- A growing number of researchers claim a mysterious "dark
energy," which most cosmologists believe fills space, might not exist.
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- Instead, they say, the laws of gravity might need some
correction.
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- A supercomputer-produced cross-section of part of the
universe shows galaxies as brighter dots along filaments of matter, with
a sea of "dark energy" filling in between the galactic islands.
But some researchers question whether the dark energy exists. (Credit James
Wadsley, McMaster University, Hamilton, Ontario)
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- Scientists have accepted the existence of the enigmatic
energy since 1998, when astronomers found the universe is expanding faster
and faster. The best explanation seemed to be that it's filled with some
unseen substance that repels itself, and thus pushes relentlessly outward.
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- Cosmologists dubbed the stuff "dark energy."
But the dark energy itself has no obvious explanation, leaving scientists
puzzled.
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- In a new paper, three researchers say they can account
for the accelerated expansion by tweaking the laws of gravity, with no
need for dark energy.
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- These modifications, some theorists argue, are natural
results of certain advanced theories of space and time that include extra,
unseen dimensions. But the idea remains speculative.
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- Nonetheless, "the current accelerated expansion
of the Universe can be explained without resorting to dark energy,"
wrote Olga Mena of Fermi National Accelerator Laboratory in Batavia, Ill.,
and colleagues in the paper. It appeared in the Feb. 3 issue of the research
journal Physical Review Letters.
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- The proposal bears an odd parallel to another modified-gravity
theory that has emerged in recent years, and which seeks to explain another
dark entity: "dark matter."
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- Not unlike dark energy, dark matter is an unseen substance
that astronomers believe pervades the cosmos, but it is different. Dark
matter, which would comprise more than 90% of the weight of the universe,
is thought to betray its existence through its gravitational pull on nearby
objects.
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- Somewhat similarly to what is happening with dark energy,
some cosmologists have also devised modified-gravity theories in past years
to explain these phenomena. But so far these alternative dark-matter explanations
have left most scientists unconvinced.
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- To make things more convoluted, Mena's alternative-gravity
proposals are different from those that have been used to explain away
dark matter, she said: the two aren't even compatible. But she added that
her research is still preliminary, and her team has a range of possibilities
to explore, which could be compatible.
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- The ideal would be to devise a model that combines the
goals of both sets of alternative-gravity theories-eliminating the need
for dark anything, Mena said: "That would be a dream."
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- The accelerated cosmic expansion, which prompted the
dark energy idea, was detected in 1998 through observations of distant
exploding stars known as supernovae. Two separate groups found supernovae
that were dimmer, and thus further away, than they should have been if
the universe was expanding at a steady rate, as was then believed.
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- The key to the proposal from Mena's team is that gravity
is modified in such a way that the change is noticeable only at the largest
distance scales-the only scales over which the accelerated expansion is
evident.
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- The modifications are arranged so that gravity becomes
weaker as space becomes more "flat." A flat space is one in which
ordinary laws of geometry apply.
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- The universe isn't flat over most of the distance scales
astronomers work with, because as Einstein showed, massive things like
galaxies, stars and even planets bend space and time. But over vast distances,
such as across the observable universe, these fluctuations are averaged
out. Tests have found that on these scales, space is flat or nearly so.
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- But why should the laws of gravity be changed?
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- It turns out that there are justifications for doing
so, beyond just explaining away the inconvenient cosmic acceleration, said
Fermilab's José Santiago, a co-author of the paper with Mena. These
arise from some advanced theories of physics that propose the existence
of extra dimension beyond those we can see.
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- The most popular of these theories are the various versions
of string theory. These attempt to solve the mysteries of physics by bridging
the two most successful modern theories of physics in modern times, general
relativity and quantum mechanics. The two seem to give solid explanations
for the behavior of things over vast distances and tiny spaces, respectively.
But they conflict with each other.
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- String theory claims all the particles of nature are
actually different vibrations of unseen, tiny loops called "strings."
The theory mathematically fixes the major inconsistencies between the relativity
and quantum mechanics, and shows an underlying unity in nature's various
forces to boot.
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- But it works only if the strings have several extra dimensions
in which to vibrate beyond those we see. Different versions of string theory
propose the existence of 10 or 26 dimensions. It's thought that the ones
we don't see may be hidden from our view because they're rolled into tiny
balls.
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- Exactly how these dimensions would be rolled or crumpled
up is unknown.
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- But some researchers have calculated "that some
specific compactifications of extra dimensions can lead to such modifications
of gravity" as the one in the no-dark-energy proposal, Santiago wrote
in an email.
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- His team isn't the first to say this could solve the
accelerating universe problem; Fermi's Sean Carroll and others have made
similar proposals. But Mena said the new paper contains some problems with
earlier proposals, which conflict with the action of gravity at shorter
distances.
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- The team also conducted some rather grueling calculations
to approximate just how much acceleration the new model predicts, Mena
added, and compared the result to the supernova data. "It provides
an excellent fit," she said.
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