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Speaker 1: Hey, Orgy, I think I may have lost something?

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Speaker 3: Well, oh, did you lose your mind again?

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Speaker 1: Of that happened a long time ago. This is something new?

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Speaker 3: Is it important?

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Speaker 1: It's actually kind of a big deal.

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Speaker 3: Is it like really small and easy to lose, like

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Speaker 3: your keys or your wedding bed.

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Speaker 1: No, it's much more embarrassing because it's really enormous, like

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Speaker 1: cosmically large, like most of the stuff in the universe.

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Speaker 3: And you can't find it.

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Speaker 1: I've been looking everywhere for it.

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Speaker 3: Well, I guess you know what to do.

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Speaker 1: Oh yeah, what's that?

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Speaker 3: You know? Give it a cool sounding name, and then

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Speaker 3: ask the federal government to help you find it.

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Speaker 1: Do you think anybody would actually fall for that?

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Speaker 3: I think they already have. Hi am Jorham, a cartoonist

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Speaker 3: and the creator of PhD comics.

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Speaker 1: Hi. I'm Daniel. I'm a particle physicist and a professor

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Speaker 1: at UC Irvine, and I am actually paid to hunt

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Speaker 1: for missing things.

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Speaker 3: Hmm. Interesting, You're like the Lost and Found of the

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Speaker 3: Universe department.

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Speaker 1: That sounds very unglamorous. I like to think of myself

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Speaker 1: more of the Sherlock Holmes of the universe.

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Speaker 3: I see you're not in like in the basement office

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Speaker 3: with a little window that people go there and claim

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Speaker 3: lost things.

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Speaker 1: That's right. I'm not sitting here surrounded by people's lost

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Speaker 1: purses or socks.

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Speaker 3: Where do all the socks and the dryers go? That's

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Speaker 3: what I want to know. We should have a whole

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Speaker 3: episode about that.

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Speaker 1: They're orbiting the Earth in the ho zone layer.

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Speaker 3: Well that socks may they're all connected by wormholes? You

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Speaker 3: think dryers, all that spinning and that heat, could that

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Speaker 3: create a wormhole?

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Speaker 1: Keep running your dryer let us know.

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Speaker 3: But welcome to our podcast Daniel and Jorge Explain the Universe,

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Speaker 3: a production of iHeartRadio.

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Speaker 1: Which we operate as a sort of lost and found

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Speaker 1: of ideas about the universe. Everything we have found about

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Speaker 1: the universe and all the ideas we have lost. Everything

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Speaker 1: we do know about what's going on out there, the

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Speaker 1: size of the universe, the number of dimensions of space,

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Speaker 1: whether or not there are alien craters on other planets

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Speaker 1: looking through their telescopes at us. Every deep and enormous

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Speaker 1: question about the universe you might consider we talk about

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Speaker 1: here on the podcast.

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Speaker 3: Yeah, because it is a huge universe full of amazing

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Speaker 3: and wonderful things that seem to be screaming at us

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Speaker 3: for us to find them. We get light from distant

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Speaker 3: stars all the time at all times, radiation, cosmic rays,

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Speaker 3: and it's all coming to us sort of one thing

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Speaker 3: for us to find out and what's out there and

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Speaker 3: to discover and learn about how it all works.

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Speaker 1: You make the universe sounds sort of cooperative, like it

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Speaker 1: wants to play a role in our science and be helpful.

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Speaker 3: To need the universe right, I mean, it's constantly shouting

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Speaker 3: at us, isn't it?

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Speaker 1: It is? But sometimes these clues are very, very subtle

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Speaker 1: and frustratingly difficult to grasp. I feel like the universe

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Speaker 1: is sort of playing cat and mouse with us, sometimes

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Speaker 1: hiding all the best bits.

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Speaker 3: It's like a koi. Universe means like it wants us

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Speaker 3: to find it, but it's not telling us everything that

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Speaker 3: it can about itself.

