Astronomers announced this week the sharpest results yet on the properties of dark energy. Hundreds of scientists from the Sloan Digital Sky Survey III (SDSS-III) collaborated to make the largest-ever, three-dimensional map of distant galaxies. The scientists then used this map to make one of the most precise measurements yet of the dark energy currently driving the accelerated expansion of the universe.
“We have spent a decade collecting measurements of 1.2 million galaxies over one quarter of the sky to map out the structure of the universe over a volume of 650 cubic billion light-years,” says Jeremy Tinker of New York University, a co-leader of the scientific team that led this effort. “This map has allowed us to make the best measurements yet of the effects of dark energy in the expansion of the universe. We are making our results and map available to the world.”
These new measurements were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) program of SDSS-III. Shaped by a continuous tug-of-war between dark matter and dark energy, the map revealed by BOSS allows astronomers to measure the expansion rate of the universe and thus determine the amount of matter and dark energy that make up the present-day universe. A collection of papers describing these results was submitted this week to the Monthly Notices of the Royal Astronomical Society.
BOSS measures the expansion rate of the universe by determining the size of the baryon acoustic oscillations (BAO) in the three-dimensional distribution of galaxies. The original BAO size is determined by pressure waves that travelled through the young universe up to when it was only 400,000 years old (the universe is presently 13.8 billion years old), at which point they became frozen in the matter distribution of the universe. The end result is that galaxies are preferentially separated by a characteristic distance, that astronomers call the acoustic scale.
The size of the acoustic scale at 13.48 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen. Measuring the distribution of galaxies since that time allows astronomers to measure how dark matter and dark energy have competed to govern the rate of expansion of the universe.
Ariel Sanchez of the Max-Planck Institute of Extraterrestrial Physics led the effort to estimate the exact amount of dark matter and dark energy based on the BOSS data and explains: “Measuring the acoustic scale across cosmic history gives a direct ruler with which to measure the universe’s expansion rate. With BOSS, we have traced the BAO’s subtle imprint on the distribution of galaxies spanning a range of time from 2 to 7 billion years ago.”Rita Tojeiro of the University of St. Andrews is the other co-leader of the BOSS galaxy clustering working group along with Tinker. “If we were to scale the volume of our survey to a cube 1 mile on each side, then the visible part of an individual galaxy would be about 1 millimetre across,” she explains. “Our challenge in the analysis of this map was equivalent to measuring the distances between all the pairs of galaxies separated by 100 yards throughout a cubic mile of space!”
To measure the size of these ancient giant waves to such sharp precision, BOSS had to make an unprecedented and ambitious galaxy map, many times larger than previous surveys. At the time the BOSS program was planned, dark energy had been previously determined to significantly influence the expansion of the universe starting about 5 billion years ago. BOSS was thus designed to measure the BAO feature from before this point (7 billion years ago) out to near the present day (2 billion years ago).
Florian Beutler of University of Portsmouth, who led two of the papers that were submitted this week, says “If dark energy has been driving the expansion of the universe over that time, our maps tells us that it is evolving very slowly if at all: the change is at most 20 percent over the past seven billion years.”
Jose Vazquez of Brookhaven National Laboratory, who combined the BOSS results with other surveys and searched for evidence for any indication of unexplained physical phenomena in the results, says “Our latest results tie into a clean cosmological picture, giving strength to the standard cosmological model that has emerged over the last eighteen years.”
Tojeiro summarises that “we see a dramatic connection between the sound wave imprints seen in the cosmic microwave background 400,000 years after the Big Bang to the clustering of galaxies 7-12 billion years later. The ability to observe a single well-modelled physical effect from recombination until today is a great boon for cosmology.”
The map also reveals the distinctive signature of the coherent movement of galaxies toward regions of the universe with more matter, due to the attractive force of gravity. Crucially, the observed amount of infall is explained well by the predictions of general relativity. This agreement supports the idea that the acceleration of the expansion rate is driven by a phenomenon at the largest cosmic scales, such as dark energy, rather than a breakdown of our gravitational theory.
Tinker concludes that “BOSS has marked an important cosmological milestone, combining precise clustering measurements of an enormous volume with extensive observations of the primary cosmic microwave background to produce a firm platform for the search for extensions to the standard cosmological model. We look forward to seeing this program extended with the coming decade of large spectroscopic surveys.”