How are galaxies clustered together? What is fueling the accelerating expansion of the universe? Just what is dark energy?
These are the big questions that scientists working at the Baryon Oscillation Spectroscopic Survey (BOSS) are asking. Brookhaven National Lab is a member of BOSS, the largest of the four surveys that make up the Sloan Digital Sky Survey III, which maps the sky over the Northern Hemisphere with New Mexico’s 2.5-meter Sloan telescope in an attempt to define dark energy and measure its effects.
Building on the legacy of the Sloan Digital Sky Survey (SDSS) and SDSS-II, the SDSS-III collaboration is working to map the Milky Way, search for extrasolar planets, and solve the mystery of our expanding universe.
BOSS studies dark energy—the mysterious force thought to be responsible for the universe’s accelerating expansion—by mapping the distribution of luminous red galaxies and quasars. The clusters and voids of galaxies currently found throughout space are a result of sound waves that spread through the early universe, like ripples through a pond. Those fluctuations imprinted patterns on the cosmic microwave background—the residual radiation from the Big Bang spread out through the universe as a faint glow of light. These patterns, called "baryon acoustic oscillations," account for the way galaxies are distributed and can help pinpoint the origin of cosmic acceleration and test different theories of dark energy.
To survey the sky, BOSS uses two spectrographs, which split incoming light into two cameras, one for red light and one for blue light. These cameras allow scientists to measure the redshift of light from distant galaxies—as an object moves away from a fixed point, the wavelength of any light it emits stretches and becomes redder. The team at BOSS targets luminous red galaxies and records the spectrum of light they give off to identify the patterns in space that can help reveal the nature of dark energy.
In addition to targeting luminous red galaxies and contributing images of the night sky to the Sloan Digital Sky Survey III, Brookhaven scientists use BOSS’s wide-field camera to map thousands of quasars at a time. Quasars are extremely bright masses of light and energy, making them ideal backlights for a picture of the universe as it existed 11 billion years ago.
Using the BOSS cameras, Brookhaven scientists are pioneering a technique to measure the “Lyman-alpha forest” using a kind of reverse astronomy that measures the expansion of the early universe more accurately than ever before. Instead of the standard approach of measuring light emitted from galaxies, Lyman-alpha measurements focus on the intergalactic hydrogen gas that blocks the light coming from the brightest beacons out there – quasars. BOSS researchers map the irregular patterns in the light from distant quasars, which are created when the light passes through hydrogen gas clouds. It’s a bit like determining the shape of someone’s hand by watching a shadow puppet show.
When the BOSS survey is complete, the team will have mapped 160,000 quasars and 1.5 million luminous galaxies.