The Sloan Digital Sky Survey III surveyed 14,000 square degrees of the sky, more than a third of its total area, and delivered over a trillion pixels of imaging data. This image shows over a million luminous galaxies at redshifts indicating times when the universe was between seven and eleven billion years old, from which the sample in the current studies was selected. Image: David Kirkby of the University of California at Irvine and the SDSS collaboration. |
Since
2000, the three Sloan Digital Sky Surveys (SDSS I, II, III) have
surveyed well over a quarter of the night sky and produced the biggest
color map of the universe in three dimensions ever. Now scientists at
the U.S. Department of Energy’s Lawrence Berkeley National Laboratory
(Berkeley Lab) and their SDSS colleagues, working with DOE’s National
Energy Research Scientific Computing Center (NERSC) based at Berkeley
Lab, have used this visual information for the most accurate calculation
yet of how matter clumps together—from a time when the universe was
only half its present age until now.
“The
way galaxies cluster together over vast expanses of the sky tells us
how both ordinary visible matter and underlying invisible dark matter
are distributed, across space and back in time,” says Shirley Ho, an
astrophysicist at Berkeley Lab and Carnegie Mellon University, who led
the work. “The distribution gives us cosmic rulers to measure how the
universe has expanded, and a basis for calculating what’s in it: how
much dark matter, how much dark energy, even the mass of the hard-to-see
neutrinos it contains. What’s left over is the ordinary matter and
energy we’re familiar with.”
For
the present study Ho and her colleagues first selected 900,000 luminous
galaxies from among over 1.5 million such galaxies gathered by the
Baryon Oscillation Spectrographic Survey, or BOSS, the largest component
of the still-ongoing SDSS III. Most of these are ancient red galaxies,
which contain only red stars because all their faster-burning stars are
long gone, and which are exceptionally bright and visible at great
distances. The galaxies chosen for this study populate the largest
volume of space ever used for galaxy clustering measurements. Their
brightness was measured in five different colors, allowing the redshift
of each to be estimated.
“By
covering such a large area of sky and working at such large distances,
these measurements are able to probe the clustering of galaxies on
incredibly vast scales, giving us unprecedented constraints on the
expansion history, contents, and evolution of the universe,” says Martin
White of Berkeley Lab’s Physics Division, a professor of physics and
astronomy at the University of California at Berkeley and chair of the
BOSS science survey teams. “The clustering we’re now measuring on the
largest scales also contains vital information about the origin of the
structure we see in our maps, all the way back to the epoch of
inflation, and it helps us to constrain—or rule out—models of the very
early universe.”
After
augmenting their study with information from other data sets, the team
derived a number of such cosmological constraints, measurements of the
universe’s contents based on different cosmological models. Among the
results: in the most widely accepted model, the researchers found—to
less than two percent uncertainty—that dark energy accounts for 73% of
the density of the universe.
The
team’s results are presented January 11 at the annual meeting of the
American Astronomical Society in Austin, Texas, and have been submitted
to the Astrophysical Journal. They are currently available online at http://arxiv.org/abs/1201.2137.
The power of the universe
“The
way mass clusters on the largest scales is graphed in an angular power
spectrum, which shows how matter statistically varies in density across
the sky,” says Ho. “The power spectrum gives a wealth of information,
much of which is yet to be exploited.” For example, information about
inflation—how the universe rapidly expanded shortly after the big
bang—can be derived from the power spectrum.
Closely
related to the power spectrum are two “standard rulers,” which can be
used to measure the history of the expansion of the universe. One ruler
has only a single mark—the time when matter and radiation were exactly
equal in density.
“In
the very early universe, shortly after the big bang, the universe was
hot and dominated by photons, the fundamental particles of radiation,”
Ho explains. “But as it expanded, it began the transition to a universe
dominated by matter. By about 50,000 years after the big bang, the
density of matter and radiation were equal. Only when matter dominated
could structure form.”
