Primary Topic
This episode delves into the enigmatic and pervasive substance known as dark matter, which composes a significant portion of our universe yet remains largely unseen and undetected.
Episode Summary
Main Takeaways
- Dark matter constitutes about 27% of the universe, influencing its structure profoundly but remaining invisible because it doesn't interact with light.
- The episode highlights historical research, like Zwicky's inference of dark matter from galaxy cluster movements and Rubin's rotational curves of galaxies.
- Contemporary research utilizes gravitational lensing to study dark matter, revealing more complexities and anomalies that challenge existing theories.
- The scientific community remains open to various possibilities for what dark matter might be, from weakly interacting massive particles (WIMPs) to axions.
- The discussion emphasizes the humility and curiosity required in scientific pursuits, particularly in fields dealing with unknown aspects of nature.
Episode Chapters
1: Introduction
Hosts introduce the topic of dark matter, its importance, and historical context. Key focus on how little humanity knows about the universe.
- Emily Kwong: "You're listening to short wave from NPR."
2: Historical Insights
Exploration of early dark matter research by Zwicky and the advancements by Rubin and Ford, setting the foundation for understanding galaxy formation.
- Rebecca Ramirez: "Studying the coma cluster, a group of about a thousand galaxies."
3: Modern Research
Discussion on the methods and challenges of current dark matter research, including the use of gravitational lensing and advanced simulations.
- Priyamvada 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."
Actionable Advice
- Foster curiosity about the universe: Engage with educational content and discussions about cosmology to broaden understanding.
- Participate in citizen science projects related to astronomy to help gather data for scientific research.
- Encourage STEM education by supporting initiatives and programs that promote science and research.
- Stay informed about the latest scientific discoveries through reliable sources to appreciate the evolving nature of science.
- Promote public understanding of science by discussing complex topics like dark matter in accessible terms.
About This Episode
The universe is so much bigger than what people can see. Visible matter — the ground, the Sun, the screen you're reading this on — makes up only about 4 or 5 percent of our known universe. Dark matter makes up much more of the universe. It's all around us even though we can't see it. So what is it? What's it made out of? How do we even know it exists? Host Emily Kwong and Rebecca Ramirez try to find out with the help of astrophysicist Priyamvada Natarajan.
People
Priyamvada Natarajan, Rebecca Ramirez, Emily Kwong
Content Warnings:
None
Transcript
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Regina Barber
Hey, everyone, it's Regina Barber, and I'm checking in from Starship shortwave 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 shortwave producer and now showrunner Rebecca Ramirez, and it's hosted by Emily Kwong. I'll let them take it from here.
Emily Kwong
You're listening to short wave from NPR.
Hey, everybody, Emily Kwong here with short wave producer Rebecca Ramirez. Hi, lady.
Rebecca Ramirez
Hello.
Emily Kwong
What you got for us today?
Rebecca Ramirez
Well, Emily, as you know, I'm kind of in love with the mysteries of our universe and how little we know know about the universe.
Emily Kwong
Ooh, are we playing in outer space today?
Rebecca Ramirez
Everywhere. Because I want to talk to you about dark matter, which is all around us.
Emily Kwong
Okay. Dark matter. What is that?
Rebecca 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 Priyanvada Natarajan, a professor of astronomy and physics over at Yale, say all of that is only somewhere around four 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.
Rebecca Ramirez
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.
Emily Kwong
Yes, I remember Priya from our black holes episode.
Rebecca Ramirez
Yeah, she was great. And you know, this four to 5% number, I don't know about you, but it's just so humbling every time I think about it.
Priyamvada Natarajan
We are so anthropocentric, like, 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 like a tiny fraction.
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Wow.
Emily Kwong
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. It is real.
Rebecca Ramirez
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.
Priyamvada Natarajan
And 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.
Rebecca 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.
Emily Kwong
You're listening to short wave, the daily Science podcast from NPR.
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Emily 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?
Rebecca Ramirez
Well, dark matter comes from decades of scientists taking these measurements and needing to explain why they don't match expectations.
