When Branes Collide.

Stringing together a new theory for the origin of the universe

For an eternity, our universe lay dormant --a frozen, featureless netherworld. Then, about 15 billion years ago, the cosmos got an abrupt wake-up call.

A parallel universe moving along a hidden dimension smacked into ours. The collision heated our universe, creating a sea of quarks, electrons, protons, photons, and other subatomic particles. It also imparted microscopic ripples, like ocean waves crashing on a shore.

These ripples generated tiny fluctuations in temperature and density, the seeds from which all cosmic architecture --from stars to gargantuan clusters of galaxies to galactic super clusters --ultimately arose.

This model for the evolution of the cosmos, first presented at a cosmology meeting at the Space Telescope Science Institute in Baltimore last April, has been widely discussed and debated ever since. Although the hypothesis sounds like science fiction, some scientists say it's the first serious challenge to the reigning model of the birth of the universe.

According to the standard theory, the universe was born some 15 billion years ago in a hot, expanding fireball, an event scientists call the Big Bang. The universe then underwent a brief spurt of faster-than-light expansion, called inflation, before settling down to the much slower, steady expansion observed today.

"After many years in which we had a single model--[the Big Bang combined with] inflation--for the universe's beginning, we now have an alternative," comments theorist Mario Livio of the Space Telescope Science Institute, one of the organizers of the April meeting on this topic.

"The reason that this is important is that in spite of its attractive features, inflation theory has not been tested observationally in any detail," he notes. Livio adds that the new model "provides us with a potential true test that can distinguish between it and inflation."

"I don't think it's by any means yet a real rival to inflation, but I think it is a model well worth pursuing," says Alan H. Guth of the Massachusetts Institute of Technology, one of the developers of the inflation model.

Despite its name, nothing goes bang in the Big Bang theory. The cataclysm it proposes wasn't anything like a bomb exploding into preexisting space, since all space was contained inside the infant universe. Rather, the Big Bang refers to the event when the immense energy in the infant universe drove it to expand.

In the new hypothesis, however, "our universe begins in a static, featureless state" that persisted for eons, notes Paul J. Steinhardt of Princeton University. That dormant period may have lasted a hundred trillion trillion years. Then, there really was a bang--a giant collision that heated the cosmos to a high temperature. This collision sparked the steady expansion of the universe, and over time, gravity molded gas clouds into stars and galaxies--equivalent to what happens in the widely accepted Big Bang scenario.

To generate that all-important collision, the new model presupposes hidden dimensions and myriad universes floating through space like parallel plates. By chance, one of those plates whacked into the one destined to become our universe.

"It's a very radical idea we have," admits Burt A. Ovrut of the University of Pennsylvania in Philadelphia. "The old idea was that the universe started out at some time zero and ballooned outwards in a burst of inflation. We're now proposing that 'time zero' was just a marker, that the universe really existed long before that."

Steinhardt, Ovrut, and their colleagues Justin Khoury of Princeton and Neil Turok of the DAMTP in Cambridge, England, call their model the ekpyrotic universe, from the Greek word for conflagration.

"We might have used the term `Big Bang', but that name was taken," jokes Ovrut.

If a theory ain't broken, why fix it? Even in its most primitive form, which does not include inflation, the Big Bang theory correctly predicts the cosmic abundance of helium and deuterium and the temperature of the radiation left over from the birth of the universe.

The classical Big Bang picture was first proposed in the late 1920s. Two decades ago, researchers realized that the scenario needed to be modified.

In its original form, the model would lead to a universe vastly different from the one we live in. For instance, the theory doesn't provide a way for stars, galaxies, and larger structures to arise, notes Steinhardt. Moreover, the Big Bang model would tend to produce a cosmos whose composition and density would vary widely from place to place and whose overall geometry would be warped or curved.

That's in stark contrast to numerous observations, which reveal a universe that is the same, on the large scale, in all directions and has just the right amount of matter and energy to keep it perfectly flat.

