What triggered the Big Bang? According to a new theory, our universe crashed into another three-dimensional world hidden in higher dimensions. "The model suggests a radically different view of cosmic history in which the key events shaping the structure of the universe occurred before the Big Bang," says cosmologist Paul Steinhardt. |
Before the Big Bang
Maverick cosmologists contend that what we think of as the moment of creation was simply part of an infinite cycle of titanic collisions between our universe and a parallel world
The Catholic Church, which put Galileo under house arrest for daring to say that Earth orbits the sun, isn't known for easily accepting new scientific ideas. So it came as a surprise when Pope Pius XII declared his approval in 1951 of a brand new cosmological theory—the Big Bang. What entranced the pope was the very thing that initially made scientists wary: The theory says the universe had a beginning, and that both time and space leaped out of nothingness. It seemed to confirm the first few sentences of Genesis.
Eventually, astrophysicists followed the pope's lead, as evidence for the Big Bang became too powerful to ignore. They accepted the notion that the entire observable universe—100 billion galaxies, each stuffed with 100 billion stars, stretching out more than 10 billion light-years in all directions—was once squashed into a space far smaller than a single electron. They bought the idea that the cosmos burst into existence precisely 13.7 billion years ago and has been expanding ever since. But even now, many astrophysicists are still uncomfortable with the implication that the Big Bang marked the beginning of time itself. And the theory has yet to yield a satisfactory answer to a key question: What made the Big Bang go bang?
Cosmologists Paul Steinhardt and Neil Turok have a radical idea that could wipe away these mysteries. They theorize that the cosmos was never compacted into a single point and did not spring forth in a violent instant. Instead, the universe as we know it is a small cross section of a much grander universe whose true magnitude is hidden in dimensions we cannot perceive. What we think of as the Big Bang, they contend, was the result of a collision between our three-dimensional world and another three-dimensional world less than the width of a proton away from ours—right next to us, and yet displaced in a way that renders it invisible. Moreover, they say the Big Bang is just the latest in a cycle of cosmic collisions stretching infinitely into the past and into the future. Each collision creates the universe anew. The 13.7-billion-year history of our cosmos is just a moment in this endless expanse of time.
The hidden dimensions and colliding worlds in the new model are an outgrowth of superstring theory, an increasingly popular concept in fundamental physics. Scientists currently rely on two mutually incompatible theories—relativity and quantum mechanics—to describe the most massive objects in the universe on the one hand and subatomic particles on the other. For nearly a century, theorists have attempted to come up with a single model and a single set of equations that melds the two views of physics. Superstring theory is an evolving attempt to do just that: explain matter, energy, space-time, and the basic forces of nature in one framework.
String theory is hellishly complex. In order to make it work, theorists have to assume that space isn't merely three-dimensional, the way it appears to our puny human senses, but rather that it has up to 10 spatial dimensions. Just as a bedsheet hanging on a clothesline appears to be a two-dimensional object hanging in a three-dimensional world, all of space-time would be suspended in a higher-order space. In keeping with this two-dimensional analogy, string theorists describe our observable universe as a membrane—"brane" for short—flapping in the breezes of the actual 10-dimensional cosmos.
Physicists are just beginning to poke and prod at the big implications of superstring theory. That's what Burt Ovrut of the University of Pennsylvania was doing during a 1998 cosmology conference at the Newton Institute of Mathematical Sciences in Cambridge, England. He asked: If we live on a brane that's wafting through multidimensional space, why shouldn't there be other such branes floating around out there? Nothing in the theory ruled out this possibility. And if other branes exist, they could interact. It would be fascinating, Ovrut proposed during his talk, to consider what might happen if they did.
The idea intrigued Steinhardt, a professor at Princeton University who was sitting in the audience. If the interaction between branes was a collision, it would trigger a fantastically powerful reaction, Steinhardt guessed, given the immense amounts of matter and energy in each one. The crash would release so much energy, in fact, that it might be comparable to another energy release he was already quite familiar with: the Big Bang.
Meanwhile, Turok, a professor at Cambridge University, was sitting in the same audience having similar thoughts. After the lecture both men approached Ovrut to discuss their ideas. "It was clear that a collision of branes would be a dramatic event," Turok says. "People had talked about it in a mathematical way before, but nobody had thought of it as a real, physical process."
Steinhardt, Turok, and Ovrut, along with Steinhardt's graduate student Justin Khoury, decided to see what implications colliding branes might have for cosmology. They weren't driven by idle curiosity alone. Steinhardt, in particular, had been growing increasingly disenchanted with the conventional Big Bang model. The problem wasn't just that the theory required that time and space have a beginning but also that the more cosmologists tried to refine their model, the messier it seemed.
