Mapping a Million Galaxies

Yuanchi Zhao | yuanchi.zhao@yale.edu December 1, 2010

Mapping a Million Galaxies

Echoes of the early universe are still detectable today within the its current structure. Waves moving through the young universe changed matter densities, affecting the distribution of galaxies in the present universe. These waves provide an indirect tool for characterizing dark energy, which scientists theorize to be responsible for accelerating the universe’s expansion.

The Baryon Oscillation Spectroscopic Survey (BOSS), part of the Sloan Digital Sky Survey III (SDSS-III), searches for traces of these waves in order to understand dark energy. One member of this collaboration is cosmologist Nikhil Padmanabhan, Assistant Professor of Physics and Astronomy. He notes that in cosmology, “the simpler the experiment, the more precision [we get in our measurements].”

Indeed, the principle behind BOSS is simple. According to the Big Bang model, after temperatures had fallen enough for protons and neutrons to form the young universe consisted of charged particles and photons. Because of scattering, light remained trapped within this primordial plasma. Since matter tends to clump due to gravitational attractions, light, through scattering, tends to spread matter apart. These competing forces created perturbations throughout this early universe, generating waves traveling at more than half the speed of light. Such oscillations—sound waves—created regions of high and low density matter. Continuing cooling of the universe allowed electrons to combine with protons, forming neutral atoms. The neutrally charged matter in the universe allowed light to escape and travel in all directions. But more importantly for BOSS, differences in matter densities, caused by the oscillations, have remained, manifesting today as the clustering of galaxies in the universe.

BOSS will use baryon oscillations as a standard ruler: by measuring the size of baryon oscillations for various ages of the universe, cosmologists can compare baryon oscillation sizes at various redshifts to determine the expansion rate of the universe.

How do astronomers at SDSS-III measure baryon oscillations when they turn the telescope—situated at Apache Point Observatory in New Mexico—to the skies? Because there were many points of perturbation in the young universe, the effect of baryon oscillations on the universe is faint. “The way to see the effect is to look at it statistically,” says Padmanabhan. Upon graphing the probability of finding other galaxies versus distance from a single galaxy, baryon oscillations appear, according to Padmanabhan, “as a bump in probability. At some radius, you will have a very small excess number of galaxies some distance away.” This is the ruler BOSS will use to gauge the universe’s expansion.

Padmanabhan and others at BOSS have worked to understand and correct for systemic errors in their measurements. BOSS’s measurements will be accurate to within 0.1%. For additional accuracy, BOSS will survey 1.5 million galaxies. The survey, which began last September, has already scanned 40,000 galaxies.

By determining the universe’s expansion rate through baryon oscillation measurements, cosmologists will better understand dark energy. Recent observations have shown that the expansion of the universe is accelerating. The concept of dark energy driving the universe’s expansion satisfies the current standard model of the universe. Scientists still do not know, however, what dark energy is; they can only theorize about what it does. By providing a wealth of data on the universe’s expansion, BOSS provides constraints on dark energy’s properties. Such improved data are essential to developing theories on dark energy. According to Padmanabhan, “if someone develops a model, we can check it.” With current theory attributing 72% of the mass-energy content of the universe to dark energy, there is clearly a tremendous knowledge deficit of the universe that BOSS will help reduce.