Most Direct Evidence of Dark Energy Detected

Ker Than
for National Geographic News
August 11, 2008
A new color-coded image represents the first visual evidence of the existence of dark energy, a mysterious force that astronomers think is causing the expansion of the universe to speed up.

"This is the first time when we actually see the effect of dark energy in a picture," said study leader István Szapudi of the University of Hawaii. "This is the most direct evidence of dark energy."

(Related: "At Ten, Dark Energy 'Most Profound Problem' in Physics" [May 16, 2008].)

The new image reveals the spectral fingerprints created by dark energy as it stretches huge supervoids and superclusters, structures that are roughly half a billion light-years across.

Superclusters are filled with dense clusters of galaxies, while supervoids are made up of mostly empty space.

According to the team, there is only a 1-in-200,000 chance that their detection of dark energy's fingerprints happened randomly.

Hot and Cold

Using mapping data gathered by the Sloan Digital Sky Survey, the team found evidence that dark energy alters ancient microwave radiation as it passes through superclusters and supervoids.

This diffuse radiation, called the cosmic microwave background, is the faint buzz of microwaves left over after the big bang.

Theory had predicted that as the universe expands at a constant rate, microwaves should gain energy as they enter a supercluster and lose an equal amount of energy as they leave.

The reverse should happen as microwaves travel through supervoids.

But if dark energy is causing the universe's expansion to accelerate, superclusters and supervoids should flatten out over time relative to the radiation.

Microwaves passing through superclusters would thus keep some of the gained energy as they leave, while microwaves passing through supervoids would lose energy.

The effect would create hot and cold regions in maps of the cosmic microwave background, a temperature difference that is visible in the new image as red and orange or blue areas.

Such an influence of dark energy on the microwave background has been observed before. But previous studies gave only a 1-in-20 chance that the effect was real and not the result of random temperature fluctuations.

Szapudi's team examined 50 superclusters and 50 supervoids and found with a high degree of precision that the amount of microwave heating and cooling is consistent with an accelerating universe.

"The most plausible explanation is dark energy," Szapudi said.

(Related: "Future Universe Will 'Stop Expanding,' Experts Suggest" [June 4, 2007].)

The research will be detailed in an upcoming issue of the Astrophysical Journal Letters.

Stronger Case

Adam Riess is an astrophysicist at the Space Telescope Science Institute in Baltimore, Maryland. His team published one of the first papers on dark energy more than a decade ago.

Riess said that while the new study strengthens the case for dark energy's existence, it sheds little light on the nature of the mysterious force.

"All of the studies of this generation are good enough to just see the presence of dark energy, but usually not enough to nail down its properties to great precision," Riess said. "I think this study falls into that category."

Robert Kirshner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, agreed.

"These measurements are too new for us to know if they will help determine the properties of dark energy," Kirshner said.

"But it is always good to have another route to show that dark energy is real."

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