DOE Pulse
  • Number 378  |
  • December 17, 2012

BOSS uses quasars to probe dark energy up to 11.5 billion years in the past

Neutral hydrogen gas “backlit” by distant quasars (red dots) leaves its signature on the quasar spectra as a forest of shifted absorption lines (inset). (Zosia Rostomian and Nic Ross, Berkeley Lab; Springel et al, Virgo Consortium.)

Neutral hydrogen gas “backlit” by distant
quasars (red dots) leaves its signature on
the quasar spectra as a forest of shifted
absorption lines (inset). (Zosia Rostomian
and Nic Ross, Berkeley Lab; Springel et al,
Virgo Consortium.)

The Baryon Oscillation Spectroscopic Survey (BOSS), led by scientists from DOE's Lawrence Berkeley National Laboratory and their colleagues in the third Sloan Digital Sky Survey, recently announced the first major result of a new technique for studying dark energy. Instead of plotting the positions of stars or galaxies, the BOSS “Lyman-alpha” result is based on mapping the density of intergalactic hydrogen gas, using the spectra of over 48,000 quasars with redshifts up to 3.5 – active galaxies whose light originated up to 11.5 billion years in the past. By the time BOSS finishes its five-year survey, its collection of distant quasars will have grown to more than 150,000.

Most of the BOSS effort is devoted to mapping 1.5 million visible galaxies, distributed in netlike tendrils and voids throughout the universe. These regularly spaced peaks in density, called baryon acoustic oscillations (BAO), also map underlying invisible dark matter. BAO originated in primordial density variations, “sound waves,” rippling through the hot soup of matter and radiation that constituted the early universe. The echoes of those acoustic oscillations are detectable today as minute variations in the temperature of the cosmic microwave background radiation.

Thus the spacing of visible and invisible matter provides a cosmic ruler for calibrating the rate of cosmic expansion all the way back to the CMB, tracing the struggle between gravitational attraction and propulsive dark energy wherever BAO can be measured. BOSS uses the 2.5-meter Sloan Foundation telescope at New Mexico’s Apache Point Observatory, and the range of useful galaxies is limited to redshifts of about 0.7, beyond which most ordinary galaxies become too faint for BAO.

“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 BOSS principal investigator David Schlegel of Berkeley Lab's Physics Division. Quasars are bright, but they’re too sparse to measure BAO directly. There’s another way they reveal BAO at high redshifts.

As the light of a quasar passes through clouds of intergalactic gas on its way to Earth, its spectrum accumulates a crowd of hydrogen absorption lines, a thicket of lines known as the Lyman-alpha forest. In the cleanest spectra, each line reveals where the light has passed through an intervening gas cloud. The different prominences and redshifts of the individual absorption lines in a single quasar’s spectrum reveal how the gas density varies along the line of sight.

With enough quasars to backlight the intervening gas clouds, close enough together and covering a wide enough expanse of sky, their distribution can be mapped in three dimensions. The idea was advanced in the early 2000s by Patrick McDonald, then at the Canadian Institute for Theoretical Astrophysics, and Martin White, both now at Berkeley Lab.

“When I presented this idea to a conference of cosmologists in 2003, they thought it was crazy,” says White, the chair of the BOSS science survey teams. “Nine years later, BOSS has shown that it’s an amazingly powerful technique. It has succeeded beyond our wildest dreams.”

Schlegel says, “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.”

Previous measurements with Type Ia supernovae made the plausible assumption that the universe is flat and interpreted the changing relation between brightness and redshift in ever-more-distant supernovae as evidence of decelerating expansion in the early universe. BAO measures the speed of expansion directly, without assumptions about flatness and the relationship of supernova brightness and redshift.

“In our own time, expansion is accelerating because the universe is dominated by dark energy,” says Schlegel. “How dark energy effected the transition from deceleration to acceleration is one of the most challenging questions in cosmology.”

[Paul Preuss, 510.486.6249,]