"Many and strange are the universes that drift like bubbles in the foam upon the river of time." --Arthur C. Clarke.
Our universe, physicists believe, began about 15 billion years ago in a Big Bang. This was an enormous explosion, far more violent than that of a nuclear bomb, an intense flash of energy that created the cosmos. But what existed before the Big Bang? What produced it, or caused it to occur?
Today a number of investigators are giving new insights that address these questions. Their work gives a far-reaching, and rather mind-boggling, conclusion: There is no reason to think that our universe is the only one. The processes that produced our own Big Bang very likely have occurred in other places and times. This means there could be other universes. Indeed, these could exist in large numbers.
For a long time, we have appreciated that our sun and galaxy are not unique. Rather, they are each no more than one among a vast multitude of similar objects. Now the same may be true of our universe.
Contemplating one universe is hard enough. Thinking about several at once is new ground even for scientists, who are tiptoeing through brave new theoretical worlds of "space-time foam," "false vacuums" and "baby universes."
The definition of a universe is simple; it consists of everything that exists. Our own universe includes every galaxy and quasar, even beyond the limits of what astronomers can see with their best telescopes. It includes stars and planets within these galaxies, as well as forms of matter that emit no light. Space itself is part of the universe, as is the energy that fills this space. And other universes would be similarly rich.
But the very phrase, "other universe," carries a contradiction. If the universe includes everything, then how could there be another everything? The answer lies in the fact that there are other, far different, concepts of space. Such concepts would allow other universes to exist, each formed from its own Big Bang--yet none of them could receive light or other signals from any of the others. They could exist, but we would never see them.
New concepts of space, indeed, are at the root of efforts that are pursuing the question: What came before the Big Bang? Two of the leaders in these efforts are Stephen Hawking of Cambridge University in England and Alexander Vilenkin of Tufts University in Medford, Mass. They begin by proposing, in effect, that space itself is radioactive. It does not give off radiation like a nuclear reactor, but rather continually produces tiny entities that Hawking calls "baby universes."
As Vilenkin describes it, space as we see it is like an apparently smooth ocean seen from an ocean liner. Up close, however, the surface of the sea is full of waves and foam. Similarly, at sufficient magnification we would see the baby universes forming and dissolving in space like tiny bubbles, forming what he calls "space-time foam." "The universe comes out of this," he declares.
Under this theory, a baby universe usually flashes momentarily into existence and then winks away. But sometimes it acts as a seed, capable of growing into a full-fledged universe. This can happen because a baby universe can consist of a most unusual form a space: "false vacuum." It has bizarre properties because it contains, for a very brief instant, a great deal of energy within a very small volume. Sidney Coleman of Harvard has studied these properties.
False vacuum, says Coleman, has a pressure that is very large, and negative. In this regard, false vacuum differs from anything we know. Air in a tire has positive pressure. True vacuum, which is produced by pumping air out of a chamber, has zero pressure. But false vacuum has a pressure that is much less than zero. A bubble of false vacuum, surrounded by true vacuum, would tend to collapse like a submarine that dives too deeply in the sea.
But false vacuum can counter this with another strange feature: anti-gravity. Its gravity does not act to hold it together, but tends to force the bubble to expand. This means that there is a tug of war between this anti-gravity, which makes the bubble expand, and negative pressure that tends to make it collapse.
What happens next? Two separate groups have addressed this question, one led by Alan Guth of Massachusetts Institute of Technology and the second by Willy Fischler and Joseph Polchinski at the University of Texas. They find that usually the negative pressure wins, and the bubble then quickly winks out of existence. But sometimes the anti-gravity wins. Then the anti-gravity takes hold with great strength, elbowing aside the negative pressure. This leads to the "inflationary scenario" for the formation of a universe, which Guth has put forward for several years.
In this concept, the anti-gravity drives an enormous and very rapid expansion of the bubble. It balloons from microscopic size to the dimensions of a cantaloupe. As it inflates in this fashion it cools, and reaches conditions where it ceases to remain in a false vacuum. It loses its anti-gravity and its negative pressure.
