The Sloan Digital Sky Survey, or SDSS-III, has 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.
The results, published in six related papers, are the culmination of more than two years of work by the team of scientists and engineers behind the Baryon Oscillation Spectroscopic Survey, or BOSS, one of the SDSS-III's four surveys of the sky.
"There has been a lot of talk about using galaxy maps to find out what is causing accelerating expansion," said David Schlegel of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, the principal investigator of BOSS. "We have been making a map, and now we're using it. We are starting to push our knowledge out to the distances when dark energy turned on."
One of the most amazing discoveries of the last two decades in astronomy is that our universe not only is expanding but doing so faster and faster. Brian P. Schmidt, who graduated from the University of Arizona with a double major in astronomy and physics in 1989, shared last year's Nobel Prize in Physics for this discovery.
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 over 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 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 1 million galaxies over six years. Galaxy spectra allow astronomers to measure the distance of individual galaxies and make 3-dimensional maps of the universe.
The maps analyzed in the papers just published 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 their light has traveled more than 6 billion years to reach the Earth – nearly half the age of the universe.
"By looking both very far and at lots of galaxies, SDSS has compiled the largest catalog of massive galaxies yet," said Ramin Skibba, a postdoctoral fellow at the UA's Steward Observatory and a co-author of one of the publications. "The advantage is that those tend to be in the densest regions of the universe, and they're very bright."
Maps of the universe like BOSS's show that galaxies and clusters of galaxies are clumped together into walls and filaments, with giant voids between. 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 young universe.
"Imagine tossing a rock into a lake making ripples," explained Xiaohui Fan, a professor of astronomy at the UA's Steward Observatory and a member of the SDSS-III advisory committee. "As the universe cooled down after the Big Bang, these ripples became a frozen wave pattern in the universe."
Billions of years later, the record of these waves can still be read in our universe as it expands.
"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," said Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics, director of SDSS-III and a pioneer in baryon oscillation surveys for nearly a decade.
Eisenstein started the project during his tenure as a professor at the UA's Steward Observatory, one of the SDSS partnering institutions. Working with Eisenstein, UA graduate students Xiaoying Xu and Kushal Mehta, co-authors of the paper, carried out detailed theoretical modeling and analysis that are crucial for interpreting the BOSS results.
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.
"Galaxies are distributed along that frozen wave pattern, so by looking at these patterns expanding over time, we are measuring how the universe is expanding. This pattern is used as a standard ruler, a cosmic yardstick so to speak," explained Fan. "The measurements allow us to get a more precise idea of what the expansion rate is and we get a better idea of relativity. These results enable physicists to test their theories on general relativity and dark energy."
"The signal is very subtle, and and requires a great many of galaxy measurements," said Skibba, an expert on galaxy evolution. "To do this, you have to really understand how galaxies evolve, and much of that work has been done here at the UA."
In addition to providing highly accurate distance measurements, the BOSS data also enable a stringent new test of General Relativity, explained 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 centers of the clusters," she said. "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," said 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," said 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, said 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."
Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the participating Institutions, the National Science Foundation and the U.S. Department of Energy Office of Science.