The best measurement for the expansion rate of the early universe has come from astronomers from the third Sloan Digital Sky Survey (SDSS-III). Using quasars that can be a trillion times brighter than the sun, they mapped variations of density within hydrogen gas and determined through redshift measurement the structure of the cosmos when it was around three billion years old, a quarter of its present age.
Using the Baryon Oscillation Spectroscopic Survey (BOSS), Timothee Delubac of the Centre de Saclay of France and his team published data that suggested the expansion rate with an accuracy of 2.2 percent. According to their calculations using 150,000 quasars, 10.8 billion years ago the universe was expanding every 44 million years by a rate of one percent, or 42 miles per second at a distance of a million light years at redshift 2.34.
BOSS charted the early universe in an attempt to discover more about dark energy, and the new findings built upon the previous analysis made by postdoctoral fellow Andreu Font-Ribera of the Physics Division of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory in Berkeley, California. Last year, his team published findings that resulted from measurements utilizing BAO, or baryon acoustic oscillations, to determine how ordinary matter was distributed through the universe within galaxies, quasars, and hydrogen, and the imprint was also present in the spread of invisible dark matter.
Font-Ribera presented the new information to the April 2014 American Physical Society meeting in Savannah, GA. After his findings last year compared quasar distribution to measure their distance, the new report was an analysis of more data published by Timothee Delubac and his team, which focused on patterns within hydrogen gas to further their measurements. Font-Ribera’s spectra analysis of quasar to quasar distance, called cross-correlation, was a different method than that employed by Delubac, who used the autocorrelation technique called the Lyman-alpha forest to increase their findings.
Baryon acoustic oscillations were created by pressure waves in the early universe, and after the expansion of 380,000 years after its birth, light and matter became disentangled and the microwave radiation now acts as a record of its density. As the light from quasars travels through hydrogen gas, the denser patches of space absorb more light, and these absorption lines in the spectrum make it possible to locate the dense areas by their redshift. Astronomers call this the Lyman-alpha forest due to the sheer number of these lines, making them incredibly useful in determining the expansion rate.
With enough quasar spectra measured, the position of the gas clouds can be determined and revealed in three dimensions, both in reference to each quasar as well as transversely within the patches shown by other spectra. It is from these maps that astronomers calculate the BAO signal, which Delubac and his team gathered from 150,000 quasars. Three years previously, the maps were being made from 14,000 quasars, and two years ago they were working with 48,000 when they detected baryon acoustic oscillations.
Delubac’s team combined their autocorrelation results with the cross-correlations determined by Font-Ribera and his team, and resulted in the constraints for the BAO scale as well as the early universe’s expansion rate, called the Hubble parameter. With measurements so precise that even astronomers were surprised by their accuracy, they have also been able to report that considering new discoveries about dark energy that the curvature of the universe is very flat. According to Delubac, these findings are an anchor to further knowledge about the formation of the cosmos.
By Elijah Stephens
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