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."