When Dark Energy Turned On
The Sloan Digital Sky Survey (SDSS-III) announced the most accurate measurements yet of the distances to galaxies in the faraway universe, giving an unprecedented look at the time when the universe first began to expand at an ever-increasing rate. Scientists from the University of Portsmouth and the Max-Planck Institute for Extraterrestrial Physics will present the new results in a press conference at 10:00 BST on Friday, March 30th at the National Astronomy Meeting in Manchester.
The results are available in six related papers posted to the arXiv preprint server and are the culmination of more than two years of work by the team of scientists and engineers behind the Baryon Oscillation Spectroscopic Survey (BOSS), one of the SDSS-III’s four component surveys.
“There’s been a lot of talk about using galaxy maps to find out what’s causing accelerating expansion,” says David Schlegel of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, the principal investigator of BOSS. “We’ve been making a map, and now we’re using it – starting to push our knowledge out to the distances when dark energy turned on.”
“The result is phenomenal,” says Will Percival, a professor at the University of Portsmouth in the United Kingdom, and one of the leaders of the analysis team. “We have only one-third of the data that BOSS will deliver, and that has already allowed us to measure how fast the Universe was expanding six billion years ago – to an accuracy of two percent.”
One of the most amazing discoveries of the last two decades in astronomy, recognized with the 2011 Nobel Prize in Physics, was that not only is our universe expanding, but that this expansion is accelerating – not only are galaxies are becoming farther apart from each other, they are becoming farther apart faster and faster.
What could be the cause of this accelerating expansion? The leading contender is a strange property of space dubbed “dark energy.” Another explanation, considered possible but less likely, is that at very large distances the force of gravity deviates from Einstein’s General Theory of Relativity and becomes repulsive.
Whether the answer to the puzzle of the accelerating universe is dark energy or modified gravity, the first step to finding that answer is to measure accurate distances to as many galaxies as possible. From those measurements, astronomers can trace out the history of the universe’s expansion.
BOSS is producing the most detailed map of the universe ever made, using a new custom-designed spectrograph of the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico. With this telescope and its new spectrograph, BOSS will measure spectra of more than a million galaxies over six years. The maps analyzed in today’s papers are based on data from the first year and a half of observations, and contain more than 250,000 galaxies. Some of these galaxies are so distant that their light has travelled more than six billion years to reach the earth – nearly half the age of the universe.
Maps of the universe like BOSS’s show that galaxies and clusters of galaxies are clumped together into walls and filaments, with giant voids in between them. These structures grew out of subtle variations in density in the early universe, which bore the imprint of “baryon acoustic oscillations” – pressure-driven (acoustic) waves that passed through the early universe.
Billions of years later, the record of these waves can still be read in our universe. “Because of the regularity of those ancient waves, there’s a slightly increased probability that any two galaxies today will be separated by about 500 million light-years, rather than 400 million or 600 million,” says Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics, director of SDSS-III and a pioneer in baryon oscillation surveys for nearly a decade. In a graph of the number of galaxy pairs by separation distance, that magic number of 500 million light years shows up as a peak, so astronomers often speak of the “peak separation” between galaxies. The distance that corresponds to this peak depends on the amount of dark energy in the Universe. But measuring the peak separation between galaxies depends critically on having the right distances to the galaxies in the first place.
That’s where BOSS comes in. “We’ve detected the peak separation more clearly than ever before,” says Nikhil Padmanabhan of Yale University, who along with Percival co-chairs the BOSS team’s galaxy clustering group. “These measurements allow us to determine the contents of the Universe with unprecedented accuracy.”
In addition to providing highly accurate distance measurements, the BOSS data also enable a stringent new test of General Relativity, explains Beth Reid, a NASA Hubble Fellow at Lawrence Berkeley National Laboratory. “Since gravity attracts, galaxies at the edges of galaxy clusters fall in toward the centres of the clusters,” Reid says. “General Relativity predicts just how fast they should be falling. If our understanding of General Relativity is incomplete, we should be able to tell from the shapes we see in BOSS’s maps near known galaxy clusters.”
Reid led the analysis of these “redshift space distortions” in BOSS. After accounting for the effects of dark energy, Reid’s team found that the rate at which galaxies fall into clusters is consistent with Einstein’s predictions. “We already knew that the predictions of General Relativity are extremely accurate for distances within the solar system,” says Reid, “and now we can say that they are accurate for distances of 100 million light-years.
We’re looking a billion times further away than Einstein looked when he tested his theory, but it still seems to work.”
What’s amazing about these results – six papers covering the measurements of cosmic distance and the role of gravity in galaxy clustering – is that they all come together to tell the same story. “All the different lines of evidence point to the same explanation,” says Ariel Sanchez, a research scientist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and lead author on one of the papers. “Ordinary matter is only a few percent of the universe. The largest component of the universe is dark energy – an irreducible energy associated with space itself that is causing the expansion of the universe to accelerate.”
But this is just the beginning, says BOSS principal investigator Schlegel. “For the past 13 years, we’ve had a simple model of how dark energy works. But the truth is, we only have a little bit of data, and we’re just beginning to explore the times when dark energy turned on. If there are surprises lurking out there, we expect to find them.”
Image Caption: The record of baryon acoustic oscillations (white rings) in galaxy maps helps astronomers retrace the history of the expanding universe. (Credit: E. M. Huff, the SDSS-III team, and the South Pole Telescope team. Graphic by Zosia Rostomian.)
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