In the Beginning. According to Genesis, the world was created in six days - but according to modern astronomers, it didn't even take six minutes. The Big Bang, a jazzy name for the idea that the entire universe expanded rapidly out of a single point, is orthodox astronomy's current creation myth. Even when astronomers realized the Big Bang had big problems, they corrected it with a new idea called "inflation" that is just another story about how the universe expanded from a single point.

But now the unthinkable has happened: a new theory has arisen that explains the origin of the universe not by inflating points but by colliding universes. This new "ekpyrotic model" is exotic, imagining vast "branes" moving and colliding in five dimensional space, but might solve some of the problems of the origin of the universe ... and in so doing, shed light on the fundamentals of time and space.

First, A Little Background. The power of the Big Bang model is that it explains the extraordinary fact that the stars are rushing away from us in terms of our understanding of how things fall down. Albert Einstein, exploring the consequences of his new theory of gravity called general relativity, was shocked to find that his equations predicted the universe would expand.  Astronomers did not believe the stars were rushing away, so Einstein added a special "cosmological constant" to his equations to make the unwanted expansion go away.

But Einstein was too hasty. As telescopes and methods improved, astronomers soon found that stars in distant galaxies appeared to be receding from us, in direct proportion to their distance. They tried for decades to account for this expansion with a vast range of theories ranging from fantastic antimatter explosions to steady state creation of matter. The Big Bang at first seemed a long shot, predicting that space should expand and that the sky should glow with the aftereffects of the explosion.  But this prediction proved correct: the sky does glow, faintly, with microwaves called the cosmic background radiation . And so, emboldened by both theory and evidence, astronomers settled upon the Big Bang as the best available explanation.

But there's a big problem with the Big Bang: the universe appears to be too damn flat --- more homogenous than milk at truly astronomical distances. That's fine for milk --- the molecules in the milk can mix freely until the milk reaches smoothness, or equilibrium --- but not fine for the universe. The universe's parts are too separated and the speed of light too slow for the universe to smooth itself out in the time it's had to expand. SO, if the Big Bang happened as predicted, there would be "hot spots" in both the distribution of matter and in the cosmic background --- which we don't see. A big mystery. Astrophysicists like mysteries --- they're paid to figure them out. So a lot of people began thinking very hard about the problem.

Inflating the Universe. Enter a man named Guth, who came up with the leading contender to fix this problem: inflation theory . The idea, you see, is to smooth out the hot spots by blowing them up. At a very early point in its history, the universe was very hot, very dense, very small and --- you guessed it, very homogeneous, in effective equilbrium. Guth proposed that the physics of the hot-dense-small universe caused it to expand violently, suddenly, expansively, under the influence of a force called inflaton. A region smaller than a proton blew up to a size greater than the entire visible universe, making the universe appear, as far as light can shine, to be almost completely flat.

Now the challenge: making the physics of inflation work. For this, astrophysicists, the surveyors of the unimaginably large, turned to particle physicists, probers of the preturnaturally small. Physicists have been plugging away for quite some time trying to come up with a simple description of the universe that can fit on a T-shirt --- for aesthetic or marketing reasons no one can say. Maxwell did it for electricity and magetism in the 1800s, and a trio of bright particle physicists did it in the 1960's for electromagetism and the so-called "weak" force that governs certain kinds of radioactivity. But gravity --- Einstein's baby, "matter tells space how to bend and bent space tells matter how to move" --- and the "strong force" --- which binds the nuclei of atoms together --- resisted physicists' advances.

Tying Up A Few Loose Ends. The best luck was had by a German mathematician called Kaluza, who in the mid 1920's wrote Einstein's equations down in five dimensions instead of four and found that --- voila! --- electromagnetism just "fell out". Of course, the extra dimension was a bit worrisome, and no-one paid attention to another German mathematician, Klein, who pointed out that we could make the extra dimension go away by "rolling it up" --- just in the same way that the surface of a garden hose is 'effectively' one dimensional because its second dimension a short small circuit insignificant with respect to its great length.

Fast forward fifty years. In the absence of a unified theory, a lot of bright people, including luminaries like Richard Feynman, worked to reconcile electromagnetism, the strong force, relativity and quantum mechanics with the virtual zoo of particles flying out of modern particle accelerators --- pions, mesons, the W, the Z --- and produced another alphabet soup of theories called QED and QCD and so on. As they did so, a few people began to realize that the old ideas of Kaluza and Klein didn't look so crazy after all. If you imagined that this zoo of particles were not points, but strings, then all of forces could easily be unified and all particles easily accounted for. True, the mathematics required extra dimensions, just as Kaluza predicted --- but now instead of five dimensions, it was ten or even sixteen --- yet the mathematics practically demanded that most of these dimensions roll up into invisibility, just as Klein predicted.

Dozens, hundreds of versions of these "string" and "gauge" theories proliferated --- SO(32) symmetry, heterotic strings, bosonic strings, and so on. Many of these theories were of very great delight to astrophysicists, providing them with candidate sources for the "inflaton" field. Many of these theories were of very great disappointment to particle physicists, experimental physicists, and Richard Feynman, because while the theories were mathematically very pretty, there was no known way to test them.

Enter the Branes. One candidate for a unified theory proposed an unusual configuration of all those rolled up dimensions: two universes, living "side by side", unable to influence each other at all, or at the very most through gravity alone. Our universe is like the surface of an ocean, its four-dimensional physics determined by a five-dimensional reality beneath the waves; out there may be other surfaces parallel to our own that we never see. Elegant, yes; but as unprovable as all other string and gauge theories. Nothing distinguished this theory from a dozen others --- until a man named Ovrut and a group of physicists working with him realized that these surfaces, these "branes" could collide, and violently --- producing a universe not entirely unlike the Big Bang that we see.

The Ekpyrotic Model of the origin of the universe postulates that our universe was a vast, formless void, virtually empty of matter and life until it "collided" with another brane in a unvisualizable space of higher dimensions. The collision heated the universe intensely over a vast period of time, triggering the expansion we now call the Big Bang. The latest version of the model, proposed by Steinhardt and Turok, turns this expansion into a cycle: the universe continues to expand until all matter has cooled and all particles are separated, leaving a vast formless void, virtually empty of matter and life, awaiting a collision with another brane that will start the whole process over again.

So in one sense the ekpyrotic model is nothing new. It is based on known mathematics. It predicts the traditional Big Bang. And it takes its very name from the ancient Greek idea that the universe was created in a vast conflagration of fire. Yet there is one key difference - the ekypyrotic model makes predictions. Testable predictions about how the matter in the early universe will be distributed --- similar enough to what is known to be interesting, but different enough from the inflationary model that they can be told apart.

In other words, if the ekpyrotic model is true, we will be able to see the signature of the theory of everything written in the very background of the sky.

With Microwave Eyes. And with that, we can see the excitement. Astrophysicists, fighting for years to explain unnatural smoothness, at last found a theory from particle physics that just might explain what they see. Particle physicists, struggling for years to explain nature with exceptional elegance, at last found an elegant theory that can be tested against evidence. And as a result, the universe is stranger than we imagined. It is more than length and width and depth and time: it has a strange fifth dimension separating us from a world which is a mirror of our own --- a fifth dimension which we can never see, except when we look close at each particle running through its tiny subatomic courses; and a mirror world we can never know, except when we look up at the sky with microwave eyes and wonder at the heavens' unnatural calm.

-The Centaur
Renaissance Engineer