Cosmic Lens Used to Probe Dark Energy for First Time

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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

NEWS RELEASE: 2010-272 Aug. 19, 2010

Cosmic Lens Used to Probe Dark Energy for First Time

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-272&cid=release_2010-272

PASADENA, Calif. -- Astronomers have devised a new method for measuring perhaps the
greatest puzzle of our universe -- dark energy. This mysterious force, discovered in 1998, is
pushing our universe apart at ever-increasing speeds.

For the first time, astronomers using NASA's Hubble Space Telescope 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. This may eventually lead to an
explanation of what the elusive phenomenon really is.

"We have to tackle the dark energy problem from all sides," said Eric Jullo, an astronomer at
NASA's Jet Propulsion Laboratory, Pasadena, Calif. "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.

Scientists aren't clear about what dark energy is, but 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.

In 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 Jullo. "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."

Other authors of the paper include Jean-Paul Kneib and Carlo Schimd of the Université de
Provence, France; Anson D'Aloisio of Yale University; Marceau Limousin of Université de
Provence and University of Copenhagen, Denmark; and Johan Richard of Durham University,
United Kingdom.

The Hubble Space Telescope is a project of international cooperation between NASA and the
European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the
telescope. The Space Telescope Science Institute, operated for NASA by the Association of
Universities for Research in Astronomy, Inc. in Washington, conducts Hubble science operations.
More information is online at http://www.nasa.gov/hubble .

The California Institute of Technology in Pasadena manages JPL for NASA. More information is
at http://www.jpl.nasa.gov .

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