Scale of Universe Measured with 1-Percent Accuracy

[Buster's Uncle]

New Cosmic Distance Measurement Points the Way to Elusive Dark Energy
Scientific American
By Clara Moskowitz  January 8, 2014 2:45 PM



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."


http://news.yahoo.com/cosmic-distance-measurement-points-way-elusive-dark-energy-194500871.html

[Buster's Uncle]

Scale of Universe Measured with 1-Percent Accuracy
SPACE.com
by Miriam Kramer, Staff Writer  January 8, 2014 2:20 PM



An artist's concept of the latest, highly accurate measurement of the universe from the Baryon Oscillation Spectroscopic Survey



WASHINGTON — An ultraprecise new galaxy map is shedding light on the properties of dark energy, the mysterious force thought to be responsible for the universe's accelerating expansion.

A team of researchers working with the Baryon Oscillation Spectroscopic Survey (BOSS) has determined the distances to galaxies more than 6 billion light-years away to within 1 percent accuracy — an unprecedented measurement.

"There are not many things in our daily lives that we know to 1-percent accuracy," David Schlegel, a physicist at Lawrence Berkeley National Laboratory and the principal investigator of BOSS, said in a statement. "I now know the size of the universe better than I know the size of my house."

Scientists working with BOSS mapped the locations of 1.2 million galaxies and found that their new measurements support the idea of the "cosmological constant" — an idea first proposed by Albert Einstein. This idea suggests that dark energy has remained constant throughout the history of the universe.

"We don’t yet understand what dark energy is, but we can measure its properties," Daniel Eisenstein, a Harvard University astronomer working with the survey, said in a statement. "Then, we compare those values to what we expect them to be, given our current understanding of the universe. The better our measurements, the more we can learn."

The new results, presented by Schlegel here today (Jan. at the 223rd meeting of the American Astronomical Society, also provide one of the best-ever determinations of the curvature of space, researchers said. In short, the universe appears to be quite "flat," meaning that its shape can be described well by Euclidean geometry, in which straight lines are parallel and the angles in a triangle add up to 180 degrees.

"One of the reasons we care is that a flat universe has implications for whether the universe is infinite," Schlegel said. "That means — while we can't say with certainty that it will never come to an end — it's likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe."

BOSS astronomers use a spectrograph on the Sloan Foundation's 2.5-meter telescope at Apache Point Observatory in New Mexico to make their observations.

"On a clear night when everything goes perfectly, we can add more than 8,000 galaxies and quasars to the map," Kaike Pan, who leads the team of observers working with the spectrograph, said in a statement.

The team has presented other results from the galaxy survey. However, the new map extends the survey's reach, more than doubling the size of the examined area and thereby allowing for more precise measurements. The newly presented data also include earlier results that surveyed nearby galaxies, BOSS officials said.

"Making these measurements at two different distances allows us to see how the expansion of the universe has changed over time, which will help us understand why it is accelerating," explained University of Portsmouth astronomer Rita Tojeiro, who co-chairs the BOSS galaxy clustering working group along with Jeremy Tinker of New York University.


http://news.yahoo.com/scale-universe-measured-1-percent-accuracy-192046198.html