SCIENCE

Measuring the expansion history of the universe

Baryon acoustic oscillation is cosmologists’ shorthand for the periodic clustering (oscillation) of matter (baryons), which originated as pressure (acoustic) waves moving through the hot, opaque, liquid-like early universe. The pressure differences resulted in differences in density and left their signature as small variations in the temperature of the cosmic microwave background. Later – because the denser regions formed by the pressure waves seeded galaxy formation and the accumulation of other matter – the original acoustic waves were echoed in the net-like filaments and voids of the clustering of galaxies and in variations in the density of intergalactic hydrogen gas.

The oscillations repeat at about 500-million-light-year intervals, and because this scale is firmly anchored in the cosmic microwave background it provides a ruler – a very big one – to measure the history of the expanding universe. With this cosmic yardstick it will be possible to determine just how fast the universe was expanding at the redshift of the objects in the BOSS survey – in other words, how the expansion rate has changed over time. (Redshift is the degree to which the light from an object speeding away from the viewer is shifted toward the red end of the spectrum.) Knowing whether expansion has accelerated at a constant rate or has varied over time will help decide among the major theories of dark energy.

Zooming in on the map slice shows areas with more gas (red) and less gas (blue) as revealed by correlations of the Lyman-alpha forest data from the spectra of thousands of quasars. A distance of one billion light years is indicated by the scale bar. (Anže Slosar and BOSS Lyman-alpha cosmology working group. Click on image for best resolution.)

Over its five-year extent, BOSS is using two distinct methods to calibrate the markings on the cosmic yardstick. The first method, well tested, will precisely measure 1.5 million luminous red galaxies at “low” redshifts around z = 0.7 (z stands for redshift). The second method will eventually measure the Lyman-alpha forest of 160,000 quasars with high redshifts around z = 2.5. These redshifts correspond to galaxies at distances of 2 to 6 billion light years and quasars at 10 to 11 billion light years.

Lyman-alpha is the name given to a line in the spectrum of hydrogen, marking the wavelength of light emitted when an excited hydrogen electron falls back to its ground state; it’s a strong signal in the light from quasars. As the quasar’s light passes through intervening clouds of hydrogen gas, additional lines accumulate where the gas clouds absorb the signal, echoing it but shifting by different degrees according to factors including the redshift of the gas cloud and its density. The spectrum of a distant quasar may have hundreds of lines, clumped and blended into a messy, wiggly structure in the spectrum: this is what astronomers call the Lyman-alpha forest.

“In theory, you can turn any of these absorption lines directly into redshifts and locate the gas cloud precisely,” says Bill Carithers of Berkeley Lab’s Physics Division, who concentrates on extracting relevant information from the noisy data that comes straight from the telescope. “But in practice only the spectra of the very brightest quasars are clean enough to make things that simple.”

Carithers says that “while a very long exposure could improve the signal-to-noise ratio, that comes at a price. We need lots and lots of quasars to make a map. We can only afford to spend so much telescope time on each.”

Since the heart of BAO is the correlation distance among density oscillations, the trick turns out to be not overconcentrating on individual spectra but instead measuring the correlations among them. “For any correlation distance, many quasars will contribute,” says Carithers, “so the noise will average and the signal will get stronger. We can say, ‘I’ll use my data, noise and all.’”

If the attempt to measure density variations in the intergalactic gas is indeed successful, what will the BAO correlation signal from the Lyman-alpha forest look like? Shirley Ho of Berkeley Lab’s Physics Division, working with Slosar and Berkeley Lab’s Martin White, developed simulations to find out.

“”We modeled what you would see when you have a BOSS-like data set, and through the simulations we understand the possible sources of systematics when we try with real data to detect the acoustic peak from the Lyman-alpha forest, the signature of baryon acoustic oscillations,” Ho says. Comparing the real data to the simulation confirms whether the search is working as hoped.

With Peter Nugent, who heads the Computational Cosmology Center at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), Ho established a 30-terabyte BOSS Project Directory to store the Lyman-alpha simulations, plus the entire Lyman-alpha raw data set as it arrives. The directory also contains a subset of galaxy data and is available to all BOSS collaborators and to the public. The total BOSS data set is stored in a dedicated cluster supercomputer nicknamed Riemann.