energy,” Guth jokes. “But I was.” During inflation, he says, “the total energy of the system is conserved.” The enormous positive energy that builds up with ferocious speed during inflation creates a precisely balancing amount of negative gravitational energy (the ordinary attractive kind).  “And so the net probably is zero,” Guth says.
 
Eventually (meaning in a very tiny fraction of a second), the false vacuum would have decayed, Guth realized. As this happened, the enormous energy that had been accumulating within the false vacuum would have been suddenly released, creating an exploding fireball of radiation and hot particles.
 

In other words, the big bang. “Inflation,” Guth concludes, “supplies the beginning to which the standard big bang theory is the continuation.”

INFLATION'S AMAZING EXPLANATORY POWER
But inflation theory does much more than explain where the big-bang fireball came from. It also explains a fundamental quality of our universe that is ignored by the big bang theory. In short, observations show that the texture of the cosmos is smooth overall but lumpy at smaller scales. The smoothness is evidenced by the cosmic microwave background energy, which appears to be at almost exactly the same temperature throughout the cosmos. Yet any look through a powerful telescope reveals the lumpiness amidst the homogeneity: Galaxies, clusters, and superclusters.

According to inflation theory, prior to the bout of exponential growth, the contents of the universe were packed together into an exceedingly tiny space — close enough for everything to come to a uniform temperature and density. (In the big bang theory, the contents of the universe were too loosely packed for this to happen.) Then, as the universe inflated, quantum fluctuations spontaneously wrinkled the fabric of spacetime. These wrinkles were almost instantly stretched to enormous size by inflation and thereby ironed permanently into warp and woof of the universe.  (According to Neil Turok, this idea first was proposed by none other than Stephen Hawking.) After the big bang, the imperfections formed regions of slightly higher gravity on an otherwise remarkably homogenous background. This caused matter to coalesce into the galaxies and clusters that punctuate the uniformly cold and dark cosmic background like a diamond necklace glittering against a black dress.

Most models of inflation make another significant prediction, but one that has not yet been conclusively borne out by astronomical observations: The geometry of spacetime should be very nearly precisely flat. If observations show otherwise — if they show that the cosmos is even slightly curved into an “open” geometry — then either the theory is wrong or scientists will have to find a way to alter it so that inflation can produce an open universe. It may be difficult to imagine curved space, but remember that Einstein told us that both matter and energy exert a gravitational pull by bending spacetime into a kind of well. So comets plunge toward the sun because they are “falling” into the star’s gravitational well.

If the density of matter in the universe were high enough, its collective gravity would curve space such that the cosmos, if viewed from outside, would look like a sphere. In this case, the universe would be described as “closed.” Moreover, there would be enough gravity to overcome the kinetic energy of outrushing matter as the universe expanded. In this case, the ratio of gravitational to kinetic energy, a term called “omega,” would be greater than 1. Ultimately the cosmos would decelerate, stop expanding, and then actually contract in a “big crunch.”

You can rest assured that such cosmic overcrowding wouldn't happen for at least 60 billion years, Guth says. And besides, astronomical observations show that there isn't nearly enough matter in the universe for this to be a risk worth worrying about.
But observations do suggest that the universe could be open. In this case, the density of matter would be too low for gravity to overcome the universe’s expansion. Thus, omega would be less than 1, the curvature of space would be open — something like a horse’s saddle — and the universe would continue expanding forever.

In the flat universe predicted by most models of inflation, the cosmos would be perched precariously between closed and open. Omega would have to equal 1 to within one part in 100,000, meaning that gravity and expansion would be, for all intent and purposes, equal. In this case, the universe would continue expanding forever; the expansion would forever slow but never quite reach zero.

Inflation predicts flatness because regardless of the shape of the infant universe, the enormity of inflation would drive the part of the universe we can observe today toward perfect flatness. To understand why, imagine an ant on the surface of an inflating balloon. As the balloon inflates to enormous size relative to the ant, the part of the surface the ant can see appears flat, just as the surface of the Earth seems flat to us.

INFLATION'S COMPLICATIONS
Much to the dismay of inflationary theorists, however, a wide variety of astronomical observations indicate that the density of matter in the universe is somewhere between 50 to 80 percent below that required for flatness. Given these numbers, therefore, the universe looks quite open. But last spring, theorists regained hope for a flat universe when new observations showed that the rate of cosmic expansion was accelerating, not slowing as had been expected. If these observations are right, something must be pushing the very fabric of spacetime apart. And that something, many cosmologists believe, is a theoretical form of energy called the cosmological constant.

Since energy is equivalent to mass, the repulsive energy of the hypothesized cosmological constant must, like any mass, exert gravity. And if there were precisely enough of this gravity, it would balance the books: Although the universe might fall far short of the mass density, and thus gravity, required for flatness, the gravity provided by the cosmological constant would make up the difference.

In other words, the universe would be flat after all, and inflation mavens like Guth and Linde would smile because observations would be in accord with their theories.

But there are quite a few “ifs” in this scenario, Turok skeptically points out. The new data on cosmic expansion might, in fact, be wrong. And even if the data are right, and even if a cosmological constant existed, its energy might add up to an omega less than 1, making the universe open. And that would mean trouble for standard theories of inflation. Which is exactly why Turok and Hawking have been pursuing their theory: The pea instanton, it turns out, produces an open universe through inflation.

There are other complications with inflation as well. For example, in their equations, cosmologists must enter by hand a very precise value for the scalar fields that drive the exponential growth. Without this fine tuning, the scalar fields crash like a breaking wave, and the false vacuum is churned into a frothing mess of true-vacuum bubbles. This picture of the universe looks nothing like our own.

In the eyes of many scientists, such problems haven’t invalidated the basic theory. Instead, they’ve spurred creativity. Cosmologists have proposed a raft of variations on the original inflationary theme, including hybrid inflation, hyperextended inflation and even “supernatural” inflation. Perhaps the most profound variation is “eternal inflation,” a concept championed by Guth, Linde and many other cosmologists. Guth explains it this way:

After about 10-30 seconds of inflation, half of the original region of false vacuum would have decayed into a normal vacuum. But that would have left half of the universe still in a false vacuum, meaning it continued inflating. Theoretical calculations (ones that Turok finds quite suspect) show that the rate of this inflation actually would have been much greater than the rate at which the false vacuum decayed. Simple logic therefore dictates that if some areas were inflating much faster than other were decaying, inflation would have outpaced decay. And to this day, some part of the universe must be continuing to inflate. In other words, once inflation gets going, it’s eternal.

If this is right, we live in a region in which the false vacuum decayed, giving rise to a standard big bang. But other areas of the universe must still be inflating. And if a proposal from Andre Linde called “chaotic inflation” is right, there would be still other areas in which inflation never really took off in the first place. The result would be the multiverse, a living, growing, multi-branching entity with new regions budding out of old ones, some buds going nowhere and others growing to colossal size.
 

Copyright 1999 - Tom Yulsman

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