Astronomers have defined the scale of the universe to within 1 percent accuracy, allowing them to better understand the enigmatic nature of dark energy and its ability to accelerate the expansion of the cosmos.
The Baryon Oscillation Spectroscopic Survey (BOSS) Collaboration is the largest program in the Sloan Digital Sky Survey-III, and researchers from the University of Utah contributed to its findings. The new measurement allows for a much more accurate picture of the universe and how it’s expanding.
"One-percent accuracy in the scale of the universe is the most precise such measurement ever made," says BOSS’s principal investigator, David Schlegel, a member of the Physics Division of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. "Twenty years ago, astronomers were arguing about estimates that differed by up to 50 percent. Five years ago, we’d refined that uncertainty to 5 percent; a year ago it was 2 percent. One-percent accuracy will be the standard for a long time to come."
BOSS involved scientists from across the country, including two U. scientists, Adam Bolton and Kyle Dawson, both assistant professors in the Department of Physics and Astronomy.
Bolton helped develop the software that allowed computers to analyze the light wavelengths from more than 1 million galaxies surveyed.
By looking at the light emitted from a galaxy, one can tell how far away it is because of the Doppler effect. In essence, it’s like hearing an ambulance siren: the faster it’s moving, the lower the pitch. The same is true for lightwaves, and scientists were looking at red-shift variations because the visible light spectrum stretches from blue to red.
"Our universe is expanding from the Big Bang, and those galaxies and stars and quasars that are farther away move away faster and are shifted toward longer wavelengths," Bolton said.
The gravitational pull of all of the galaxies in the universe should mean that the universe would be contracting. However, the universe is expanding, and doing so at an increasing rate. Scientists now say that expansion is being propelled by dark energy, but they don’t have a clear understanding of exactly what dark energy is. They hope this new measurement will help them better understand its behavior and its effect on the universe’s expansion.
When the universe was forming about 13.7 billion years ago, it started as electrons and protons. Eventually, it cooled down enough that galaxies could begin forming. The locations of where those galaxies formed depended on the density of the matter in locations throughout the universe. If one area was even slightly more dense than a neighboring area, galaxies would group together in those dense areas because of gravitational forces pulling in on each other.
"Small distinctions from the early universe begin to get amplified, and you have areas that have lots of galaxies and some that don’t have galaxies," Bolton said. "Length scales of the characteristics between those very early overdensities and underdensities are preserved and propagated down to present day. The relative density between these regions have been amplified greatly."
By measuring the Baryon acoustic oscillations — basically densities as indicated by the light wavelengths emitted — researchers were able to create a "standard ruler" for distances in the universe — though it’s in millions or billions of light years as opposed to inches or meters. The new measure will allow astronomers to create better models to understand how dark energy is accelerating the expansion of the universe, said Dawson, who oversaw the steps ranging from equipment at the observatories to getting data to scientists.
"What we want is the underlying model that describes that behavior, that’s the fundamental purpose," Dawson said.
Much like trying to understand the weather history in the desert by looking at the rings of a tree, the survey looks at galaxies from different epochs. This portion of the survey looked at galaxies that existed when the universe was about 8 billion years old; the light from those galaxies took about 5.5 billion years to reach Earth’s observatories, Dawson said. The goal is to be able to go back to 12 billion years ago.
The U. will have an even larger role in SDSS-IV, which is planned to begin in summer of 2014.
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