Massive 3D map of the universe tracks dark energy like never before

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This is one slice through the map of the large-scale structure of the universe from the Sloan Digital Sky Survey and its Baryon Oscillation Spectroscopic Survey, with each dot indicating the position of a galaxy 6 billion years into the past(Credit: Daniel Eisenstein and the SDSS-III collaboration)

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An international consortium of astronomers says that it's made the most precise measurements to date of dark energy, while pinpointing the positions of 1.2 million galaxies in the largest ever 3D map of the universe. The map covers over a quarter of the sky, charting over a volume of 650 cubic billion light years.

The project was prepared and conducted over five years by hundreds of scientists from the Sloan Digital Sky Survey III (SDSS-III) in its Baryon Oscillation Spectroscopic Survey (BOSS) program, which explored how dark energy drives the expansion of the universe. The SDSS-III collaboration is a multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5m wide-angle optical telescope, located at Apache Point Observatory in New Mexico. In short, the researchers surveyed a section of the sky to measure redshift, which happens when astronomical objects move away from us, shifting light to the red end of the spectrum as its wavelengths grow longer. It's a key concept in charting the expansion of the universe.

Dark energy is the mysterious, repulsive force that astronomers believe is causing the universe to expand at an accelerating rate. The map is shaped by the competing forces of dark energy and dark matter — a theoretical material that doesn't emit light or energy — and it allows researchers to measure the universe's expansion rate. From that, they're then able to determine the amount of dark energy and matter that make up the universe.

"Dark energy is 69 percent of the 'energy density' in the universe today," David Schlegel, astrophysicist at Lawrence Berkeley National Laboratory and principal investigator for BOSS, told Gizmag. "What that means is that it's dominating how the universe is evolving today, and we think will eventually tear it apart such that we can no longer see any other galaxies.

The expansion rate of the universe is determined by the size of the baryonic acoustic oscillations (BAO) in the 3D arrangement of galaxies. BAO are fluctuations or pressure waves in the density of visible matter, akin to sound waves that leave an acoustic imprint. Researchers looked at the pressure waves and their imprints after the big bang, which were frozen in the distribution of matter of the 400,000-year-old universe. They could then see a sharp connection between that newly born universe and the clustering of galaxies 7 to 12 billion years later.

"One of our main results is exactly this – the expansion rate of the universe, which we measure at several different times," said Schlegel. "The present-day universe is expanding by 1 percent in each direction every 145 million years. If there were no dark energy, it would be expanding more slowly."

The map is able to measure how much the galaxies and stars cluster together as a function of time, and with such accuracy that researchers are now able test general relativity at a cosmological level. They can also see galaxies moving toward regions of the universe with more matter, pulled there through the attractive force of gravity.

"The force of gravity - general relativity - is usually measured and tested on scales of miles, thousands of miles, or millions of miles in the solar system," added Schlegel. "With these new maps, we're seeing what gravity is doing on scales that are billions of times larger than that. Some theories have conjectured that the gravitational force would be different on those large scales, but we see the same gravitational force as described originally by Einstein at work on those largest of scales."

Results of the map project were described in a series of papers submitted to the Monthly Notices of the Royal Astronomical Society.

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