Astronomy news
Void alternative to dark energy may be ruled out
10 July 2011
Astronomers using data from the Hubble Space Telescope may have ruled
out one alternative theory to dark energy after recalculating the
expansion rate of the universe to unprecedented accuracy.
Astronomers using data from the Hubble Space Telescope may have ruled
out one alternative theory to dark energy after recalculating the
expansion rate of the universe to unprecedented accuracy.
Studies have shown that the universe appears to be expanding at an
increasing rate. Some believe that is because the universe is filled
with a dark energy that opposes 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 possibly 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."
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