By Ron Cowen
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 a cosmic mirage — if humans live in a special place in the universe with a peculiar distribution of matter.
If Earth and its environs are centered in a vast, billion-light-year-long bubble, relatively free of matter, in turn 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.