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Speaker 1: Yeah, it doesn't just come right out and tell us

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Speaker 1: it's deep nature. It sometimes seems to make sense, and

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Speaker 1: then you dig deeper and discover oh my gosh, it's

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Speaker 1: bonkers underneath. But there is a lot of fascinating information

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Speaker 1: in those little hints that do arrive here on Earth.

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Speaker 3: Yeah, and it's pretty amazing that, you know, if you

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Speaker 3: think about it, that we're sitting on this little rock,

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Speaker 3: floating through space in a corner of the galaxy, in

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Speaker 3: a little corner of the universe, and somehow, from this

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Speaker 3: light that we're getting from distant stars, we can somehow

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Speaker 3: piece together the whole structure almost of the universe.

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Speaker 1: I like how you just say somehow, you know, it's

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Speaker 1: like the information comes and dot dot dot, we know

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Speaker 1: these things. You just like, somehow my entire career right there.

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Speaker 3: Yeah, somehow it goes to the lost and Found department

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Speaker 3: in the basement and we get a little memo about

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Speaker 3: what happened.

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Speaker 1: That's right. Somehow, because the government decides to fund the

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Speaker 1: basic science research, we have uncovered some truths about the universe.

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Speaker 3: I guess what I mean by somehow thought dot dot

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Speaker 3: is you know, mostly engineering and then some science involved.

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Speaker 1: Some happy collaboration between scientists and engineers.

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Speaker 3: But yeah, we are learning more and more about the universe.

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Speaker 3: And there's a big question, especially a big question about

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Speaker 3: the universe, about what it's made out of and how

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Speaker 3: it's what structure it has out there in the galaxies

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Speaker 3: and also in between galaxies. What does it all look

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Speaker 3: like how is it all put together?

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Speaker 1: It's really one of the biggest questions in the universe,

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Speaker 1: and one of the first questions you might ask, which

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Speaker 1: is what is in the universe? What's the universe made

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Speaker 1: out of? Where is all the stuff out there? And

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Speaker 1: what kind of stuff is it? Really it's sort of

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Speaker 1: shocking and amazing that we don't know the answer to

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Speaker 1: that very basic question about the nature of our own reality.

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Speaker 3: Yeah, because the universe sort of pulled the fast one

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Speaker 3: on us, right like it did this big reveal twist

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Speaker 3: somewhere a few decades ago where we thought we knew

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Speaker 3: what the universe was made out of. It was stars

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Speaker 3: and matter and atoms and things like that, But suddenly

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Speaker 3: we learned that that was only like five percent of it.

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Speaker 3: You know. It's kind of like when you're watching a

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Speaker 3: show and Sony you realize that you still have like

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Speaker 3: ninety five episodes ago.

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Speaker 1: It's sort of like the universe was wearing a mask

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Speaker 1: and then somebody pulled it off and we discovered, oh

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Speaker 1: my gosh, there's a lot more to it. So much

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Speaker 1: for the universe being helpful and revealing, right.

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Speaker 3: Well, I think it just wants you to keep watching.

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Speaker 3: I guess it's trying to parse out the information, trying

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Speaker 3: to keep it interesting.

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Speaker 1: Maybe the showrunners of the Universe are just trying to

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Speaker 1: dole out the reveals a little bit at a time.

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Speaker 3: Show runners. Huh, so you're a multi theist.

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Speaker 1: I think it's got to be a committee. I mean, whoo,

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Speaker 1: there's so much to do.

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Speaker 3: Oh that's not a flattering, I guess description of you.

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Speaker 3: It looks like it was made by a committee of

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Speaker 3: the universe. Looks like it had too many writers.

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Speaker 1: Well, it does sometimes seem inconsistent, you know.

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Speaker 3: Well, it is sort of a mysterious question. What is

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Speaker 3: the universe made out of? And as we've learned in

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Speaker 3: the last few decades, it's not made out of just

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Speaker 3: stars and atoms and elements. It's mostly made out of

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Speaker 3: other things, namely dark matter and dark energy.

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Speaker 1: The kind of stuff that makes up you and me

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Speaker 1: and hamsters and ice cream and baloney sandwiches is actually

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Speaker 1: quite unusual in the universe. Most of the stuff that's

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Speaker 1: out there is not stars and gas and dust and

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Speaker 1: all the visible stuff. It's something else, something dark, something

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Speaker 1: we've only recently discovered exists out there.

