The Great Space Coaster: Expansion of the Universe Now Measured in an Era before Dark Energy Takes Over

An illustration showing how SDSS-III was able to measure the distant universe

An illustration showing how SDSS-III was able to measure the distant universe.
Light rays from distant quasars (dots at left) are partially absorbed as they pass through clouds of intergalactic hydrogen gas (center). When the light arrives at the spectrograph of the Sloan Foundation 2.5-Meter Telescope (square at right), some has been absorbed, leaving behind a record in the form of a "forest" of small absorption lines in the observed spectrum. These lines can be interpreted to make a map of the gas along the line of sight between us and the quasar.
By examining light from thousands of quasars all over the sky, astronomers can make a detailed three-dimensional map of the distant universe. In this illustration, the dots at the far left are quasars, and the thin lines show light rays that left those quasars more than 10-billion years ago. Yellow dots are quasars that had been measured by prior projects of the Sloan Digital Sky Survey. By measuring the spectra from ten times as many quasars in this range (red dots), BOSS can reveal the large-scale structure of the early universe in much greater detail.

CREDIT: Zosia Rostomian, LBNL; Nic Ross, BOSS Lyman-alpha team, LBNL; and Springel et al, Virgo Consortium and the Max Planck Institute for Astrophysics

13 November 2012

13 November 2012 — For the past five-billion years, the expansion of the universe has been powered by a mysterious repulsive force known as "dark energy." Now, thanks to a new technique for measuring the three-dimensional structure of the distant universe, scientists in an international team within the Sloan Digital Sky Survey (SDSS-III), including an astronomer at Penn State University, have made the first measurement of the rate of this cosmic expansion as it was just three-billion years after the Big Bang.

"Observations in the past 15 years have revealed that the expansion rate of the universe is accelerating," said Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State, a coauthor of the study. "Most cosmological models predict that when the universe was young, dark energy had little influence on the expansion; at that time the evolution of the large-scale structure of the universe was dominated by gravitation, which is an attractive force that acted to slow the expansion. The new SDSS-III observations are an important probe of this early era." Schneider is the Sloan Digital Sky Survey's survey coordinator and scientific publications coordinator.

"If we think of the universe as a roller coaster, then today we are rushing downhill, gaining speed as we go," said Nicolas Busca of the Laboratoire Astroparticule et Cosmologie of the French Centre National de la Recherche Scientifique (CNRS), one of the lead authors of the study." Our new measurement tells us about the time when the universe was climbing the hill -- still being slowed by gravity." These research results are detailed in a paper submitted to the journal Astronomy & Astrophysics. A preprint of the paper is posted on the preprint site.

A graph showing how the universe's expansion rate has changed over the last 10-billion years
A graph showing how the universe's expansion rate has changed over the last 10-billion years.
Until recently, three-dimensional maps by BOSS and other surveys were able to measure the regular distribution of galaxies back to only about five-and-a-half-billion years ago, a time when the expansion of the universe was already accelerating.
The numbers along the bottom of the graph show the time in the universe's past, in billions of years. The vertical scale (y-axis) shows the expansion rate of the universe; higher means the universe was expanding faster. These older measurements appear as data points toward the right of the graph.

The new SDSS-III measurements, shown as the data point to the far left, have now probed the structure of the early universe at a time when expansion was still slowing down.

CREDIT: Zosia Rostomian, LBNL, and Nic Ross, BOSS Lyman-alpha team, LBNL

The new measurement is based on data from the Baryon Oscillation Spectroscopic Survey (BOSS), one of the four surveys that make up SDSS-III. It uses a technique pioneered by the SDSS in 2005 called "baryon acoustic oscillations" (BAO). The BAO technique uses small variations in matter left over from the early universe as a "standard ruler" to compare the size of the universe at various points in its history.

Using that new standard ruler to observe a part of the universe that now is so very far away requires new techniques because objects like galaxies there are so faint when observed from Earth. Instead, the new technique makes use of the clustering of intergallactic hydrogen gas in the distant universe. Astronomers can see this gas because it absorbs some light from the quasars lying behind the gas. Measurements of the spectrums of these quasars reveal not only the light emitted by the quasar, but also what happened to that light in its long journey to Earth. A quasar's spectrum reveals how the intervening gas absorbs some of the quasar's light. Measuring this absorption -- a phenomenon known as the Lyman-alpha Forest -- yields a detailed picture of the gas between Earth and the quasar.

"It's a cool technique, because we're essentially measuring the shadows cast by gas along a single line billions of light-years long," said Anze Slosar of Brookhaven National Laboratory." The tricky part is combining all those one-dimensional maps into a three-dimensional map. It's like trying to see a picture that's been painted on the quills of a porcupine." For the first time, the team's new map is large enough to detect the subtle variations of baryon acoustic oscillations in the faraway "Lyman-alpha forest" gas. It measures the Lyman-alpha forest using light from 50,000 quasars all over the sky.

The team's new measurement of the baryon acoustic oscillation peak, combined with measurements of the same peak at other points in the universe's history, paints a picture of how the universe has evolved. The picture that emerges is consistent with the current scientific understanding of the universe -- that dark energy is a constant part of space throughout the cosmos. What is fascinating about the new result is that, for the first time, it reveals how dark energy worked at a time before the universe's current acceleration started.

The BOSS measurements show that the expansion of the universe was slowing down 11-billion years ago due to the mutual gravitational attraction of all of the galaxies in the universe -- but that as the universe expanded, the constant repulsive force of dark energy began to dominate as matter was diluted by the expansion of space. Thus, more than eighty years after Edwin Hubble and Georges Lemaitre first measured the expansion rate of the nearby universe, the SDSS-III has made the same measurement of the expansion rate of the universe as it was 11-billion years ago.

"No technique has ever been able to probe this ancient era before," said BOSS principal investigator David Schlegel of the Lawrence Berkeley National Laboratory. "Back then, the expansion of the universe was slowing down; today, it's speeding up. How dark energy caused the transition from deceleration to acceleration is one of the most challenging questions in cosmology."

SDSS-III will continue to learn more about dark energy as it collects more than a million and a half galaxies and more than 160,000 quasars by the end of the survey in 2014. By the time SDSS-III is complete, it will have helped to transform the Lyman-alpha-forest technique from a risky idea into a standard method by which astronomers explore the nature of the faraway universe.

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.

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