Is Earth at the heart of a giant cosmic void?

12 November 2008 by Marcus Chown
NewScientist | Magazine issue 2682.

IT WAS the evolutionary theory of its age. A revolutionary hypothesis that undermined the cherished notion that we humans are somehow special, driving a deep wedge between science and religion. The philosopher Giordano Bruno was burned at the stake for espousing it; Galileo Galilei, the most brilliant scientist of his age, was silenced. But Nicolaus Copernicus's idea that Earth was just one of many planets orbiting the sun - and so occupied no exceptional position in the cosmos - has endured and become a foundation stone of our understanding of the universe.

Could it actually be wrong, though? At first glance, that question might seem heretical, or downright silly. But as our cosmic horizons have expanded over the centuries so too has the scope of Copernicus's idea. It has morphed into the Copernican, or cosmological, principle: that nothing distinguishes the position of Earth's galaxy from any other place in the entire universe. And that idea, some cosmologists point out, has not been tested beyond all doubt - yet.

That could be about to change. A new generation of experiments might shore up the cosmic orthodoxy - or blow it out of the water. That unexpected alternative, some people go so far as to say, might be no bad thing at all.

The modern-day Copernican principle amounts to two assumptions. First, that averaged over large enough scales the universe is homogeneous, having essentially the same properties in all locations. Second, that the universe is isotropic, or appears to have the same properties when viewed in any direction from every location. These two ideas are intimately related, but logically separate (see diagram). They were introduced into cosmology not because of any observational evidence, but to save face. In 1917, Albert Einstein had applied his theory of gravity - general relativity - to the dynamics of the universe. Without the simplifying assumptions of homogeneity and isotropy, Einstein's fiendishly complex equations proved impossible to solve.

Even with those assumptions, Einstein's initial insistence that we live in an unchanging universe led him to the wrong solutions. By dropping the "unchanging universe" requirement a few years later, cosmologists created the picture that became the kernel of today's phenomenally successful big bang model. In this picture, the universe started out as a single, infinitely hot and dense point in space, and has since been expanding - initially rapidly, but gradually more slowly as gravity has exerted its pull on the mass of the cosmos.

All seemed well, with evidence in support of the big bang model piling up throughout the 20th century. Then, in 1998, astronomers studying stellar explosions known as type 1a supernovae made a sensational discovery. These supernovae are thought to be uniformly bright, so that the fainter they appear to us, the farther they must be away. But measurements showed that the most distant supernovae did not fit in: they were a lot fainter than they should have been, and seemed impossibly far away. Some time over the past few billion years, they must have begun to race away from us ever faster. Rather than the universe's expansion slowing down, it looked like it was speeding up.

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