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 reigned 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.
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