The universe may not have started with a bang

The Universe began 15 billion years ago in a fantastic explosion. It's the story we've been taught since first grade science, and cosmologists have held dear the aptly named big bang model for more than a half century. But as knowledge grows, more elaboration is needed, and this wonderfully successful picture of the universe is now showing some cracks.

Paul Steinhardt, a physics professor at the University, has been keeping an eye on the cracks for some time. A member of the Old Guard of modern cosmology, he has been a key contributor through much of the development of the current version of the big bang model.

"Certain things about it work wonderfully well, but certain things about it seem quite mysterious. And the challenge is explaining the mysterious things while keeping the good things," he said.

Using the latest methods in theoretical physics, Steinhardt and his collaborators have come up with an entirely new perspective on the creation, one which promises to shed new light on some of the most basic questions in science.

Big bang busts

Steinhardt insists that the big bang model is not easily upended or dismissed. Like all good science, it explains what we see.

It predicts, for example, that the universe should be expanding, which turns out to be the case. It asserts that the intense heat of the early universe appears today as an all encompassing, ambient glow of radiation — a cosmic microwave background (CMB) — discovered by Arno Penzias and Robert Wilson in the 1960s. It additionally requires the creation of the nuclear elements which, in turn, gave birth to galaxies, stars and you and me.

The big bang also appeals to our common sense. It doesn't take much to picture an explosion, even if it is a simplification of what really happened.

But the big bang theory has problems. It does not entirely explain the universe as we see it, and it is fundamentally incomplete.

First, the big bang does not explain why, when viewed as a whole, the universe looks the same in every direction. Observations show that the CMB varies an astronomically small amount across the entire sky.

This might seem unremarkable, except that distant regions of the universe have never been in contact. That is to say, they are so distant that light hasn't had time to connect one to the other. According to relativity, heat couldn't have flowed back and forth to equalize the temperature.

The question then becomes: Why should two causally disconnected regions — as the jargon goes — be so similar?

It is as if a group of 100 people were filling 100 buckets with water. But in this case, they never discussed beforehand about how high they would each fill their respective bucket. And yet, in the end, they all just happened to be very nearly equally full.

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But at the same time, everywhere we look we see matter in small lumps: galaxies, clusters of galaxies, stars and planets.

So there is a paradox: The universe has to be at once uniform and lumpy. It is not clear how this fits into the traditional big bang picture.

Physicists also observe space-time to be flat, as opposed to curved. This is, in a sense, like comparing a flat model of the Earth to a curved model. But, unlike the ancient debate over the Earth in which a flat model seemed most reasonable, to scientists today a curved model of the universe is most appealing.

Steinhardt explained why it is surprising that the universe is flat. "Flat is an unstable condition," he said. "So if there were ever any deviation from perfect flatness, the universe would get more and more curved with time. We seem to have been in a very precarious balance for the past 14-or-so billion years, and the big bang fails to explain why."

Still deeper, more fundamental problems crop up as the clock turns back toward the moment of creation. As the universe gets smaller and smaller, the temperature heads toward infinity, and space is compressed into a point — the cosmic singularity.

This singularity is the beginning of time, and the universe emerges from it full of energy and radiation. It is here, when the universe was somewhere around 10 to 43 seconds old, that the equations of general relativity blow up, and the rich theory upon which the big bang is based breaks down.

Physicists have attempted to address these problems over the years. The best fix, the inflationary model, was originally proposed in the early 1980s by Alan Guth at MIT. He claimed that, during the universe's first instants of life, it entered a phase of hyperexpansion. In less than 10 to 35 seconds, it expanded from the size of a proton to that of a grapefruit. Space was violently stretched flat, and matter diluted to virtual uniformity.

At the same time, random density fluctuations burgeoned to cosmological scale and seeded the future lumpiness of matter. These early fluctuations reveal themselves in patterns within the CMB, which current observations are beginning to confirm.

The question asked by Steinhardt and his team — including Justin Khoury GS and former Princeton physicist Neil Turok — was, "Can I dream up a different theory that can produce fluctuations that look basically the same?"

If inflation were the only way to predict the patterns we see, then we would have renewed confidence in its validity. But, as it turns out, there is an alternative, and what better than competition to get the mental juices flowing.

String theory

Enter string theory, the starting point for Steinhardt's new model. Within the context of this new theory, physicists have started to imagine the three spatial dimensions of our universe as a membrane — or "brane" for short — within some higher dimensional region. The easiest analogy is to imagine a flat sheet. Our entire universe is contained on that sheet, and matter and energy are constrained to its surface. But the sheet as a whole is free to move within the higher dimensional space.

Steinhardt and his collaborators set out to reconstruct the creation in extra dimensions. The lowest energy and therefore most likely configuration would be two flat, parallel, facing branes, one of which we currently occupy. String theory predicts the presence of a force that would draw them together. Eventually, they would collide and bounce off each other, with a portion of the energy of impact converted into matter and radiation. After the bounce, the branes would move apart, and with time, through continued mutual attraction, come back together and begin the process anew: a cyclic universe.

This new model addresses some of the big bang model's old problems. At the end of each cycle, the branes are flat and empty, so this would explain why the universe is flat. Moreover, because every part of each brane is hit by a common object, the energy of the overall collision is spread uniformly, and with it matter and radiation.

However, as they approach, the branes develop random fluctuations that causes it to change shape slightly. So different areas impact at slightly different times. The result is small scale differences that will go on to become the lumps of the universe. And the overall heat of this collision produces the observed CMB, tempered by tiny fluctuations.

Additionally, there is never a point singularity, since both branes start out as sheets. At the same time, since the energy of the collision is finite, the temperature never becomes infinite.

A cyclic universe also has some unique properties. While the branes are apart, everything seems to follow the big bang model closely. But after some time, the energy keeping the branes apart begins to take over, and the universes enter a phase of accelerating expansion, much like what we see today.

After a few trillion years or so, all of the matter in our universe — or on our brane — will have been spread so thin that the average density is, for all intents and purposes, zero. By then, the branes will have been stretched to the point where they are extremely flat, and we are back to where we started at the end of the last cycle. The branes then accelerate toward each other, collide and create the next hot, expanding universe.

By the time they collide, the universes are nearly empty, so each cycle is indistinguishable from the others. As a result, it is impossible to say when the whole process began. And, since the branes never completely contract, they get bigger every cycle. This extra-dimensional cosmos could be infinitely old, infinitely large, and it will exist forever.

The cyclic model does not merely reproduce the observable consequences of inflation. Fortunately for its creators, it does give testable predictions that are markedly different. The inflationary model baths the universe in gravitational waves — ripples in the fabric of space and time — which would form a gravitational background radiation. Steinhadt's model, on the other hand, predicts virtually none.

Unfortunately, we do not yet have the technology to directly test such predictions. Gravitational wave detectors, such as the LIGO project at CalTech, are under construction, but may not be sensitive enough to detect the gravitational waves.

There is, however, a group at Princeton led by Professors Lyman Page and Suzanne Staggs using very precise measurements of the CMB to look for the effects of a gravitational waves.

In the meantime, Steinhardt and his collaborators have shown that inflation is not unique and have given us a radically different perspective on the universe. And in doing so, they have given string theory a chance to flex its predictive muscle. Results are just a matter of time. And, at least the way Steinhardt sees it, time is something we have plenty of.