Dark energy has been described as the greatest puzzle of our universe. This mysterious force, discovered in 1998, is pushing the universe apart at ever-increasing speeds and astronomers have now devised a new method of measuring it. Using NASA's Hubble Space Telescope, astronomers were able to take advantage of a giant magnifying lens in space – a massive cluster of galaxies – to narrow in on the nature of dark energy. Their calculations, when combined with data from other methods, significantly increase the accuracy of dark energy measurements and may eventually lead to an explanation of what the elusive phenomenon really is.
Although scientists aren't clear about what dark energy is, they do know that it makes up a large chunk of our universe – about 72 percent. Another chunk, about 24 percent, is thought to be dark matter, also mysterious in nature but easier to study than dark energy because of its gravitational influence on matter that we can see. The rest of the universe, a mere four percent, is the stuff that makes up people, planets, stars and everything made up of atoms.
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Cosmic lensIn their new study, the science team used images from Hubble to examine a massive cluster of galaxies, named Abell 1689, which acts as a magnifying, or gravitational, lens. The gravity of the cluster causes galaxies behind it to be imaged multiple times into distorted shapes, sort of like a fun house mirror reflection that warps your face.
Using these distorted images, the scientists were able to figure out how light from the more distant, background galaxies had been bent by the cluster -- a characteristic that depends on the nature of dark energy. Their method also depends on precise ground-based measurements of the distance and speed at which the background galaxies are traveling away from us. The team used these data to quantify the strength of the dark energy that is causing our universe to accelerate.
"What I like about our new method is that it's very visual," said Eric Jullo, an astronomer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "You can literally see gravitation and dark energy bend the images of the background galaxies into arcs."
According to the scientists, their method required multiple, meticulous steps. They spent the last several years developing specialized mathematical models and precise maps of the matter – both dark and "normal" – constituting the Abell 1689 cluster.
"We can now apply our technique to other gravitational lenses," said co-author Priya Natarajan, a cosmologist at Yale University, New Haven, Conn. "We're exploiting a beautiful phenomenon in nature to learn more about the role that dark energy plays in our universe."
"We have to tackle the dark energy problem from all sides," said Jullo. "It's important to have several methods, and now we've got a new, very powerful one." Jullo is lead author of a paper on the findings appearing in the Aug. 20 issue of the journal Science.