Scientists using BOSS, the Baryon Oscillation Spectroscopic Survey, have used a new mapping technique to probe the period before dark energy began to speed up the expansion of the universe.
The team has examined the spectra of over 48,000 quasars with redshifts up to 3.5, meaning that light left them up to 11.5 billion years ago.
“No technique for dark energy research has been able to probe this ancient era before, a time when matter was still dense enough for gravity to slow the expansion of the universe, and the influence of dark energy hadn’t yet been felt,” says BOSS principal investigator David Schlegel.
“In our own time, expansion is accelerating because the universe is dominated by dark energy. How dark energy effected the transition from deceleration to acceleration is one of the most challenging questions in cosmology.”
BOSS studies dark energy by mapping baryon acoustic oscillations (BAO) – the large-scale network of variations in the distribution of visible galaxies and clouds of intergalactic gas, which also reveal the presence of dark matter.
The regular spacing of peaks in matter density originated in primordial density variations, and provide a cosmic ruler for calibrating the rate of expansion wherever BAO can be measured.
The BOSS team is focusing on quasars.
“Quasars are the brightest objects in the sky, and therefore the only credible way to measure spectra out to redshift 2.0 and beyond,” says Schlegel. “At these redshifts there are a hundred times more galaxies than quasars, but they’re too faint to use for BAO.”
As the light of a quasar passes through clouds of intergalactic gas on its way to Earth, its spectrum develops a plethora of hydrogen absorption lines known as a Lyman-alpha forest. Ideally, each absorption line in the ‘forest’ reveals where the quasar’s light has passed through an intervening gas cloud, showing how gas density varies with distance along the line of sight.
With enough quasars, close enough together and covering a wide expanse of sky, the distribution of intervening gas clouds can be mapped in three dimensions.
“We don’t use the specific information in a single line of sight, we look at the correlations among many,” says Berkeley Lab’s Bill Carithers. “BOSS is the first to do this because we have enough quasars – with too few, you can’t see the pattern.”
In the end, after data processing and generation of mock spectra on the Riemann Linux cluster of computers provided by Berkeley Lab’s High-Performance Computing Services Group, the competing analyses of the Lyman-alpha forest of over 48,000 quasars all gave similar results.
“We are seeing back to the matter-dominated universe, when expansion was decelerating and dark energy was hard to see,” says Martin White of Berkeley Lab.
“The transition from decelerating expansion to accelerating expansion was a sharp one, and now we live in a universe dominated by dark energy. The biggest puzzle in cosmology is, why now?”