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Universe as Expanding Phase Boundary

This article comes from New Scientist [No2173/13 February 1999] [TEC] [Written by Marcus Chown]

In the beginning, space was filled with a liquid hovering below its normal freezing point. Supercooled liquids like this are on a hair-trigger: the merest nudge is enough to set off a runaway frenzy of freezing. That nudge might be provided by a dust-like impurity in the liquid or perhaps by a small region which by hance is a little colder than the rest. Whatever it was, something triggered the cosmic liquid, seeding a crystal that grew explosively, racing outwards.

Does this scenario ring any bells? According to Michael Grady from the University of New York College at Fredonia, it should. He is convinced that the seeding of the crystal is nothing other than the big bang, which spawned our Universe.

As Grady sees it, all sorts of mysterious observations fall into place if our Universe was brought into being by such a phase nucleation event, triggering a transition from liquid to solid in a pre-existing fluid.

If this extraordinary idea hasn't fazed you yet, hold onto your hat, For the liquid Grady has in mind is unlike any liquid you have ever imagined. Instead of the familiar three space dimensions and one time dimension of the world we see. Grady's liquid would have filled four dimensions of space and one of time - a total of five dimensions. "For want of a better word I call it 'protospace'," says Grady.

So why do we see only three space dimensions? Grady's answer is that we are stuck on the expanding solid surface. Imagine an ordinary crystal growing in a familiar three-dimensional liquid. The boundary between the crystal and the liquid is two dimensional, like the surface of an expanding soap buble. But in the strange liquid envisaged by Grady - one with four space dimensions - the phase boundary is a three-dimensional surface, something that is impossible to visualise. "That is our Universe," says Grady. "We think we're in a three-dimensional Universe but we're actually riding the surface of a four-dimensional bubble."

So what about time? In Grady's version of the Universe, time comes in two distinct varieties. First there is the "universal time" which ticks away in the bulk liquid. This time is completely hidden from us because our Universe exists only on the surface. The second kind is the time we experience. This, Grady believes, arises as we are carried along the fourth space dimension, perpendicular to the phase boundary. "What we perceive as time is actually the extra space dimension," says Grady. "It is difficult from the other space dimensions because it extends out of the phase boundary and so is inaccessible."

Grady says he has not had any direct reaction to his idea from any other physicists. No matter. According to Martin Rees of the University of Cambridge, such ideas are not without value. "It's good to float alternatives to conventional cosmology because it tests the limits of plausibility," he says. "But if you think hard about any of them, you can always find an inconsistency."

The idea that the Universe is an expanding phase boundary first occurred to Grady in the mid-1980s. It was the recent claim that the expansion of the Universe is actually accelerating, or at least not slowing down, that spurred Grady to develop his ideas and submit a paper to the journal Physics Letters A. "The phase boundary would in the early Universe have undergone accelerated expansion," says Grady. "Eventually, if the fluid dissipates energy, the expansion would settle down to a constant rate."

Grady says he has gone to such lengths to concoct an alternative picture of cosmology because it explains many of puzzling things about our Universe in a very natural way. Take the so-called horizon problem - the fact that reions of the Universe that are today on opposte sides of the sky have the same temperature. According to Einstein's famous speed limit, different parts of the Universe cannot behave in synchrony unless light has had time to travel between them - which means that regions on opposite parts of the sky should have been out of touch with each other when their temperature was set in the very early Universe.

Cosmologists have had to come up with the bizarre idea of inflation - a mind-bogglingly rapid idea of inflation - to account for this. Grady's explanation is more straightforward. "The seed that formed the Universe was born with a uniform temperature for the simple reason that the fluid that existed before had had time to reach a uniform temperature.

Another cosmological puzzle that Grady's model explains is the "flatness problem" - the fact that the Universe today is balanced on the knife-edge between one that will expand forever and one that will eventually re-collapse. It's a puzzle, because this requires the expansion rate to have been ridiculously fine-tuned in the early Universe. But in Grady's model "the expansion of the boundary is naturally fine-tuned because a phase transition occurs only at a ctirical temperature when the energy of the two phases is equal".

Despite the apparent successes, however, not everything in Grady's garden is rosy. He cannot, for instance, explain the uniformity of the cosmic microwave background radiation, the microwave afterglow of the big bang which still permeates all of space. "This is my most serious problem," he admits. "But the idea explains so many other things that I'm hopeful things will eventually fall into place."