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Speaker 3: Yeah, and so dark energy is this sort of the

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Speaker 3: phenomenon where the universe is expanding faster and faster every second.

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Speaker 3: But dark matter is especially sort of weird and concerning

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Speaker 3: because it's matter, it's stuff. It's like exerting gravity. You

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Speaker 3: can feel it, but we can't see it, which means

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Speaker 3: we don't know where it is exactly.

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Speaker 1: We know that dark matter is some kind of stuff.

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Speaker 1: There's something out there exerting gravity on the rest of

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Speaker 1: the universe, and we can sort of tell that it exists,

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Speaker 1: but we don't know what it is. And we've done

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Speaker 1: a lot of podcasts digging into what it might be.

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Speaker 1: Is it axions, is it black holes? Is it whimps?

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Speaker 1: Is it something else? Something weird? And they are even

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Speaker 1: more crazy ideas we haven't yet covered.

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Speaker 3: It's sort of like we know there's an elephant in

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Speaker 3: the room or in the galaxy or the universe, but

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Speaker 3: we don't know what this elephant is doing, or you know,

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Speaker 3: what is it like striking a pose? Is it jogging?

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Speaker 3: Is it sleeping. It's kind of a big mystery, like

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Speaker 3: we know it's there, but we sort of don't know

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Speaker 3: what it's doing or what structure it has.

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Speaker 1: Yeah, you can ask so many fascinating questions. So far,

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Speaker 1: we've mostly focused on what is dark matter? Is it

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Speaker 1: this kind of particle? Is it that kind of particle?

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Speaker 1: But equally interesting without even knowing what it's made out of,

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Speaker 1: is just wondering like where is the dark matter? Is

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Speaker 1: it here with us in this room? Is it out

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Speaker 1: there in deep space? Does it form planets and other

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Speaker 1: kinds of structure? Is it smoothly spread out throughout the universe?

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Speaker 1: Where in the universe is all of this stuff?

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Speaker 3: So today on the podcast, we'll be asking the question,

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Speaker 3: how do we know where dark matter is? It sounds

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Speaker 3: like you lost you lost it again. Then I feel

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Speaker 3: like we said of knew where it was, but now

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Speaker 3: we don't know where it is.

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Speaker 1: It is a really important question because knowing where the

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Speaker 1: dark matter is can help us get clues about what

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Speaker 1: it might be, because certain kinds of dark matter might

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Speaker 1: clump up in different ways, and other kinds of dark

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Speaker 1: matter might not.

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Speaker 3: Oh yeah, is it chunky or smooth? I guess it's

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Speaker 3: a big question about the peanut butter dark matter of

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Speaker 3: the universe.

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Speaker 1: Well, I hope the showrunners don't disagree about that, because

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Speaker 1: then you might get chunky parts of the universe and

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Speaker 1: creamy parts of the universe.

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Speaker 3: You're not a creamy peanut butter fan.

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Speaker 1: It's all about the chunks, man, It's all about the chunks.

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Speaker 3: Well, to each their own may there's a different flavor

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Speaker 3: of dark matter.

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Speaker 1: For every tape Nutella there you go. Nutella is the

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Speaker 1: best flavor of dark flavor.

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Speaker 3: And everyone gets a heart attack from being surrounded by

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Speaker 3: all this saturated fat. But yeah, it's a big question.

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Speaker 3: Where is dark matter and sort of like what structure

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Speaker 3: it has in the universe. Is it sort of smooth

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Speaker 3: out there like a big cloud or is it a chunky?

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Speaker 3: Is it in like strands? Is it in clumps? What's

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Speaker 3: it doing out there?

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Speaker 1: And this is something that scientists are eager to figure

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Speaker 1: out because they just want to know where all of

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Speaker 1: this stuff is. They're trying to develop a picture of

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Speaker 1: the universe both visible and invisible, and of course the

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Speaker 1: invisible stuff much harder to see, but since there's much

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Speaker 1: more of it than there is the visible stuff, it's

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Speaker 1: a very important question.

