Catching Early Universe Echoes with BOSS

This guest post is written by BNL cosmologist Anže Slosar. Slosar, who joined Brookhaven's physics department in 2009, received his Ph.D. from Cambridge University in 2003. He previously worked at Lawrence Berkeley National Laboratory, Oxford University, and the University of Ljubljana in Slovenia.

Brookhaven Lab is involved in three cosmology experiments aimed at unraveling deep mysteries and inner workings of the universe. Two of these -- the Dark Energy Survey (DES) and Large Synoptic Survey Telescope (LSST) -- won't see "first light" (the beginning of commissioning) until 2012 and 2017, respectively. But the third, the Baryonic Oscillation Spectroscopic Survey (BOSS) is already taking data.

BOSS is an instrument capable of capturing the spectra -- "fingerprints" of radiation emitted -- of 1,000 astronomical objects at the same time. Over the course of its operation, it will take spectral snapshots of around 1 million luminous red galaxies (LRGs) and about 150,000 high-redshift quasars -- those most distant from our position on Earth, whose light comes from a very early time in the universe.

BOSS makes these distant measurements using a specially designed metal plate placed in the focal plane of a 2.5-meter telescope located at the Apache Point Observatory in New Mexico, part of the Sloan Digital Sky Survey (SDSS). This metal plate has 1,000 holes drilled at the positions of known objects. Optical fibers plugged into these holes lead the light of the astronomical objects into a very sensitive spectrograph. The holes must be drilled very accurately (to within a micron) and their positions must take into account not only the positions of the objects in the sky, but also nuances like sky diffraction at the time of observation, which depend on how high above the horizon the telescope is pointing.

The light collected from LRGs and quasars allows astronomers to reconstruct a three-dimensional map of the universe on very large scales: 8 billion to 20 billion light years. Where there are more galaxies, the universe is denser, so the measurements from LRGs can directly trace the cosmic structure. Quasars, on the other hand, are used as backlights to illuminate what is called the "cosmic neutral hydrogen" between us and a given quasar. In denser parts of the universe, there is more neutral hydrogen and hence more absorption in the spectra of quasars.

Why do we want a three-dimensional map of the universe?

In the early universe, all the usual matter that now surrounds us was in a plasma state, because the universe was very hot and dense. This state was similar to the stuff that makes up our sun and is characterized by protons, electrons, and photons in thermal equilibrium. In such a hot state, small density perturbations did not collapse under their own gravity, but instead exhibited propagation of acoustic waves -- sound, but with very, very low frequencies of around 3x10-17 Hertz.

As the universe cooled and hydrogen and photons decoupled, the remnants of these sound waves were frozen into the cosmic structure. These remnants remain detectable in the distribution of matter in the universe even today. In particular, they cause a bump in the correlation of galaxies at distances around 150 megaparsecs -- that is, about 500 million light years, or the distance that the sound in the early universe has managed to travel. This means that if we take one galaxy and count the number of galaxies in surrounding it spherical shells, we would see a small increase in the number of galaxies in the shell at around 150 megaparsecs with respect to the neighboring two shells.

Of course, counting galaxies around any given one galaxy is not enough, and in practice one needs to take into account every galaxy around every other galaxy in order to gather enough sensitivity to detect this effect. Measuring the angular scale of this effect as a function of redshift allows us to measure the distance to different redshifts in our universe, and hence characterize the content of our universe -- including the mysterious dark energy that is key to understanding the accelerating expansion of the universe today.