| A globular cluster floating in the galactic halo of the Milky Way, only 7,000 light-years from Earth, is home to some of the oldest stars ever seen with an optical telescope. Globular clusters, which can contain up to 1 million densely packed low-mass stars, are the progeny of the universe's first stars, huge hydrogen fireballs that flamed out as supernovas. For a close-up of this cluster, see below. |
Photograph courtesy of NOAO/AURA/NSF.
Fourteen billion years ago, give or take an aeon, the Big Bang created the universeor did it? If by universe you mean an abysmally black void with no stars, galaxies, planets, or the slightest promise of life, then the Big Bang's your baby. But if you mean the starry cosmos we see around us today, a universe that includes at least one planet with life, the Big Bang was a bust. For 100 million years after time began, the universe was less interesting than a mud puddle. Only a handful of elementsmostlyhydrogen and helium, along with faint traces of lithium and berylliumricocheted through fathomless, unending gloom. Had anyone been around at the time to bet on the future, the smart money would have been on more of the same: darkness, emptiness, death. Yet improbably, miraculously, the universethe ultimate dark horsebeat those odds. It was reborn. A 100-million-year-long night ended when clouds of hydrogen collapsed and ignited. In the blast furnaces of the first stars, atoms were crushed, burned, and transmuted into more complex particles, like the carbon in the paper of this page or in the hand that's holding it. That momentwhen the universe first lit upwas nothing less than a second creation, the one that really counts.
Astronomers, however, haven't yet been able to see that cosmic dawn, because it's well beyond the range of any existing telescope. "Ah, but a man's reach should exceed his grasp," wrote Robert Browning, "or what's a heaven for?" Or, as astrophysicist Tom Abel might say, "What's a computer for?" Abel has not extended our grasp to heaven, but he has reached out to the time and place when light transfigured a murky universe. He and two colleagues did it with software, using an astonishing computer program to resurrect a long-ago epoch dominated by giant balls of flaming hydrogen hundreds of times bigger than the sun and millions of times brighter. They were the first stars, unlike any in the universe today. They created everything necessary for all future stars, as well as the essential elements of life as we know it on Earth. These fireballs blazed for about 3 million years and then died in a chorus of detonations, long before anything we now see in the sky existed. And their death created life. Parts of those earliest giant stars are in our blood, bones, and skin. It may even be that a bit of the stardust of which the Earth is made was shot into the void by the explosions that ripped apart the earliest stars.
In a nondescript concrete building on the campus of Pennsylvania State University, a six-foot-tall matrix of animate stardust named Tom Abel is looking much farther out into the universe than any telescope ever has. Abel is 32, born and educated in Germany. His English is fluent. He wears an untucked, loose-fitting white shirt, black jeans, sandals, and has a gold earring in his left ear. When he's not re-creating a universe, he likes to sky-divehis Web site features a photo of him plunging from a plane; he appears to be screaming.
Now he's on solid ground, observing a cosmos on the flat-panel display of his desktop computer. He has a god's-eye view of the universe 100 million years after the Big Bang, when it was just a thirtieth of its current size, and when a cosmic dark age was about to end. Multicolored eddies of hydrogen gas fill the monitor, like psychedelic cigarette smoke, color-coded to show different densities. Tens of thousands of years flash by in a few seconds as the churning gas clouds begin to coalesce into a nascent fireball. At the moment, the monitor encloses a virtual space-time (see "What is Space-Time?," below) 20,000 light-years across, about one-fifth the diameter of our own galaxy. But Abel can zoom in on smaller domains in his creation.
The ability to shift focus effortlessly over enormous scales of space and time is the simulation's key feature. Every image that appears on the computer monitor is built from a grid consisting of thousands of individual cells. Within each cell, the computer solves dozens of equations involving gravity, heat flow, and collisions between atoms and molecules. When Abel chooses to zoom in on some region, even if it's only a thousandth of the size of his previous view, the resolution of the image doesn't change or become fuzzy like a magnified photo. The underlying grid still consists of thousands of cells, with the physics meticulously worked out in each one. The simulation is like an extraordinary microscope that never goes out of focus, no matter how large or small the target object. What drives this impressive programming is not computing power but a detailed understanding of stellar physics from the level of molecules all the way up to gravitational interactions spanning light-years.
