Unreal: A portrait of the cosmos


Author: Chet Raymo'58, '64Ph.D.

“One can’t believe impossible things,” says Alice in Through the Looking-Glass. “I daresay you haven’t had much practice,” replies the Red Queen. “When I was your age I always did it for a half an hour a day. Why, sometimes I’ve believed as many as six impossible things before breakfast.”

Well, I haven’t had breakfast yet this morning, but I’ve been reading physicist Brian Greene’s best-selling book The Fabric of the Cosmos, and here are a few impossible things he asks us to believe:

  • Fourteen billion years ago the entire universe we observe today—containing upward of 100 billion galaxies—was contained in a speck much smaller than the period at the end of this sentence.

  • The speck suddenly expanded, in an inflation that lasted for a billion billion billion billionth of a second, bringing the observable universe of matter and energy into existence.

  • In some versions of the inflation scenario, the part of the universe we observe is just a tiny fraction of what exists. Vast realms of galaxies are too far away for their light to have yet reached us.

  • The visible stars and galaxies are just the tip of the cosmic iceberg. Space is also filled with mysterious dark matter and dark energy that make up most of what exists. We know this stuff is out there, by its effect on the luminous galaxies, but so far no one knows what it is.

  • The universe—including you and me—is made of string. A special kind of string, to be sure, vibrating linear mathematical entities in 10 spatial dimensions that give rise in their various excitations to all the known elementary particles.

  • And why, if space has 10 spatial dimensions, do we observe only three? Because the extra dimensions are curled up too small—a hundred-billion-billion times smaller than an atomic nucleus—to be presently observable with even the most powerful particle accelerators.

I could go on, listing not just six impossible things but dozens. One puts down a book like Greene’s—and there are many others in which the new cosmologists try to explain what they are up to—wishing one had spent more time practicing with the Red Queen.

* * *

These Alice-in-Wonderland_ _stories are not just idle speculations. The fundamentals of big-bang cosmology have been put to the test, and so far they pass with flying colors. Let me mention just two stunning confirmations.

First, if the universe began with a big bang, as physicists say, then the flash of that primeval event should still fill the entire universe with a diffuse microwave radiation. Inflationary theory predicts exquisitely detailed variations in the temperature of the radiation across the dome of the sky. The radiation has now been mapped with special satellite telescopes—first in 1992 with COBE (the Cosmic Background Explorer) and more recently with WMAP (the Wilkinson Microwave Anisotropy Probe)—and the agreement of theory and observation is so close as to take one’s breath away.

Second, just this past year a group of European theoretical physicists, after 20 years of preparation, modeled the big bang in a month-long run on one of the most powerful supercomputers in the world, tracking trillions of simulated particles. They plugged in what we think we know about the relevant physics and let the computer spin out a universe from the first moments of creation to the present day. And—this is the kicker—the simulated universe looks stunningly like the one we live in, the same wisps and streamers of galaxies. Crucially, the computer model seems to confirm the existence of the so-called dark matter and dark energy that supposedly account for most of “what’s there.”

Humans have had creation stories since the dawn of time, and until recently the stories belonged mostly to the theologians. But in recent decades something remarkable has happened. A fleet of extraordinary space telescopes has revealed details of the universe that were previously invisible. Giant particle-accelerating machines on Earth reproduce ever more closely conditions that existed in the earliest universe. Supercomputers make it possible to mathematically model the creation itself, at least back to the first trillion trillion trillionth of a second. Cosmology—the story of the origin and evolution of the universe—has become an experimental science.

What the physicists are telling us may seem impossible to believe, but it becomes increasingly apparent that they are doing something right. Albert Einstein’s wife, Elsa, was once asked if she understood her famous husband’s theory of relativity. She replied, “Oh, no, although he has explained it to me many times—but it is not necessary to my happiness.” Big-bang cosmology, with its strange subatomic world of multidimensional vibrating strings, may not be necessary to our happiness, but it is certainly essential information if we are to understand where we came from and who we are.

* * *

It has been just over a century since Einstein’s anno mirabilis, his year of miracles. During 1905, the relatively unknown Swiss patent clerk published four astonishingly original papers in the prestigious journal Annalen der Physik, any one of which might have won him a Nobel prize. One of those papers established the theory of special relativity, linking space and time into a seamless space-time fabric and asserting the equivalence of matter and energy. Another showed that light consists of small indivisible packets of energy called quanta. Later, Einstein expanded his special theory of relativity to include gravity, showing that this fundamental force of the universe can be understood as a curvature of four-dimensional space-time. It is perhaps not too much to say that Einstein single-handedly established the agenda for the two great pillars of 20th-century physics: general relativity (gravitation) and quantum physics, the sciences of the very large and the very small.

One prediction of Einstein’s gravity theory was that the universe could not be stable; it must expand or contract. Einstein was so skeptical of this possibility that he added a fudge factor to his theory—a “cosmological constant,” a sort of negative gravity, a push to balance the pull—to allow for a universe that didn’t stretch itself thin or collapse into a heap.

