High-Resolution Dark Matter Map Fans Flames of Speculation about
Dark Energy
One of the most massive structures in our universe has
been revealed in greater detail than ever before, thanks to a new
analysis by astronomers.
"Galaxy clusters are the largest construction projects ever undertaken
by our universe, yet they are almost entirely invisible to us." said
lead author Dan Coe at NASA's Jet Propulsion Laboratory. Galaxies
and galaxy clusters are made up primarily of "dark matter", an
invisible and unknown substance. Astronomers infer the existence
of dark matter due to its "gravitational lensing" effect which distorts
the images of more distant galaxies as if one were looking through a
glass lens.
Abell 1689 is among the most powerful gravitational lenses in our sky,
producing multiple images of many background galaxies. "The
lensed images are like a big puzzle. Here we have figured out,
for the first time, a way to arrange the mass of A1689 such that it
lenses all of these galaxies to their observed positions." By
using all of this available information, Coe was able to produce a
higher resolution map of the dark matter distribution than possible
before.
The key to this breakthrough came from an advance in the field of
Mathematics by Edward Fuselier. Coe and Fuselier were both
finishing their doctoral theses when Coe searched for the necessary
mathematical technique, learned of Eddie's new method, and contacted
him. "Thanks to in large part to Eddie's contributions, we have
finally 'cracked the code' of gravitational lensing."
Hinting at Early Dark Energy?
Based on their higher resolution mass map, Coe and collaborators
confirm previous results showing that the core of A1689 has a much
higher mass density than expected for a cluster of its total
mass. (Expectations are based on computer simulations of
structure growth such as the Millennium simulation.)
A denser core implies that A1689 formed earlier than expected when the
universe was more dense. "A1689 appears to have been well fed at
birth by the high mass density surrounding it. This has given it
a chubby belly which it has carried with it through its adult life."
A handful of other well-studied clusters have been found to have
similarly "chubby bellies". If all of these clusters formed
earlier than expected, one possible explanation would be "early dark
energy".
Dark energy is a mysterious repulsive force that appears to drive
structures away from one another, inhibiting them from merging and
growing. Additional dark energy early in the history of the
universe would have stunted the growth of all galaxy clusters.
Thus, galaxy clusters would have had to have started forming much
earlier (by billions of years) in order to reach the numbers that we
observe today.
However more data is needed to confirm this speculation. It may
not be a coincidence that these well studied clusters have overly dense
cores. Such dense cores make for stronger gravitational lenses
which are the most interesting for astronomers to study in detail.
Astronomers can expect more conclusive results from CLASH, the Cluster
Lensing and Supernova survey with Hubble. This large project will
dedicate the Hubble Space Telescope to observing galaxy clusters for a
total of one month over the next three years.
"We have selected twenty-five galaxy clusters free of this lensing
bias, and we will study these clusters in as much detail as we have
A1689." CLASH will help astronomers work toward their recently
prioritized goals for the next decade: studying the natures of dark
matter and dark energy.
Dan Coe 11/11/10
dcoe(at)stsci.edu
