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Defined Space

NM telescope puts dark energy on the map

September 18, 2012, 12:00 am

The nature of dark energy—the mysterious force causing the universe to expand at an ever-increasing speed—is one of science’s biggest puzzles.

But now, there’s a map for that.

Using a telescope at Apache Point Observatory near Cloudcroft, the Sloan Digital Sky Survey III has created the biggest map of the universe ever made. In its finest detail, the map would require digital space dwarfing the most ambitious Best Buy TV wall—half a million high-def flat screens. The telescope, which was designed specifically for the SDSS project, has a uniquely wide field of view: at any one time, it can see in great detail the amount of the sky that would be taken up by six full moons laid out side by side. The famous Hubble Space Telescope’s field of view is a mere “pinpoint” by comparison, SDSS III spokesman Jordan Raddick tells SFR.

On a clear night at Apache Point Observatory, as winds up to 35 miles per hour whip across the telescope’s mountainside perch, two SDSS III observers add to this intergalactic quilt, patch by patch. Seated in a control room in a nearby building, the observers point the telescope lens toward a certain set of coordinates, then run—literally, for optimal viewing time is precious—outside and up to the telescope, with only headlamps and starlight illuminating their path. They heft into place a new 300-pound aluminum cartridge fitted with 1,000 fiber-optic cables positioned specifically for both the hour of night and time of year. The telescope camera takes four 15-minute exposures—then the process starts again.

“On a good night, we do that nine times,” says SDSS III site manager Mark Klaene.

The most recent result, released in August, is not only a highly detailed visual depiction of more of the sky than ever captured before. It also records precise locations of stars and galaxies billions of light years away—in other words, cosmic features as they looked billions of years ago, in the early days of the universe. Studying them is key to understanding how the cosmos has expanded since then, under the influence of the elusive phenomenon known as dark energy.

Physicists believe that when the universe first formed 13.7 billion years ago, it was a ball of free-moving subatomic particles (protons, neutrons and electrons) in a superheated gas-like state called a plasma. The plasma was so hot that those particles were moving at a frequency faster than visible light —the universe was completely dark. 

About 380,000 years after it formed, the universe cooled enough for the particles to form atoms, and move at a frequency of visible light. Those atoms have slowed down since, but they’re still moving—now at the frequency of sound waves. The SDSS I, the current project’s predecessor, was first to clearly detect that signal—called the cosmic radiation background—which turned out to be a critical baseline for analyzing the SDSS III data. 

Since those very early days, the universe has expanded at an accelerating speed as the significance of dark energy has grown.

“This gives us a measurement from before we think dark energy should have mattered at all,” says SDSS III data coordinator Mike Blanton. “It gives us a baseline to compare [dark energy] models to, because we have a measurement from before we think dark energy kicks in.” 

With the new SDSS III data—only a third of which has been analyzed so far—astronomers can measure the locations of distant galaxies by comparison with the cosmic radiation background, and find out how quickly they are moving as the universe expands. The locations are accurate to about 1-2 percent, whereas previous methods only got to within about 10 percent accuracy—not close enough to match up with any theoretical models that try to explain dark energy.

David Schlegel, principal investigator for the Baryon Oscillation Spectroscopic Survey, a key SDSS III study, says that, as they examine the SDSS III data, he and his colleagues are hoping for a dark energy breakthrough—which may come in the form of an observation that doesn’t fit with anything physicists might expect.

“The most fun is when you find something that is kind of startlingly wrong, like the discovery of dark energy in the first place,” Schlegel says. “It’s just more fun when the universe throws something like that at you.” 

Editor's note: A previous version of this story identified SDSS III data coordinator Mike Blanton as Mark Blanton. The error has been corrected.


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