Under the BigBOSS proposal, the NOAO Mayall Telescope (left) would be modified (inset) to cover a wider field of view, and it would be equipped with a new spectrographic instrument of unprecedented astrophysical grasp, capable of precisely measuring nearly 5,000 galaxies or stars simultaneously. (Mayall Telescope photo by Pete Marenfeld, NOAO/AURA/NSF) |
The National Optical Astronomy Observatory (NOAO), the National Science
Foundation’s (NSF’s) research and development center for ground-based
astronomy, has announced its conditional approval of the BigBOSS
Collaboration’s proposal to use 500 nights of valuable observing time on the
NOAO 4?meter Mayall Telescope on Kitt
Peak, Arizona. The
time would be used to build the biggest-ever map of the universe, for
investigating the mysterious dark energy that permeates the universe.
Key to the successful inauguration of BigBOSS will be construction of a
remarkable new spectrographic instrument capable of making simultaneous
measurements of thousands of astronomical objects. The instrument will be
available to all users of the Mayall telescope, and the data is expected to be
available in an archive for all astronomers and the public. The BigBOSS
Collaboration plans to seek funding needed for this instrumentation and
associated software from NSF and the U.S. Department of Energy.
“Approving BigBOSS to use the Mayall Telescope is the first step toward an
ambitious program to explore the expansion of the universe in detail,” says the
BigBOSS Collaboration’s principal investigator, cosmologist David Schlegel of
the Physics Division at DOE’s Lawrence Berkeley National Laboratory (Berkeley
Lab).
“BOSS” stands for Baryon Oscillation Spectroscopic Survey. In the course of
observations over five years, the BigBOSS program will target 50 million
objects and find precise locations for almost 20 million galaxies and quasars,
reaching back 10 billion years to the youthful universe. The BigBOSS map will
encompass 10 times the volume of the current best map of the universe, now
being assembled by the Sloan Digital Sky Survey III’s BOSS project, whose first
data were released to the world January 11, 2011, on their public website, http://www.sdss3.org/.
The goal of both BOSS and BigBOSS is to examine the expansion history of the
universe and study the nature of dark energy; both projects are led by Berkeley
Lab. The idea for BigBOSS emerged when studies for the proposed Joint Dark
Energy Mission space satellite indicated that very large numbers of
high-redshift galaxies could be observed and accurately measured using
reliable, economical ground-based telescopes.
BigBOSS, enabled by Kitt
Peak’s Mayall Telescope
and using the new spectrographic instrument, will reach much farther in space
and farther back in time than BOSS, across wider reaches of the sky. Important
contributions to the new instrument will come from BigBOSS’s 35 collaborating
institutions in the U.S. and
abroad, including institutions in France,
the United Kingdom, China, Spain,
and Korea.
Schlegel says, “By measuring baryon acoustic oscillation, BigBOSS will study
dark energy and can even test whether General Relativity is valid. What’s more,
the BigBOSS instrument will give the astronomical community an unprecedented
opportunity to make millions of observations for projects not connected to our
primary effort.”
Baryon
oscillation as a ruler to measure the universe
Baryon acoustic oscillation is cosmology-speak for the way galaxies tend to
bunch up at roughly 500-million-light-year intervals. These density
oscillations had their origin in the pressure waves that moved through the
liquid-like plasma of the early, hot universe. When the growing universe
“decoupled”—cooled down enough so that light and matter could go their separate
ways—the density oscillations were recorded in the cosmic microwave background,
where they can still be read today.
Since those regions denser in matter became the seeds of today’s galaxies
and groups of galaxies, the cosmic microwave background provides the starting
point for a natural ruler to measure how the universe has expanded since
decoupling. The greater the number of galaxies and quasars that can be used to
measure density fluctuations accurately over time, the more accurate the cosmic
ruler will be. This is the primary purpose of BigBOSS and its new
spectrographic instrument.
“BigBOSS Collaboration members Mike Sholl of the Univ.
of California at Berkeley’s Space Sciences Laboratory and Ming
Liang of NOAO discovered that the 4-meter Mayall was capable of a three-degree
field of view—much, much larger than had previously been recognized,” says BigBOSS
director Michael Levi of Berkeley Lab’s Physics Division.
Arjun Dey, BigBOSS Collaboration member and NOAO astronomer, says, “BigBOSS
will provide a much-needed unique and powerful scientific capability for the
venerable Mayall Telescope. Its ability to obtain measurements of nearly 5,000
galaxies or stars simultaneously will enable ground-breaking studies into the
nature of dark energy and the structure of our Milky Way galaxy, and will also
provide an instrument of unprecedented astrophysical grasp for the U.S.
astronomical community. Re-instrumenting and repurposing existing telescopes
like the Mayall provide the most cost-effective way of addressing the most
important scientific questions of our time.”
Levi adds, “To enable this science, a new astronomical CCD is being
developed at Berkeley Lab’s microsystems laboratory under the direction of
BigBOSS instrument scientist Natalie Roe. The new CCD will be supersensitive in
the red and infrared wavelengths needed to image very distant objects.”
Measuring the redshift of each galaxy reveals how much the universe has
expanded since its light left that galaxy. A red shift of 0.5, for example,
means the universe has expanded 50% since the emission of the light. Comparing
how distance varies with redshift for many millions of galaxies at different
times in the history of the universe will allow precise calibration of the
spacing of density oscillations at different epochs.
The
mystery of dark energy
Dark energy was discovered as a result of comparing the
brightness and redshift of individual Type
Ia supernovae, which revealed
that the universe is expanding at an accelerating rate. Dark energy has
“negative pressure”—that is, by stretching space it counteracts the mutual
gravitational attraction of all the matter in the universe, which would
otherwise slow down expansion. Although dark energy is thought to constitute
some 70% of the density of the universe, its nature is unknown.
Theories of dark energy abound, falling into two broad camps. Either dark
energy is constant and acceleration is steady, or dark energy varies in time,
perhaps even in space. A third possibility is even more radical: dark energy is
an illusion, brought on because Einstein’s General Theory of Relativity, the
best explanation of gravitation we have, is wrong or incomplete. With a bigger
map of the universe and a more precise measurement of its expansion history,
BigBOSS will go a long way toward providing the data needed to choose among
these possibilities.
Far beyond dark energy and the measurement of baryon acoustic oscillations,
the BigBOSS instrument and the publicly available databases BigBOSS creates
will have a major scientific impact on astronomy. The biggest-ever galactic
survey will provide new data on cosmological questions including the large- and
small-scale structure of the universe, neutrino mass, warm dark matter, and the
geometry of space. BigBOSS will provide an unparalleled resource for studying
the evolution of galaxies, including our own. It will provide a wealth of new
data on quasars. And it will be available for studying such topics as galaxy
clusters, planetary nebulae, giant stars, binary stars, and a host of other
individual observing programs.