"Each time we had to stop our research, it wasn't because we didn't have a big enough computer," says Abel. "It was because we ran out of physics. Greg Bryan, who's now at Oxford, wrote the program that allows us to focus in on regions as they collapse. We don't lose any resolution. If the largest volume of our computer simulations contained the entire Earth, our smallest grid would be the size of a red blood cell in your body."
Abel, Bryan, and physicist Michael Norman at the University of California at San Diego have been working on this desktop universe for seven years. Their work has not only transformed astronomers' views of how the universe first lit up, it also marks a major change in how astronomers do their work.
Until recently, cosmology was primarily driven by observationsan astronomer detected a mysterious phenomenon and then tried to figure out what it was. In the late 1950s, for example, astronomers discovered powerful sources of radio waves all across the sky. They were labeled quasars, for quasi-stellar radio sources. But years passed before anyone understood that they are the bright cores of very young galaxies.
The work of Abel, Norman, and Bryan reverses that process, which happens to make cosmology more predictive. The universe, when modeled on a computer, becomes a laboratory where astronomers can test their theories. By creating models of stars that lie beyond the range of today's telescopes, Abel's group can tell astronomers what they might find 10 years from now. So what will they see when telescopes are better?
Abel shifts in his seat and starts the simulation with a click of his mouse. It begins like this: The monitor shows a dark cosmosthe universe as it was 13 million years after the Big Bang. Like an ocean surface, the darkness conceals powerful currents. Something stirs in the depths. Dark matter, which had been randomly scattered throughout the cosmos, starts to clump together. Astronomers estimate that this mysterious material accounts for more than 90 percent of the matter of the universe. They know it's out there only because they can detect its effect on the motions of galaxies. But they can't see it directly, and they don't know what it's made of.
The screen fast-forwards 90 million years. It's still dark, but now the simulation shows a universe with structure. Strands of dark matter stretch across the cosmos like a giant web. And there's another component, almost unnoticeable, floating like the thinnest of mists through space: a gas, mostly hydrogen. Pulled by gravity, the gas begins to form clouds around the densest regions of dark matter. Each cloud is stocked with enough matter to make 100,000 suns. By comparison, our Milky Way galaxy contains a few hundred billion suns.
Abel lets another 50 million years slide by, and then zooms in on one of the hydrogen clouds, narrowing his field of view from 18,000 light-years to 1,800 light-years across. If we were looking through the porthole of a rocket ship instead of at a computer monitor, we would have to travel faster than the speed of light for the view to change so rapidly. The cloud itself fills a third of the screen; it's about 600 light-years wide and still pregnant with enough hydrogen to make 100,000 suns. And now, for the first time since the fires of creation cooled after the Big Bang, warmth enters the universe: The entire cloud has started to glow. In the simulation, it looks almost like a flower, an orange poppy, fragile and small against the black velvet of dark matter and empty space. The glow is from gases compressed and heated by gravity as they fall toward the cloud's center, where the first star will be born. The heating is simple backyard physicscompressed gases get hot, like air pumped into a bicycle tire. But it's backyard physics on a titanic scale: Every hundred years, enough gas to make Earth's sun plunges into this embryo.
|A hubble deep field image, composed of 342 exposures covering a tiny speck of the sky, offers a keyhole view of galaxies stretching to the visible horizon of the universe. Astronomers use such images to search for relics of the early universe. "It would have been amazing to have been an astronomer then," says Penn State astronomer Niel Brandt. "You could have looked up and seen quasars with your naked eye." |
Photograph courtesy of NASA/STSCI/HST.