But in making this change, the false vacuum releases an enormous burst of energy. This energy takes the form of very hot particles, which are produced in vast quantities. There are enough of them, in fact, to form all the stars and galaxies in the new universe, once these particles have the chance to cool.
The rapid inflation of the false vacuum, followed by this release of energy, constitutes the Big Bang. The newly-born universe, formed in this fashion, will then settle into a long era of expansion. Our own universe has been expanding in this manner for about 15 billion years.
All this amounts to creating a universe out of nothing, which appears impossible. But Steven Weinberg at the University of Texas notes that this is not a problem. To appreciate why not, he points out that our universe indeed had a great deal of energy. But it also had a great deal of gravity--and gravity has negative energy. This counterbalances the other forms of energy, so that the total amount of energy in the universe, including gravity, could be very small. It could even be zero.
These ideas also hold in looking at the formation of the universe. From the Big Bang to the present day, there has always been enough negative energy, in the form of either anti-gravity or ordinary gravity, to balance the energy of every star and galaxy out to the farthest distances.
In the cosmic balance accounts, every atom in a sense is paid for with an appropriate amount of this negative energy. The universe thus can have a total of zero energy, and would always have done so. In Guth's words, "The universe is the ultimate free lunch."
But where do the multiple universes come from? As the false vacuum inflates, it can readily produce new baby universes that act as seeds for the formation of other universes. Those seeds take the form of new bubbles of false vacuum, which inflate in their turn. And while they do this they produce their own seeds for yet other universes. The result is a chain reaction, in which the formation of one universe gives rise to many more.
"That's something that seems to almost always happen in inflationary scenarios," says Guth. "Once the process has begun it seems like it goes on forever, continually spinning off new universes as pieces of the false vacuum."
This means that our own universe could have formed in the course of such a chain reaction. It would trace its ancestry to a much earlier baby universe that would have formed out of nothing and turned into a cosmic seed. Similarly, other seeds might be sprouting this very minute . . . anywhere . . . perhaps within your own living room.
Could we see such things as they happen? Lawrence Susskind of Stanford University offers words of caution: New universes, he says, "don't form in our space. It doesn't make sense to ask where in space these things happen. These new universes are not immersed in some other space." Susskind adds that in no way could we observe them; in particular, we could not determine how far away they are. If a universe formed in your living room, you could never tell.
Indeed, between us and these other universes would lie what Guth calls "a state of absolute nothingness--the absence of matter, energy, space or time." Space itself, as we know it, would come into existence with the inflation of the initial cosmic seed of false vacuum.
If these ideas are correct, though, there could be many universes, but they need not be strange, as Arthur C. Clarke wrote. They could have their own galaxies, stars, planets--and physicists.
In the words of Edward Tryon of Hunter College, who helped initiate some of these ideas, "Our universe is simply one of those things that happen from time to time."
Some researchers believe that so-called baby universes are forming all the time out in space. Here is a general look at the theory:
A. Imagine a universe as the circle above, with the swirls representing galaxies within it. The magnified image symbolizes baby universes, which would be microscopic in size, continually forming and dissolving in space.
B. Usually, a baby universe will disappear as fast as it appears. But occasionally, the theory goes, it can take form of a bubble of a false vacuum (a momentary event in which a great deal of energy is contained in a small volume). That bubble is theoretically capable of becoming the seed for a full-fledged universe.
C. The force that would make the bubble of a false vacuum expand is called anti-gravity. (Usually, that force is countered by negative pressure, which makes it collapse.) But if the anti-gravity force does prevail, it would cause the bubble to expand very rapidly, releasing vast amounts of new energy that form stars and galaxies.
D. Eventually, the connection-or wormhole-would pinch off and the false vacuum would cool and turn into space as we know it. Then, the baby universe would become a newly formed universe.
Source: Research News, National Science Foundation