Posted on 04/26/2005 3:46:54 PM PDT by neverdem
n a kind of belated birthday present to Albert Einstein, whose theory of relativity is 100 years old this year, astronomers say they have confirmed an essential but previously unconfirmed prediction of general relativity, namely that the entire universe can act as a magnifying lens.
The light from distant quasars, enigmatic and violent galaxy-birthing events on the shores of time, some 10 billion light-years away, has been magnified by the gravitational force of lumps and irregularities in the structure of the nearby cosmos. So the quasars appear slightly brighter in telescopes than they actually are, according to a multinational team of researchers led by Dr. Ryan Scranton of the University of Pittsburgh.
They reached that conclusion after sifting a mountain of data about 13 million galaxies and other celestial objects, obtained by the Sloan Digital Sky Survey, a continuing effort to remap the heavens.
The magnification, they said, confirms the dark picture cosmologists have built up in the last few years, in which the atoms that make up stars and people are overwhelmed by clouds of mysterious dark matter and that matter is in turn overwhelmed by something even stranger, so-called dark energy, which seems to be wrenching space and time apart faster and faster, taking the galaxies for a potentially fatal ride into endless cold and loneliness.
"This is all hanging together," said Dr. Scranton, lead author of a paper that will be published in The Astrophysical Journal and is being posted today on the physics Web site, www.arXiv.org. The astronomers said that cosmic magnification gave them a new way to weigh the universe and to investigate its evolution.
Dr. Robert Nichol, a member of the team from the University of Portsmouth in England, said, "In this year of Einstein, it is a wonderful demonstration of the power of general relativity as it shows that light travels to us on a very 'bumpy road' from these quasars."
Dr. Tony Tyson, a gravitational lensing expert at the University of California, Davis, said: "It is nice to close the loop on Einstein here. This all traces back to Einstein's prediction of light bending."
Einstein's theory, promulgated in 1915, and spectacularly supported by observations during a solar eclipse in 1919, ascribes the effect we call gravity to the warping of space-time geometry by matter or energy. As one consequence, the theory says, lumps of mass, like a star, a galaxy or a whole cluster of galaxies with their attendant clouds of dark matter, can act as a gravitational lens, magnifying very distant objects.
In recent decades astronomers have recorded instances in which the images of galaxies or quasars have been distorted into arcs or rings or even split into multiple images by the gravity of intervening galaxies, so-called gravitational lenses. And they have seen individual stars appear to flare as their light is magnified by the gravity of a passing star.
Astronomers have observed a distortion in the shapes of distant background galaxies, known as cosmic shear, because of the large-scale structure of the universe, but until now they have not been able to make a reliable measurement of the cosmic magnification factor.
Now, in the Sloan survey, said Dr. Nichol, "we see the summed magnification of all quasars by the intervening masses in the universe." He estimated that mass, the contents of the universe out to about 2.5 billion light-years, as roughly equivalent to 100 billion billion Suns.
"This is a measurement that's been hanging around in the background," said Dr. Scranton, who added that earlier attempts at the measurement had resulted in "a huge mess" because data about the heavens lacked the needed precision. The results, he said, did not match the standard cosmological theory or one another.
The present calculation was based on a sample of 13 million galaxies and 200,000 quasars derived from the mammoth Sloan survey. Begun in 1998 with a custom built telescope at Apache Point Observatory in New Mexico, the survey was created to measure the colors and brightnesses of several hundred million objects over a quarter of the sky and map the distances to a million galaxies and quasars.
"We have the biggest set of quasars ever assembled," Dr. Scranton said. The quasars, thought to be black hole fireworks in young galaxies, are all out about 10 billion light-years away, their light has been on the way to us since the universe was 4 billion years old.
The Sloan galaxies are in front of them, roughly 2.5 billion light-years away.
Because quasars are wildly erratic and far, far away, there is no way to tell by how much any one of them has been magnified. As a result astronomers resorted to statistical methods, looking for correlations between the numbers of quasars and the locations of galaxies on the sky.
Quasars too faint to show up normally should pop into view near galaxies that have amplified the quasars' light, swelling the counts. But, as Dr. Scranton explained, there is a competing effect at work. The bending of light rays by those same galaxies will also cause the quasars to appear to be displaced outward slightly from the galaxy lowering their apparent density on the sky.
The two effects can only be separated and the magnification confirmed after the quasars and galaxies have been sifted and resifted according to their colors and apparent brightnesses by powerful computer codes.
The detection of the magnification is a triumph of computer science as well as astronomy, the astronomers say.
Because both dark and visible matter contribute to gravity and thus to the cosmic magnification, Dr. Scranton said, astronomers can use the effect to investigate the dark side of the universe, looking into questions like how galaxies form and whether galaxies and dark matter coincide.
So far, he said, "The galaxies basically trace the dark matter very clearly."
He added, "It would be nice if it were more exotic, but its pretty much another brick in the structure," he added, referring to the so-called concordance cosmology of dark matter and dark energy.
Dr. Max Tegmark, a cosmologist at the Massachusetts Institute of Technology, called the detection of cosmic magnification "a big deal," and said, "I think it will emerge as a powerful cosmological tool."
Dr. Tyson, however, said he doubted that cosmic magnification would emerge as an important cosmological tool. It might be easier, he said, to get information from cosmic shear, the distortion of distant galaxies, noting that there are many more of those than of quasars. Dr. Tyson heads a group that is planning to build a large telescope and camera, known as the Large Synoptic Survey telescope, to do just that.
But, he added, referring to the prospects for cosmic magnification, "I would be very pleased to eat my hat on that one." The Sloan survey, he said, shows what you can do with a large well-controlled astronomical survey.
"The sky's the limit," he said.
Fermilab/Photo Researchers
The Sloan Digital Sky Survey telescope was designed to create a map of the sky.
Joerg Colberg, Ryan Scranton and Robert Lupton/S.D.S.S.
A grid of points representing background quasars and, above, the same grid after being "gravitationally lensed" by the cluster of galaxies in the center of the panel.
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The light from distant quasars, enigmatic and violent galaxy-birthing events on the shores of time, some 10 billion light-years away, has been magnified by the gravitational force of lumps and irregularities in the structure of the nearby cosmos.
This article is so poorly written I don't know if I should scratch my watch or wind my butt.
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