In 1980, Guth amended the Big Bang theory to account for these discrepancies. Refined by several researchers over the past 2 decades, Guth's model posits that the infant cosmos underwent a brief but enormous episode of inflation, ballooning at a rate faster than the speed of light. In just [10.sup.-32] seconds, the universe expanded its girth by a factor of about 100 trillion trillion, more than it has in the billions of years that have elapsed since.

The inflation model accomplishes several feats (SN: 12/19&26/98, p. 392). It explains why widely separated parts of the universe--regions so far apart that all communication between them is impossible--can nonetheless look as similar as the closest of neighbors. Inflation theory suggests that when the universe began, these regions were indeed neighbors and then rapidly spread far apart.

Inflation also makes the universe flat. Any curvature to space-time would have been stretched out by this era of faster-than-light expansion.

Furthermore, the ballooning would have provided a way for chance subatomic fluctuations in the early universe to inflate to macroscopic proportions. Over time, gravity could then have molded these variations into the spidery network of galaxies and voids seen in the universe today.

The Big Bang model combined with inflation matches several important observations, including the detailed structure of the radiation called the cosmic microwave background, which is left over from the universe's birth. Data gathered by several balloon-borne and ground-based telescopes fit the predictions of the inflation model (SN: 4/28/01, p. 261).

Yet some cosmologists view inflation as a mysterious, ad hoc device. For instance, notes Steinhardt, no one knows what type of force triggered the onset of inflation or what ended it. "We've been searching for several years to find either a more natural way of incorporating inflation or an alternative model based on new physics," he says.

Inflation, Steinhardt says, is based on quantum field theory, which views every elementary particle as a point-like object. In the past decade, however, physicists have begun thinking about elementary particles in a new way, based on a model called string theory.

According to this view, electrons, quarks, and all the other elementary particles in the universe behave as point particles when observed at a distance, but each is actually composed of tiny loops or strings of energy. The different vibrations of a string, like the different notes that can be plucked on a violin, correspond to different particles.

"It's a beautiful idea because it says that all of the particles we see actually arise from a single object--string," says Ovrut.

Each string vibrates in a space-time that has 11 dimensions--7 dimensions beyond the usual 3 of space and 1 of time (SN: 2/19/00, p. 122). The newest twist on string theory, dubbed M theory, allows for more-complex objects: surfaces rather than just strings. These surfaces are known as membranes, or just branes.

Many physicists are studying branes in the hope of linking gravity and the other fundamental forces of nature to the elementary particles that communicate these forces. According to Steinhardt and his colleagues, certain types of branes may turn out to have profound consequences for cosmology.

Instead of working with the 11 dimensions implied by M theory, the researchers have focused on branes that exist in 5 dimensions. In this model, the other 6 dimensions are tightly curled up and can be ignored. Certain branes that exist in this abstract five-dimensional space can be represented by infinitely long, parallel planes and seem to have a close correspondence to our universe.

In this construct, our cosmos could have plenty of company. Other would-be universes--also represented by branes--may be floating through the fifth dimension. These branes would remain invisible because particles and light can't travel through the fifth dimension. However, gravity can couple matter across that dimension, and collisions between branes are possible.

In the ekpyrotic scenario, the fifth dimension is finite in size and bounded on either side by a three-dimensional brane. One of these boundary branes was the surface that was to become our own cosmos, and the other represents another universe. In the version of the theory first described last April, a third brane peels off the opposing boundary brane and bangs into ours. In the collision, it melds with our brane, igniting the Big Bang.

"There is a certain sense in which this is like two pieces of putty slamming into each other and heating up," says Ovrut.

Critics of the scenario, as well as Steinhardt's team, have noted that the universe created by the impact contracts rather than expands. If so, it wouldn't have generated a cosmos like ours.

In a modified version of the ekpyrotic theory, posted Aug. 26 on the Internet (http://xxx.lanl.gov/abs/hep-th/0108187), Steinhardt, Nathan Seiberg of the Institute of Advanced Study in Princeton, N.J., and their collaborators say such concerns are now unwarranted. According to their calculations, the new model can produce a collision without having to rely on one invisible brane peeling off from another.