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Speaker 3: So, as usual, we were wondering how many people out

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Speaker 3: there had thought about this question of the location and

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Speaker 3: structure of dark matter. So Daniel went out there to

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Speaker 3: ask people on the internet. How do we know where

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Speaker 3: dark matter is?

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Speaker 1: So, if you'd like to put your brain to the

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Speaker 1: test and answer questions that leading physicists don't know the

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Speaker 1: answer to, please write to me two questions at Danielandjorge

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Speaker 1: dot com.

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Speaker 3: Here's what people had to say.

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Speaker 5: Well, dark matter reacts with gravity, so by looking out

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Speaker 5: into the universe we can sort of detect where the

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Speaker 5: dark matter is because of its effect on gravity.

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Speaker 3: I know that it's biogravity.

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Speaker 6: Is the gravitational lensing that makes it possible to have

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Speaker 6: like some kind of ideas where it could be dark

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Speaker 6: matter in the universe. The distortion of light suggests that

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Speaker 6: there are more matter out there that we can actually

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Speaker 6: see whenever it like bands.

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Speaker 7: Well, once again, I assume this has to do with

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Speaker 7: the part with the examples and instances you're talking of

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Speaker 7: these massive tubs of argon. If I'm not misticn that

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Speaker 7: are deep underground.

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Speaker 8: We cannot see what dark matter itself is. Light travels

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Speaker 8: straight through it, but we can see other planets and

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Speaker 8: we can see light being distorted by the gravitational effects

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Speaker 8: of what dark matter is itself.

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Speaker 9: I'd say we know where the dark matter is because

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Speaker 9: we can see the gravitational force it exerts on the

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Speaker 9: visible matter around it. Next to that, it also bends

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Speaker 9: light from distant galaxies when it comes towards us. So

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Speaker 9: I'd say gravity is the usual suspect for us knowing

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Speaker 9: where dark matter is.

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Speaker 1: Who is DM?

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Speaker 2: I think I've heard that dark matter is everywhere, that

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Speaker 2: it just kind of permeates the universe, so all over

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Speaker 2: the place.

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Speaker 10: So my guess is for what we know dark matter is,

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Speaker 10: or how we know it is, if it gives off

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Speaker 10: gravitational waves because it interacts with stuff through gravity, then

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Speaker 10: we would use something that detects gravitational waves to see

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Speaker 10: either how far away it is or where it is

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Speaker 10: in respect to something else.

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Speaker 11: I think we know where dark matter is by looking

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Speaker 11: for localized gravitational effects like lensing or relative velocities that

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Speaker 11: aren't completely explained by matter we can observe in other ways.

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Speaker 3: All right, pretty interesting answer is a lot of people

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Speaker 3: went with gravity, which is sort of true, right, Like

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Speaker 3: that's how we initially discovered dark matter is through gravity.

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Speaker 1: Yeah, that's basically the only way we can sense dark matter,

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Speaker 1: and so gravity is basically the answer. Gravity is the

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Speaker 1: reason we know dark matter exists, and dark matter is

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Speaker 1: basically an explanation for otherwise unexplainable gravity. So yeah, gravity

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Speaker 1: is basically our portal into the dark universe.

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Speaker 3: And I like this person who said who is dark matter?

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Speaker 3: It's like who is new phone?

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Speaker 1: I think that's because in the question I wrote DM

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Speaker 1: instead of dark matter, assuming that everybody would know what

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Speaker 1: DM meant.

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Speaker 3: Right, who doesn't know what DM is?

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Speaker 1: Of sliding into some of these dms with that one,

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Speaker 1: I guess.

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Speaker 3: You are being a physicist using acronyms on people who

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Speaker 3: had no idea what those acronyms are. But yeah, a

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Speaker 3: lot of people seem to sort of have a basic

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Speaker 3: idea that it's through gravity. But the picture is a

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Speaker 3: little bit more complicated than that, right, I mean, we

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Speaker 3: sort of know that it's there because of gravity, but

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Speaker 3: instead of finding out exactly where it is or whether

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Speaker 3: it clumps or or smooth, that's much harder because we

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Speaker 3: can't see it right exactly.

