NATIONAL HARBOR, Md.—More galaxies are separated by about 490 million light-years than by any other large distance, astronomers have found in the most precise measurement yet of this key cosmic length scale. Using this scale, researchers calculated astronomical distances with a record low level of 1 percent uncertainty in a measurement that helps clarify what is behind the unexplained dark energy causing the universe's expansion to accelerate.
The separation of 150 megaparsecs, or nearly 490 million light-years, is an artifact of the birth of the universe, which created tiny ripples in the density of matter that caused material in some spots to clump together into the seeds of galaxies. When the universe was young and very hot, these overdense spots could not contain their own pressure, so they emitted sound waves into space that traveled until the universe cooled down and neutral atoms formed. "Basically every one of these regions in the universe has thrown off a sound wave which has traveled off for a distance that is 150 megaparsecs today," says Daniel Eisenstein of Harvard–Smithsonian Center for Astrophysics. And where those sound waves stopped, they gave the matter there a kick that made it, too, more likely to form the seed of a galaxy. Eisenstein, who directed the Sloan Digital Sky Survey (SDSS) III that collected the data used for the measurement, presented the results today here at the 223rd meeting of the American Astronomical Society.
These periodic ripples in the spread of galaxies across space are called baryon acoustic oscillations, and they have profound implications for understanding the evolution of the universe. Using such oscillations, astronomers can compare the present-day separation of galaxies with the size of these ripples just after the universe was born to learn how space has stretched over time. "It's really allowing us to track the expansion history of the universe very accurately," Eisenstein says. This, in turn, constrains the properties of the dark energy that is driving this accelerating expansion.
The new measurement comes from a project called BOSS (Baryon Oscillation Spectroscopic Survey), which used maps of the locations of 1.2 million galaxies captured by SDSS-III. Baryon acoustic oscillations were first measured in 2005 by teams from SDSS and another project called the 2dF Galaxy Redshift Survey. "The original measurements were only bare detections," says Matthew Colless of the Australian National University, who led the 2dF experiment. "The latest BOSS results are real precision cosmology." Colless is now working on two experiments, the 6dF Galaxy Survey and WiggleZ Dark Energy Survey, which are measuring baryon acoustic oscillations at nearer and farther distances, respectively, than SDSS-III can reach. For the range it covers, however, SDSS-III achieves significantly more precision. "It's definitely an impressive milestone and it provides one of the present cornerstones of our cosmology," says one of the co-discoverers of dark energy, Adam Riess of the Space Telescope Science Institute in Baltimore, who was not involved in BOSS. "These measurements are very difficult and dark energy is very mysterious, so we are trying very hard to understand it."
What dark energy is and whether it changes over time are two of the greatest unsolved mysteries in physics. If it is a static property, it behaves very much like the "cosmological constant" conceived by Albert Einstein when he formulated his equations of general relativity back in the early 20th century. Einstein originally added the constant term to force his equations to predict a static universe, but discarded the concept once scientists realized the cosmos was actually expanding. In the 1990s, however, scientists resurrected the idea when they discovered dark energy.
The latest baryon acoustic oscillation measurement fits extremely well with predictions based on the cosmological constant. "The data today is consistent with the cosmological constant, but the goal of improving the precision is to try to test that further," Eisenstein says. If dark energy is a cosmological constant, scientists are no closer to knowing why it exists at all. Some researchers are hoping subtle deviations from the cosmological constant model will crop up in measurements like these to point the way toward a deeper explanation of dark energy. "As for how big deviations to expect," Riess says, "that is a real problem—we just don't know."