How did you get into cosmology?
[Honestly], I was pretty bored with school and I was thinking about the most challenging thing I could do. At that point, the most challenging thing I could come up with was astrophysics. I had no idea what astrophysics really was but it sounded really hard, like rocket science. One of the earliest memories I have of something that really made me fall into physics is learning about the idea that light gets bent by gravity. I had no idea what I was thinking or talking about then, I was 12 years old, but it was the first time I was trying to understand the universe in a really conceptual way. That ended up becoming what I’m doing now in my daily research.
What is dark matter?
If you trace the [speed] of stars in the galaxy, as you move outward from the center, the curve is basically flat. Things that are close to the center [of the galaxy] and things that are at the edge of the galaxy are all moving at the same velocity. Gravitationally, that makes no sense at all, given what we can observe. The only way it can happen is if there’s other stuff we can’t see that is contributing to the gravitational field of the galaxy. We call this dark matter. Dark energy is basically the opposite. Instead of something helping the universe collapse onto itself, dark energy is pushing things apart. Dark matter is helping things collapse together faster and dark energy is slowing that collapse.
What is the Dark Energy Survey?
We basically spent six years, since 2012, looking at an area of the sky about 5,000 square degrees, which is about an eighth of the total sphere of the sky. We took pictures of every part of that area ten times. [The dark energy camera] used a new type of [detection device] that was particularly sensitive to red wavelengths. This is important when you want to look at things that are very far away [almost 10 billion light-years].
What do you do with pictures from the Dark Energy Survey?
I work on something called gravitational lensing, where we look at all of these galaxies in this part of the sky and we use the shapes of the galaxies to reconstruct a map of all the mass in the universe. Photons that are streaming toward us from these far-away objects, like everything else in the universe, interact gravitationally with everything they pass by. As a photon goes by something that has a large gravitational field, the path of the photon will appear to be deflected. We can’t see dark matter…. but the deflection of photons by the gravity of dark matter allows us to create a huge, 5000-square degree map of where dark matter is in this part of the universe.
What is the goal of the survey?
If we look at this dark matter distribution as a function of time or distance from us, that tells us about how the universe is expanding. These new experiments we’ve been building, the Dark Energy Survey [and others] are designed to give us a handle on what dark matter and dark energy are. [These are] things that must exist for [our cosmological model of the universe] to work, but which we’ve never directly detected in a physics experiment.
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