An artist's conception of how BOSS uses quasars to measure the distant universe. Light from distant quasars is partly absorbed by intervening gas, which is imprinted with a subtle ring-like pattern of known physical scale. Astronomers have now measured this scale with an accuracy of two percent, precisely measuring how fast the universe was expanding when it was just 3 billion years old. (Illustration by Zosia Rostomian, Lawrence Berkeley National Laboratory, and Andreu Font-Ribera, BOSS Lyman-alpha team, Berkeley Lab.) (Click here for best resolution.)
Berkeley Lab scientists and their colleagues in BOSS study quasars in a new way, yielding a precise determination of expansion
The Baryon Oscillation Spectroscopic Survey (BOSS), the largest
component of the third Sloan Digital Sky Survey (SDSS-III), pioneered
the use of quasars to map density variations in intergalactic gas at
high redshifts, tracing the structure of the young universe. BOSS charts
the history of the universe’s expansion in order to illuminate the
nature of dark energy, and new measures of large-scale structure have
yielded the most precise measurement of expansion since galaxies first
formed.
The latest quasar results combine two separate analytical techniques.
A new kind of analysis, led by physicist Andreu Font-Ribera of the U.S.
Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley
Lab) and his team, was published late last year. Analysis using a tested
approach, but with far more data than before, has just been published
by Timothée Delubac, of EPFL Switzerland and France’s Centre de Saclay,
and his team. The two analyses together establish the expansion rate at
68 kilometers per second per million light years at redshift 2.34, with
an unprecedented accuracy of 2.2 percent.
“This means if we look back to the universe when it was less than a
quarter of its present age, we’d see that a pair of galaxies separated
by a million light years would be drifting apart at a velocity of 68
kilometers a second as the universe expands,” says Font-Ribera, a
postdoctoral fellow in Berkeley Lab’s Physics Division. “The uncertainty
is plus or minus only a kilometer and a half per second.” Font-Ribera
presented the findings at the April 2014 meeting of the American
Physical Society in Savannah, GA.
BOSS employs both galaxies and distant quasars to measure baryon
acoustic oscillations (BAO), a signature imprint in the way matter is
distributed, resulting from conditions in the early universe. While also
present in the distribution of invisible dark matter, the imprint is
evident in the distribution of ordinary matter, including galaxies,
quasars, and intergalactic hydrogen.
“Three years ago BOSS used 14,000 quasars to demonstrate we could
make the biggest 3D maps of the universe,” says Berkeley Lab’s David
Schlegel, principal investigator of BOSS. “Two years ago, with 48,000
quasars, we first detected baryon acoustic oscillations in these maps.
Now, with more than 150,000 quasars, we’ve made extremely precise
measures of BAO.”
The BAO imprint corresponds to an excess of about five percent in the
clustering of matter at a separation known as the BAO scale. Recent
experiments including BOSS and the Planck satellite study of the cosmic
microwave background put the BAO scale, as measured in today’s universe,
at very close to 450 million light years – a “standard ruler” for
measuring expansion.
BAO directly descends from pressure waves (sound waves) moving
through the early universe, when particles of light and matter were
inextricably entangled; 380,000 years after the big bang, the universe
had cooled enough for light to go free. The cosmic microwave background
radiation preserves a record of the early acoustic density peaks; these
were the seeds of the subsequent BAO imprint on the distribution of
matter.
Quasars extend the standard ruler
Previous work from BOSS used the spectra of over a million galaxies
to measure the BAO scale with a remarkable one percent accuracy. But
beyond redshift 0.7 (roughly six billion light years distant), galaxies
become fainter and more difficult to see. For much higher redshifts like
those in the present studies, averaging 2.34, BOSS pioneered the
“Lyman-alpha forest” method of using spectra from distant quasars to
calculate the density of intergalactic hydrogen.
As the light from a distant quasar passes through intervening
hydrogen gas, patches of greater density absorb more light. The
absorption lines of neutral hydrogen in the spectrum (Lyman-alpha lines)
pinpoint each dense patch by how much they are redshifted. There are so
many lines in such a spectrum, in fact, that it resembles a forest –
the Lyman-alpha forest.
With enough good quasar spectra, close enough together, the position
of the gas clouds can be mapped in three dimensions – both along the
line of sight for each quasar and transversely among dense patches
revealed by other quasar spectra. From these maps the BAO signal is
extracted.
Although introduced by BOSS only a few years ago, this method of
using Lyman-alpha forest data, called autocorrelation, by now seems
almost traditional. The just-published autocorrelation results by
Delubac and his colleagues employ the spectra of almost 140,000
carefully selected BOSS quasars.
