Trespassing on Einstein's Lawn (10 page)

When the universe's energy hits rock bottom in the true vacuum, all the kinetic energy of the inflaton field pours out into the nascent universe, heating it to a thousand trillion trillion degrees and flooding it with radiation. What were once tiny quantum fluctuations in the density of matter and energy on the order of 10
^-33
centimeter are now stretched out to astronomical proportions, creating subtle ripples and valleys throughout space, laying a gravitational blueprint for what will eventually become a network of stars and galaxies.

The hot universe continues to expand, propelled by inflation's inertia. For the first 380,000 years, a dense, hot plasma permeates space, so dense that even light can't shine through it. Any photon that tries to muddle through is quickly thwarted by a rogue proton or electron. But as the universe expands, the scalding temperature cools, and particles slow down long enough to bond together. As matter organizes itself into nuclei and then atoms, the photons are set free from the opaque plasma and stream out into the universe on their own. This first generation of emancipated photons are the same ones that constitute the CMB, a snapshot of the universe from the time they were first released.

While the photons travel unimpeded, the matter particles begin to collect in the overdense regions carved out by the quantum fluctuations, setting off a chain reaction of gravitational collapse. Matter piles upon matter, its temperature steadily rising until, after 200 million years, it triggers nuclear fusion. Suddenly the landscape changes dramatically: stars burst forth from the darkness and burn scattered across the sky. The chain reaction continues its cascade—stars coalesce into galaxies, galaxies into clusters, clusters into superclusters.

All the while the universe continues to expand at a leisurely rate. Eventually one particular star is born, and around this star some rocky debris assembles into a planetary system. On one particular rock, the third from the star, elements such as oxygen, hydrogen, and carbon come together, elements forged in the furnaces of other stars gone supernova, spewed remnants that journeyed through empty space to one day land on a lucky planet where they would combine in just the right way within just the right environment for life to emerge from some primordial goo, to grow and replicate and evolve until—voilà—we are born.

Only the story doesn't end there. For if inflation really happened, it didn't happen just once. While the history of our humble universe was unfolding, something much, much bigger was going on. Thanks to quantum randomness, the false vacuum from which our universe was born couldn't decay everywhere at exactly the same rate. While one region of the false vacuum rolled downhill to form our universe, other regions were left behind. Eventually they, too, would decay, forming other universes, permanently disconnected from our own. While these regions decayed, still others were left behind, and when in turn they plummeted toward universehood, still more lingered in their wake, and so on and so forth, ad infinitum. No matter how many universes it creates, there's always more false vacuum left over, and the creation process never ends. Inflation is eternal.

If we allow inflation to occur even once in our cosmic history, we are suddenly stuck with an infinite number of universes beyond our own, an ever-growing multiverse, a meta-Universe with a capital
U
composed of causally disconnected small-
u
universes, one sprouting from the next in a ceaseless process of birth and reproduction. While
they are all governed by the same fundamental laws of physics, each universe is born with its own local laws: its own geometry, its own array of physical constants, its own set of particles, its own spectrum of force strengths, and its own unique history. Reality as a whole begins to resemble a vast cosmic patchwork quilt, wildly diverse and fast approaching infinity.

With the WMAP data, cosmologists now had in their hands a detailed map of the microwave sky, one that revealed subtle fluctuations from the uniform temperature, hot and cold spots that differed by a mere one part in a hundred thousand. The spots had been formed when the dense plasma still permeated the infant universe, imprints of a struggle between gravity and electromagnetism. While gravity had tried to squeeze the plasma tighter, electromagnetic radiation had tried to expand it, resulting in a tug-of-war that compressed and expanded the plasma like an accordion. When it compressed, it heated up ever so slightly, and when it expanded, it cooled, leaving subtle hot and cold spots for WMAP to find some 14 billion years later.

