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BOSS: Dark Energy and the Geometry of Space

The SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS) will map the spatial distribution of luminous red galaxies (LRGs) and quasars to detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Sound waves that propagate in the early universe, like spreading ripples in a pond, imprint a characteristic scale on cosmic microwave background fluctuations. These fluctuations have evolved into today's walls and voids of galaxies, meaning this baryon acoustic oscillation (BAO) scale (about 150 Mpc) is visible among galaxies today. This concept is illustrated below (some of the relative scales have been exaggerated for illustration purposes).

The baryon acoustic
oscillation imprint a fixed scale in the universe just 300,000 years
after the Big Bang (shown as white circles). These ripples in the
density of matter even remain today in the distribution of galaxies
and BOSS is designed to measure the size of these ripples better
than any other existing survey.
An illustration of the concept of baryon acoustic oscillations, which are imprinted in the early universe and can still be seen today in galaxy surveys like BOSS
(Illustration courtesy of Chris Blake and Sam Moorfield).
Power Spectrum of LRGs.
Comparison of the power spectrum of SDSS-II LRGs and BOSS DR9 CMASS galaxies. Solid lines show the best-fit models. From Anderson et al. 2012.

These baryon acoustic oscillations have now been measured in the distribution of galaxies as illustrated on the right, where we show the power spectrum of galaxy fluctuations, as a function of scale (shown here as a wave number, k). We have removed the smooth component to more clearly show the oscillations, which are the BAO signal of interest. The two panels show the previous results from SDSS-II DR7 LRGs (top) and the new results from the z = 0.55 BOSS DR9 sample (CMASS, bottom). The reduction in errors and the better fit to the cosmological model (solid line) of the BOSS data compared to the SDSS-II is clearly evident. When the survey is completed, the errors on the BOSS measurements will be further reduced, and we will also measure the BAO at several other redshifts.

Using the acoustic scale as a physically calibrated ruler, BOSS will determine the angular diameter distance with a precision of 1% at redshifts z = 0.3 and z = 0.55 using the distribution of galaxies. It will also measure the distribution of quasar absorption lines at z = 2.5, yielding a measurement of the angular diameter distance at that redshift to an accuracy of 1.5%. It will also measure the cosmic expansion rate H(z) with 1-2% precision at the same redshifts. These measurements will provide demanding tests for theories of dark energy and the origin of cosmic acceleration.

BOSS at a glance
Dark time observations
Fall 2009 - Spring 2014
1,000-fiber spectrograph, resolution R~2000
Wavelength: 360-1000 nm
10,000 square degrees
Redshifts of 1.5 million luminous galaxies to z = 0.7
Lyman-α forest spectra of 160,000 quasars at redshifts 2.2 < z < 3
Summary statistics of the DR9 release
Summary statistics of the DR10 release

In addition to constraining cosmological models, BOSS will deliver an outstanding sample of galaxies and quasars ideally suited to probing the formation and evolution of galaxies in the Universe. For example, we show below an example z =0.56 galaxy spectrum for early BOSS data which shows the level of spectral detail we can expect on these intermediate redshift galaxies. By studying the various key absorption features in this spectrum (labeled and shown with red tickmarks), we can estimate the age and metallicity of the stars in these galaxies, and thus determine how and when they formed. The sheer size of BOSS, coupled with the BOSS spectrograph resolution, will greatly enhance this area of science.

A randomly
selected DR10 BOSS galaxy spectrum.
A randomly selected spectrum from the dr10 BOSS data, showing absorption (red) and emission (blue) lines. Click on the image to go to this object's page on SkyServer.

For a detailed description of BOSS, see Section 3 of the Project Description PDF.

Working with BOSS data

Data Release 10 is the latest public release of BOSS spectra. Data Release 9 was the first public release of BOSS spectra. Prior to that, DR8 included all BOSS imaging.

BOSS papers

BOSS has submitted and/or published several papers which are listed on the BOSS Publications page.