'Dark Energy' Might Not
Exist Say Scientists

Special to World Science

A growing number of researchers claim a mysterious "dark energy," which most cosmologists believe fills space, might not exist.
Instead, they say, the laws of gravity might need some correction.
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)
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. 
Cosmologists dubbed the stuff "dark energy." But the dark energy itself has no obvious explanation, leaving scientists puzzled.
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.
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.
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.
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." 
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. 
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.
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.
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."
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.
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. 
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. 
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.
But why should the laws of gravity be changed? 
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.
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.
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. 
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.
Exactly how these dimensions would be rolled or crumpled up is unknown. 
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.
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. 
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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