Big Bang evidence found

Posted: May 2, 2001

Several teams of astronomers released new data this week from a suite of groundbased and balloon-borne instruments that they believe is the strongest evidence yet for the leading model for the formation of the universe.

The findings, announced Sunday at a meeting of the American Physical Society in Washington, DC, has provided solid evidence that the universe underwent a brief but powerful period of expansion, called inflation, in the first instant after the Big Bang.

New map
Old map
Better maps of the microwave sky and an analysis of a larger section of those mapsare two factors that have led to an improved picture of the kind of universe in which we live. The top map shows the previously analyzed section (rectangle), and the new expanded region of analysis (ellipse). The bottom image shows a map used in the previous analysis, which was released last year.
"With these new data, inflation looks very strong," said John Carlstrom, a University of Chicago astronomy professor who led one of the research teams who presented their results at the conference. "It's always been theoretically compelling. Now it's on very solid experimental ground."

Inflation models were first proposed two decades ago in an attempt to explain some of the puzzling aspects of the universe that conventional models of the Big Bang failed to describe. Those aspects included the uniformity of the universe over large length scales despite the short fundamental scale of gravity, the large number of photons in the universe, and physical processes that generated fluctuations in the density of the universe that led to the formation of the first physical structures. While the inflation model worked well theoretically, only recently have astronomers been able to look for evidence that would support the model.

The new evidence in support of the inflation model comes from new measurements of the cosmic microwave background (CMB), radiation visible in all directions that dates back to about 500,000 years after the Big Bang, when the universe cooled to the point where protons and electrons could combine to form hydrogen atoms. The CMB initially appeared uniform, although studies dating back to NASA's Cosmic Background Explorer (COBE) spacecraft a decade ago have shown small variations in it which have been construed as evidence for inflation.

The latest observations, using instruments based at the South Pole and on balloons flying over Antarctica and the continental United States, have given astronomers their sharpest, most sensitive measurements to date of the CMB. Capable of seeing variations as small as 100 millionths of a degree in the temperature of the CMB, the new data have revealed a series of three progressively fainter peaks in the temperature of CMB, a harmonic structure predicted by inflation models.

"The early universe is full of sound waves compressing and rarefying matter and light, much like sound waves compress and rarefy air inside a flute or trumpet," explained Paolo deBernardis of the University of Rome La Sapienza, one of the members of the Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERanG) team. "For the first time the new data show clearly the harmonics of these waves."

BOOMERanG used a microwave telescope suspended from a balloon that flew 37,000 meters above Antarctica for ten and a half days in late 1998, circumnavigating the continent as it studied the CMB. Initial results from BOOMEanG, released a year ago, provided some evidence for the inflation model, as astronomers were able to detect one peak in the data. More detailed analysis performed over the last year was able to detect two additional fainter peaks in the data.

"Using a music analogy, last year we could tell what note we were seeing -- if it was C sharp or F flat," explained Andrew Lange of Caltech. "Now, we see not just one, but three of these peaks and can tell not only which note is being played, but also what instrument is playing it -- we can begin to hear in detail the music of creation."

The BOOMERanG results are supported by observations from two other instruments: the Millimeter Anisotropy Experiment Imaging Array (MAXIMA), another balloon-borne detector that flew over Texas in 1998, and the Degree Angular Scale Interferometer (DASI), a groundbased instrument located at the South Pole. Those instruments also see the harmonic peaks in the CMB consistent with inflation.

"We're very excited about this because the results give us a strong indication of the magnitude of temperature fluctuations on this scale," said University of Minnesota professor Shaul Hanany, one of the leaders of the MAXIMA project. "The observed magnitude is consistent with what inflation predicts."

"If no peaks had shown up, inflation would have difficulties," added Carlstrom, who led the DASI team. "We'd be back to the drawing board."

In addition to supporting inflation, the data from DASI also provides a measure for the amount of ordinary matter in the universe: just 4.5 percent of the overall mass and energy of the universe, based on the ratio of the intensities of the first two peaks in the CMB. The rest, astronomers believe, is tied up in either dark matter or a mysterious dark energy that may explain the accelerating expansion rate of the universe today.

"The big bang framework and Einstein's general relativity have passed a major new test," said Michael Turner of the University of Chicago.

More work is needed to better understand not only the variations in the CMB but also why the universe underwent such a major growth spurt in an almost infinitesimal fraction of a second. Scientists are already analyzing data from a second MAXIMA balloon flight in 1999, and another BOOMERanG balloon flight is in the planning stages. In addition to the Earth-based observations, a new spacecraft, the Microwave Anisotropy Probe (MAP), will soon study the CMB to resolutions of one millionth of a degree from the Earth-Sun L2 Lagrange point, 1.5 million kilometers from the Earth. The spacecraft is scheduled for launch on a Delta 2 from Cape Canaveral on June 30.

"This is just the beginning," said Turner. "Not only will we be able to test inflation, but we will be able to learn about its underlying physical cause."