Worlds Without End

Marrying particle physics (the study of the very small) to cosmology (the study of the very large), Andrei Linde argues that our universe is just one of many.

November/December 2007

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Worlds Without End

Photo: Fred Mertz

How did the universe begin? Was there always a yesterday, and will there always be a tomorrow? Scientists long assumed that our universe started some 13.7 billion years ago with a bang—the Big Bang—created by a single, expanding fireball. This is still the vision offered in astronomy textbooks that have yet to be revised. But the theory has flaws. Over the past three decades, a pioneering group of physicists including Stanford professor Andrei Linde invented a new model called inflationary cosmology, hailed by many as the most important development in cosmological thinking since the Big Bang itself.

Inflationary cosmology has some jaw-dropping implications: that ours may be but one universe in an eternally self-replicating “multiverse”; that each universe has its own laws of physics; that our universe might collapse in a Big Crunch many billion years from now. There is another startling notion—that scientists, applying the principles of inflationary cosmology, might one day be able to create a universe in the lab. (Linde calls that idea “extremely speculative” but has been known to ask, not entirely in jest, how we can be sure our universe isn’t the tinkering of a physicist from some other universe.)

In recent years, important aspects of inflationary cosmology have been borne out empirically; other theories remain works in progress. To those who might wonder why we should care about scenarios that sound like science fiction or folly, Linde’s response is simple. Cosmological theories and observations are crucial if we care to learn the fate of the universe, and of humankind.

“Cosmology” comes from the Greek “cosmos,” meaning beauty or harmony. From the time it was introduced by the Russian mathematician Alexander Friedmann in 1922, the beauty of the Big Bang theory captivated cosmologists, physicists and even religious leaders with its epoch vision of creation.

As Linde puts it: “It was a bang. And it was big.” The theory held that a fireball of extremely hot, dense matter expanded from a singularity. As the universe cooled over the eons, it became less energetic, and along the way produced the first planets, stars, galaxies and, eventually, you and me.

Empirical support for the Big Bang comes from Hubble’s law, which demonstrated that galaxies are indeed moving apart from one another. Space itself is expanding. Like dying embers, leftover radiation from this cosmic explosion has also been detected. It was discovered inadvertently by Arno Penzias and Robert Wilson, when their new microwave receiver at Bell Labs began picking up interference in 1964. They thought it was from pigeons nesting in the dish. In fact, their precise instruments had detected the echo of the Big Bang. The men would share a Nobel Prize for their work. More support came in the late 1990s from the Hubble Space Telescope and several other NASA experiments.

So why did the Big Bang fizzle?

“We assumed that the entire universe was created at the same moment,” says Linde. “It wasn’t.”

For all its explanatory power and beauty, the Big Bang cannot answer several vexing questions. What banged? Why did it bang when it did, and seemingly everywhere at once? What existed before it banged? And why does the visible universe appear flat and largely homogeneous when general relativity suggests that space should be curved? These are just some of the questions that have become central to efforts in particle physics to explain the construction of our world. Until the early 1980s, these questions seemed intractable.

Enter inflationary cosmology. Instead of an expanding ball of fire, inflation suggests that the early universe exploded faster than the speed of light from a size smaller than that of a proton in a fraction of a second. Like a vast bed sheet snapped taut, this exponential stretching effectively flattened out the visible universe, so that things look uniform in all directions. Thereafter, the universe evolved along the lines that the Big Bang predicts. At first, inflation was seen as a phase of the Big Bang. Only recently has the Big Bang become part of inflationary theory.

The first inflationary model was developed in 1979 by Alexei Starobinsky at the L. D. Landau Institute for Theoretical Physics in Moscow, but his model was rather complicated, and it remained relatively unknown outside the Soviet Union. A simpler model was independently developed by Alan Guth, a postdoctoral physicist working at the Stanford Linear Accelerator Center. Guth, now a professor at MIT, theorized that the early universe could have been supercooled, allowing for a period of exponential growth, or inflation, in a phase preceding the Big Bang.

Guth had been working on cosmology only six months when he had his breakthrough one sleepless night in the fall of 1979. “My diary for that night read ‘spectacular realization.’ I knew instantly that, if accurate, this theory would be potentially very important,” Guth recalls. “The next day I set a personal speed record biking back to SLAC.” The young postdoc’s insight solved many important problems with the Big Bang and popularized inflation among Western cosmologists for the first time. Yet his original model is now termed “old inflation” because it did not adequately explain how inflation ended. That puzzle has been Linde’s field—and his discovery moved inflationary cosmology from an interesting theory to a plausible roadmap for creation.

