NASA's Hubble Rules Out One Alternative to Dark Energy

WASHINGTON -- Astronomers using NASA's Hubble Space Telescope have
ruled out an alternate theory on the nature of dark energy after
recalculating the expansion rate of the universe to unprecedented accuracy.

The universe appears to be expanding at an increasing rate. Some
believe that is because the universe is filled with a dark energy
that works in the opposite way of gravity. One alternative to that
hypothesis is that an enormous bubble of relatively empty space eight
billion light-years across surrounds our galactic neighborhood. If we
lived near the center of this void, observations of galaxies being
pushed away from each other at accelerating speeds would be an illusion.

This hypothesis has been invalidated because astronomers have refined
their understanding of the universe's present expansion rate. Adam
Riess of the Space Telescope Science Institute (STScI) and Johns
Hopkins University in Baltimore, Md., led the research. The Hubble
observations were conducted by the SHOES (Supernova Ho for the
Equation of State) team that works to refine the accuracy of the
Hubble constant to a precision that allows for a better
characterization of dark energy's behavior. The observations helped
determine a figure for the universe's current expansion rate to an
uncertainty of just 3.3 percent. The new measurement reduces the
error margin by 30 percent over Hubble's previous best measurement in
2009. Riess's results appear in the April 1 issue of The Astrophysical Journal.

"We are using the new camera on Hubble like a policeman's radar gun to
catch the universe speeding," Riess said. "It looks more like it's
dark energy that's pressing the gas pedal."

Riess' team first had to determine accurate distances to galaxies near
and far from Earth. The team compared those distances with the speed
at which the galaxies are apparently receding because of the
expansion of space. They used those two values to calculate the
Hubble constant, the number that relates the speed at which a galaxy
appears to recede to its distance from the Milky Way. Because
astronomers cannot physically measure the distances to galaxies,
researchers had to find stars or other objects that serve as reliable
cosmic yardsticks. These are objects with an intrinsic brightness,
brightness that hasn't been dimmed by distance, an atmosphere, or
stellar dust, that is known. Their distances, therefore, can be
inferred by comparing their true brightness with their apparent
brightness as seen from Earth.

To calculate longer distances, Riess' team chose a special class of
exploding stars called Type 1a supernovae. These stellar explosions
all flare with similar luminosity and are brilliant enough to be seen
far across the universe. By comparing the apparent brightness of Type
1a supernovae and pulsating Cepheid stars, the astronomers could
measure accurately their intrinsic brightness and therefore calculate
distances to Type Ia supernovae in far-flung galaxies.

Using the sharpness of the new Wide Field Camera 3 (WFC3) to study
more stars in visible and near-infrared light, scientists eliminated
systematic errors introduced by comparing measurements from different telescopes.

"WFC3 is the best camera ever flown on Hubble for making these
measurements, improving the precision of prior measurements in a
small fraction of the time it previously took," said Lucas Macri, a
collaborator on the SHOES Team from Texas A&M in College Station.

Knowing the precise value of the universe's expansion rate further
restricts the range of dark energy's strength and helps astronomers
tighten up their estimates of other cosmic properties, including the
universe's shape and its roster of neutrinos, or ghostly particles,
that filled the early universe.

"Thomas Edison once said 'every wrong attempt discarded is a step
forward,' and this principle still governs how scientists approach
the mysteries of the cosmos," said Jon Morse, astrophysics division
director at NASA Headquarters in Washington. "By falsifying the
bubble hypothesis of the accelerating expansion, NASA missions like
Hubble bring us closer to the ultimate goal of understanding this
remarkable property of our universe."

For images and more information about this study, visit:

http://www.nasa.gov/hubble

and

http://hubblesite.org/news/2011/08

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 STScI
conducts Hubble science operations. STScI is operated for NASA by the
Association of Universities for Research in Astronomy, Inc., in Washington.

Source: NASA

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