- 19:40 23 May 2008
- NewScientist.com news service
- Zeeya Merali
Supervoids of empty space (blue) and superclusters of galaxies (red) were slightly cooler or warmer, respectively, than expected in maps of the cosmic microwave background because of dark energy (Illustration: Granett/Neyrinck/Szapudi/NASA/SDSS)
Some had hoped it might be just an illusion. But it looks like dark energy is real and here to stay, as astronomers "image" the mysterious entity in action.
In 1998, astronomers found that distant supernovae were dimmer, and thus farther away, than expected. This suggested that the expansion of the universe is accelerating – and "dark energy" was named as the culprit.
Since then, astronomers have struggled to explain what dark energy actually is – leading some to speculate that it may not exist at all. Some have claimed that uneven distributions of matter within the universe could distort our measurements of the distance to supernovae, fooling us into thinking that they are farther away than they are.
To find an independent check on the existence of dark energy, István Szapudi at the University of Hawaii in Honolulu, US, and colleagues turned to the cosmic microwave background (CMB) – relic radiation from the big bang. Detailed maps of the CMB show hot and cold spots that reflect variations in the density of the early universe.
When dark energy was proposed, astronomers realised that it should create additional temperature bumps on the map.
This extra dark energy effect is generated because the temperature of a photon zipping across the universe can be changed depending on whether it has passed through a region of dense matter or a sparser region.
Shallow well
A photon gains energy when it enters a dense region with enhanced gravity – such as a galaxy cluster – as though it is falling into a well. When it leaves the cluster and climbs back out of the gravitational well, it loses energy.
In a universe without dark energy, the energy gained and lost during the crossing would be equal and would cancel out. But in the presence of dark energy, the universe expands quickly enough to stretch the gravitational well while the photon is still inside. This makes the well shallower and easier for the photon to climb out.
That means that a photon travelling through a cluster gains more energy than it loses, giving it a little energy kick so that it creates a hotter spot than would be expected on images of the CMB. Similarly, a photon that has passed through a void would leave a cold spot.
It's tough to detect this effect because dark energy gives only a slight nudge to the temperature, which is easily swamped by the normal temperature variations seen in the CMB, says Szapudi.
Extreme density
To get around this, his team looked at regions of extremely high and extremely low density, where you would expect to see the biggest effect.
Using data from the Sloan Digital Sky Survey, they chose over 3000 superclusters of galaxies and 500 "supervoids" of relatively empty space, and they found that the regions did indeed tally with enhanced hot and cold spots in the CMB.
Other teams have reported signs of this effect in the past, but those have been open to alternative explanations, says Szapudi. By contrast, his calculations suggest that there is less than a 1 in 200,000 chance that the match up his team saw is down to anything other than dark energy.
"We have shown the imprint on the CMB of dark energy at work," says Szapudi. "In this sense, we have imaged dark energy."
The finding should rule out any notion that dark energy is an illusion, he says. "We've really attacked the dark energy question in a different way from supernovae measurements. It's difficult to argue that an illusion could be responsible for this effect."
Warped space
Thomas Buchert at the University of Lyon in France, one of the physicists who suggested that dark energy may be an illusion, is impressed by the thoroughness of Szapudi's work. "People have discussed this effect, but this is the first really clear-cut signal," he says.
However, Buchert is not ready to abandon alternatives to dark energy just yet. He points out that a similar effect might be produced if space-time is significantly warped around the voids and clusters - something that isn't usually considered in standard calculations.
He also notes that Szapudi's team observed a temperature effect that was stronger than expected from dark energy. "This could be a hint that there may be something else happening here that's worth investigating," he says.
