Dark energy 'imaged' in best detail yet

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London, May 24 : Scientists have imaged dark energy, one of the most mysterious matters in the universe, in best detail yet.

According to a report in New Scientist, Istvan Szapudi from the University of Hawaii in Honolulu, US, and colleagues, did an independent check on the existence of dark energy.

They 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.

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," said Szapudi.

To get around this, his team looked at regions of extremely high and extremely low density, where one can 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.

Szapudi's 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," said Szapudi. "In this sense, we have imaged dark energy," he added.


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