BOSS and Pleiades figure out the universe

Astronomy these days is something of a tag-team event involving both observers and theorists. We got a look at how it works at the most recent Astronomy on Tap Seattle event at Bad Jimmy’s Brewing Company in Ballard.

Case in point: for a couple of decades cosmologists had been using the cold dark matter theory to explain how the universe evolved from a hot, dense, uniform place right after the Big Bang to the web of galaxies that we see today. The theory worked pretty well, but there were a couple of catches: it predicted that dwarf galaxies would have large central bulges of stars and increasingly dense dark matter at their cores. Neither prediction matched with the observations.

Figuring it out


Look! Up in the sky! Prof. Fabio Governato makes a point during his Astronomy on Tap talk Feb. 17 at Bad Jimmy’s. Photo: Greg Scheiderer.

Dr. Fabio Governato, a research professor in the Department of Astronomy at the University of Washington, said he and a few colleagues, after downing several beers each during an escape from a boring conference, decided to figure out this anomaly. Governato‘s talk at Astronomy on Tap Seattle was titled, “Dark Matter, Black Holes, and other reasons to work with NASA’s fastest supercomputer: Pleiades.”

Eventually, they hit upon the idea that supernova explosions in the dwarf galaxies might push away gas and thus retard star formation, and may also blow dark matter away as well.

“This is very simple physics,” Governato said, “but the problem was to find a numerical experiment that you could run with computers that shows clearly” how it works. They used millions of hours on supercomputers, like NASA’s Pleiades, adding the supernovae into the mix and tweaking the idea until the computer simulation of the cold dark matter theory turned out dwarf galaxies that matched what we actually observe. Their paper about the work was published in the journal Nature, and Governato has some humorous tales about the twists and turns between the work, the publication, and ultimate acceptance of the findings.

His talk also used interesting and sometimes humorous animations to make points. Governato’s movie of a dwarf galaxy formation based on the work is posted below.



Dr. John Parejko, holding a sample of the metal plates used in the BOSS survey, answers questions after his talk. Even pooches love Astronomy on Tap! Photo: Greg Scheiderer.

Dr. John Parejko works on the observation side of the equation. Parejko recently was with the Sloan Digital Sky Survey out of New Mexico. His talk was titled, “Detect the Ancient Universe Like a BOSS.”

“BOSS is measuring distances to millions of galaxies to find wiggles from the early universe, but that doesn’t make a very good acronym,” Parejko quipped. BOSS actually stands for Baryon Oscillation Spectroscopic Survey.

The wiggles or oscillations are evidence of interactions that happened right after the Big Bang.

“Patterns in that hot, dense plasma persist to today in the distribution of galaxies in the universe,” Parejko said.

“These are not gravitational waves,” he noted, as the discoveries from LIGO were fresh in the news. “These are actually the interaction between the dark matter and the baryons very early in the universe.”

The process was simple enough, as they took spectra of galaxies and computed their redshifts to precisely determine distances. The challenge was that they had to look at a lot of galaxies, and over the years BOSS examined about a third of the sky and took images of about two million galaxies, measuring the redshifts of about half of those. Using the redshift to pin down distances to and between galaxies, and examining the patterns that emerge, helps astronomers figure out galaxy formation and learn how dark energy is causing the expansion of the universe to speed up.

Part of the tool that BOSS uses is made at the University of Washington, where telescope plates are created for the project. Each metal plate, about three feet wide, has a thousand holes drilled into it, each one corresponding to a specific object in the sky. Humans plug a fiberoptic cable into each hole by hand, and the cable collects the light from targeted galaxies.

Birthday party!

Astronomy on Tap Seattle is organized by astronomy graduate students at the University of Washington. The next event, scheduled for March 23, will celebrate the first birthday of the program. Speakers will update the subjects of their talks from the first year. Attendees will be able to purchase a commemorative AoT beer glass and sample Bad Jimmy’s barrel-aged Big Sipper, a Scotch ale named as a salute to Astronomy on Tap.