Font-Ribera and his colleagues determine BAO using even more BOSS
quasars in a different way. Quasars are young galaxies powered by
massive black holes, extremely bright, extremely distant, and thus
highly redshifted. Instead of comparing spectra to other spectra,
Font-Ribera’s team correlated quasars themselves to the spectra of other
quasars, a method called cross-correlation.
“Quasars are massive galaxies, and we expect them to be in the denser
parts of the universe, where the density of the intergalactic gas
should also be higher,” says Font-Ribera. “Therefore we expect to find
more of the absorbing gas than average when we look near quasars.” The
question was whether the correlation would be good enough to see the BAO
imprint.
Indeed the BAO imprint in cross-correlation was strong. Delubac and
his team combined their autocorrelation results with the
cross-correlation results of Font-Ribera and his team, and they
converged on narrow constraints for the BAO scale. Autocorrelation and
cross-correlation also converged in the precision of their measures of
the universe’s expansion rate, called the Hubble parameter. At redshift
2.34, the combined measure was equivalent to 68 plus or minus 1.5
kilometers per second per million light years.
“It’s the most precise measurement of the Hubble parameter at any
redshift, even better than the measurement we have from the local
universe at redshift zero,” says Font-Ribera. “These results allow us to
study the geometry of the universe when it was only a fourth its
current age. Combined with other cosmological experiments, we can learn
about dark energy and put tight constraints on the curvature of the
universe – it’s very flat!”
David Schlegel remarks that when BOSS was first getting underway, the
cross-correlation technique had been suggested, but “some of us were
afraid it wouldn’t work. We were wrong. Our precision measures are even
better than we optimistically hoped for.”
*****
“Quasar-Lyman α Forest Cross-Correlation from BOSS DR11: Baryon
Acoustic Oscillations,” by Andreu Font-Ribera, David Kirkby, Nicolás
Busca, Jordi Miralda-Escudé, Nicholas P. Ross, Anže Slosar, Éric
Aubourg, Stephen Bailey, Vaishali Bhardwaj, Julian Bautista, Florian
Beutler, Dmitry Bizyaev, Michael Blomqvist, Howard Brewington, Jon
Brinkmann, Joel R. Brownstein, Bill Carithers, Kyle S. Dawson, Timothée
Delubac, Garrett Ebelke, Daniel J. Eisenstein, Jian Ge, Karen Kinemuchi,
Khee-Gan Lee, Viktor Malanushenko, Elena Malanushenko, Moses Marchante,
Daniel Margala, Demitri Muna, Adam D. Myers, Pasquier Noterdaeme,
Daniel Oravetz, Nathalie Palanque-Delabrouille, Isabelle Pâris, Patrick
Petitjean, Matthew M. Pieri, Graziano Rossi, Donald P. Schneider, Audrey
Simmons, Matteo Viel, Christophe Yeche, and Donald G. York, has been
submitted to the Journal of Cosmology and Astropartical Physics and is now available online at arxiv.org/abs/1311.1767.
“Baryon Acoustic Oscillations in the Lyα forest of BOSS DR11
quasars,” by Timothée Delubac, Julian E. Bautista, Nicolás G. Busca,
James Rich, David Kirkby, Stephen Bailey, Andreu Font-Ribera, Anže
Slosar, Khee-Gan Lee, Matthew M. Pieri, Jean-Christophe Hamilton,
Michael Blomqvist, William Carithers, Daniel J. Eisenstein, J.-M. Le Go,
Daniel Margala, Jordi Miralda-Escudé, Adam Myers, Pasquier Noterdaeme,
Nathalie Palanque-Delabrouille, Isabelle Pâris, Patrick Petitjean,
Nicholas P. Ross, Graziano Rossi, David J. Schlegel, David H. Weinberg,
and Christophe Yèche, has been submitted to Astronomy & Astrophysics and is available on arXiv.org as of late Monday, April 7, and before then at www.sdss3.org/science/lyaauto.pdf.
The SDSS-III version of this release may be found at www.sdss3.org/press/precise.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’s Office of Science. This
research used resources of the National Energy Research Scientific
Computing Center (NERSC), which is supported by the Office of Science.
Visit SDSS-III at 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, University of Cambridge, 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)
provides high-end scientific production computing resources for DOE’s
Office of Science researchers, supporting work in a wide range of
disciplines that span the DOE missions. For more information visit www.nersc.gov/.
DOE’s Office of Science is the single largest supporter of basic
research in the physical sciences in the United States and is working to
address some of the most pressing challenges of our time. For more
information, please visit the Office of Science website at science.energy.gov/.
