On the dark side
Astrophysicist Josh Frieman, PhD’89, works on the dark side, studying the night sky for insight into the accelerating expansion of the universe.
Josh Frieman, PhD’89, will spend the next five years photographing the night sky with a really big camera. In August the 570-megapixel Dark Energy Camera, built at Fermilab and mounted on the Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, began taking about 400 images of the southern sky every night. Each image captures the light of approximately 100,000 distant galaxies. The project, the Dark Energy Survey, is the largest-yet extragalactic survey and will record information on more than 300 million galaxies. Led by Frieman, UChicago professor of astronomy and astrophysics and Fermilab staff scientist, the survey enlists more than 200 scientists from 25 organizations.
The main goal of the survey is to understand why the expansion of the universe is speeding up: whether a mysterious dark energy pervades the universe or if something is amiss with the law of gravity on cosmic scales. To that end, it is measuring the history of cosmic expansion, or how fast the universe is expanding today compared to its rate billions of years ago. The survey is also measuring the history of large-scale structures: organizations of cosmic elements like clusters of galaxies, superclusters, and filaments. Galaxies tend to clump together, but the strength of that tendency changes over time. “There’s this competition between gravity”—particularly the gravity of dark matter—“which is making galaxies attract to each other, and dark energy, which is pushing them apart,” Frieman says. Studying this competition and the historical rates of expansion should explain more about the properties of dark energy.
Trained as a theoretical cosmologist, Frieman has worked increasingly with survey data. He previously led the Sloan Digital Sky Survey (SDSS-II) Supernova Survey, a three-year project that discovered and measured more than 500 type Ia supernovae—exploding stars that grow as bright as an entire galaxy for a short time and can be used to measure cosmic distances. Frieman is struck, he says, by the knowledge that can be gained by simply looking at the sky. “What’s remarkable to me is that just by taking pictures, we can learn so much about how the universe has evolved.” In an interview with the Magazine, adapted and edited below, he talked about the cosmos scientifically and philosophically.
It really wasn’t until I was in college at Stanford that I caught fire with cosmology. An eminent cosmologist from Oxford, Dennis Sciama, came and gave a colloquium on the history of the universe. That was eye-opening to me, the notion that cosmology, in a way, was like archaeology on the grand scale, and that we could use the observed universe, galaxies and how they’re distributed in space, similarly to how pottery shards are used by an archaeologist, to figure out what the universe looked like billions of years ago.
In the dark
We don’t know what dark matter is, but we know that it obeys the ordinary laws of gravity—or we think it does. So there are experiments going on to try to detect particles of dark matter, since that’s one of the leading ideas of what dark matter could be. It could just be clouds of elementary particles zipping around. Dark energy—we call them both dark because they don’t emit or interact with light—is something much stranger, and it would make up about 70 percent of the universe. Unlike dark matter, it doesn’t hold stuff together. Dark energy pushes stuff apart.
Filled with emptiness
One idea for what dark energy could be is the energy of empty space. If you imagine taking this coffee cup, well, it’s kind of dirty, but it’s filled with molecules of air, right? And imagine I sealed it, attached it to a pump to a vacuum and pumped out all of the particles that were there. It would be totally empty space. In classical physics, if there are no particles in there, there’s no energy. But according to the laws of quantum mechanics, even if there are no particles in there, empty space itself can still have energy.
Before the beginning
Currently the laws of physics can take us back very close to the big bang—a tiny fraction of a second, we think, after the big bang. There is strong evidence that the early universe was very hot and very dense, and that’s really what we should call the big bang. Now whether we trace that back to a single point in time, and whether it traces back somehow beyond that point in time, that becomes much more speculative because the laws of physics break down before we get there. And then there comes this question of what do we even mean by time when we get to this point? Because our classical notions of space and time themselves break down. There are certainly physicists who have worked on theories of “what happens before the big bang,” ideas that before the big bang, maybe there was a previous universe that contracted and then bounced and led to the current expansion. That’s certainly possible. It may be possible to theorize about that in a consistent way given the laws of physics, but at this point it’s very speculative.
Where we are
Copernicus showed that we are not at the center of the universe. Now with Hubble and others, we know that not only is the sun not at the center of the universe, the sun is just one of tens of billions of stars in this rather ordinary galaxy that’s one of billions of galaxies that are flying apart from each other due to the expansion of the universe. So in fact there is no center of the universe. And now with dark matter and dark energy, we’re not even made of the stuff that most of the universe is made of. It’s like we’re this little spray on the big ocean of the universe.
On the other hand, I think what counters that is the sense that there’s a real unity to the cosmos. I find some strange comfort in the fact that we’re all made of stuff that was produced in the supernovae. Most of the elements in our bodies and that we construct the world out of were forged in nuclear reactions in stars when they exploded, and then were spread throughout space. So we actually have this very strange, very direct physical connection to this universe that we’re studying. And the fact that we’ve been able to evolve and develop technologies to understand that universe doesn’t give us power over the universe, but I think for me, there’s comfort in that understanding of how we got here.