Having an idea is one thing, proving it quite another. But Grady points out that his theory makes some predictions which could be used to put it to the test. For instance, he says, banging heavy ions together at close to the speed of light might concentrate enough energy in a local region of space to temporarily re-melt the solid phase. "It would change the rate at which the pahse boundary advanced - in other words, the rate at which time passes locally," says Grady. "And this might be noticeable in particle events triggered by the ion-ion collision."

Another of Grady's predictions emerges from the possibility that the phase-nucleating event that was the big bang might not have been alone. "If the seed was some kind of dust-like impurity, or eddy current, we might expect other seeds - perhaps concentrated in a small region of the fluid," he says. "It raises the possibility of collisions between universes, rather like collisions between soap bubbles."

What woud such a collision look like in our Universe? Grady says the area of contact between colliding crystals would first appear as a point. Then it would become an expanding spherical surface, rather like the shell of a supernova explosion. Associated with this expanding spherical shell would be dramatic dislocations of the phase boundary. There would create large amounts of matter and antimatter, and copious radiation which would be further increased when some of the matter and antimatter annihilated.

According to Grady, the most dramatic effect of a collision between our Universe and another would be that everything from our Universe in the interior of the expanding shell would be destroyed and replaced by matter from the other universe. "It's just like one soap bubble colliding with another," he says. "The portion of the surface where one bubble touches the other eventually pops and is replaced by the other bubble."

In fact, says Grady, there is some tentative evidence that this could have happened in the past. If matter in the form of galaxies was created in this way, it should be distributed as if on the surface of giant bubbles, the remains of the joining boundaries of the now merged universes. "This is exactly what astronomers observe," says Grady. "And there is no real explanation within the conventional big bang theory."

So is there any other evidence of universe-crunching collisions? "It's conceivable that prodigiously luminous quasars are where small universes are colliding," says Grady. "The fact quasars existed only in the early Universe implies our Universe formed from a tight group of initial crystals which very quickly merged."

It's amazing that an idea as simple as an expanding crystal could explain so many puzzles. But cosmologists are unlikely to abandon their current theories in favour of Grady's unless he can turn up some overwhelming evidence in its favour. Grady points out that there is one dramatic ways that his theory could make itself felt: there could be another crystal growing right next to us in the fluid that fills protospace. "If it collided with us, there would be no warning," he says. "It would be like amini big bang going off in our neighbourhood." There is only one problem. If it did happen, it's unlikely that any of us would survive to witness his triumph.

Michael Grady's idea that the Universe is a giant crystal growing in a five-dimensional liquid scores successes in the subatomic realm as well as on the cosmic scale. Take quantum fluctuations, the random seethings of the sea of the vacuum. According to Grady, these fluctuations are simply the random sloshing back and forth of heat in the bulk of the fluid. "These heat fluctuations continually buffet the phase boundary in which we live," he says.

Grady also thinks he can make sense of the behaviour of the subatomic particules that form the building blocks of our world. There particles come in two types: "fermions" such as electrons, which obey the Pauli exclusion principle forbidding two particles from occupying the same quantum state; and "bosons" such as photons, which observe no such restrictions. Grady believes bosons are "phonons", or vibrations of the crystal lattice, while fermions are defects of the lattice known as "screw dislocations". "Think of the planes of a crystal as the stacked floors of a multistorey parking lot," says Grady. "A screw dislocation is like the spiral ramp connecting the floors."

Two identical screw dislocations obey the Pauli exclusion principle becuase they repel each other when forced together. And a mirror-image pair of screw dislocations, differing only in the sense in which they spiral, behave just like a particle and its antiparticle. When they meet, they cancel each other and are annihilated in a burst of energy. The opposite of this process is "pair production", in which two screw dislocations of opposite sense pop into existence if vibrational energy is supplied to the lattice.

According to Grady, screw dislocations even obey Einstein's special theory of relativity, which the speed of sound in the solid acting like the speed of light in our Universe. The link between the speed of sound and the rate at which screw dislocations can travel was first shown by the Russian physicists J. Frenkel and T. Kontorowa in 1938. According to their picture, as a screw dislocation approaches this limiting speed it compresses in the direction of motion by exactly the amount predicted by Einstein. "At the speed of sound, the energy, and hence the effective mass, of the dislocation becomes infinite," says Grady. "Fermionic matter is therefore prevented from travelling faster than light."