The fluctuations were fingerprints, forensic evidence of the universe's beginning. It was hard to believe that what looked like a bunch of random blotches on the map actually contained detailed information about the universe's origin, its makeup, and its evolution. With hard data like WMAP, cosmology had gone from a speculative science to a rigorous field on par with astronomy and astrophysics. The golden age had dawned, and the cosmologists were ready to party.

In fact, they already had one planned. Searching around online, I found that they were preparing for a big four-day conference the following month at the University of California, Davis. I had to be there.

I called my father. “Four days of sun and cosmology!” I said. “If we want to understand the origin of the universe, these are the people who can explain it. You've got to come with me!”

He sighed. “I wish I could. There's no way I can get out of work on such short notice. You should go, though! Be a journalist. You'll take lots of notes for me.”

I hung up, unsure whether I really wanted to go by myself. This
had always been intended as a joint mission; going solo didn't feel quite right. I mean, crashing physics conferences was
our
thing—if doing something once could count as a “thing.” Of course, I had interviewed Fotini Markopoulou on my own, but that didn't seem quite the same at the time. After all, the idea had been to get some journalism credentials just so we could worm our way into more conferences. Thinking back, I wasn't entirely sure how those credentials were going to get my father anywhere, but it had seemed like a minor technicality, a bridge we'd cross when we came to it. Now I began to wonder if the five seconds I had given to devising that plan hadn't been enough. It probably should have occurred to me the moment I decided to spontaneously will myself into a science journalism career that my father's trajectory and mine would soon diverge, two blissfully unaware parallel world lines that unknowingly find themselves moving through a curved space, looking over to see the other receding into the distance despite all our best efforts to move in a straight line.

But what choice did I have? I was a twenty-two-year-old kid who could afford the time to pursue a quixotic dream. It was like taking time to backpack through Europe, except that you couldn't pay me enough to wear a backpack, let alone fill it with months' worth of cute outfits and shoes. So I chose ultimate reality. And what did I have to give up? A job working for an ontologically questionable magazine in some guy's apartment. For my father, it was totally different. He had already chosen his path. He had a family and a house and a career. He was in the hospital every day, morning to night, helping to save people's lives. He wasn't going to just up and ditch his job. Even if he wanted to, there was no way in hell my mother was going to let him.

I was just going to have to go it alone, I thought, and know that I was doing it for the both of us. I sent an email to the people in charge of press registration.
I'm a freelance physics writer
, I told them.
I write for
Scientific American. It was vaguely true, I figured, if you ignored my use of the present tense. My profile of Markopoulou had been published in the previous month's issue. The conference organizers granted me a press pass right away, and I booked my flight to California.

* * *

A few weeks later I arrived in Davis and thawed my Brooklyn skin in the warm California air. Each morning I walked several blocks from my hotel to the UC campus for eight hours of physics lectures punctuated by coffee breaks and lunches. As each speaker took to the stage I hunkered down in my seat with my notebook, furiously scrawling, struggling to keep pace while trying to cut through layers of jargon and figure out what the hell everyone was talking about. I couldn't get enough. In a world in which I didn't belong and didn't speak the language, I had never felt more at home.

That's not to say I didn't stand out. My gender, age, and questionable occupation were all liabilities. Still, I did my best to blend in. I covered my tattoos with slacks and button-down shirts. I wore loafers. I tried to lie low.

Most of the talks focused on what the WMAP data meant for our understanding of the universe. To start, it pinned down the universe's age to 13.7 billion years. Even better, it decided the universe's geometry.

Because gravity can confer a net curvature to the overall shape of space, there were three possible cosmic geometries: the universe could be positively curved, like the surface of a sphere; negatively curved, like the inverted swoop of a saddle; or flat, like ordinary, Euclidean space where parallel lines never diverge nor meet.

The best way to find the geometry of a space is to draw a big triangle on it and add up its angles. If they sum to more than 180 degrees, you'll know the space is positively curved; if they sum to less, the curvature is negative.