Linde’s 1983 “chaotic inflation” theory did away with the assumption that the Big Bang encompassed every part of the early universe. Instead, he argued that expansion could happen in any point of space given the right potential energy. This energy is measured in terms of a “scalar field” that can be pictured as a kind of parabola—a U-shaped figure, like a skateboard ramp. Differing values of the scalar field give rise to different masses of particles.

Linde theorized that our universe started at a high state of energy (say, halfway up one side of the ‘U’) and gradually fell toward equilibrium (the bottom of the ‘U’) as the scalar field’s energy dissipated, giving up its energy in the form of new elementary particles—the stuff of our universe and the building blocks of life.

Inflation answers many of the unresolved questions of the Big Bang and predicts the observable universe that we see when we look up on a clear night. Although there have been many models of inflation over the decades, the most enduring is Linde’s chaotic inflation.

His theory of “eternal chaotic inflation” followed in 1986. “Instead of a universe with a single law of physics, eternal chaotic inflation predicts a self-reproducing, eternally existing multiverse where all possibilities can be realized,” Linde says.

Eternal inflation predicts that in some other universe, you are reading this article right now. Not someone like you with a different haircut or middle name, but you, or at least a person who is entirely indistinguishable from you. Not on another planet like Earth, but an exact replica of Earth. This is the power of an infinite ‘multiverse,’ as emphasized by physics professor Alex Vilenkin of Tufts University. In yet another universe this article is in German, in another you just today found the cure for cancer, figured out how to travel faster than light, and discovered the secret to immortality. In a far distant universe, this article just won the Pulitzer Prize. While in still other universes, the laws of physics are entirely alien and no life as we know it could ever exist.

“Our cosmic home grows, fluctuates and eternally reproduces itself in all possible forms, as if adjusting itself for all possible types of life that it can support,” Linde explains.

Eternal chaotic inflation answers the great unanswered question from the Big Bang: what came before? According to Linde’s theory, there has always been a yesterday and there will always be a tomorrow. Our universe grew out of a quantum fluctuation in some pre-existing region of the space-time continuum. “Each particular part of the [multiverse] may stem from a singularity somewhere in the past and it may end up in a singularity somewhere in the future,” Linde says. Some parts may stop their expansion and contract, but inflation ends at different times in different places. There are always parts of the multiverse that are still inflating, with universes like ours eternally being produced.

Chaotic inflation holds that the early universe was anything but harmonious. Chaos reigned. Each region of space varied wildly from its neighbors. “Inflation did not occur everywhere at once,” explains Vilenkin. “But those domains where inflation does take place become exponentially large and dominate the universe.”

Consider a table littered with M&Ms. Under chaotic inflation, certain candy pieces would have just the right potential, or scalar field value, to expand to the size of a universe billions of light years across. Other M&Ms lack this potential and are destined to stay small forever. In this analogy, each color also represents unique laws of physics. Now, consider the perspective of an ant on top of an expanding green M&M. What would it see? As its universe expanded, it would become convinced that all of creation was a homogeneous green, that being the only color visible. After billions of years, all the other M&Ms are effectively invisible and unreachable, but they still exist.

On a very large scale then, the universe is irregular. Some M&Ms are shrinking into oblivion, others expanding to infinity, while still others are created of every conceivable size and property—some with peanuts, others with people. Our universe is homogeneous, but the multiverse is not. “Parallel lines can intersect far enough away. The laws of physics can change,” says Linde. “We just can’t see it when they do. We’re ants on a vast balloon.”

Chaotic inflation can make a big impact on a lay audience. “If someone had told me this 25 years ago, I would have said, go to sleep, and come back tomorrow when you’re not drunk,” Linde concedes.

“After hearing Andrei speak, a person’s view of the universe is never the same,” says Pat Burchat, chair of the physics department. “After his talk at a Stanford Fellows dinner a few years ago, my husband said he was physically dizzy—the ideas of universes spawning universes, infinite in size, expanding at exponential rates, is mind-boggling to anyone who tries to absorb it all.”

Stanford physics professor Shamit Kachru co-wrote two 2003 papers with Linde. “The importance of chaotic inflation to theoretical physics today and in the foreseeable future cannot be overstated,” Kachru says. “Of developments in theoretical physics during the second half of the 20th century, inflation ranks in the top two. I think the lessons of inflation in general and Andrei’s work in particular will have a profound influence on the way we think of physical law for generations to come.”

This does not mean that inflation is without its critics, or that it is understood by a wide audience. “It took two years to develop this theory, and ten more years to explain it,” Linde notes. “Still, the only reason I can think of that it wasn’t developed earlier is a psychological attachment to the Big Bang.”

