Our view of dark energy, the mysterious force that is shoving the universe
apart, just got a little clearer. By observing the way large clumps of mass
distort their local space-time into enormous cosmological lenses, astronomers
have zoomed in on a quantity that describes how dark energy works.
“We have established the potency of a brand new technique to address this
very fundamental problem,” said astrophysicist Priyamvada Natarajan of Yale
University, co-author of a paper in the Aug. 20 Science describing the
new results. Combined with earlier experiments, the new results lead to
significantly more accurate measurements of dark energy’s properties, and could
ultimately help explain what the bizarre stuff really is.
Dark energy was first proposed in 1998 to explain why the universe is
expanding at an ever-increasing rate. Astronomers suggested that some kind of
force, dubbed “dark energy” because of the shroud of mystery it hides in, works
against gravity to push matter apart.
Although earlier experiments convinced astronomers the enigmatic stuff
exists, not much else is known about it. Dark energy makes up the majority of
the mass and energy in the universe, about 72 percent. Another 24 percent is
thought to be dark matter, which is easier to study than dark energy because of
its gravitational tugs on normal matter. The regular matter that makes up
everything we can see, including atoms, stars, planets and people, comprises
just 4 percent of the universe.
Dark energy also helps explain the geometry of the
universe, and how the shape of the universe has changed over time. In the
new study, Natarajan and her colleagues used Hubble Space Telescope images of a massive
cluster of galaxies called Abell 1689 to get a clear view of the way space-time
is shaped behind the cluster.
This galaxy cluster contains so much matter — both dark matter and the
regular type — that light passing through it is distorted into long, stringy
arcs. The cluster acts as a gigantic magnifying glass called a gravitational
lens, and produces multiple, distorted images of the galaxies behind it.
For the first time, Natarajan said, “we were able to exploit this beautiful,
clean phenomenon to characterize this lens so well that we could then map dark
energy.”
Natarajan and her colleagues carefully measured the way each image was
distorted to determine how far the background galaxies were from the lens. They
then combined that information with data on how far the galaxies are from Earth
to come up with a parameter that describes the density of dark energy in the
universe, and how the density changes with time.
“Knowing exactly where the object is, and knowing about the big lump that is
causing the bumps in space-time, allows us to accurately calculate the light
path,” Natarajan said. “The light path depends on geometry of space-time, and
dark energy manifests itself there. That’s how we get at it.”
This technique had been attempted before with a different cluster, but
without much success. But because Abell 1689 is one of the most massive lenses
around, it made more than 100 images of the galaxies behind it. “You want the
oomphiest lens, the most massive, dramatic, extreme lens,” Natarajan said. Abell
1689’s extreme mass allowed the team to measure many more galaxies than ever
before, and gave them a better picture of the cluster itself.
Natarajan hopes to apply the same technique to other massive clusters in the
future. “What is fantastic about this technique is it’s really rich,” she said.
“With just one cluster we can get a lot of stuff out. The prospects of applying
this technique to many clusters, and to add to the statistical power, is very
tantalizing.”
“This method looks to be quite a promising addition to the cosmography
toolkit,” commented Stanford astrophysicist Phil Marshall, who was not involved
in the new study. “It’s impressive how well they do with just one cluster.”
The results confirm what astronomers already thought they knew about dark
energy, but with much greater accuracy, said study co-author Eric Jullo of NASA’s Jet
Propulsion Lab. The new measurements suggest that dark energy has had the same
density for the entire history of the universe.
“That’s weird,” Jullo said. Imagine the universe as a balloon full of gas, he
suggests. When the balloon gets bigger, the gas inside should spread out and get
less dense. But dark energy seems to stay the same no matter how big the balloon
is. “We don’t know why this happens,” he said. “That’s why there is this race
now, with many techniques and this one in particular, in trying to measure how
dark energy density evolves with time.”
Ultimately, astronomers will have to throw the kitchen sink at dark energy to
figure out what it’s made of. Every technique to measure dark energy has its own
set of problems and errors. Using many different techniques can make each
technique’s shortcomings less important.
“The power is in combination,” Natarajan said.
Image: NASA/ESA/Jullo/Natarajan/Kneib
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