NASA's Galaxy Evolution Explorer Helps Confirm Nature of Dark Energy

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News release: 2011-149 May 19, 2011

NASA's Galaxy Evolution Explorer Helps Confirm Nature of Dark Energy

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-149&cid=release_2011-149

PASADENA, Calif. -- A five-year survey of 200,000 galaxies, stretching back seven billion
years in cosmic time, has led to one of the best independent confirmations that dark energy is
driving our universe apart at accelerating speeds. The survey used data from NASA's space-
based Galaxy Evolution Explorer and the Anglo-Australian Telescope on Siding Spring
Mountain in Australia.

The findings offer new support for the favored theory of how dark energy works -- as a constant
force, uniformly affecting the universe and propelling its runaway expansion. They contradict an
alternate theory, where gravity, not dark energy, is the force pushing space apart. According to
this alternate theory, with which the new survey results are not consistent, Albert Einstein's
concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at
great distances.

"The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into
the sky faster and faster," said Chris Blake of the Swinburne University of Technology in
Melbourne, Australia. Blake is lead author of two papers describing the results that appeared in
recent issues of the Monthly Notices of the Royal Astronomical Society. "The results tell us that
dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we
wouldn't be seeing these constant effects of dark energy throughout time."

Dark energy is thought to dominate our universe, making up about 74 percent of it. Dark matter,
a slightly less mysterious substance, accounts for 22 percent. So-called normal matter, anything
with atoms, or the stuff that makes up living creatures, planets and stars, is only approximately
four percent of the cosmos.

The idea of dark energy was proposed during the previous decade, based on studies of distant
exploding stars called supernovae. Supernovae emit constant, measurable light, making them so-
called "standard candles," which allows calculation of their distance from Earth. Observations
revealed dark energy was flinging the objects out at accelerating speeds.

Dark energy is in a tug-of-war contest with gravity. In the early universe, gravity took the lead,
dominating dark energy. At about 8 billion years after the Big Bang, as space expanded and
matter became diluted, gravitational attractions weakened and dark energy gained the upper
hand. Billions of years from now, dark energy will be even more dominant. Astronomers predict
our universe will be a cosmic wasteland, with galaxies spread apart so far that any intelligent
beings living inside them wouldn't be able to see other galaxies.

The new survey provides two separate methods for independently checking the supernovae
results. This is the first time astronomers performed these checks across the whole cosmic
timespan dominated by dark energy. The team began by assembling the largest three-
dimensional map of galaxies in the distant universe, spotted by the Galaxy Evolution Explorer.
The ultraviolet-sensing telescope has scanned about three-quarters of the sky, observing
hundreds of millions of galaxies.

"The Galaxy Evolution Explorer helped identify bright, young galaxies, which are ideal for this
type of study," said Christopher Martin, principal investigator for the mission at the California
Institute of Technology in Pasadena. "It provided the scaffolding for this enormous 3-D map."

The astronomers acquired detailed information about the light for each galaxy using the Anglo-
Australian Telescope and studied the pattern of distance between them. Sound waves from the
very early universe left imprints in the patterns of galaxies, causing pairs of galaxies to be
separated by approximately 500 million light-years.

This "standard ruler" was used to determine the distance from the galaxy pairs to Earth -- the
closer a galaxy pair is to us, the farther apart the galaxies will appear from each other on the sky.
As with the supernovae studies, this distance data were combined with information about the
speeds at which the pairs are moving away from us, revealing, yet again, the fabric of space is
stretching apart faster and faster.

The team also used the galaxy map to study how clusters of galaxies grow over time like cities,
eventually containing many thousands of galaxies. The clusters attract new galaxies through
gravity, but dark energy tugs the clusters apart. It slows down the process, allowing scientists to
measure dark energy's repulsive force.

"Observations by astronomers over the last 15 years have produced one of the most startling
discoveries in physical science; the expansion of the universe, triggered by the Big Bang, is
speeding up," said Jon Morse, astrophysics division director at NASA Headquarters in
Washington. "Using entirely independent methods, data from the Galaxy Evolution Explorer
have helped increase our confidence in the existence of dark energy."

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations
and data analysis. NASA's Jet Propulsion Laboratory in Pasadena, manages the mission and built
the science instrument. The mission was developed under NASA's Explorers Program managed
by the Goddard Space Flight Center, Greenbelt, Md. Researchers sponsored by Yonsei
University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France
collaborated on this mission. Caltech manages JPL for NASA.

Graphics and additional information about the Galaxy Evolution Explorer are online at
http://www.nasa.gov/galex/ and http://www.galex.caltech.edu .

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

Trent Perrotto 202-358-0321
Headquarters, Washington
trent.j.perrotto@nasa.gov

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