Winston Churchill once said, “I pass with relief from the tossing sea of Cause and Theory to the firm ground of Result and Fact.” So, too, was Linde relieved when several NASA experiments confirmed predictions offered by inflationary cosmology.

Inflation predicts that there should be small variations in the intensity of the cosmic background radiation: the echo of the Big Bang should be louder in one direction than another. This nonuniformity is exactly what the NASA Cosmic Background Explorer satellite found in 1990. The slight disparities that COBE detected occurred when our universe was a mere 10-30 seconds old. (In 2006, the leaders of the COBE experiment received the Nobel Prize for their discovery.) Those results were confirmed by the Wilkinson Microwave Anisotropy satellite (WMAP) in 2003. WMAP also verified, within 1 to 2 percent, the prediction that the universe is flat. So even though Earth isn’t flat, the universe is. The Planck Surveyor scheduled for launch in 2008 promises even more accurate measurements.

“It’s incredibly exciting that inflation now has experimental verification. I’m still in awe,” Guth says. This does not mean that inflationary theory has been proven. At present, though, no other theory can simultaneously explain why the universe is so homogeneous and predict the ripples in background radiation. “There’s an ongoing war of ideas, but so far no other theory has replaced inflation’s depth,” says Linde. “If I see something better, I’ll be the first to switch.”

A paradoxical feature of inflationary theory is that a mere hundred-thousandth of a gram of matter would suffice to create an eternal, self-reproducing universe. “It looks like cheating, but that’s how the inflationary theory works,” Linde says. “All the matter in the universe gets created from the negative energy of the gravitational field.”

We have a lot of matter around. Does that mean we can create a new universe in a lab? Physicists remain divided on such bold speculations. “It is possible in principle,” says Vilenkin at Tufts. “It’s all up in the air,” says Guth at MIT. “It seems a little less likely now than it did 10 years ago. Even if it is possible, though, it’s still far from practical.”

Vilenkin and his collaborators theorize that a universe could be created from “false vacuum bubbles,” more popularly known as dark energy. WMAP has verified that our universe is comprised of 74 percent dark energy, 22 percent dark matter and only 4 percent visible matter. The gravitational pull from dark energy speeds up expansion of the universe. However, Linde and his colleagues argue that this acceleration eventually will end and our part of the universe will decay and collapse. “It would be nice to create a new universe, jump there and escape” the end of days, Linde says.

Just because it might be theoretically possible doesn’t make it easy. Researchers would be unable to verify their experiment’s success because of the rapid separation of the new universe from our own universe.

Still, creating a universe is not purely an academic question. The closer physicists get to figuring out how to make a universe, the more they understand our own. “It’s natural to be passive when dealing with cosmological problems,” says Linde. “The universe is a big place, existing on an utterly inhuman scale. But nevertheless we want to be active, not mere observers. If we can create a new universe, however improbable this might seem, then we should know more about it and think about its possible implications.”

The technology to set up this ultimate experiment is still generations away. And that may be a good thing, according to Linde. “It’s easy to imagine technology, but that does not automatically give you wisdom. I’d like to spend whatever years I have left to get a little more wisdom, and then to concentrate on the technology of turning dreams into reality.”

When the Large Hadron Collider in Switzerland comes on line next year—the biggest linear accelerator in the world, with a 17-mile circumference, more than eight times the size of Stanford’s accelerator—there are plans to detect mini black holes. This will be done when the Large Hadron Collider smashes together protons moving at 99.9 percent of the speed of light, in an effort to recreate conditions a fraction of a second after the Big Bang.

No laboratory on Earth yet has the capability to create miniature black holes—although Linde says the question is “at least discussed from time to time.” But experiments at the Large Hadron Collider should shed new light on the interplay between gravity, mass and quantum phenomena. This could lead to breakthroughs in the physical sciences, and give physicists a new appreciation for the relationship between quantum physics and cosmology that could further our understanding of both.

During the last 25 years, inflationary theory describing creation of our world has evolved from the stuff of science fiction to the standard cosmological paradigm. Today, a small army of physicists around the world is working on the theory of an inflationary multiverse consisting of different universes with different laws of physics. The day may be approaching when physicists start debating not how our universe was created, but how to create a new one to test their predictions.

Like Linde, one can only hope that in that brave new world, a seed of wisdom has kept apace with advances in theoretical physics.

SCOTT SHACKELFORD is a second-year Stanford Law student and a PhD candidate in international relations at the University of Cambridge.

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