An artist's conception of the measurement scale of the universe. Baryon
acoustic oscillations are the tendency of galaxies and other matter to
cluster in spheres, which originated as density waves traveling through
the plasma of the early universe. The clustering is greatly exaggerated
in this illustration. The radius of the spheres (white line) is the
scale of a “standard ruler” allowing astronomers to determine, within
one percent accuracy, the large-scale structure of the universe and how
it has evolved. (Image by Zosia Rostomian, Lawrence Berkeley National
Laboratory)
The Baryon Oscillation Spectroscopic Survey makes the most precise calibration yet of the universe’s “standard ruler”
Today the Baryon Oscillation Spectroscopic Survey (BOSS)
Collaboration announced that BOSS has measured the scale of the universe
to an accuracy of one percent. This and future measures at this
precision are the key to determining the nature of dark energy.
“One-percent accuracy in the scale of the universe is the most
precise such measurement ever made,” says BOSS’s principal investigator,
David Schlegel, a member of the Physics Division of the U.S. Department
of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
“Twenty years ago astronomers were arguing about estimates that differed
by up to fifty percent. Five years ago, we’d refined that uncertainty
to five percent; a year ago it was two percent. One-percent accuracy
will be the standard for a long time to come.”
BOSS is the largest program in the third Sloan Digital Sky Survey
(SDSS-III). Since 2009, BOSS has used the Sloan Foundation Telescope at
the Apache Point Observatory in New Mexico to record high-precision
spectra of well over a million galaxies with redshifts from 0.2 to 0.7,
looking back over six billion years into the universe’s past. Schlegel
says, “We believe the BOSS database includes more redshifts of galaxies
than collected by all the other telescopes in the world.”
BOSS will continue gathering data until June, 2014. However, says
Martin White, a member of Berkeley Lab, a professor of physics and
astronomy at the University of California at Berkeley, and chair of the
BOSS science survey team, “We’ve done the analysis now because we have
90 percent of BOSS’s final data and we’re tremendously excited by the
results.”
Baryon acoustic oscillations (BAO) are the regular clustering of
galaxies, whose scale provides a “standard ruler” to measure the
evolution of the universe’s structure. Accurate measurement dramatically
sharpens our knowledge of fundamental cosmological properties,
including how dark energy accelerates the expansion of the universe.
Combined with recent measures of the cosmic microwave background
radiation (CMB) and supernova measures of accelerating expansion, the
BOSS results suggest that dark energy is a cosmological constant whose
strength does not vary in space or time. Although unlikely to be a flaw
in Einstein’s General Theory of Relativity, the authors of the BOSS
analysis note that “understanding the physical cause of the accelerated
expansion remains one of the most interesting problems in modern
physics.”
Among other cosmic parameters, says White, the BOSS analysis “also
provides one of the best-ever determinations of the curvature of space.
The answer is, it’s not curved much.”
Calling a three-dimensional universe “flat” means its shape is well
described by the Euclidean geometry familiar from high school: straight
lines are parallel and triangles add up to 180 degrees. Extraordinary
flatness means the universe experienced relatively prolonged inflation,
up to a decillionth of a second or more, immediately after the big bang.
“One of the reasons we care is that a flat universe has implications
for whether the universe is infinite,” says Schlegel. “That means –
while we can’t say with certainty that it will never come to an end –
it’s likely the universe extends forever in space and will go on forever
in time. Our results are consistent with an infinite universe.”
The BOSS analysis is based on SDSS-III’s Data Releases 10 and 11 (DR
10 and DR 11) and has been submitted for publication in the Monthly Notices of the Royal Astronomical Society; the analysis is available online at http://arxiv.org/abs/1312.4877.
Ripples in a sea of galaxies
The BOSS analysis incorporates spectra of 1,277,503 galaxies and
covers 8,509 square degrees of the sky visible from the northern
hemisphere. This is the largest sample of the universe ever surveyed at
this density. When complete, BOSS will have collected high-quality
spectra of 1.3 million galaxies, plus 160,000 quasars and thousands of
other astronomical objects, covering 10,000 square degrees.
Periodic ripples of density in visible matter (“baryons,” for short)
pervade the universe like raindrops on the surface of a pond. Regular
galaxy clustering is the direct descendant of pressure waves that moved
through the hot plasma of the early universe, which was so hot and dense
that particles of light (photons) and particles of matter, including
protons and electrons, were tightly coupled together. Invisible dark
matter was also part of the mix.
By 380,000 years after the big bang, however, the temperature of the
expanding mixture had cooled enough for light to escape, suffusing the
newly transparent universe with intense radiation, which in the 13.4
billion years since has continued to cool to today’s faint but pervasive
cosmic microwave background.
Minute variations in the temperature of the CMB record periodicity in
the original density ripples, of which the European Space Agency’s
Planck satellite has made the most recent and most accurate measures.
The same periodicity is preserved in the clustering of the BOSS
galaxies, a BAO signal which also mirrors the distribution of underlying
dark matter.
Regular clustering at different eras, starting with the CMB,
establishes the expansion history of the universe. BOSS collaborator
Beth Reid of Berkeley Lab translates the two-dimensional sky coordinates
of galaxies, plus their redshifts, into 3-D maps of the density of
galaxies in space.
“It’s from fluctuations in the density of galaxies in the volume
we’re looking at that we extract the BAO standard ruler,” she says. “To
compare different regions of the sky on an equal footing, first we have
to undo variations from atmospheric effects or other patterns caused by
how we observe the sky with our telescope.” The results depend crucially
on accurate measures of redshifts, which disclose the galaxies’
positions in space and time. But galaxies don’t move in lock step.
