REGINA BARBER: Hey, everyone. It's Regina Barber, and I'm checking in from Starship Short Wave. For this week's edition of space camp, we're revisiting an episode that's one of my personal favorites, all about dark matter. It features former Short Wave producer and now show runner Rebecca Ramirez, and it's hosted by Emily Kwong. I'll let them take it from here.
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ANNOUNCER 1: You're listening to Short Wave from NPR.
EMILY KWONG: Hey, everybody. Emily Kwong here with Short Wave producer Rebecca Ramirez. Hi, lady.
REBECCA RAMIREZ: Hello.
KWONG: What you got for us today?
RAMIREZ: Well, Emily, as you know, I'm kind of in love with the mysteries of our universe and how little we know about the universe.
KWONG: Ooh, are we playing in outer space today?
RAMIREZ: Everywhere because I want to talk to you about dark matter, which is all around us.
KWONG: OK. Dark matter. What is that?
RAMIREZ: So there's regular matter, all the things that are visible to you and me-- the clothes you're using to soundproof your closet for recording, all the stars, planets, you get the idea. And scientists like Priyamvada Natarajan, a professor of astronomy and physics over at Yale, say all of that is only somewhere around 4% to 5% of our universe.
PRIYAMVADA NATARAJAN: So it's just the tip of the iceberg. In fact, the bulk of the matter in the universe is actually dark matter that is invisible to us, and the reason that it's invisible to us is because it doesn't interact with light in any way at all.
RAMIREZ: Hence, it's all around us, possibly even occasionally floating through us, and you didn't realize because you can't see it, and because scientists like Priya think it doesn't really interact with normal matter either.
KWONG: Yes, I remember Priya from our black holes episode.
RAMIREZ: Yeah, she was great. And you know, this 4% to 5% number, I don't know about you, but it's just so humbling every time I think about it.
NATARAJAN: We are so anthropocentric. We believe that we are so important and so significant. In the grand scheme of things, I think it's a very sobering, humbling kind of number to keep track of that everything that we can see and perceive is just a tiny fraction.
BARBER: Wow, I'm ready. I'm ready to be humbled by dark matter and leave it to you to immediately wax poetic about something humanity knows very little about.
RAMIREZ: It is real, and scientists like Priya are able to study dark matter indirectly doing lots of observations, modeling, running simulations, and doing very intense calculations. So there are a couple things researchers know, like it has mass and--
NATARAJAN: Dark matter, we believe, consists of particles that were created likely in the very early universe, but particles that have very peculiar properties. They interact only via gravity, and if at all, they interact with each other beyond gravity, we think it's extremely weakly.
RAMIREZ: Like Priya said, everything about dark matter is peculiar, even the story of how researchers found it. So we start with those first inklings and follow the dark matter trail to see where current research is going.
BARBER: You're listening to Short Wave, the daily science podcast from NPR.
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KWONG: All right, Rebecca, so a lot of the universe is dark matter, this thing we can't see. And since we can't see it, how do we know that it exists?
RAMIREZ: Well, dark matter comes from decades of scientists taking these measurements and needing to explain why they don't match expectations.
NATARAJAN: The first hints came from Zwicky in the 1930s when he measured the motions of galaxies in galaxy clusters.
RAMIREZ: Zwicky was studying the coma cluster, a group of about 1,000 galaxies. And like the name suggests, the galaxies are all sort of clustered together because of gravity. And Fritz Zwicky is this astronomer who just wants to know, what's the mass of a galaxy?
KWONG: Yeah, casual. That's what I spend my time thinking about, too.
RAMIREZ: Same. So I'll spare you the math, but to calculate how massive the galaxies are in this cluster, Zwicky first had to figure out how quickly the galaxies were moving. And when he went to record their movement, he noticed something really weird.
NATARAJAN: So they whizzed around much faster than the amount of matter that you see in the light in terms of the stars would suggest.
KWONG: I'm gathering that he found all of this extra mass that was unaccounted for that made things move much faster.
RAMIREZ: Exactly.
NATARAJAN: And so Zwicky proposed that there was probably this dark material that was sitting there that was providing extra gravity.
KWONG: Ah, OK.
RAMIREZ: And the extra gravity from all of this invisible matter should be so powerful that it bends light, what's called gravitational lensing. But back in the 1930s, the telescopes available were just not precise enough to pick up any of these distortions that Zwicky suggested should happen. And so the idea kind of goes nowhere until the 1970s,
KWONG: Aside from the greatest music of our time, what happens in the 1970s?
RAMIREZ: Astronomer Vera Rubin comes along. She was interested in the rotations of spiral galaxies, and to figure out how a galaxy is rotating, scientists have to look at individual stars in a galaxy once closer to the center and also further out.
KWONG: So she was looking at galaxies at a much smaller scale than Zwicky, who was looking at whole clusters of galaxies?
RAMIREZ: Right. So what scientists expect is there will be more mass at the center of the galaxy, so stars closer to the center will move more quickly than stars at the edges of the galaxy because of all of the gravity exerted by these inner objects, like the planets in our solar system.
KWONG: The sun's gravity pulling on the planets is why Mercury, an inner planet, moves more quickly than Neptune, an outer planet. So it sounds like they were expecting to see the same thing with other stars.
RAMIREZ: Exactly. That's what they knew, so it's what they expected. But what Vera and her collaborator Kent Ford found was the opposite, these unexpectedly fast star speeds at the edges.
NATARAJAN: They were whizzing around much faster than the gravity that you would infer from the visible material alone, and so this was suggestive that on the scale of individual galaxies, that there ought to exist a repository of dark matter that these stars were actually responding to that you could measure.
KWONG: There's that dark matter popping its head up again.
RAMIREZ: Mm-hmm.