Instead, one of the boundary branes moves slowly but steadily toward the other, attracted by an exchange of lower-dimension branes between the two. As the boundary brane moves, it shrinks the fifth dimension. When the two boundary branes touch, the fifth dimension collapses completely, an event the researchers call the Big Crunch.

As in the earlier version of the theory, the collision triggers the Big Bang. However after the impact, the two boundary branes bounce off each other and move apart, recreating the fifth dimension. This rebound starts the expansion of our universe.

In either version of the theory, the laws that govern elementary particle physics require that the boundary branes be flat as a pancake before they collide and that they stay that way afterwards. Consequently, the universe generated by the collision is flat. An episode of inflation isn't needed to stretch out any curvature since none ever existed.

Because the impact is so uniform--exactly the same force is applied up and down the flat boundary between the two branes--widely separated parts of the universe get the same kick and thus evolve in exactly the same way after the collision. This accounts for the uniformity of distant reaches of the cosmos without having to invoke an episode of inflation.

Due to quantum effects, which make the boundary between the branes slightly uneven, some parts of our brane would be struck ever so slightly earlier or later than other parts. This would create tiny temperature differences within the struck brane that, like those in the standard Big Bang model, become the seeds for galaxy formation. The collision also causes the brane to stretch or expand, accounting for the expansion of the universe observed today.

The researchers "make a graceful transition from the Big Crunch to the Big Bang," says David N. Spergel of Princeton University. "This is arguably a `new ekpyrotic universe' that appears to be more elegant than the old model."

According to Steinhardt, the ekpyrotic theory does everything that Big Bang plus inflation accomplishes. "It's just that we happened to discover one theory first--20 years ago," he says.

"What [the ekpyrotic theory] has going for it is a much closer relationship to string theory than any formulation we currently have of inflation," says Guth. "String theory is simply the only hope we currently have for a quantum theory of gravity, and obviously gravity has to be quantized to be consistent with the rest of what we know about physics."

Nonetheless, "I'm still somewhat skeptical about the whole thing," Guth adds. "They need to make very strong assumptions about the initial conditions--they're really starting out with a universe that's already infinite and uniform."

Another developer of the inflation model, Andrei Linde of Stanford University, takes a much dimmer view of the new work and has posted several papers on the Internet lambasting the ekpyrotic model. He says that to produce galaxies, Steinhardt and his colleagues have to choose a highly specialized, unrealistic form of interaction between branes. Moreover, Linde claims that the branes in the ekpyrotic model are not truly uniform in structure and therefore can't account for the large-scale uniformity of the universe.

"Instead of a theory, we have only wishful thinking," he says.

Steinhardt and his colleagues have posted responses on the Internet.

Making a universe in ekpyrotic theory requires patience, notes Ovrut. Because the attractive force between branes is so small, they move at a snail's pace, and it could take an extraordinarily long time for a collision to occur, he says.

In effect, says Ovrut, the new theory replaces the very short growth spurt of inflation with a very long lead time for a collision.

As a bonus, he notes, the collision described by ekpyrotic theory not only generates cosmic structure, it also creates the known families of quarks and other fundamental particles.

"What's very beautiful about these brane models is that one can actually compute the spectrum of [elementary] particles, and what you get is something like our real world," notes Ovrut.

At least one empirical test of the ekpyrotic theory may soon be possible. The test would examine gravitational waves, the radiation produced when massive objects accelerate.

Big Bang plus inflation predicts that gravitational waves can have extremely long wavelengths, while the ekpyrotic theory does not. Long-wavelength gravitational waves would leave a distinctive fingerprint on the cosmic microwave background.

Future experiments with a new generation of space, balloon-borne, and ground-based telescopes may be able to detect that fingerprint, says Ovrut.

Other aspects of the ekpyrotic model are still being scrutinized.

"I worry a lot about the details," says Ovrut. "This is a theory that's really still in its infancy."