How a Bunch of Tiny Robots Will Help Crack Dark Energy
Automating the tedious work of manually resetting thousands of fiber-optic cables at a time.
The task of the Dark Energy Spectroscopic Instrument (DESI) currently under development at the Lawrence Berkeley National Laboratory is the careful three-dimensional mapping of one-third of the sky to a depth of nearly 10 billion years, or when the universe was only two billion years old. (Recall that cosmological distance maps to cosmological history.) Set for a 2018 deployment at the Kitt Peak National Observatory in Arizona, the instrument/experiment will run for five years, functioning as a 3D analog to the 2D Dark Energy Camera, often noted as the world's most powerful digital camera.
Wrangling detailed spectroscopic data for 25 million galaxies and quasars is no simple task, however, and requires the precise manipulation of 5,000 individual optical fibers—what are essentially just tiny telescopes—each one registering the light signature of a target galaxy and passing it onward to a spectrograph, where it's broken down into its different wavelength components. Inconveniently, this has historically meant the tedious manual resetting of each individual fiber every time some new target is to be imaged, a literal unplugging and replugging that, in the case of experiments like the Sloan Digital Sky Survey and the Baryon Oscillation Spectroscopic Survey, may require thousands of by-hand manipulations in a given day. It sounds pretty miserable.
DESI will pass this work off to a dense array of tiny robots, as described at Fermilab's Symmetry Breaking, each one about 8 inches long and just a few millimeters across and each one sporting a tiny motor of the sort usually reserved for medical equipment, like insulin pumps and surgery robots.
"The fibers direct light to the 30 cameras and spectrographs we have connected to the whole rig," explains David Schlegel, an astrophysicist and DESI project team member, in a Berkeley Labs Q&A. "We'll observe these galaxies for about 20 minutes, then we will point the telescope in a new direction and 5000 little robots will rearrange these fibers to look at a new collection of up to 5000 galaxies, one fiber per galaxy. In other words, the positions of the optical fibers mimic the positions of the galaxies so that each fiber collects light from one galaxy. Point the telescope in a different direction and the fibers need to take on a new configuration."
By carefully mapping the positions of galaxies through time, astrophysicists will get an even more high-resolution picture of the the universe as it evolved. This should in turn reveal information about a crucial driver of that evolution: dark energy, the omnipresent repulsive force of empty space thought to be behind the universe's ever-accelerating expansion and its eventual bleak end as a cold, hollow void.
"The [sky] map close to us is stretched out a lot more than it should be because of this acceleration due to dark energy," Schlegel explains. "And then in the early universe, when there's not as much dark energy, it shouldn't be as stretched out, but this is where we don't have much data yet. Depending on when dark energy was pushing the universe apart, it will push apart different parts of the map. This data will help us eliminate a number of theories about the way dark energy works." This is the motivation for DESI and its predecessors.
The project is currently in the CD-2 phase of reviews and approvals by the US Department of Energy, and further approvals for the project's final construction and operation are still a ways off. Here's hoping things stay on track for one of the universe's biggest questions.