February 11, 2008
New Cosmic Theory Unites Dark Forces
The two biggest mysteries in cosmology may be one. A new theory says that dark matter and dark energy could arise from a single dark fluid that permeates the whole universe. And this could mean Earth-based dark matter searches will come up empty.
Dark matter, as originally hypothesized, is extra hidden mass that astrophysicists calculate is necessary for holding together fast-turning galaxies. The most popular notion is that this matter is made of some yet-to-be-identified particle that has almost no interactions with light or ordinary matter. Yet it seems to be everywhere, acting as a scaffolding for galaxy clusters and the whole structure of the universe.On the other hand, dark energy is needed to explain the more recently-discovered acceleration of the universe's expansion. It supposedly exists all throughout space, delivering a pressure that counteracts gravity.
It's counterintuitive that one substance could be both a gravitational anchor for galaxies and anti-gravity force for the universe. However, HongSheng Zhao of the University of St Andrews in Scotland claims that a fluid-like dark energy can act like dark matter when its density becomes high enough.
"Dark energy is a property of the vacuum — of fields that we do not easily see," Zhao told Space.com. "From it, we can derive the dark matter effect."
Zhao compares this dark fluid to Earth's atmosphere. Atmospheric pressure causes air to expand, but part of the air can collapse to form clouds. In the same way, the dark fluid might generally expand, but it also could collect around galaxies to help hold them together.
Zhao is not the first theorist to try to bring dark energy and dark matter under the same framework.
The type of dark fluid that Zhao is looking at is similar to one that Pedro Ferreira of the University of Oxford and his colleagues devised a few years ago.
"[Our theory] involves positing a preferred time direction, in some sense a special time frame," Ferreira said. "It has the interesting effect of modifying Einstein's theory of general relativity."
The idea is similar to the "ether," an invisible medium that physicists once thought light waves travelled through. Einstein's relativity did away with the need for such a medium, but cosmologists have recently found that an ether-like substance can mimic dark matter.
The presence of such a substance changes the way gravity works. This is most noticeable in the distant outskirts of a galaxy, where the galaxy's gravitational pull would be expected to be small, but the ether makes it much stronger.
The ether "effectively softens space-time in regions of low [gravitational] acceleration making it more sensitive to the presence of mass than usual," Ferreira explained.
Zhao has refined this approach and found that it can match a lot of astronomical data, as reported in a recent article in Astrophysical Journal Letters.
"I like [Zhao's model] because it shows that these theories are predictive and, if worked out in detail, can be tested properly against experiment," Ferreira said.
For one, Zhao's fluid divides itself into a dark energy part and a dark matter part with the same ratio that is seen from observations (dark energy is about 75 percent of the universe's mass-energy content, while dark matter is about 21 percent and normal matter makes up the last 4 percent).
Although the fluid is all around us, Zhao found that it does not affect the motion of Earth or the other planets, which is "reassuring," he said, because data shows that our solar system obeys traditional gravity to very high accuracy.
But the fluid does affect the speed at which galaxies can rotate. Some 75 years ago, astronomers noticed that galaxies were turning faster than would be expected from the amount of normal light-emitting matter they contained. The answer seemed to require some form of unseen dark matter.
However, Zhao has shown that his fluid can keep galaxies from flying apart just as well as dark matter can.
Zhao has also tested his model against the bullet cluster of galaxies, where a massive collision appears to have stripped hot gas from its dark matter envelope. This "naked" dark matter was seen as iron-clad proof for traditional dark matter theories, but Zhao claims that his fluid can reproduce the same effect.
Christian Boehmer from University College London thinks it "compelling" that Zhao's model can reproduce so much galaxy data.
If the dark fluid is mimicking dark matter, then scientists are searching in vain for the elusive dark matter particle, often called a WIMP (for weakly interacting massive particle).
Currently, several experiments are trying to detect a rare collision of a WIMP on Earth or observe gamma rays from distant WIMP self-annihilations.
"Direct detections will be more difficult," Zhao said. WIMPs may still exist, but there won't be as many of them as predicted.
Without WIMPs to worry about, the dark fluid could make scientists' jobs easier.
But not many cosmologists are ready to abandon dark matter just yet. The dark fluid idea is still fairly new, so some issues have yet to be worked out, whereas dark matter is a fairly mature theory.
"The current [dark matter] model provides the best fit to the data and is therefore the best model at hand," Boehmer said.
However, Boehmer agrees that having two unknowns — dark matter and dark energy — make up 95 percent of the universe is a bit embarrassing for cosmology.
"Frankly speaking, these are just fancy words we use to name something we do not understand," he said.
If a simpler model (with a single word) can explain all the data, then cosmologists will gladly accept it, Boehmer said.