SYDNEY: Astronomers say they have detected the clearest evidence yet for the mysterious dark energy that stretches and flattens out the universe.

The research, led by astrophysicist Dr István Szapudi of the Institute for Astronomy at the University of Hawaii in Honolulu, is published online on the arXiv.org physics website ahead of publication in Astrophysical Journal Letters.

Little known but pervasive

Dark energy is a little understood type of energy that pervades space and tends to increase the rate of expansion of the universe.

The researchers looked for signs of it in the Cosmic Microwave Background Radiation (CMB) as mapped by the Sloan Digital Sky Survey (SDSS), which aims to map one-quarter of the universe. The CMB is made up of the relict microwaves left over when photons interacted with newly recombined atoms soon following the Big Bang.

While the CMB has remained relatively unchanged since the beginning of the universe, subtle variations can illuminate how radiation travels through regions of space.

Like a watery pudding, the universe is split on a massive scale into dense and less dense areas. Dense regions where galaxies cluster are called superclusters and relatively empty areas of space are called supervoids.

The researchers looked at 50 of the largest superclusters and 50 of the largest voids and measured the strength of the microwave radiation travelling through these regions.

Dark energy in action

As expected, the survey showed that microwaves were stronger if they passed through a supercluster and weaker if they passed through a supervoid.

"We were able to image dark energy in action, as it stretches huge supervoids and superclusters of galaxies," Szapudi said.

"When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster. Later, as it leaves the supercluster, it should lose exactly the same amount of energy," he said. "But if dark energy causes the universe to stretch out at a faster rate, the supercluster flattens out in the half-billion years it takes the microwave to cross it. Thus, the wave gets to keep some of the energy it gained as it entered the supercluster."

"Good stuff"

Scott Croom, an astrophysicist at the University of Sydney in Australia, said the research was "good stuff" and the "best and most clear-cut detection" of dark energy made so far, using the biggest available galaxy survey.

He said that one of the exciting things about this research is that it's the first obvious visual signal of the evidence for dark energy. "You basically see a hole or deficit in the CMB in terms of temperature – the photons have lost energy [going into the supercluster] so they're longer in wavelength and cooler in effective temperature."

He added that while there were different ways of looking for dark energy, using the CMB was "quite a nice one" because it's independent of other methods like looking at the brightness of supernovas, or using standard universal rulers (things we know the size of).

"Dark energy is such a serious question, and raises more questions than answers, so that cross-checking is an important part of the process of understanding it," said Croom.