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Speaker 1: We can't see it in the way that we can

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Speaker 1: see the other kinds of matter. It doesn't give off light,

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Speaker 1: it doesn't reflect light. Remember that dark matter is a

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Speaker 1: bit of a confusing name because dark matter is not

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Speaker 1: actually dark. It's transparent. It's invisible. It's not like a

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Speaker 1: cloud of dark matter between you and another star would

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Speaker 1: block your view of that star. If that were true,

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Speaker 1: would be much easier to see dark matter than it

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Speaker 1: is today. Instead, light passes right through dark matter.

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Speaker 3: So give us, maybe start us off with a refresher

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Speaker 3: of dark matter, or DM as the physicists call it.

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Speaker 3: You know, what do we know about it?

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Speaker 1: Well, we know that dark matter is about twenty five

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Speaker 1: percent of the energy budget of the universe. That means

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Speaker 1: if you take like a cubic light year of space,

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Speaker 1: or any volume of space, then twenty five percent of

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Speaker 1: the energy in that space is devoted to the mass

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Speaker 1: of dark matter, whereas five percent of the energy budget

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Speaker 1: of any chunk of space on average, you know, averaging

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Speaker 1: over big distances, is devoted to making things like stars

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Speaker 1: and galaxies and dust and giraftes and all that kind

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Speaker 1: of normal matter made out of atoms. And that means

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Speaker 1: that there's a huge amount of dark matter that a galaxy,

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Speaker 1: for example, is mostly dark matter. That the universe, the

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Speaker 1: stuff in the universe, the matter, the physical form of

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Speaker 1: the universe, is mostly dark matter. So we've been studying

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Speaker 1: the universe for thousands of years looking up at the

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Speaker 1: sky wondering how things work, and only recently have we

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Speaker 1: discovered that we've been missing most of it. So that's

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Speaker 1: pretty exciting. And we know that dark matter is not

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Speaker 1: made out of atoms, not made out of the kind

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Speaker 1: of stuff that you and I are made out of.

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Speaker 1: If it were, then it would probably interact with light.

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Speaker 1: And we can also do some careful accounting from the

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Speaker 1: very beginning of the universe, where we know something about

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Speaker 1: how much material there was to make atoms, we can

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Speaker 1: kind of account for where all of that went. So

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Speaker 1: we're pretty sure. We're almost certain that dark matter is

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Speaker 1: some kind of matter that doesn't give off light or

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Speaker 1: reflect light. It must be made out of something else.

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Speaker 1: And we know that it doesn't move very fast. We

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Speaker 1: call it cold dark matter because if it did, it

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Speaker 1: would spread out much more throughout the universe.

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Speaker 3: Yeah, and it's kind of interesting because I think I

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Speaker 3: almost feel like, in a way, physicists called this thing

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Speaker 3: or named it a little bit too early, you know

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Speaker 3: what I mean, Like we gave it a name like

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Speaker 3: dark matter, maybe a little too early, Like maybe you

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Speaker 3: should have kept going and say that you know, twenty

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Speaker 3: five percent of the universe is just something that we

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Speaker 3: don't understand, or something that is not like the rest

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Speaker 3: of the stuff in the universe.

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Speaker 1: Well, that doesn't work as well in grand proposals as

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Speaker 1: a nice ZINGI phrase like dark matter.

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Speaker 3: But I yeah, but I know, But I guess the

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Speaker 3: point is that really we don't know that much about it.

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Speaker 3: I mean, we sort of know its presence, or at

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Speaker 3: least we know it's the effect on the rest of

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Speaker 3: the universe, but we don't even know if it's matter.

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Speaker 1: Right, Well, we know that it generates gravity, which suggests

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Speaker 1: that its curves space according to general relativity, and so

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Speaker 1: either our understanding of how space curves is wrong, or

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Speaker 1: it's some new kind of energy in matter that does

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Speaker 1: curve space. And it's possible that we don't understand gravity.

