The Dark Energy Spectroscopic Instrument, known as DESI, is attached to a 50-year-old telescope at the Kitt Peak National Observatory in southern Arizona.
It’s in the middle of a project with a very big goal: measuring the expansion of the universe. That’s right, the entire universe. And, it turns out, it’s not expanding as fast as it used to be.
Arjun Dey, Ph.D. is a project scientist for DESI, and he joined The Show to talk about it.
Full conversation
ARJUN DEY: The whole project is essentially a giant cosmic cartography project, designed to kind of map out not just the 3D structure of the universe, but also how that's evolved in time over the last 11 billion years or so of the universe's history
LAUREN GILGER: 11 billion years or so. OK. We'll talk a little bit more about the, how you're doing this in a moment. But am I right, you're using like a 50 year old telescope there to do this? Like this is kind of interestingly combining old and new technology.
DEY: That's exactly right. I mean, it was a telescope that was built more than 50 years ago now, and it was built like a battleship, so it was before we had a lot of modern tools to really engineer things just right. And so it was hugely over-engineered for the time. And as a result, it ended up being one of the few telescopes on the planet that could hold an instrument that was as massive as the one that we designed to do this experiment.
GILGER: Wow. OK. OK. Tell us because it's in the name, what is dark energy?
DEY: Well, it's a very difficult question to answer because we don't know a lot about the universe. So everything that we're made out of, atoms, molecules, et cetera, all of that material constitutes about 4% of the energy density of the present day universe. And the rest of the 96% is at the moment unknown. We understand that about another 26% or so behaves more or less like normal matter, it has gravity. But the remaining 70% or so is stuff that really behaves like a negative gravity on the universe. It has negative pressure, it's pushing the universe outwards and causing the expansion of the universe to accelerate. And that's what we call dark energy, but we don't really know what it is. And, and this experiment is really designed to try and measure the expansion rate of the universe very precisely and see whether that gives us clues of distinguishing between different theories that tell us more about what dark energy might actually be.
GILGER: OK. So you're in year three of a five-year project here. Let's talk a little bit about what you found so far because it sounds like there are some hints at some big discoveries potentially here in terms of how fast or if the universe is continuing to expand.
DEY: That's right. It's super-exciting. So for, you know, dark energy was really only discovered in about 1998. So we haven't known about it very long, but we've sort of assumed that it was basically something that was a constant. That is the energy density didn't change as a function of time as the universe expanded. And that was a theory that essentially fell right out of Einstein's theory of general relativity. And it wasn't a very, it wasn't something that was loved greatly, but it worked and it gave us a fairly predictive model of the universe.
What we're finding with the DESI data and in combination with other measurements is that it's likely that this dark energy, whatever it is, has changed as a function of time. And that there was, it was more pushy if you will in the past and has become less pushy into the present. And we don't know how it will evolve beyond that into the future. But hopefully, the measurements that we'll make by the time we're done with this project will tell us more about its evolution.
GILGER: So then let's spend the last few minutes here talking about the big kind of why part of this, the, the what does it all mean part, right. I mean, I think in general, like people are interested in the idea of the universe, we want to know where we come from, right? Is that what you're trying to answer, that question in mapping it is, is that the goal?
DEY: Absolutely. I mean, I think it's a very human desire to know what our surroundings are, what, you know, where we live, how we got here, where we're going. I think this is very much part of that exploration. It is of course disconcerting to most scientists that 96% of the energy density of the present day universe is stuff we don't understand. So it would be nice to know that I think we've made a lot of progress in understanding or at least making more precise the limits of our knowledge. And I think what we're trying to do with this experiment is to see whether, you know, it changes the way in which we think about physics. There may be new physics that's lurking which we haven't yet discovered and which this experiment will allow us to explore.
GILGER: So you have talked about this in terms of art, right? Like this idea that what you're trying to do is similar to an artist trying to paint something that is 3D in real life in in two dimensions, on a flat surface, right? Talk a little bit about that comparison. I thought that was so interesting.
DEY: Well, I think it's one of these long standing questions of how you represent nature. And, you know, for example, Cezanne experimented with this, it was the origins of Cubism, in the very early days of trying to describe something in three dimensions and sort of put it on a 2D canvas. Here, we have a more complex problem, which is seeing something that is evolving in time. You know, Picasso also struggled with this. He tried to depict it in these different facets of of, of his canvas.
We're trying to describe the universe, that's the way it is today. But also because of the finite speed of light, we are seeing every single thing we see in the universe at some instant in the past. And so the more distant things we look at, the further into the past we're exploring them. And so trying to visualize this in space I think is one of the interesting questions of how we do this project. This project is giving us the data to do it. But how we then communicate that I think is going to be an interesting challenge and a fun project I feel, right?
GILGER: You're talking about this, this crazy kind of mind-bending idea that if you're looking at something in, in the sky that is, you know, 2 million light years away, it's, you're seeing it as it was 2 million years ago. Like it's a, it's not a, it's not a picture taken in real time, I guess in that sense.
DEY: That's right. And so at the very limits of our survey, you know, we basically can see all the way back until the universe was completely opaque. And that point was about 300,000 years after the Big Bang, almost 13 billion years ago. And so with this project, we're trying to reach back to about 11 billion years, you know, while matter, the first galaxies had formed, but the large-scale structure of the universe was still not completely in place the way we see it in the present day universe. So the the current universe is very textured. Galaxies form filaments and clusters and shells and so on in, you know, over hundreds of millions of light years. And we're part of that structure and it's that structure that allows us sort of identifying that structure and measuring it precisely that allows us to see how this universe has evolved with time.
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