Listening to the talks, I learned that the microwave background radiation provided the perfect cosmic triangle, with the WMAP satellite sitting at its pointed tip. The paths of two incoming photons from opposite sides of a hot or cold spot could be used to form two equal sides of a long, thin triangle, their length given by the light's travel time since the photons were freed simultaneously from the plasma. The length of the triangle's third side was determined by the distance a sound wave can travel in 380,000 years—that is, the stretch of space that the accordion compressed or expanded to form the hot or cold spot in the first place.

Knowing the lengths of all three sides, physicists used some basic trigonometry to calculate the angles at the triangle's base: 89.5 degrees apiece, summing to 179. Now they just needed the third angle—the one at the near tip. If the photons had traveled in straight lines to get there, the angle would be 1 degree, bringing the total to a flat 180. If their paths had bent outward as they journeyed through a positively curved universe, the angle would be larger, and if their paths had bent inward due to negative curvature, it would be smaller. According to WMAP, the third angle was precisely 1 degree. Way to call it, Euclid.

There was just one problem. The universe's geometry is determined by the amount of mass—or, using E = mc
²
, energy—it contains. As Wheeler would say, mass tells space how to curve. A flat universe requires a critical density of mass to flatten it, the equivalent of an average of six hydrogen atoms per cubic meter. It didn't sound like much. You'd think there'd be more than enough stuff out there, considering all the galaxies swirling around. But there's not. Not even close.

Ordinary matter—particles such as protons, electrons, and quarks—accounts for a pathetic 4 percent of the total you'd need. Our planet, the stars, ourselves, everything we see and know, is virtually negligible in the cosmic scheme of things, the sad, shining tip of a larger, darker iceberg.

So what else was out there? The physicists had a few ideas.

For one, they already knew that there's more to matter than meets the eye, thanks to the simple fact that galaxies aren't bursting at the seams, sending billions of rogue unshackled stars flying off in all directions. Somehow gravity is holding them together in tight spiral and elliptical formations, despite the fact that the total mass of all the stars in a given galaxy doesn't provide nearly enough gravity to do the trick. Something else had to be lurking there, hidden in the dark spaces between stars or encircling each galaxy like an invisible fence, preventing stars from wandering off. In order to provide the necessary gravity but also to have remained unseen all this time, it had to be something sturdy and solid, like matter, but indiscernible to electromagnetism. Dark.

Astronomers calculated how much of this dark matter was skulking out there, but when you add it to the ordinary luminous stuff,
you're still only at 27 percent of the total mass and energy needed to flatten the universe. A disturbing 73 percent is still missing.

Enter dark energy. In the late 1990s, two teams of astrophysicists—one led by Saul Perlmutter, the other by Brian Schmidt and Adam Riess—went supernova hunting, hoping to measure the expansion rate of the universe. They knew it had begun in a burst of inflation, but figured it had been slowing down ever since, reined in by the grip of gravity.

Perlmutter, Schmidt, and Riess realized that the expansion history of the universe is encoded in light from exploding stars. Certain kinds of supernovae—so-called standard candles—always burn with the same intrinsic brightness, even though they appear dimmer when they're farther away. Just how dim a standard candle appears reveals exactly how far away it is. As its light travels through an expanding space, it gets stretched out, its wavelengths shifted toward the red end of the electromagnetic spectrum. This redshift measures how much the universe expanded during the time it took the light to reach us. By collecting light from many standard candles at varying distances, the teams mapped a history of the universe's expansion. Only it wasn't slowing down at all. It was speeding up.

What could accelerate the universe's expansion in spite of gravity's best efforts to slow it down? Some mysterious dark force had to be out there, permeating the emptiness of interstellar space, hiding in the depths of the vacuum, pushing it apart, exerting a kind of antigravity, causing spacetime to expand faster and faster. Exactly how much of this dark energy is out there according to the supernovae? The answer was a near miracle. It was exactly the amount you need to fill that 73 percent hole and flatten the universe.