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Speaker 1: I mean, it's certain that we don't have complete understanding

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Speaker 1: of gravity. There are alternative ideas to explain dark matter

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Speaker 1: using variations on gravitational theory, but none of them can

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Speaker 1: really explain everything that we see. And so you're right

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Speaker 1: that we're not one hundred percent certain that it's matter.

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Speaker 1: But it's the simplest explanation that fits all of the data.

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Speaker 1: A new kind of particle that only interacts gravitationally explains

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Speaker 1: basically everything that we see out there in the universe,

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Speaker 1: from the ripples in the earliest light to the structure

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Speaker 1: of the universe today to the rotations of galaxies. So

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Speaker 1: we're not certain, but it's the best candidate.

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Speaker 3: Maybe should have called it dark probably matter or dark

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Speaker 3: most likely matter DMML DMLM. But we, as you said,

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Speaker 3: so far, we only know about it because of its

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Speaker 3: gravitational effects, right, But we sort of know quite a

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Speaker 3: few things about sort of generally where it is.

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Speaker 1: We do have a good idea of where it might

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Speaker 1: be because of its gravitational effects. Right, it is invisible, It

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Speaker 1: doesn't give off light or interact with any other kind

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Speaker 1: of force. But gravity is local. Right, if you are

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Speaker 1: close to something, it tugs on you more strongly than

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Speaker 1: if you're far from something. So if you're measuring the

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Speaker 1: gravity of an object, if you can only tell that

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Speaker 1: something that's there because of its gravity, you can get

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Speaker 1: an idea for where it is based on what it

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Speaker 1: tugs on. For example, we can see the black holes

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Speaker 1: are there because of the way the stars move around

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Speaker 1: them without actually directly seeing black holes, and so gravity

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Speaker 1: definitely can give you a picture as to where things

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Speaker 1: are in the universe, and we have a rough idea

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Speaker 1: for where dark matter is. We think that dark matter

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Speaker 1: is mostly lined up with the normal matter. That where

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Speaker 1: you see a galaxy is where there's a huge clump

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Speaker 1: of dark matter. So every galaxy, we think, for example,

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Speaker 1: is embedded in a huge cloud. We call it a

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Speaker 1: dark matter halo for every galaxy.

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Speaker 3: Yeah, it's like where you see regular stars and planets,

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Speaker 3: you see dark matter. Or it's almost like the opposite, Right,

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Speaker 3: it's like where you see dark matter is where all

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Speaker 3: the stars and planets formed in.

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Speaker 1: A way, Yes, stars are more like the tracers for

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Speaker 1: the rest of stuff. Right, dark matter leads the way.

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Speaker 1: There's more dark matter than everything else, and so it's

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Speaker 1: actually like where the dark matter started clumping is where

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Speaker 1: the normal matter fell into it because of its gravity

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Speaker 1: and then formed galaxies and stars and all kinds of

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Speaker 1: stuff that we can see. So you know how when

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Speaker 1: the military is fighting at night, they shoot bullets and

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Speaker 1: then occasionally, like one out of every thousand bullets is

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Speaker 1: a tracer. It's like glows, so they can see where

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Speaker 1: they're shooting. Stars are sort of like that. They follow

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Speaker 1: the dark matter, and they give us a clue as

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Speaker 1: to where that dark matter is. And that's why we

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Speaker 1: think that most galaxies are embedded in this cloud, this

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Speaker 1: halo of sort of spherical, sort of a little bit

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Speaker 1: elliptical dark matter that goes well beyond actually where the

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Speaker 1: stars are.

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Speaker 3: Yeah. I've heard this sort of analogy that regular matter

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Speaker 3: like planets and stars is sort of like the sprinkling

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Speaker 3: on the icing of a cupcake. Like not just like

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Speaker 3: in terms of our relative importance to that and the

443
00:21:02,359 --> 00:21:04,240
Speaker 3: size of the universe, but also kind of like you know,

444
00:21:04,320 --> 00:21:07,000
Speaker 3: sprinkles stick to the icing in a cupcake. You know,

445
00:21:07,040 --> 00:21:09,359
Speaker 3: you can't sort of have sprinkles anywhere else. They're like,

446
00:21:10,119 --> 00:21:11,760
Speaker 3: you know, the sprinkles sort of tell you where the

447
00:21:11,920 --> 00:21:12,280
Speaker 3: icing is.