The original Big Bang model was simple: a hot dense knot of energy burst outward, congealed into matter, and kept expanding. But by the 1980s, astrophysicists had embraced a more complex elaboration of the Big Bang known as inflation. Ironically, one of the theorists who developed this idea was Steinhardt. Inflation theory postulates that in the first hundred-millionth of a billionth of a billionth of a billionth of a second of its life, the universe expanded as though it were turbocharged, swelling much faster than the speed of light, before settling down to a more sedate rate of growth. The only way that could have happened is if there had been some incredible energy source pervading the newborn cosmos and blowing it apart. We don't see anything like that in the universe today, however, so cosmologists had to assume the potent energy field existed for only a fraction of a second after the Big Bang and then vanished.
Conjuring up new, unknown energy fields goes against both common sense and one of the most cherished scientific doctrines. A principle known as Occam's razor says the simplest possible explanation for natural phenomena is usually right. Perhaps the best-known example is the Earth-centered cosmology of Ptolemy, which dominated Western science for 1,000 years. When Ptolemaic theorists discovered that the planets did not appear to be moving in a simple pattern around Earth, they added epicycles—tiny circular movements on top of the grand orbital circles. Closer examination showed that this didn't quite explain observations either, so the theorists added epicycles on top of epicycles until the model did work. The final result was also very complex. Then Copernicus came along with the idea of a sun-centered cosmology, and Johannes Kepler realized that planets actually move in ellipses. Suddenly, planetary motions made sense without the complexity of epicycles, and the old theory was dropped.
IN THE BEGINNING . . .
Is the universe infinite or finite? Is it eternal or will there be an end of time? Did it arise from something else, or did it simply pop out of nothingness—creation ex nihilo? Cosmologists have wrestled with these questions since Edwin Hubble first uncovered evidence of cosmic expansion in 1929. For more than half a century, the standard answer has been that our universe began as a single burst of energy—the Big Bang. Recent elaborations have answered some questions but not the biggest ones. A radical new cosmology proposes that our universe is just a tiny fraction of a vast, higher-dimensional realm and that the Big Bang is one step in an endless cycle of creation.
—Alex Stone
According to the reigning Big Bang theory, the universe began as an infinitely hot, dense dot. Within a tiny fraction of a second, the cosmos underwent a period of runaway expansion, called inflation. Over billions of years, the universe cooled, giving rise to galaxies, stars, and planets. Today, 13.7 billion years after the Big Bang, the universe continues to expand and
in fact is speeding up under the influence of a mysterious energy force. If things keep going this way, the future of the universe looks bleak: Stars will burn out, galaxies will disintegrate, and the universe will end eternally dark and lifeless. This theory leaves many unknowns hanging. It does not explain why the Big Bang happened and what, if anything, existed before. It also does not explain the nature of the unidentified energy field that is causing our universe to accelerate.
CYCLIC MODEL
To address some of the limitations and paradoxes of the Big Bang model, cosmologists Paul Steinhardt and Neil Turok have developed a new cosmology that views the visible universe as one small part of a much larger reality, most of which exists in other dimensions that we cannot perceive. Our universe exists on a three-dimensional membrane (represented by the flat panels at right) that lies right next to another membrane. Every trillion years or so, the two membranes collide, unleashing a firestorm of energy analogous to the Big Bang. As in the earlier model, the universe cools, gives rise to galaxies, and eventually expands to near emptiness. In this version, however, another collision between membranes then restarts the whole cycle of creation. Thus, time and space are both infinite.
Inflation seemed like a necessary complexity. Without it, the universe would look very different—for instance, galaxies on one side of the universe would be distributed differently from galaxies on the other side, which they don't appear to be. As inflation caught on, however, some cosmologists grumbled about epicycles. Then the Big Bang got even more complicated. Starting about five years ago, astronomers measuring the expansion rate of the universe discovered that billions of years after the Big Bang—long after inflation had died out—cosmic expansion started speeding up again.
Theorists invoked another unknown energy field, called dark energy, to account for that cosmic acceleration. "This wasn't really predicted at all," says Steinhardt. "We can fit it into the model, but we don't know what this so-called dark energy is. The standard model is definitely becoming more encumbered with time. It may still be valid, but the fact that we have to keep adding things is a bad sign."
Astronomical evidence clearly indicates that the observable universe has been expanding for the past 13.7 billion years. In the inflationary Big Bang model, the universe was hot and dense at the outset, and then immediately went through a period of hyperexpansion. Steinhardt and his colleagues considered a very different possibility: What if the universe actually started out cool and vacuous?
If that were the case, the idea of branes colliding in a hidden dimension might provide a simpler explanation for the ongoing expansion. To find out whether the idea made sense, the pair took on the daunting task of mastering the equations of superstring theory and applying them to their theory. For simplicity, the researchers assumed that the branes were flat and parallel to each other. They also assumed that the branes contained no matter.
That didn't mean the branes were voids: Quantum theory asserts that even the total vacuum of empty space is seething with "virtual" subatomic particles that constantly wink in and out of existence. In aggregate, these virtual particles add up to a huge amount of latent energy—which, according to Einstein's theory of special relativity, is equivalent to an astounding amount of mass. So a crash between two empty branes would still be a collision of gigantic proportions.
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