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The universe is flat, after all

South Pole balloon flight confirms a Euclidean law

By Diane Ainsworth, Public Affairs
Posted May 10, 2000

The first detailed images of the universe in its infancy, obtained by an 800,000-cubic-meter balloon carrying microwave detectors, has settled a longstanding debate over the shape of the universe.

It's flat.

Data from 3 percent of the sky, taken during a 5,000-mile journey around the Antarctic, provided Andrew Jaffe, an astrophysicist at Berkeley, and an international team of scientists with tens of thousands of pixels and close to one billion measurements of cosmic microwave background radiation, which filled the universe shortly after the Big Bang. The data support the widely held view that the universe is, indeed, flat, and not curved.

"Wow, wow and wow," said Michael Turner, an astrophysicist at the University of Chicago. "Wow number one: Euclid was right, the universe is flat. Wow number two: inflation, our boldest and most promising theory of the earliest moments of creation, passed its first very important test. And wow number three: this is just the beginning. We are on our way to a better understanding of the universe back through time, when the largest structures in the universe were protons."

The project, called BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), obtained images from an extremely sensitive microwave telescope that was suspended from a balloon circumnavigating the South Pole in late 1998. A map published in the April 27 issue of Nature, and results in the May 8 issue of Astrophysical Journal Letters show the most detailed glimpse yet of the primordial universe, revealing the shape of the cosmos and the distribution of matter shortly after its birth.

Today, the universe is filled with galaxies and clusters of galaxies. But 12 to 15 billion years ago, just after the Big Bang, it was very smooth, incredibly hot and dense. The intense heat that filled the embryonic universe is still detectable today as a faint glow of microwave radiation that is visible in all directions. That radiation, known as the cosmic microwave background, was the centerpiece of NASA's Cosmic Background Explorer, which discovered the first evidence of structures, or spatial variations, in this background radiation.

BOOMERANG, so named because it circled and returned to its original departure site, mapped tiny temperature differences in the cosmic background radiation that was present when the universe was smooth and filled only with protons, electrons and other charged particles. From a map of these temperature fluctuations, the researchers were able to derive a "power spectrum," a curve that registers strength of the fluctuations on different angular scales, and which contains information on such traits of the universe as its geometry and how much matter and energy it contains.

"The data are remarkably clear," said Jaffe, a member of the university's Center for Particle Astrophysics and Space Sciences Laboratory. "You can write down everything you know about the data and then calculate the most likely power spectrum, a task that is conceptually simple but computationally challenging."

Background radiation in the early universe, about 300,000 years after a violent subatomic explosion burst from the vacuum of space and spewed a primordial soup of boiling particles outward, cooled to about minus 450 degrees Fahrenheit and began to form clusters of matter. BOOMERANG was able to detect minute temperature variations of less than 1/1000th of a degree in what was once a hot plasma sheet of almost perfectly uniform temperature.

"We were able to see temperature variations that differed by only hundreds of millionths of a degree," said Turner. "The pattern of hot and cold spots on the universe showed us that the shape of the universe is undeniably flat."

"This is good confirmation of that standard cosmology, and a large triumph for science," said Paul Richards, a Berkeley professor of physics on the MAXIMA experiment, "because we are talking about predictions made well before the experiment, about something as hard to know as the very early universe."

The findings bring astrophysicists closer to a theory of space and time in which inflation caused the universe to stretch rather than balloon into a sphere.

New snapshots of the cosmic microwave background will help solve the riddle of how much light matter and dark matter exists in today's universe.



May 10 - June 6, 2000 (Volume 28, Number 33)
Copyright 2000, The Regents of the University of California.
Produced and maintained by the
Office of Public Affairs at UC Berkeley.
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