universe_mass_010307 A team of researchers that looked at data from more than 140,000 galaxies says the universe is far from heavy and has a density of next to nothing.
"It's about 300 billion billion billion times less dense than water," said John Peacock of the University of Edinburgh, "or one ten-thousandth of an ounce in a volume the same size as the Earth."
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Peacock and his colleagues got the answer using an instrument on the Anglo-Australian Telescope that can detect the light from 400 galaxies simultaneously over a field of view four times as wide as the full Moon. The research is part of a massive effort to create a three-dimensional map of the universe.
The instrument shows the "
redshift" of each galaxy (i.e., the extent to which galactic light turns reddish as the galaxy moves away from the observer), thereby suggesting the galaxies' relative distance to Earth. That determination leads to the calculation of each galaxy's density and mass, and also those properties of the universe. The results were published in the March 8, 2001, issue of the journal Nature.
Supercluster collapse
The analysis also claims to clarify astronomers' understanding of the structure of the universe throughout which galaxies are sprinkled not randomly, but in clusters that glom together in "superclusters."
Peacock and his colleagues found that superclusters form, as suspected, from the inward gravitational collapse of galaxies upon one another.
To get the density figure, the team looked at "peculiar" velocities created when the superclusters tug and pull on nearby galaxies whizzing by. That led them to calculate that the universe is one third as populated as the calculated density that would halt an expanding universe. Most theorists now agree the universe is expanding as a result of the Big Bang -- the explosive instant thought to have given birth to the universe.
"The actual figure is tiny," said Peacock, "about 3 times 10^{-27} kilograms per cubic meter." That's much, much lighter -- a fraction of a three kilograms with 27 zeroes in front of the decimal point -- than a dust speck in a bucket the size of a wide-screen TV for a typical home.
Survey finished later this year
The results come from an ambitious effort, called the
2dF Galaxy Redshift Survey, to detect light simultaneously from hundreds of galaxies at a time. Eventually, the survey, conducted by British and Australian researchers, will yield an inventory of about 10 percent of the galaxies in Earth's cosmic neighborhood. It will map one-tenth of the galaxies within 2 billion light-years of Earth (a light-year is the distance light travels in a year -- about 6 trillion miles or 9.7 trillion kilometers).Last year, project scientists used earlier data from the survey to show that the universe will
continue to expand forever, rather than end in a catastrophic collision like the opposite of the Big Bang. Peacock is optimistic that future analyses of 2dF data will yield the "fluctuation spectrum" -- jargon for how clumpy the distribution of galaxies is in the universe.
"This can not only give us another independent way to measure the mass density," he said, "it can also tell us what fraction of this mass is in the form of ordinary matter (particles like electrons and protons -- the same as you are), as opposed to exotic elementary particles left over as relics from the earliest phases of the Big Bang."
The survey, which should be done later this year after completing the logging of data on 250,000 galaxies, has involved a vast array of astronomical talent, including instrument designers and engineers, observational astronomers and theorists who have cranked out statistics on the results. (Other sky surveys currently under way include the 2MASS Redshift Survey and the Sloan Digital Sky Survey.)
"The end result is something that no individual could have produced unaided," Peacock said.
Marc Davis of the University of California, who wrote an accompanying article in Nature on the new study, heralded the analysis as moving cosmology to a new and more sophisticated level of analysis.
"Cosmology, long considered a branch of philosophy rather than physics because of the dearth of data, has made dramatic progress in the past few years and is now entering an era of large-scale studies and precision measurements," he said.
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