April 8, 2014
Scientists Make The Most Precise Measurment Of Early Universe Yet
Lawrence LeBlond for redOrbit.com - Your Universe Online
Two recent studies of the early Universe using the Baryon Oscillation Spectroscopic Survey (BOSS), a component of the third Sloan Digital Sky Survey (SDSS-III), have made the most accurate measurement of the rate of expansion in the early Universe to date.
BOSS has pioneered the use of quasars to map density variations in intergalactic gas at high redshifts to trace the structure of the young Universe. BOSS helps scientists chart the history of the growing Universe in order to bring light to dark energy. The latest rounds of research have made the most precise measurement of expansion since the first galaxies formed some 13 billion years ago.
The most recent quasar analyses have combined two separate techniques to gain a better understanding of how fast the early Universe was dispersing.
The first never-before-used technique was employed by physicist Andreu Font-Ribera of the US Dept. of Energy’s Lawrence Berkeley National Laboratory and published in a study last year. The second technique was an already-tested analytical approach, but using far more data than ever before to make a solid measurement. The approach was used by Timothee Delubac, of EPFL Switzerland and France’s Centre de Saclay, and colleagues. Both analyses together have established that the expansion rate of the early Universe is 68 kilometers (42.25 miles) per second per million light years at redshift 2.34 – the analyses has an margin error of only 2.2 percent.
"This means if we look back to the universe when it was less than a quarter of its present age, we'd see that a pair of galaxies separated by a million light years would be drifting apart at a velocity of 68 kilometers a second as the universe expands," said Font-Ribera, a postdoctoral fellow in Berkeley Lab's Physics Division. "The uncertainty is plus or minus only a kilometer and a half per second." Font-Ribera presented the findings at the April 2014 meeting of the American Physical Society in Savannah, GA.
The teams relied on analyses of baryon acoustic oscillations (BAO), which BOSS picks up while analyzing quasars and galaxies. BAO is a signature imprint in the way matter is distributed and is a result of conditions in the early Universe. While BAO is present in the distribution of invisible dark matter, the imprint is also found in the distribution of ordinary matter, including galaxies, quasars and intergalactic hydrogen.
"Three years ago BOSS used 14,000 quasars to demonstrate we could make the biggest 3-D maps of the universe," says Berkeley Lab's David Schlegel, principal investigator of BOSS. "Two years ago, with 48,000 quasars, we first detected baryon acoustic oscillations in these maps. Now, with more than 150,000 quasars, we've made extremely precise measures of BAO."
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To better understand how BAO helps scientists make accurate measurements of the early Universe’s expansion, we need to understand more about where BAO comes from.
BAO directly descends from pressure waves, or sound waves, that move through the early Universe when particles of light and matter were inextricably entangled. At about 380,000 years after the Big Bang, the Universe had cooled enough to allow light to escape. The cosmic microwave background radiation preserves a record of the early acoustic density peaks, which were the seeds of the subsequent BAO imprint on the distribution of matter, according to the LBNL scientists.
The researchers from both teams were able to extract the BAO signal from three-dimensional maps that determined the position of gas clouds using quasar spectra that existed in close proximity. Although fairly new for BOSS, this method of autocorrelation is now almost traditional in nature. The results from Delubac and colleagues employed the spectra of nearly 140,000 carefully selected BOSS quasars.
Font-Ribera and his team determines BAO using even more BOSS quasars in a much different way.
Quasars, which are young galaxies powered by massive black holes, are extremely bright, extremely distant, and highly redshifted. Instead of comparing spectra to other spectra, Font-Ribera’s team correlated quasars themselves to the spectra of other quasars, known as cross-correlation.
"Quasars are massive galaxies, and we expect them to be in the denser parts of the universe, where the density of the intergalactic gas should also be higher," said Font-Ribera. "Therefore we expect to find more of the absorbing gas than average when we look near quasars."
The question was whether the correlation would be good enough to see the BAO imprint? Font-Ribera answered that the BAO imprint in cross-correlation was definitely strong.
Delubac and Font-Ribera then combined their data and converged on narrow constraints for the BAO scale. Their converging analyses made a precise measurement of the speed at which the early Universe is expanding, known as the Hubble parameter.
"It's the most precise measurement of the Hubble parameter at any redshift, even better than the measurement we have from the local universe at redshift zero," said Font-Ribera. "These results allow us to study the geometry of the universe when it was only a fourth its current age. Combined with other cosmological experiments, we can learn about dark energy and put tight constraints on the curvature of the universe – it's very flat!"