Teams from Fermilab and Berkeley Lab used galaxies from wide-ranging SDSS Stripe 82, a tiny detail of which is shown here, to plot new maps of dark matter based on the largest direct measurements of cosmic shear to date. Image: SDSS |
Two teams of physicists at the U.S. Department of
Energy’s Fermilab and Lawrence Berkeley National Laboratory (Berkeley Lab) have
independently made the largest direct measurements of the invisible scaffolding
of the universe, building maps of dark matter using new methods that, in turn,
will remove key hurdles for understanding dark energy with ground-based
telescopes.
The teams’ measurements look for tiny distortions
in the images of distant galaxies, called “cosmic shear,” caused by the
gravitational influence of massive, invisible dark matter structures in the
foreground. Accurately mapping out these dark-matter structures and their
evolution over time is likely to be the most sensitive of the few tools
available to physicists in their ongoing effort to understand the mysterious
space-stretching effects of dark energy.
Both teams depended upon extensive databases of
cosmic images collected by the Sloan Digital Sky Survey (SDSS), which were
compiled in large part with the help of Berkeley Lab and Fermilab.
“These results are very encouraging for future
large sky surveys. The images produced lead to a picture that sees many more
galaxies in the universe and sees those that are six time fainter, or further
back in time, than is available from single images,” says Huan Lin, a Fermilab
physicist and member of the SDSS and the Dark Energy Survey (DES).
Melanie Simet, a member of the SDSS collaboration from the University of
Chicago, will outline the new techniques for improving maps of cosmic shear and
explain how these techniques can expand the reach of upcoming international sky
survey experiments during a talk at the
American Astronomical Society (AAS) conference in Austin, Texas. In her talk she
will demonstrate a unique way to analyze dark matter’s distortion of galaxies to
get a better picture of the universe’s past.
Eric Huff, an SDSS member from Berkeley Lab and the University of California
at Berkeley, will present a poster describing the full cosmic shear measurement,
including the new constraints on dark energy, at the AAS conference.
Several large astronomical surveys, such as the
Dark Energy Survey, the Large Synoptic Survey Telescope, and the
HyperSuprimeCam survey, will try to measure cosmic shear in the coming years.
Weak lensing distortions are so subtle, however, that the same atmospheric
effects that cause stars to twinkle at night pose a formidable challenge for
cosmic shear measurements. Until now, no ground-based cosmic-shear measurement
has been able to completely and provably separate weak lensing effects from the
atmospheric distortions.
“The community has been building towards cosmic
shear measurements for a number of years now,” says Huff, an astronomer at
Berkeley Lab, “but there’s also been some skepticism as to whether they can be
done accurately enough to constrain dark energy. Showing that we can achieve
the required accuracy with these pathfinding studies is important for the next
generation of large surveys.”
To construct dark matter maps, the Berkeley Lab and
Fermilab teams used images of galaxies collected between 2000 and 2009 by SDSS
surveys I and II, using the Sloan Telescope at Apache Point Observatory in New Mexico. Berkeley Lab
also used updated calibrations from SDSS III, which continues today. The
galaxies lie within a continuous ribbon of sky known as SDSS Stripe 82, lying
along the celestial equator and encompassing 275 square degrees. The galaxy
images were captured in multiple passes over many years.
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The two teams layered snapshots of a given area
taken at different times, a process called coaddition, to remove errors caused
by the atmospheric effects and to enhance very faint signals coming from
distant parts of the universe. The teams used different techniques to model and
control for the atmospheric variations and to measure the lensing signal, and
have performed an exhaustive series of tests to prove that these models work.
Gravity tends to pull matter together into dense
concentrations, but dark energy acts as a repulsive force that slows down the
collapse. Thus the clumpiness of the dark matter maps provides a measurement of
the amount of dark energy in the universe.
When they compared their final results, both teams
found somewhat less structure than would have been expected from other
measurements such as the Wilkinson Microwave Anisotropy Probe (WMAP), but, says
Berkeley Lab’s Huff, “the results are not yet different enough from previous
experiments to ring any alarm bells.”
Meanwhile, says Fermilab’s Lin, “Our
image-correction processes should prove a valuable tool for the next generation
of weak-lensing surveys.”