Dark energy blues. Dark energy stretches the fabric of spacetime so that light passing by a massive galaxy is shifted toward the blue end of the spectrum (bottom).

Dark Energy Passes Another Test

Dark energy, the baffling antigravity force that is flinging the universe apart, is here to stay. Scientists from nearly two dozen institutions across the globe have announced what is arguably the most direct detection of dark energy to date, leaving little hope for those who prefer the simplicity of a universe without the enigmatic energy.

The researchers spotted evidence of the "integrated Sachs-Wolfe" (ISW) effect, a quirk of general relativity that causes massive objects such as galaxy clusters to increase the temperature of light that's passing nearby. According to the equations of general relativity, this can occur only if dark energy is a big fraction of all the stuff in the cosmos.

To spot the ISW effect, scientists need to compare light that passed by large chunks of mass with light that steered well clear. This job became a lot easier this spring, when the Wilkinson Microwave Anisotropy Probe (WMAP) satellite made an incredibly detailed map of the cosmic microwave background--ubiquitous cool light from the ancient universe (Science, 2 May, p. 730). Now, in a paper submitted to Physical Review Letters, scientists have taken that map and compared it with a growing survey of galaxies known as the Sloan Digital Sky Survey (SDSS). The paper shows that, indeed, the photons from the cosmic microwave background that passed close to galaxies were hotter, on average, than those that were far away from galaxies. That's clear evidence for the ISW effect, and dark energy, says team member Robert Nichol, a physicist at Carnegie Mellon University in Pittsburgh.

"It's exciting because it's more or less a direct detection of dark energy," says Wayne Hu, a cosmologist at the University of Chicago. And there might be more than mere detection up ahead. By the time the SDSS project finishes its work in 2006, it should have about two and a half times as much data as it does now--enough for team members not only to detect the ISW effect with higher confidence but also to measure how much dark energy there is and throw some light on dark energy's nature. With luck, the weird stuff that makes up three-fourths of the universe may even start to feel familiar.

Related sites
Detailed explanation of the integrated Sachs-Wolfe effect
Wilkinson Microwave Anisotropy Probe
Sloan Digital Sky Survey