Do We Live in a Giant Cosmic Bubble?

Clara Moskowitz, Staff Writer, SPACE.com 

If the notion of dark energy sounds improbable, get ready

for an even more outlandish suggestion.

Earth may be trapped in an abnormal bubble of space-

time that is particularly void of matter. Scientists say this

condition could account for the apparent acceleration of

the universe's expansion, for which dark energy currently

is the leading explanation.

Dark energy is the name given to the hypothetical force

that could be drawing all the stuff in the universe outward

at an ever-increasing rate. Current thinking is that 74

percent of the universe could be made up of this exotic

dark energy, with another 21 percent being dark matter,

and normal matter comprising the remaining 5 percent.

Until now, there has been no good way to choose

between dark energy or the void explanation, but a new

study outlines a potential test of the bubble scenario.

If we were in an unusually sparse area of the universe,

then things could look farther away than they really are

and there would be no need to rely on dark energy as an

explanation for certain astronomical observations.

"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.

"This idea that we live in a void would really be a

statement that we live in a special place," Clifton told

SPACE.com. "The regular cosmological model is based

on the idea that where we live is a typical place in the

universe. This would be a contradiction to the Copernican

principle."

Clifton, along with Oxford researchers Pedro G. Ferreira

and Kate Land, say that in coming years we may be able

to distinguish between dark energy and the void. They

point to the upcoming Joint Dark Energy Mission, planned

by NASA and the U.S. Department of Energy to launch in

2014 or 2015. The satellite aims to measure the

expansion of the universe precisely by observing about

2,300 supernovae.

The scientists suggest that by looking at a large number

of supernovae in a certain region of the universe, they

should be able to tell whether the objects are really

accelerating away, or if their light is merely being

distorted in a void.

The new study will be detailed in an upcoming issue of the

journal Physical Review Letters.

·           Top 10 Strangest Things in Space 

·           Nearby Evidence for Dark Energy 

·           Images: Hubble's New Views 

轉貼自︰

http://news.yahoo.com/s/space/20080930/sc_space/doweliveinagiantcosmicbubble;_ylt=AtqWZBdQrgSUcdOKSvjVU6Ybr7sF