There’s been something of a crisis in cosmology. A few years ago, researchers started taking measurements that disagreed with our standard understanding of how the universe expands, meaning they were in conflict with Einstein’s theory of relativity. The numbers in question have to do with the rate at which the universe is growing as well as its “lumpiness,” or how galaxy clusters and dark matter—the invisible, mysterious substance thought to hold the cosmos together—are distributed. And, if those numbers are right, that means conventional physics is at least a little bit wrong, and we’ll need a few new theories.
But anxious cosmologists the world over are breathing a sigh of relief thanks to a new map of dark matter released earlier this month by an international team of scientists, including several from the University of Toronto. Made using measurements taken by the Atacama Cosmology Telescope in Chile, the map shows that everything—clusters of galaxies, the dark matter that binds them and the voids in between—is where Einstein’s 1915 theory predicted it would be. “But, in science, you never want to just trust your equations. You want to go out there and check,” says Adam Hincks, a contributor to the project and an assistant professor at U of T’s David A. Dunlap Department of Astronomy and Astrophysics. “And that’s what we’ve been able to do, with amazing precision.” We spoke to Hincks about the invisible forces that shape the cosmos.
There’s a whole universe to cover here, but let’s start with the map. What exactly does it show?
It’s a map of the biggest structures in our universe. Stars, planets and even galaxies are considered to be pretty small, cosmologically. When you really zoom out and look at the whole universe on a large scale, it looks kind of lumpy. Those lumps are the large objects we mapped—galaxy clusters and the dark matter that holds them together.
And, for those in need of a reminder, what is dark matter?
Only about 15 per cent of the matter in the universe is made up of atoms—including stars, galaxies and us. The rest is dark matter, which doesn’t emit or absorb or scatter light, so we can’t see it. We only know it’s there because of its gravitational effect—we can see it pulling on atomic matter. It’s super important in terms of how our universe behaves. Where dark matter becomes denser, atomic matter gathers and collapses to form galaxies and, inside galaxies, stars. It creates an invisible background on top of which the matter we can see and that we’re made of forms, giving structure to the universe.
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But, if you can’t see dark matter, how can you map it?
There are different ways you can do it, but we used what’s called the cosmic microwave background, which is the very oldest light in the universe. That light has been travelling toward us from the big bang, and along the way it’s been bent by dark matter’s gravitational pull. We can measure the degree of bending, and that’s what we used to make the map.
What does the map tell us about dark matter that we didn’t know before?
Nothing, really. But it confirms that we understand, quite well, how the universe grows and evolves. By looking at the cosmic microwave background, we see what the universe looked like near the beginning, 14 billion years ago. We can take this early picture of the universe and use physics and models like Einstein’s general theory of relativity to predict how it will grow over time. Now, thanks to this map, we can check our theories, and what we found is that it matches our predictions.
Forgive my tiny social media–addled brain, but this is the analogy that comes to mind: it’s like you had a baby picture of the universe—that’s the cosmic microwave background—and you applied an aging filter to it—Einstein’s general theory of relativity—to predict what the universe would look like all grown up. And then you got a pretty detailed photo of the actual adult universe today—the map—and they matched, which confirmed that your Einstein aging filter works.
Exactly, with one amendment. We’re not looking at the universe today—it’s more like the universe a few billion years ago.
So like a picture of the universe in middle school. Got it. This isn’t the largest map of dark matter ever made, though, right?
In the 2010s, the Planck satellite, an observatory equipped with a highly sensitive telescope, mapped the distribution of dark matter over the whole sky. Our telescope is on the ground in Chile—I was actually there when it was set up, in 2007, as a graduate student from Princeton—so we can’t see the whole sky. We mapped about a quarter of it. But our telescope was significantly bigger than Planck’s, which means that our map has a lot more detail. The fact that we can successfully explain how our cosmos works with this level of precision is amazing.
There are over 160 collaborators on this project from all over the planet. What did you work on?
I wrote a lot of software to make the telescope work—how it moves, understands temperatures, collects data.
And what about U of T?
Beyond the faculty members, including me, Dick Bond and Renée Hložek, and the dozens of students and postdocs who collaborated on the project, one of our important contributions is in supercomputing. There was 15 years of data to process, and a lot of that happened in Toronto. SciNet is a supercomputing centre here that hosts an incredibly fast supercomputer called Niagara. That was a crucial resource.
On top of being a cosmologist, you’re also a Jesuit priest. How does religion inform your study of the universe?
The belief that you can observe the universe—and that it’s intelligible—is deeply connected to the biblical notion of creation, which says that God created a world that is understandable. Actually, in Greek, the word “cosmos” means “order.” A mistake we can make as human beings—in science and in religion—is to think we have all the answers. But there’s no such thing as a final answer in science. There’s always more to understand, so we roll up our sleeves and try to figure it out. People who are engaged in their faith do the same thing. Cosmology and theology are their own distinct subjects with their own methodologies, but they come together in the human enquirer. We want to know how things work and why they are the way they are.
This interview has been edited for length and clarity.