Priyamvada Natarajan
The first hints came from Zwicky in the 1930s, when he measured the motions of galaxies in galaxy clusters, Zwicky was.
Rebecca Ramirez
Studying the coma cluster, a group of about a thousand galaxies. And like the name suggests, the galaxies are all sort of clustered together because of gravity. And Fritz Wiki is this astronomer who just wants to know, what's the mass of a galaxy?
Emily Kwong
Yeah, casual. That's what I spend my time thinking about, too.
Rebecca 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.
Priyamvada Natarajan
So they whiz around much faster than the amount of matter that you see in the light in terms of the stars would suggest.
Emily Kwong
I'm gathering that he found all of this extra mass that was unaccounted for that made things move much faster.
Rebecca Ramirez
Exactly.
Priyamvada Natarajan
And so Zwicky proposed that there was probably this sort of dark material that was sitting there that was providing extra gravity.
Emily Kwong
Ah, okay, okay.
Rebecca 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.
Emily Kwong
Aside from the greatest music of our time, what happens in the 1970s?
Rebecca 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, ones closer to the center and also further out.
Emily Kwong
So she was looking at galaxies at a much smaller scale than Zwicky, who was looking at whole clusters of galaxies.
Rebecca 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.
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Right.
Emily 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.
Rebecca 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.
Priyamvada Natarajan
Edges, 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.
Emily Kwong
There's that dark matter popping its head up again.
I bet that was actually kind of alarming data back in the seventies.
Like, guys, this is completely not what we expected. Can someone check up on this galaxy?
Rebecca 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 Ken, they just want to double check. Is this a quirk of this one galaxy or a sign of something bigger?
Emily Kwong
Dark matter.
Rebecca Ramirez
Yeah, exactly. So they study another galaxy, and when they publish the data, they find the same puzzling movement. Meanwhile, other astronomers find similar evidence of straight up missing mass.
Emily Kwong
Okay, so evidence is mounting from multiple researchers at this point, evidence is mounting.
Rebecca Ramirez
Vera and Kent studied dozens more galaxies. Do years more research. Her fellow astronomers say, okay, there's gotta be something going on. And theorists, they start to put all these observations together, running simulations that confirmed that there had to be all this.
Priyamvada Natarajan
Missing matter and a ton of it 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 served 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.
Rebecca 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.
Emily Kwong
Okay. But they still have no idea what the heck dark matter actually is.
Rebecca 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.
Priyamvada Natarajan
These particles have the right properties and have the right abundance, and they fit well with the standard model of particle physics.
Rebecca 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, but they must.
Emily Kwong
Be onto something else, then, right? If there's one thing I know about scientists, it's that failure just means more room to play. Yeah. More chances to figure out what dark matter is. Made of.
Rebecca 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 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.
Priyamvada 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.
Emily Kwong
So Priya is not putting all her dark matter eggs in one basket as she researches?
Rebecca Ramirez
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.
Priyamvada 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. Okay, 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, meaning they found.
Rebecca Ramirez
Some extra mass at the inner regions of the galaxy cluster that current dark matter theory doesn't explain.
Emily Kwong
Oh, okay.
Rebecca 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.
Emily 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.
Rebecca Ramirez
Yeah. And so it's gonna honestly just take a lot more work to get to the bottom of it.
Priyamvada 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.
Emily Kwong
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.
Rebecca Ramirez
I love it so much, 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.
Priyamvada Natarajan
Life lesson, that openness and willingness to learn and not be stuck in our ways is the only way forward.
Emily Kwong
An open mind is an important part of the scientific process.
Thank you for reminding us of that today.
Rebecca Ramirez
Anytime.
Regina 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 Darken energy.
Brian Nord
Hey shortwave crew. It's Doctor 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.
Regina Barber
The episode you just heard was produced by Rebecca Ramirez with the help of Indycara. It was edited by Giselle Grayson and fact checked by indie additional producing done by Hannah Chin. Julia Carney is our space camp project manager, Beth Donovan is our senior director, and Colin 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 Barbour, and you're listening to short wave from NPR.
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