The
other cosmic ruler is also big, but it has many more than one mark in
the power spectrum; this ruler is called BAO, for baryon acoustic
oscillations. (Here, baryon is shorthand for ordinary matter.) Baryon
acoustic oscillations are relics of the sound waves that traveled
through the early universe when it was a hot, liquid-like soup of matter
and photons. After about 50,000 years the matter began to dominate, and
by about 300,000 years after the big bang the soup was finally cool
enough for matter and light to go their separate ways.
Differences
in density that the sound waves had created in the hot soup, however,
left their signatures as statistical variations in the distribution of
light, detectable as temperature variations in the cosmic microwave
background (CMB), and in the distribution of baryons. The CMB is a kind
of snapshot that can still be read today, almost 14 billion years later.
Baryon oscillations – variations in galactic density peaking every 450
million light-years or so—descend directly from these fluctuations in
the density of the early universe.
BAO
is the target of the Baryon Oscillation Spectroscopic Survey. By the
time it’s completed, BOSS will have measured the individual spectra of
1.5 million galaxies, a highly precise way of measuring their redshifts.
The first BOSS spectroscopic results are expected to be announced early
in 2012.
Meanwhile
the photometric study by Ho and her colleagues deliberately uses many
of the same luminous galaxies but derives redshifts from their
brightnesses in different colors, extending the BAO ruler back over a
previously inaccessible redshift range, from z = 0.45 to z = 0.65 (z
stands for redshift).
“As
an oscillatory feature in the power spectrum, not many things can
corrupt or confuse BAO, which is why it is considered one of the most
trustworthy ways to measure dark energy,” says Hee-Jong Seo of the
Berkeley Center for Cosmological Physics at Berkeley Lab and the UC
Berkeley Department of Physics, who led BAO measurement for the project.
“We call BAO a standard ruler for a good reason. As dark energy
stretches the universe against the gravity of dark matter, more dark
energy places galaxies at a larger distance from us, and the BAO
imprinted in their distribution looks smaller. As a standard ruler the
true size of BAO is fixed, however. Thus the apparent size of BAO gives
us an estimate of the cosmological distance to our target galaxies—which
in turn depends on the properties of dark energy.”
Says
Ho, “Our study has produced the most precise photometric measurement of
BAO. Using data from the newly accessible redshift range, we have
traced these wiggles back to when the universe was about half its
present age, all the way back to z = 0.54.”
Seo adds, “And that’s to an accuracy within 4.5 %.”
Reining in the systematics
“With
such a large volume of the universe forming the basis of our study,
precision cosmology was only possible if we could control for
large-scale systematics,” says Ho. Systematic errors are those with a
physical basis, including differences in the brightness of the sky, or
stars that mimic the colors of distant galaxies, or variations in
weather affecting “seeing” at the SDSS’s Sloan Telescope—a dedicated
2.5-m telescope at the Apache Point Observatory in southern New Mexico.
After
applying individual corrections to these and other systematics, the
team cross-correlated the effects on the data and developed a novel
procedure for deriving the best angular power-spectrum of the universe
with the lowest statistical and systematic errors.
With
the help of 40,000 central-processing-unit (CPU) hours at NERSC and
another 20,000 CPU hours on the Riemann computer cluster at Berkeley
Lab, NERSC’s powerful computers and algorithms enabled the team to use
all the information from galactic clustering in a huge volume of sky,
including the full shape of the power spectrum and, independently, BAO,
to get excellent cosmological constraints. The data as well as the
analysis output are stored at NERSC.
“Our
dataset is purely imaging data, but our results are competitive with
the latest large-scale spectroscopic surveys,” Ho says. “What we lack in
redshift precision, we make up in sheer volume. This is good news for
future imaging surveys like the Dark Energy Survey and the Large
Synoptic Survey Telescope, suggesting they can achieve significant
cosmological constraints even compared to future spectroscopy surveys.”
Animated visualizations of the luminous galaxies in the SDSS-III dataset can be accessed at http://darkmatter.ps.uci.edu/lrg-sdss.
Acoustic scale from the angular power spectra of SDSS-III DR8 photometric luminous galaxies.
