even Dark Energy is Not, a notion of some gravity: I kid you not

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" Dark energy " might not exist, scientists say

 

Feb. 14, 2006

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.

 

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 corrects 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.

 

a posting by.....bob