KWONG: I bet. That was actually kind of alarming data back in the '70s.
RAMIREZ: Mm-hmm.
KWONG: Like, uh, guys, this is completely not what we expected. Can someone check up on this galaxy?
RAMIREZ: Can you imagine? So, of course, they were cautious when they published their findings because again, the explanation is, so we think there's a lot of invisible mass around the galaxy. And so Vera and Kent, they just want to double check-- is this a quirk of this one galaxy or a sign of something bigger?
KWONG: Dark matter. [COUGHS]
RAMIREZ: Yeah, exactly. So they study another galaxy, and when they published the data, they find the same puzzling movement. Meanwhile, other astronomers find similar evidence of straight-up missing mass.
KWONG: OK, so evidence is mounting from multiple researchers at this point. Evidence is mounting. Vera and Kent studied dozens more galaxies, do years more research. Her fellow astronomers say, OK, there's got to be something going on. And theorists, they start to put all these observations together, running simulations that confirm that there had to be all this missing matter and a ton of it.
NATARAJAN: Simultaneously, completely independently, theorists who were trying to understand how galaxies form in the first place also realized that they needed an additional source of gravity to stabilize galaxies and that dark matter serve that purpose, too. So it turns out that, theoretically, there was a lot of room and, in fact, necessity for having a dark component in the universe.
RAMIREZ: The entire universe seems to be structured by dark matter. All this missing matter is key to the formation of galaxies in the first place, and researchers think dark matter is actually spread out across the entire universe like cobwebs, and galaxies seem to basically form along these filaments.
KWONG: OK. But they still have no idea what the heck dark matter actually is?
RAMIREZ: No, this is still an active area of research, and there are a few theories. So for a long time, some researchers thought dark matter was made up of this thing called Weakly Interacting Massive Particles or WIMPs for short, which theoretically would have been around since the early universe, making them a good candidate for the job.
NATARAJAN: These particles have the right properties and have the right abundance, and they fit well with the standard model of particle physics.
RAMIREZ: But if a particle is weakly interacting, they basically don't collide with each other. They're really hard to find. And despite many an experiment, researchers have come up short. No WIMPS so far.
KWONG: But they must be on to something else then, right? If there's one thing I know about scientists is that failure just means more room to play.
RAMIREZ: Yeah.
KWONG: More chances to figure out what dark matter is made of.
RAMIREZ: Exactly. And so there is another big candidate called axions, which scientists also think would have been formed in the early universe-- check-- and would have been plentiful enough to account for all this missing mass and hardly interact with all this regular matter that we can see-- another check. But researchers haven't found evidence of it either, and the uncertainty of it all, that's part of what researchers like Priya love.
NATARAJAN: I'm really thrilled that we are exploring multiple possibilities and not being fixated on one class of particle because, you know, we don't have any specific guidance necessarily to fixate on an individual set of particles.
RAMIREZ: So Priya is not putting all her dark matter eggs in one basket as she researches. No. She and many colleagues just published a paper last year, and it focused on mapping dark matter rather than focusing on one of these candidates.
NATARAJAN: Not only do we have a map of the biggest peaks, but we have the details of the incline of little mounds of dark matter, so real exquisite mapping. And so what we found is that when we then add up and look at the strength of lensing of these little lenses that are embedded in the bigger lens, they are much, much stronger than the theory predicts.
RAMIREZ: Meaning they found some extra mass at the inner regions of the galaxy cluster that current dark matter theory doesn't explain.
KWONG: Oh, OK.
RAMIREZ: So remember, initially, researchers were calculating the mass of the galaxies and saying, all this missing mass is dark matter. Well, now, factoring in the dark matter, there's still missing mass.
KWONG: Well, I love how these researchers are in the dark about dark matter and also that modern ideas about dark matter could just be totally upended with more research.
RAMIREZ: Yeah, and so it's going to honestly just take a lot more work to get to the bottom of it.
NATARAJAN: You'd have to tweak all models. All alternatives that currently exist have to be tweaked further in order to explain what we find. And that's what is really exciting-- that it opens up all these possibilities, new ways to think and imagine how you could clump dark matter in the inner parts of galaxies.
RAMIREZ: Priya is so impressively unfazed by the challenge ahead. I mean, the tone in her voice is of someone who just doesn't mind that we don't know and really, really wants to figure it out.
KWONG: I love it so much.
RAMIREZ: And I think that's what makes it an exciting time to talk about dark matter. Scientists are deeply in the discovery process, which Priya says is a very important life lesson.
NATARAJAN: That openness and willingness to learn and not be stuck in our ways is the only way forward.
RAMIREZ: An open mind is an important part of the scientific process.
KWONG: Thank you for reminding us of that today.
RAMIREZ: Any time.
BARBER: Hey, it's Regina Barber again with a reminder that we'll be back tomorrow with more regular Short Wave and back Tuesday with our next installment of the Space Camp series. Plus, I got a sneak preview for you. If you thought dark matter was wild, wait till you hear about dark energy.
BRIAN NORD: Hey, Short Wave crew, it's Dr. Brian Nord, your cosmological consultant for deep space. Dark energy makes up most of the universe, but we still know so little about it or how it even works. As you travel outward, can you send back updates on those cosmic probes? The future of the galaxies depends on it.
BARBER: The episode you just heard was produced by Rebecca Ramirez with the help of Indi Khera. It was edited by Gisele Grayson and fact checked by Indi Khera. Additional producing done by Hannah Chin. Julia Carney is our Space Camp project manager. Beth Donovan is our senior director, and Collin Campbell is our senior vice president of podcasting strategy. Special thanks to our friends at the US Space and Rocket Center, home of Space Camp. I'm Regina Barber, and you're listening to Short Wave from NPR.
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