To be continued ... infinitely
Cosmologists mute 'big-bang' theory and discover inflation as aneternal feature of the universe.
Using mountain-top telescopes, orbiting observatories, powerful particle accelerators, and boxes of blackboard chalk, scientists appear to be on the verge of solving one of humanity's most enduring puzzles: how the universe formed and what its future holds.
As data pour in, old notions of the universe's earliest moments are giving way to the new: The 'big-bang' theory is out, inflation is in. And creation may be an eternal feature of an infinite universe, spawning a limitless number of cosmic "domains," such as the expanding one we inhabit.
"This is a great time in cosmology," says an ebullient Edward Kolb, professor of astronomy and astrophysics at the University of Chicago. "We have new tools, new ideas, and we can test those ideas."
Call it the age of the inflationary universe.
In broad terms, inflation theory holds that at the moment of its birth, the part of the universe our domain inhabits reached much of its current size in a burst of growth that took but the tiniest fraction of a second. Depending on the details, the size of our domain varies. But by many accounts, it extends far beyond what astronomers can detect.
By contrast, the standard big-bang theory held that the visible universe expanded and cooled more gradually after exploding into existence from an extremely hot, dense point in space.
In many ways, the big-bang theory was reasonable, according to Stanford University physicist Andrei Linde, who has been refining his ideas about inflation for some 20 years. For example, it successfully predicted the relative abundance of light elements such as helium and hydrogen. Combined with particle physics, it appears to account for the emergence of subatomic particles.
And it implied the existence of a cold remnant of the original explosion. In 1965 Arno Penzias and Robert Wilson, researchers at Bell Laboratories, discovered the remnant, known as the microwave background radiation. For astronomers, it constitutes a cosmic Rosetta Stone that dates back to 300,000 years after the universe was born.
Big-bang theory a bust?
But Dr. Linde adds that the big-bang theory also raised some troubling questions. Among other problems, the visible universe is vastly larger than the big-bang theory predicts it should be.
It doesn't explain why our piece of the universe appears so uniform on large scales, but can still give rise to lumpiness in the form of galaxies, clusters of galaxies, and superclusters. Combined with particle physics, the big-bang theory predicted the existence of vast quantities of particles called magnetic monopoles, which are extremely heavy and should be as abundant as protons. But they're not.
It wasn't so much that the big-bang theory was wrong, researchers say. Rather, it was presenting an incomplete picture of our domain's earliest moments. The notion of an inflationary universe has begun to fill in much of the rest of the picture.
In one of its earliest formulations, inflation was invoked after the big bang as a way to prod the universe toward its present size. That solved the size problem. But it still required scientists to accept initial conditions for the big bang that seemed to defy explanation through known physics and, in any case, couldn't be tested. And it predicted that our cosmic domain would evolve a structure much different from the one seen today.
Meanwhile, other variations on inflation were being developed in which the universe first emerged, not in a big-bang explosion, but in an inflationary outburst of exponential growth. From that point on, the domain's evolution follows the physics and timeline of the big-bang theory.
As inflationary theories have evolved, proponents say, they appear to have solved several problems the big bang couldn't shake, not the least of which is that inflation's initial conditions - hence those of the universe, if inflation is correct - are grounded in current theories of particle physics.
Moreover, inflation theories make specific predictions about key features of the cosmos that ground- and space-based instruments are detecting.
For example, inflation theory predicts that as our domain took shape, matter would have been unevenly distributed. Though slight, the lumps would become the gravitational seeds around which galaxies could form and cluster.
Lumpy cosmic background
This lumpiness - the once-tiny footprints of quantum mechanical effects that grew in size as the domain expanded - should appear in the microwave background radiation as slight variations in density. Indeed, in 1992, the Cosmic Background Explorer (COBE) spacecraft found such variations, as have smaller-scale experiments.
According to University of Chicago astrophysicist Michael Turner, detailed studies of the microwave background radiation completed early last year strongly suggested that our domain is just dense enough to continue expanding forever, but at an ever-slowing pace. This so-called critical density has been one of the knottier problems in trying to reconcile inflation with current observations.
"Astronomers kept saying: We love that story, it sounds good, and we love the physics. But we aren't even close" to having enough matter to match the critical density, Dr. Turner says.
In the early 1980s, astronomers estimated that based on what they could see through their telescopes, the universe contained only about 0.5 percent of the critical density. Yet studies of the motion of galaxies indicated that these rotating pinwheels of billions of stars were under the gravitational influence of more matter than was visible. A few years ago, Turner continues, researchers calculated that this dark matter may account for another 40 percent of critical density. "Since then we've been stuck at 40 percent."
Then, last year two teams of astronomers independently reported that our domain's expansion was accelerating, not slowing. To many researchers, this betrayed the presence of energy that acted to repel objects, once distance had sufficiently weakened gravity between them.
Some insist that this energy is Albert Einstein's cosmological constant, which he included in his calculations to ensure that the universe conformed to what at the time was thought to be a steady state.
Others have dubbed the energy "quintessence" and argue that it may not be constant, but changes over time.
Whichever it turns out to be, this "dark energy" is sufficiently dense to account for the remaining 60 percent of critical density, Turner says. In cosmological jargon, our domain is "flat," just as inflation theories predict.
In addition to these discoveries, deep-sky images from the Hubble Space Telescope and large ground-based observatories are opening windows to the earliest epochs of galaxy formation. They are providing a reality check on inflation theories' predictions of how our domain could have evolved.
More data are beginning to pour in from projects such as the Sloan Deep Sky Survey, while researchers eagerly await new US and European satellites that will map the microwave background radiation with more detail than ever before.
As for the future of our corner of the cosmos? It has been spawning its own offspring ever since its own inflationary period began, according to Linde. In his version of inflation, once any part of the universe inflates sufficiently, that domain's expansion begins to trigger new inflationary domains, which in turn produce yet more.
In this fractal universe, possibilities are indeed infinite and for the universe as a whole, creation is eternal.