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).

These baryon acoustic oscillations have now been seen in the distribution of galaxies as illustrated below, where we show the power spectrum of galaxy fluctuations, as a function of scale (shown here as a wave number or k). The inset plot shows the power spectrum again but with the smooth component removed, thus demonstrating the oscillations which are the BAO signal of interest. We show two cosmological models which illustrate the location and amplitude of these oscillations can be accurately predicted and used to constrain cosmology.

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 = 0.6 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.

The power spectrum of LRGs in the original SDSS-II. (Illustration courtesy of Will Percival). See Percival et al. 2010.

BOSS at a glance
Dark time observations
Fall 2009 - Spring 2014
1,000-fiber spectrograph, resolution R~2000
Wavelengths 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

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 stacked LRG 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 (labelled and shown in grey), we can estimate the age and metallicity of the stars in these galaxies, and thus determine how and went they formed. The sheer size of BOSS, coupled with the BOSS spectrograph resolution, will greatly enhance this area of science.

A stacked spectrum from the early BOSS data.
(Illustration courtesy of Daniel Thomas).

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