A One-Percent Measure of the Universe

January 8, 2014
Kavli Institute for the Physics and Matehmatics of the Universe (Kavli IPMU)

Researchers from the Baryon Oscillation Spectroscopic Survey (BOSS) today announced that they have measured the distances to galaxies more than six billion light-years away to an unprecedented accuracy of just one percent. Their measurements place new constraints on the properties of the mysterious "dark energy" thought to permeate empty space, which causes the expansion of the Universe to accelerate.

"There are not many things in our daily lives that we know to one-percent accuracy," said David Schlegel, a physicist at Lawrence Berkeley National Laboratory (LBNL) and the principal investigator of BOSS. "I now know the size of the Universe better than I know the size of my house."

The new distance measurements were presented at the meeting of the American Astronomical Society by Harvard University astronomer Daniel Eisenstein, the director of the Sloan Digital Sky Survey III (SDSS-III), the worldwide collaboration of which BOSS is a part. They are detailed in a series of articles submitted to journals by the BOSS collaboration last month, all of which are now available as online preprints.

"Determining distance is a fundamental challenge of astronomy," said Eisenstein. "You see something in the sky -- how far away is it? Once you know how far away it is, learning everything else about it is suddenly much easier."

Throughout history, astronomers have met this challenge using many different techniques: for example, distances to planets in the Solar System can be measured quite accurately using radar, but for more distant objects, astronomers must turn to less-direct methods. Regardless of the method, every measurement has some uncertainty, which can be expressed as a percentage of the thing being measured -- for example, if you measure the distance from Washington to New York (200 miles) to within 2 miles of the true value, you have measured to an accuracy of 1%.

Only a few hundred stars and a few star clusters are close enough to have distances measured to one-percent accuracy. Nearly all of these stars are only a few thousand light-years away, and all are still within our own Milky Way galaxy. Reaching out a million times farther away, the new BOSS measurements probe far beyond our Galaxy to map the Universe with unprecedented accuracy.

With these new, highly-accurate distance measurements, BOSS astronomers are making new inroads in the quest to understand dark energy. "We don't yet understand what dark energy is," explained Eisenstein, "but we can measure its properties. Then, we compare those values to what we expect them to be, given our current understanding of the Universe. The better our measurements, the more we can learn."

Making a one-percent measurement at a distance of six billion light-years requires a completely different technique from measurements in the Solar System or the Milky Way. BOSS, the largest of the four projects that make up the Sloan Digital Sky Survey III (SDSS-III), was built to take advantage of this technique: measuring the so-called "baryon acoustic oscillations" (BAOs), subtle periodic ripples in the distribution of galaxies in the cosmos.

These ripples are imprints of pressure waves that moved through the early Universe, which was so hot and dense that particles of light (photons) moved along with the protons and neutrons (known collectively as "baryons") that today make up the nuclei of atoms. The original size of these ripples is known, and their size today can be measured by mapping galaxies.

"With these galaxy measurements, nature has given us a beautiful ruler," said Ashley Ross, an astronomer from the University of Portsmouth. "The ruler happens to be half a billion light years long, so we can use it to measure distances precisely, even from very far away."

Making these measurements required astronomers to map the locations of 1.2 million galaxies. BOSS uses a specialized instrument that can make detailed measurements of 1000 galaxies at a time. "On a clear night when everything goes perfectly, we can add more than 8000 galaxies and quasars to the map," said Kaike Pan, who leads the team of observers at the SDSS-III's Sloan Foundation 2.5-meter Telescope at Apache Point Observatory in New Mexico.

The BOSS team presented preliminary BAO measurements from its early galaxy maps a year ago, but the new analysis covers an area more than twice as large, and thus provides a much more precise measurement. The new dataset also includes the first BAO measurements from a sample of nearby galaxies. "Making these measurements at two different distances allows us to see how the expansion of the Universe has changed over time, which will help us understand why it is accelerating," explained University of Portsmouth astronomer Rita Tojeiro, who co-chairs the BOSS galaxy clustering working group along with Jeremy Tinker of New York University.

In addition, the BOSS team announced that this galaxy map enables us to test the gravity theory at cosmological scales with 10% precision. It is often discussed that modifying Einstein’s Theory of General Relativity could explain the cosmic acceleration without introducing unknown dark energy. In fact, we can investigate whether gravitational pull deviates from the theory at very large scales of about 100 million light years by analyzing how strongly galaxies are clustering via gravity. In particular, we can measure peculiar velocities of galaxies using the so-called redshift-space distortion. “Similar to how the velocity of free-falling matter is determined by the earth's gravitational pull, the velocity of the galaxy can tell us about the law of gravitation. Using this large 3D galaxy map, we can now verify Einstein’s Theory of General Relativity at a 100 million-light-year scale with 10% precision. Verifying the law of gravitation via the redshifts-space distortion plays an important and complementary role to tackle the mystery of the cosmic acceleration,” said Shun Saito, a project researcher at the Kavli IPMU who is also involved in the distance measurement.

For now, the BOSS measurements appear consistent with a form of dark energy that stays constant through the history of the Universe. This "cosmological constant" is one of just six numbers needed to make a model that matches the shape and large-scale structure of the Universe. Schlegel likens this six-number model to a pane of glass, which is pinned in place by bolts that represent different measurements of the history of the Universe. "BOSS now has one of the tightest of those bolts, and we just gave it another half-turn," said Schlegel. "Each time you ratchet up the tension and the glass doesn't break, that's a success of the model."



1. "The Clustering of Galaxies in the SDSS-III DR11 Baryon Oscillation Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release 10 and 11 Galaxy Samples"
L. Anderson, E. Aubourg, S. Bailey et al., submitted to Monthly Notices of the Royal Astronomical Society,

2. "The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Testing gravity with redshift-space distortions using the power spectrum multipoles”
F. Beutler, S. Saito, H-J. Seo et al., submitted to Monthly Notices of the Royal Astronomical Society,

This research is supported by a Grant-in-Aid for Young Scientists (Start-up) from the Japan Society for the Promotion of Science (JSPS) (No. 25887012).


Science Contacts

Shun Saito (Project Researcher at Kavli IPMU)
shun.saito _at_ ipmu.jp

Nao Suzuki (Project Assistant Professor at Kavli IPMU)
nao.suzuki _at_ ipmu.jp

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Yoshihisa Obayashi (Kavli IPMU Public Relation Office)
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Related Links

Press Release by SDSS-III (Jan. 8, 2014)

This is Your Galaxy: New Data Help Astronomers Explore the Hidden Milky Way (August 8, 2013)

About the Sloan Digital Sky Survey

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.

About Kavli IPMU

The Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) is an international research institute with English as its official language. The goal of the institute is to discover the fundamental laws of nature and to understand the universe from the synergistic perspectives of mathematics, astronomy, and theoretical and experimental physics. The Institute for the Physics and Mathematics of the Universe (IPMU) is established in October 2007 as one of the World Premier International Research Center Initiative (WPI) of the Ministry of Education, Sports, Science and Technology in Japan with the University of Tokyo as the host institution. IPMU was designated as the first research institute within Todai Institutes for Advanced Study (TODIAS) in January 2011. It received endowment from The Kavli Foundation and was renamed “Kavli Institute for the Physics and Mathematics of the Universe” in April 2012. Kavli IPMU is located on the Kashiwa campus of the University of Tokyo, and more than half of its full-time scientific members come from outside Japan.