June 17, 2008
A Special Place in the Universe
By Cowen, Ron
Scientists propose tests of Copernican principle For all the hand wringing among physicists about the nature of dark energy, the invisible stuff that appears to be revving up the rate of cosmic expansion, a nagging possibility remains. Dark energy could be acosmic mirage - if humans live in a special place in the universe having a peculiar distribution of matter.
If Earth and its environs are centered in a vast, billion-light- year-long bubble, relatively free of matter, and that bubble is surrounded by a massive, dense shell of material, then gravity's tug would cause galaxies inside the void to hurtle toward the spherical concentration of mass, say theorists Robert Caldwell of Dartmouth College and Albert Stebbins of the Fermi National Accelerator Laboratory in Batavia, Ill. That process would mimic the action of dark energy - a local observer would be tricked into thinking that the universe's expansion is accelerating.
But that scenario violates the Copernican principle, a notion near and dear to the hearts of physicists and cosmologists, including Caldwell and Stebbins. Named after the 16th century astronomer Nicolaus Copernicus, who made the then heretical proposal that Earth does not have a favored, central position in the solar system, the principle states that humans are not privileged observers in the universe, but have just as good - or bad - a vantage point as any other observer in the cosmos.
"Although the Copernican principle may be widely accepted by fiat, it is imperative that such a foundational principle be proven," Caldwell and Stebbins assert in the May 16 Physical Review Letters. The researchers suggest a concrete way to check whether our neck of the cosmic woods is different from other parts of the universe. Their test relies on the cosmic microwave background radiation that bathes all parts of the universe.
If Earthlings were at the center of a bubble, the spectrum of microwave background radiation that came directly to Earth - without reflection - would trace a blackbody radiation curve. A blackbody emits all light that falls on it, and its spectrum depends only on its temperature (2.7 kelvins for the microwave background).
But another observer, not centered in the bubble, would see an asymmetric universe, with a matter-free region - the bubble -off to one side. This lopsided distribution of matter would leave its imprint on the microwave background. The photons would have various energies, depending on whether they originated in a high- or low- density region, and the resulting spectrum would no longer look like a blackbody's.
Two observers, two separate views of the universe, and never the twain shall meet. Except that the two views are not separate, Caldwell and Stebbins calculate. Electrons floating through the universe act like tiny mirrors, reflecting some of the microwave photons seen by other observers back toward Earth. If humans live in a special place, the microwave background will contain tiny deviations from a perfect blackbody spectrum.
Those deviations would be too small to show up in the most precise measurements of the microwave background recorded so far. But a proposed satellite, the Absolute Spectrum Polarimeter, could easily detect such deviations, says Alan Kogut of NASA's Goddard Space Flight Center in Greenbelt, Md. The mission could be launched next decade.
In the same issue of Physical Review Letters, Jean-Philippe Uzan of Pierre and Marie Curie University in Paris, along with Chris Clarkson and George Ellis of the University of Cape Town in South Africa, suggest a different way to test the Copernican principle.
As dark energy speeds up cosmic acceleration, the recession velocities of galaxies would change, as indicated by a shift in wavelength of their light to redder wavelengths, the scientists note. By measuring both redshifts and distances to remote galaxies over a 10-year span, astronomers should be able to tell whether we live in a Copernican universe.
Copyright Science Service, Incorporated Jun 7, 2008
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