Could Earth be in a giant cosmic bubble?

Scientists dusted off the "bubble" theory as an alternative to dark energy to explain the apparent acceleration of the universe's expansion. Researcher Timothy Clifton of Oxford University in England, along with other Oxford researchers, say we might live in a region of space with less matter in it than normal.

If we really live in an area of the universe void of matter, then things could look farther away than they really are and the space-time itself wouldn't be accelerating. Until now there was no good way to choose between the dark energy or the void explanation, but the researchers say we might get an answer from the Joint Dark Energy Mission satellite which will launch in 2014 or 2015. This spacecraft aims to measure the expansion of the universe precisely by observing about 2,300 supernovae.

"If we lived in a very large under-density, then the space-time itself wouldn't be accelerating," said researcher Timothy Clifton of Oxford University in England. "It would just be that the observations, if interpreted in the usual way, would look like they were."

Scientists first detected the acceleration by noting that distant supernovae seemed to be moving away from us faster than they should be. One type of supernova (called Type Ia) is a useful distance indicator, because the explosions always have the same intrinsic brightness. Since light gets dimmer the farther it travels, that means that when the supernovae appear faint to us, they are far away, and when they appear bright, they are closer in.

But if we happened to be in a portion of the universe with less matter in it than normal, then the space-time around us would be different than it is outside, because matter warps space-time. Light travelling from supernovae outside our bubble would appear dimmer, because the light would diverge more than we would expect once it got inside our void.

One problem with the void idea, though, is that it negates a principle that has reined in astronomy for more than 450 years: namely, that our place in the universe isn't special. When Nicholas Copernicus argued that it made much more sense for the Earth to be revolving around the sun than vice versa, it revolutionized science. Since then, most theories have to pass the Copernican test. If they require our planet to be unique, or our position to be exalted, the ideas often seem unlikely.

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