Understanding Cosmic Sound Waves

In our April 2016 feature article, "Mapping the Universe's Ancient Sound Waves", Daniel Eisenstein (Harvard University) outlines one of the most ambitious projects to survey our universe. The motivation for mapping out millions of galaxies is simple to say, but not so easy to understand: to find the echoes of primordial sound waves that sloshed through the early universe.

The early universe was a vastly different place, hot and filled with sloshing plasma. Within this primordial soup, a single disturbance would initiate a sound wave, like a rock dropped in a pond sends out an expanding ripple. Many such disturbances would send out many such ripples. Ultimately, these ripples (aka baryon acoustic oscillations) organized the universe's large-scale structure into a sponge-like pattern. We can witness that pattern in nearby galaxies, which align along sheets and filaments surrounding vast empty voids. On the largest scales, galaxies are most likely to be separated by 500 million light-years, and that tendency is a direct result of the early sound waves.

Eisenstein created the simple animations below to visualize the effects of sound waves ringing through the early universe:

Single sound wave

A sound wave propagates outward from a single disturbance.
Daniel Eisenstein

Many sound waves

A multitude of disturbances create a corresponding multitude of overlapping sound waves.
Daniel Eisenstein

The Sloan Digital Sky Survey (SDSS) set out to find the echo of these sound waves in the structure of millions of galaxies in the nearby universe. In Data Release 12, the SDSS team released the complete data set collected as part of the Baryon Oscillation Spectroscopic Survey (BOSS).

The next two videos contain only a fraction of all the data in the BOSS project, but they nevertheless give a sense of scale.

Animated flight through the universe using data from SDSS's Data Release 7:

Another animation zooms out through SDSS data to the universe's edge. (The cosmic microwave background data comes from WMAP.)

The BOSS project collected spectra of more than 1 million galaxies, and that extraordinary feat required a remarkable set-up. For spectroscopy, SDSS uses optical fibers, flexible glass cylinders that are each several meters long and a little thicker than a human hair. Once the light enters the tip of the fiber, it is trapped inside the cylindrical walls and routed to the spectrograph.

The catch to using optical fibers is that each fiber must be positioned at each galaxy's exact location. An aluminum plate with precision-drilled holes holds the fibers in place. Workers must plug the fibers into the holes by hand. Each plate takes roughly an hour to prepare. Watch the process in this video, which shows the plugging of 1,000 fibers into a BOSS spectroscopic plate:

There's a lot more that goes into the making of each of these aluminum plates. Take a look:

Once the plates are drilled and plugged, they take their turn at the telescope. Watch a night in the life of SDSS:

See more SDSS videos on their YouTube channel. And to read more about cosmic sound waves and what they reveal about the universe's make-up, check out Sky & Telescope's April 2016 issue.