At the UK-Germany National Astronomy Meeting NAM2012, the Baryon Oscillation Spectroscopic Survey (BOSS) team has just 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 March 30, 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-metre 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 analyses, can now constrain the parameters of the standard cosmological model to an accuracy of better than five per cent. "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 new papers. "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 per cent," 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 per cent 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."