A Flat Universe?

It’s a question that plagued Einstein and every astrophysicist in his wake: What is the shape of our universe? Is it curved like the top of a ball? Does it open upward like a potato chip? Or is it perfectly flat?

The question is so big, and so mind-boggling, that until recently, it could be attacked only by theorists. But on Wednesday, astronomers released the first detailed images of the infant universe--images that provide conclusive evidence that the universe is very nearly flat.

The finding, already being celebrated by cosmologists around the world, provides the first direct evidence for many provocative and sometimes unpopular notions about how the universe formed. The images may one day provide a clear recipe for the still unknown contents of the cosmos and may help predict its eventual fate and whether it will one day violently collapse on itself.

“Five years ago, even last year, people were talking about a very curved universe. Our data says no way,” said John Ruhl, a physicist at UC Santa Barbara and part of the 36-member international team that made the finding.

“This will be reckoned as the turning point when the history books are written,” said Michael Turner, an astrophysicist at the University of Chicago. Turner was not involved in the experiment, but he had been among those arguing for the less popular case for a flat universe.

Beyond what they may say about the birth and possible death of our universe, the images are stunning physicists simply because they provide the first close-up view of what the universe was like when it was only 300,000 years old and 1,000 times smaller and hotter than it is today.

“It’s like looking at the surface of a new world,” said Craig Hogan, a cosmologist who heads the astronomy department at the University of Washington and who had been among those eagerly awaiting the images. “It’s a picture of our own past.”

The further away astronomers look in space, the further back in time they can see because of the time it takes for light to travel across space. The distant cosmic background radiation contains the oldest structures seen yet, from a time billions of years before the first stars began to form.

The observation also provides critical support for a cherished theory of how the universe formed. This previously speculative theory, called inflation, proposes that the entire universe arose from a space smaller than an atom during a violent explosion that occurred a fraction of a second after the Big Bang. Theory predicts that such an incredible expansion would stretch space out until it was flat.

When cosmologists say flat universe they mean flat in three dimensions, which is difficult to imagine, said Paolo deBernardis, an astrophysicist at the University of Rome and co-director of the project. Space, as envisioned in Einstein’s theory of general relativity, expands into a fourth dimension that we three-dimensional creatures cannot directly experience. That’s why cosmologists use two-dimensional analogies to describe the flat universe, comparing it to a sheet of cardboard.

The new images were captured with a balloon-borne telescope in a 10 1/2-day, 5,000-mile flight high over Antarctica, where constant sunshine and stable high-altitude winds can keep balloons aloft for the long periods necessary to collect data. To make its precise measurements, the telescope needed to float above most of the Earth’s distorting atmosphere.

During its flight, the telescope detected nearly imperceptible differences in the faint, cold glow of microwave background radiation that pervades the sky. The equipment was “sensitive enough to detect the heat given off by a coffee maker all the way from the moon,” said James Bock of the Jet Propulsion Laboratory, which developed the device. Like a cosmic fossil trove, this primordial light is made up of relic particles of light, or photons, from shortly after the Big Bang, about 10 billion to 15 billion years ago.

“We’re looking at the oldest photons in the universe, and they’re really starting to talk,” said Ruhl.

The background radiation was discovered in 1965. Though it was expected to reveal hints about the early universe, it took decades to develop technology that could examine it.

In 1992, NASA’s COBE (Cosmic Background Explorer) satellite detected variations in the radiation across the sky, huge ripples in the fabric of space-time. The variations were dubbed the “Face of God” because they were the first evidence of structure in an early universe that started out as a kind of hot cosmic bisque of particles and radiation.

As exciting as the finding was, the images produced by COBE were blurry and indistinct. Obtaining images of the wrinkles with more detail has been something of a scientific Holy Grail. “It’s been like an Indiana Jones movie,” said Turner. “Everybody’s trying to get to the treasure.”

The new images from the telescope dubbed Boomerang (for Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) are 40 times sharper than those obtained by COBE. “Boomerang has for the first time brought these very faint structures into sharp focus,” said Andrew Lange, an astrophysicist at Caltech and leader of the U.S. portion of the international team.

Like many scientific endeavors, the project was initially plagued by mishaps--balky hardware and a test flight in Waco, Texas, that crash-landed into a puddle--but the crucial flight in the harsh Antarctic environment was “picture perfect,” Ruhl said.

“It’s quite an achievement,” said Wayne Hu, an astrophysicist at the Institute for Advanced Study in Princeton. Hu wrote an editorial accompanying the new research paper, which appears in today’s issue of the journal Nature. “With this data, we’ve entered an era of precision cosmology,” he said.

Once astronomers knew there were wrinkles in the fabric of the universe, the critical question was how large they were. The new report shows they generally cover 1 degree of space in the sky, which is about twice the size of a full moon seen from Earth, and the exact size predicted by models of a flat universe. A curved universe would have meant the light traveling through space was also curved and the wrinkles would have been distorted in size.

“It’s undeniably the pattern of a flat universe,” Turner said.

The pictures reveal hundreds of complex structures in the universe that are the seeds in which clusters of galaxies formed, said Ruhl. It’s thought that all structures we see today were formed by gravitational attraction in areas of slightly different density and temperature in the early universe.

The images, which are expected to undergo months and years of additional analysis, may also begin to answer other looming questions about the basic contents of the universe. There is simply not enough matter to account for the universe depicted by these images. Further computation of the data on supercomputers at the Lawrence Berkeley Lab and at the University of Toronto may help prove theories that the universe is also composed of a strange form of dark matter that is yet to be detected, and dark energy, an even stranger force that acts against gravity and keeps the universe expanding.

And more work must be done before the inflation theory of the early universe is proved, Ruhl said. Many cosmologists are puzzled by a mystery in the new data. While they see one mathematical pattern that was predicted, they do not clearly see a second pattern that was also predicted. Rather than being disappointed, many theorists are excited about the puzzle because it gives them a new problem to chew on. “To me, that’s the most intriguing thing. What’s going on there?” Hu said.

Future satellite missions to examine the cosmic background radiation across the entire sky are also expected to help answer many questions--including those about the ultimate fate of the universe. Will it continue expanding forever at a stately pace? Will it simply stop one day? Or will it violently collapse in on itself in an event dubbed, in advance, “the big crunch?”

Earlier thinking was that a curved universe was destined to die in a “big crunch” and a flat universe would face the calmer fate of expanding ever more slowly and then stopping. However, the addition of the little understood dark energy means all bets for the fate of the universe are off.

“The fate of the universe is going to take a long time to answer,” said Turner. “Luckily, we have at least 200 billion years to chew on that one.”