Then, in the late 1920s, Edwin Hubble and Milton Humason, working at the Mount Wilson Observatory in California, discovered that the galaxies are in fact racing away from one another. Einstein took the train from Princeton to the West Coast to have a look at the astronomers’ data. Yes, the universe was expanding. The “cosmological constant” was, he said, the biggest mistake he ever made.

If the galaxies are moving apart, then they must have been closer together in the past. It is mathematically possible to run the movie in reverse, so to speak, and show that billions of years ago all the matter and energy of the universe was contained within a seed of infinite density. The big-bang theory was born.

Still, astronomers resisted. Few scientists liked the idea of a beginning. Even fewer liked the idea of a violent beginning. Theoreticians struggled to explain the outward flight of the galaxies without invoking a special moment of creation.

When I was a student at Notre Dame in the 1950s, two stories of the universe were in contention: the big bang and the steady state. In the latter theory, the galaxies move apart, as observation insists, but new matter continuously appears in the space between the galaxies, eventually bringing new stars and galaxies into existence. A steady-state universe has no beginning and no end, and looks pretty much the same from every time and place. It is the sort of universe a physicist can love.

But observation has not been kind to the steady-state universe. In the 1960s astronomers observed the flash of the big bang—the blaze of radiation that accompanied the universe’s birth—now much diluted by the subsequent expansion, an invisible microwave energy that uniformly fills the entire universe with precisely the spectrum predicted by big-bang theory. Most scientists were convinced.

Other evidence for a big bang soon fell into place. The relative abundance of elements in the observable universe—hydrogen, helium and so on—is exactly as predicted by big-bang cosmology. Also, since light takes time to reach us, looking at distant objects is equivalent to looking back in time. And what we see far away—most notably, the quasars—is different from what we see nearby. The universe has a demonstrable history.

So far, the big bang has survived every observational test. What once seemed unimaginable—space and time emerging from a tiny mathematical point—now seems almost commonplace.

Meanwhile, physicists were using high-energy accelerating machines to smash apart atomic nuclei. In the resulting debris they found whole families of exotic particles, with matter and energy flickering back and forth one to the other as Einstein had predicted. Quantum theory describes this subatomic realm with impressive precision.

* * *

But one very big problem still looms for theoretical cosmologists, the very problem that defeated Einstein, though he devoted much of his later life to its solution: The unification of the two great theories of the 20th century, general relativity (gravitation) and quantum physics. Each theory reigns supreme in its domain of application, gravitation on the cosmic scale, quantum theory on the scale of elementary particles. Only occasionally, as in discussions of black holes or the big bang, where cosmic quantities of matter are contained in subatomic-sized spaces, do the two theories rub against each other. The rubbing can be abrasive, and the race is on to find a unified theory of “quantum gravity,” sometimes called a Theory of Everything. The invention and verification of such a theory is the central problem of physics today. Only when such a theory is in hand will we understand the evolution of the universe in the first infinitesimal fraction of a second as it emerged from the unknowable.

Run the movie of the expanding universe in reverse. As we approach time zero, the size of the known universe becomes vastly smaller than the nucleus of an atom, the realm of quantum physics. But the universe also becomes exceedingly dense, and therefore gravity becomes exceedingly strong. With no unified theory to work with, that first trillionth trillionth trillionth of a second of the universe’s history remains uncharted territory. The true beginning—the singular instant of creation—lies tantalizingly out of reach.

Currently, the most promising approach to quantum gravity is string theory, which proposes that the ultimate elements of reality are vibrating filaments of energy. Electrons and quarks, the fundamental components of ordinary matter, and gravitons, the constituents of the gravitational field, are not points, as previously imagined, but instead have a linear dimension: metaphorically speaking, not tiny billiard balls but string. In the most popular version of the theory, the strings are very small, a hundred-billion-billion times smaller than an atomic nucleus, but it is precisely these little snips of energy—that can’t be snipped any smaller—that make possible a unification of quantum physics and general relativity.

All versions of quantum gravity, when they have battered their way to a common vision, will probably suggest that space and time, like matter and energy, come in quantized (indivisible) units, and that relationships, a sort of mathematical music, rather than things, are the fundamental elements of reality. Even those particles of matter we thought of as hard, solid “stuff” dissolve into pure song.

And it just gets weirder and weirder. Some physicists think of the universe as a hologram, a kind of grand illusion. Others talk of myriad universes that bubble from the void like bubbles from champagne, or a mirror universe on the other side of the big bang where time runs backward. Many cosmologists believe that the universe is truly infinite in extent and contains an infinitude of galaxies, including some that are exactly like the one we live in, down to the very color of the socks you are wearing at this instant.

Ordinary folks like you or me might reasonably ask, “What does any of this have to do with me?” And indeed, like Elsa Einstein, we manage to live out our lives in a universe of three space dimensions and one time dimension with no consciousness of those proposed ultra-small vibrating strings and multidimensional spaces. We have, in spite of ourselves, become more or less accustomed to the big bang, but the wildly abstract ruminations of the quantum gravity physicists seem rather like those of medieval theologians who supposedly debated how many angels can dance on the head of a pin.