“When galaxies are close together their mutual gravitational
attraction pushes them around and interferes with attempts to measure
large-scale structure,” Schlegel says. “Their peculiar motion makes it
hard to write a formula for overall gravitational growth.”
However, says Reid, “We have a very good model for what these
distortions look like. The galaxy density field shows you where there
are concentrations of matter, and the peculiar velocity field points in
the direction of the net effect of all the local over- and
under-densities.”
“The BOSS data are awe-inspiring,” says Martin White, “but many other
pieces had to be put into place before we could get what we’re after
out of the data.” Complex computer algorithms were essential for
reconciling the inherent uncertainties. “We made thousands of model
universes in a computer, and then observed them as BOSS would do and ran
our analysis on them to answer the questions of ‘What if?’”
By gauging how well their algorithms could analyze these model
universes, known as “mocks” and based on catalogues of realistic but
artificial galaxies, the experienced BOSS team was able to assess and
fine-tune the algorithms when they were applied to the real BOSS data.
The National Energy Research Scientific Computing Center (NERSC),
based at Berkeley Lab, was critical to the analysis and the creation of
the mocks. Says White, “NERSC set aside resources for us to push
analyses through quickly when we were up against deadlines. They provide
a virtual meeting place where members of the collaboration from all
around the world can come together on a shared platform, with both the
data and the computational resources they need to perform their
research.”
BOSS has now provided the most accurate calibration ever of BAO’s
standard ruler. The universe’s expansion history has been measured with
unprecedented accuracy during the very stretch of ancient time, over six
billion years in the past, when expansion had stopped slowing and
acceleration began. But accurate as they are, the new BOSS results are
just the beginning. Greater coverage and better resolution in scale are
essential to understanding dark energy itself.
The proposed Dark Energy Spectroscopic Instrument (DESI), based on an
international partnership of nearly 50 institutions led by Berkeley
Lab, would enable the Mayall Telescope on Kitt Peak in Arizona to map
over 20 million galaxies, plus over three million quasars, in 14,000
square degrees of the northern sky. By filling in the missing eons that
BOSS can’t reach, DESI could sharpen and extend coverage of the
expansion history of the universe from the first appearance of the
cosmic background radiation to the present day.
In the meantime, BOSS, ahead of schedule for completion in June,
2014, continues to be the premier instrument for mapping the universe.
***
“The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release
10 and 11 galaxy samples,” by Lauren Anderson, Eric Aubourg, Stephen
Bailey, Florian Beutler, Vaishali Bhardwaj, Michael Blanton, Adam S.
Bolton, J. Brinkmann, Joel R. Brownstein, Angela Burden, Chia-Hsun
Chuang, Antonio J. Cuesta, Kyle S. Dawson, Daniel J. Eisenstein,
Stephanie Escoffier, James E. Gunn,Hong Guo, Shirley Ho, Klaus
Honscheid, Cullan Howlett, David Kirkby, Robert H. Lupton, Marc Manera,
Claudia Maraston, Cameron K. McBride, Olga Mena, Francesco Montesano,
Robert C. Nichol, Sebastian E. Nuza, Matthew D. Olmstead, Nikhil
Padmanabhan, Nathalie Palanque-Delabrouille, John Parejko, Will J.
Percival, Patrick Petitjean, Francisco Prada, Adrian M. Price-Whelan,
Beth Reid, Natalie A. Roe,Ashley J. Ross, Nicholas P. Ross, Cristiano G.
Sabiu, Shun Saito, Lado Samushia, Ariel G. Sanchez, David J. Schlegel,
Donald P. Schneider, Claudia G. Scoccola, Hee-Jong Seo, Ramin A. Skibba,
Michael A. Strauss, Molly E. C. Swanson, Daniel Thomas, Jeremy L.
Tinker, Rita Tojeiro, Mariana Vargas Magana, Licia Verde, David A. Wake,
Benjamin A. Weaver, David H. Weinberg, Martin White, Xiaoying Xu,
Christophe Yeche, Idit Zehavi, and Gong-Bo Zhao, has been submitted to
the Monthly Notices of the Royal Astronomical Society and is available online at http://arxiv.org/abs/1312.4877.
The SDSS-III Collaboration’s press release on this analysis may be found at http://www.sdss3.org/press/onepercent.php .
Funding for SDSS-III has been provided by the Alfred P. Sloan
Foundation, the Participating Institutions, the National Science
Foundation, and the U.S. Department of Energy Office of Science. The
SDSS-III web site is http://www.sdss3.org/.
SDSS-III is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS‑III Collaboration including the
University of Arizona, the Brazilian Participation Group, Brookhaven
National Laboratory, Carnegie Mellon University, University of Florida,
the French Participation Group, the German Participation Group, Harvard
University, the Instituto de Astrofisica de Canarias, the Michigan
State/Notre Dame/JINA Participation Group, Johns Hopkins University,
Lawrence Berkeley National Laboratory, Max Planck Institute for
Astrophysics, Max Planck Institute for Extraterrestrial Physics, New
Mexico State University, New York University, Ohio State University,
Pennsylvania State University, University of Portsmouth, Princeton
University, the Spanish Participation Group, University of Tokyo,
University of Utah, Vanderbilt University, University of Virginia,
University of Washington, and Yale University.
Lawrence Berkeley National Laboratory addresses the world’s most
urgent scientific challenges by advancing sustainable energy, protecting
human health, creating new materials, and revealing the origin and fate
of the universe. Founded in 1931, Berkeley Lab’s scientific expertise
has been recognized with 13 Nobel prizes. The University of California
manages Berkeley Lab for the U.S. Department of Energy’s Office of
Science. For more, visit www.lbl.gov.
The National Energy Research Scientific Computing Center (NERSC)
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Contact:
Paul Preuss
Email: paul_preuss@lbl.gov
