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Observing the Galaxy Distribution When the Universe Was Half Its Current Age

Press Release From: Max-Planck-Institut für extraterrestrische Physik
Posted: Friday, March 30, 2012

At the U.K.-Germany National Astronomy Meeting NAM2012, the Baryon Oscillation Spectroscopic Survey (BOSS) team today announced the most accurate measurement yet of the distribution of galaxies between five and six billion years ago. This was the key 'pivot' moment at which the expansion of the universe stopped slowing down due to gravity and started to accelerate instead, due to a mysterious force dubbed "dark energy". The nature of this "dark energy" is one of the big mysteries in cosmology today, and scientists need precise measurements of the expansion history of the universe to unravel this mystery -- BOSS provides this kind of data. In a set of six joint papers presented today, the BOSS team, an international group of scientists with the participation of the Max Planck Institute of Extraterrestrial Physics in Garching, Germany, used these data together with previous measurements to place tight constraints on various cosmological models.

The BOSS survey, which is a part of the Sloan Digital Sky Survey (SDSS-III), was started in 2009 to probe the universe at a time when dark energy started to dominate. The survey will continue until 2014, collecting data for 1.35 million galaxies with a custom-designed new spectrograph on the 2.5-meter Sloan Telescope at the Apache Point Observatory in New Mexico, USA. In the first year-and-a-half, it has already mapped the three-dimensional positions of more than a quarter of a million galaxies spread across about one tenth of the sky, yielding the most accurate and complete map of the galaxy distribution up to a distance of about 6 billion light-years.

Galaxies form a "cosmic web" with a variety of structures which encode valuable information about our universe. One particular feature, the so-called "Baryonic Acoustic Oscillations" (BAO), has been subject of much interest from scientists as it provides them with a "standard rod". BAO are a relic of the early phases of the universe, when it was a hot and dense "soup" of particles. Small variations of density travelled through this "soup" as pressure-driven (sound) waves. As the universe expanded and cooled, the pressure dropped, causing these waves to stall after they had traveled about 500 million light-years. These frozen waves imprinted a particular signature on the matter distribution and are visible in the galaxy map today: it is in fact slightly more probable to find pairs of galaxies separated by this scale than at smaller or larger distances.

Measurements of the apparent size of the BAO scale in the galaxy distribution then provide information about cosmic distances. Combined with the measurement of the galaxies' redshift -- a measure for how fast they move away as a result of the cosmic expansion -- scientists can then reconstruct the expansion history of the universe.

The new BOSS data, combined with previous analyzes, can now constrain the parameters of the standard cosmological model to an accuracy of better than five percent. "All the different lines of evidence point to the same explanation," says Dr. Ariel Sanchez, scientist at the Max Planck Institute for Extraterrestrial Physics and lead author of one of the six papers released today. "The dark energy is consistent with Einstein's cosmological constant: a small but irreducible energy continually stretching space itself, driving the accelerated expansion of the universe."

Besides dark energy, the information encoded in the large-scale distribution of galaxies can be used to obtain robust constraints on other important physical parameters such as the curvature of the universe, the neutrino mass, or the phase of inflation in the very early universe. "Current observations show that the universe has to be flat, to an accuracy better than 0.5 percent," says Ariel Sanchez. "And at the same time as we measure such a global parameter on a cosmic scale, we can also get information about neutrinos on the smallest scales in the cosmos."

Neutrinos are tiny, subatomic particles. Even though a number of experiments have shown that they must have mass, scientists do not know how much they actually weigh, as it is difficult to measure this in a laboratory. However, as an additional component in the hot, early phase of the universe the neutrinos affect the growth of structures. The galaxy distribution probed by BOSS provides information about the maximum mass that these neutrinos are allowed to have. "This is really the connection of two extreme worlds, the very large and the very small", adds Ariel Sanchez.

The quality of the new data even provided the BOSS team with new clues about cosmic inflation, a period of time shortly after the Big Bang during which the universe expanded at an incredible rate. During cosmic inflation, small regions of space were blown out to form our entire observable universe. At the same time, tiny quantum fluctuations also expanded and became the seeds of the structures that the BOSS data show us today. "There is a real zoo of alternative models of inflation. With BOSS we get important new clues about the inflationary phase of the universe, which allows us to pare down the market of available models", remarks Ariel Sanchez.

So far, all measurements are highly consistent with the standard cosmological model, which is made up of a few percent ordinary matter, about a quarter of dark matter, and the rest dark energy. But Ariel Sanchez is cautious: "This is just the beginning. We can expect much tighter constraints once we have the full five years of BOSS data. There are also a number of future projects, such as EUCLID, that will provide even better measurements, bringing us one step closer to finding answers to the big open questions in cosmology."


1. The Sloan Digital Sky Survey (SDSS) has been collecting deep, multi-color images covering more than a quarter of the sky since 2000. The Max Planck Institute for Extraterrestrial Physics is a full member institution for SDSS-III. SDSS-III Website:

2. EUCLID is an ESA space mission dedicated to the analysis of Dark Energy with a major participation by the Max Planck Institute for Extraterrestrial Physics. EUCLID Website:

Fig 1: A map of the galaxies in a thin slice of the BOSS catalogue. We are at the center of the arc, outside the bottom part of the figure, and each black point is a galaxy. The red circle shows the approximate size of the BAO feature. Credit: Francesco Montesano/Max Planck Institute for Extraterrestrial Physics, Sloan Digital Sky Survey III

Fig 2: The record of baryon acoustic oscillations (white rings) in galaxy maps helps astronomers retrace the history of the expanding universe. These schematic images show the universe at three different times. The false-color image on the right shows the "cosmic microwave background," a record of what the very young universe looked like, 13.7 billion years ago. The small density variations present then have grown into the clusters, walls, and filaments of galaxies that we see today. These variations included the signal of the original baryon acoustic oscillations (white ring, right). As the universe has expanded (middle and left), evidence of the baryon oscillations has remained, visible in a "peak separation" between galaxies (the larger white rings). The SDSS-III results announced today (middle) are for galaxies 5.5 billion light-years distant, at the time when dark energy turned on. Comparing them with previous results from galaxies 3.8 billion light-years away (left) measures how the universe has expanded with time. Credit: E. M. Huff, the SDSS-III team, and the South Pole Telescope team. Graphic by Zosia Rostomian.


Dr. Hannelore Haemmerle
Press Officer
Max-Planck-Institut fuer Extraterrestrische Physik, Garching
+49 89 30000-3980

Dr. Ariel Sanchez
Max-Planck-Institut fuer Extraterrestrische Physik, Garching
+49 89 30000-3776, cell: +49 176 8006 3852

Original publications:
The BOSS team:
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: baryon acoustic oscillations in the data release 9 spectroscopic galaxy sample

Ariel G. Sanchez et al.:
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological implications of the large-scale two-point correlation function

Beth Reid et al.:
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: measurements of the growth of structure and expansion rate at z=0.57 from anisotropic clustering

Ashley J. Ross el al.:
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: analysis of potential systematics

Rita Tojeiro et al.:
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: measuring structure growth using passive galaxies

Marc Manera et al.:
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: a large sample of mock galaxy catalogues

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