* * *

There is a difference between strings and angels. No matter how weird the speculations of the physicists, at least potentially it is possible to put their ideas to an observational test. Without a test, yes, they might as well be talking angels and pins, and, for the moment, strings, multiple universes, and 11-dimensional space-time are beyond experimental verification. But that might change in the decade to come.

In 2007, the European Space Agency’s Planck satellite will test theories of the early universe by looking more precisely at the radiation left over from the big bang. In 2008 the first results from the Large Hadron Collider, the world’s most powerful particle accelerator, will be available from CERN, the European high-energy physics laboratory near Geneva, Switzerland, recreating even more closely those first moments of the big bang when quantum gravity presumably ruled. And sometime after 2011, NASA’s Laser Interferometer Space Antenna, spread out across a huge space of the solar system, could reveal the effects of quantum gravity in the early universe as ripples echoing in space-time.

Meanwhile, astronomers use the instruments we currently have on Earth and in space to map the universe in ever greater detail. Of particular interest is the precise rate of expansion of the universe. Astronomers measure the expansion rate by observing supernovas—exploding stars—in distant galaxies, and using the apparent brightness of the supernovas as distance indicators. The speed of the galaxies is measured as a shift in the wavelength of their light, in much the same way as a radar gun measures the speed of a moving car. Put the two observations together and the conclusion is inescapable: The expansion of the galaxies is _accelerating, _not slowing down as would be the case if only gravity were at work. Something is pushing the universe toward a fate of infinite dispersal, cold and dark. That mysterious “something,” spread out uniformly through the universe, is called dark energy—an unexpected re-incarnation of Einstein’s cosmological constant.

Dark energy is not to be confused with dark matter, an equally mysterious massy, non-luminous substance detectable by its gravitational effect on the clustering and motion of galaxies, almost certainly some hitherto undiscovered kind of elementary particle. It is humbling to realize that most of what exists is invisible to us and—so far—unknown.

Perhaps after all it is not so absurd to think of theoretical and experimental cosmologists as the new theologians. They are, after all, working at the very limits of the knowable—what the physicist John Wheeler has called “the flaming ramparts of the world”—asking the biggest questions of all: Why is there something rather than nothing? Whence the circumstances of life and consciousness? What will be the fate of the universe? Many centuries ago Saint Columbanus asked in a sermon: “Who shall examine the secret depths of God? Who shall dare to treat of the eternal source of the universe? Who shall boast of knowing the infinite God, who fills all and surrounds all, who enters into all and passes beyond all, who occupies all and escapes all?” Those who wish to know God, he said, must first review the natural world.

* * *

All cultures, everywhere on Earth, have stories, passed down in sacred writings or tribal myths that answer the questions: Where did the world come from? What is our place in it? What is the source of order and disorder? What will be the fate of the world and of ourselves? The new story, the scientific story, is the product of thousands of years of human curiosity, observation, experimentation and creativity. It is an evolving story, not yet finished. Perhaps it will never be finished. It is a story that began about 14 billion years ago (13.7 billion years, to give the most up-to-date estimate) with an explosion from an infinitely small, infinitely hot seed of energy, and that has provided, in this tiny corner of the universe at least, conscious creatures who match their brains against the ultimate mysteries.

The new scientific cosmology has important advantages over all the stories that have gone before. For one thing, it works. Scientists test the story in every way they can devise, in its particulars and in its totality. They build giant particle-accelerating machines to see what happened in the first hot moments of the big bang. They put telescopes into space to look for the radiation of the primeval explosion. They model the formation of stars and galaxies with supercomputers. With spectroscopes and radiation detectors they analyze the composition of stars and galaxies and compare them to our theories for the origin of the world. Always and in every way they try to prove the story wrong. And when the story fails, they change it.

The new cosmology places us squarely in a cosmic unfolding of space and time, and affirms our biological affinity to all humanity and other creatures. We are inextricably related to all of life, to the planet itself, and even to the lives of stars.

The microbiologist Ursula Goodenough, in her book The Sacred Depths of Nature, reminds us that the word religion derives from the Latin religio, to bind together again. She writes: “We have throughout the ages sought connection with higher powers in the sky or beneath the earth, or with ancestors living in some other realm. We have also sought, and found, religious fellowship with one another. And now we realize that we are connected to all creatures. Not just in food chains or ecological equilibria. We share a common ancestor . . . We share evolutionary constraints and possibilities. We are connected all the way down.” All the way down, perhaps, even to those mysteriously vibrating strings that bind together the very large and the very small. And in a universe of a hundred billion galaxies—at least!—we have only begun to understand our place in the fabric of creation.

We treasure the ancient creation stories for the wisdom and values they teach us. But only the new story has the universal authority to help us navigate the future. It may not be the “true” story, but for the time being it is certainly the truest. Of all the stories, it is the only one that has had its feet held to the fire of exacting empirical experience.

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    Chet Raymo’s newest book is Walking Zero: Discovering Cosmic Space and Time Along the Prime Meridian. He resides on the web at http://www.sciencemusings.com/.

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