448
00:21:14,800 --> 00:21:17,159
Speaker 1: Yeah, the sprinkles are sort of the stars, and the

449
00:21:17,440 --> 00:21:19,800
Speaker 1: icing is sort of dark matter, and the cupcake itself

450
00:21:19,920 --> 00:21:22,399
Speaker 1: is dark energy. That gives you sort of a sense

451
00:21:22,720 --> 00:21:26,000
Speaker 1: of the relative fractions of the energy budget of the universe.

452
00:21:26,280 --> 00:21:28,679
Speaker 3: Yeah, so I guess we are just the hanger ons

453
00:21:28,720 --> 00:21:32,119
Speaker 3: of the universe. We're just hanging onto dark matter. And

454
00:21:32,200 --> 00:21:34,480
Speaker 3: so let's get into a little bit more detail about

455
00:21:34,520 --> 00:21:37,679
Speaker 3: what we know about this halo around galaxies and also

456
00:21:37,800 --> 00:21:40,720
Speaker 3: how we know where it is. But first, let's take

457
00:21:40,720 --> 00:21:41,320
Speaker 3: out quick break.

458
00:21:45,640 --> 00:21:48,640
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525
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Speaker 3: All right, we're talking about dark matter and where exactly

526
00:25:26,680 --> 00:25:28,960
Speaker 3: it is because I guess we know it's there, but

527
00:25:29,080 --> 00:25:30,639
Speaker 3: if physicists can't find it.

528
00:25:30,640 --> 00:25:34,000
Speaker 1: It's pretty tricky to nail down an individual piece of

529
00:25:34,080 --> 00:25:37,920
Speaker 1: dark matter, for example, because it only interacts gravitationally. Imagine

530
00:25:37,920 --> 00:25:41,239
Speaker 1: trying to find like an invisible piece of sand in

531
00:25:41,280 --> 00:25:43,960
Speaker 1: your room. How would you detect it? It's gravity is

532
00:25:44,080 --> 00:25:47,480
Speaker 1: essentially nothing because gravity is a really really weak force.

533
00:25:47,840 --> 00:25:50,879
Speaker 1: All the other forces of electromagnetism, even the weak force

534
00:25:51,240 --> 00:25:54,760
Speaker 1: is much stronger than gravity, So in order to detect

535
00:25:54,800 --> 00:25:57,600
Speaker 1: something through gravity, you need to have a huge force.

536
00:25:57,680 --> 00:26:00,520
Speaker 1: You need to have like a planet sized force, Solar

537
00:26:00,560 --> 00:26:04,280
Speaker 1: system sized force. Because gravity is so weak, so when

538
00:26:04,320 --> 00:26:06,920
Speaker 1: we use gravity to look for dark matter, we can

539
00:26:07,000 --> 00:26:10,120
Speaker 1: only sort of tell the large scale structure. It's very

540
00:26:10,119 --> 00:26:13,480
Speaker 1: difficult to get a fine grained picture of where things are.

541
00:26:13,560 --> 00:26:16,439
Speaker 3: But even though it's very weak gravitation, and we do

542
00:26:16,480 --> 00:26:19,000
Speaker 3: have kind of a pretty good idea of what shape

543
00:26:19,000 --> 00:26:21,680
Speaker 3: it has in the galaxy, right like this halo, it's

544
00:26:21,720 --> 00:26:24,359
Speaker 3: not just like a blob. It has some sort of

545
00:26:24,440 --> 00:26:25,160
Speaker 3: shape to it.

546
00:26:25,200 --> 00:26:27,680
Speaker 1: That's right. It tends to be denser at the core

547
00:26:28,240 --> 00:26:31,040
Speaker 1: and thinner further out, much like the visible matter in

548
00:26:31,080 --> 00:26:33,640
Speaker 1: the galaxy. And we can tell where it is because

549
00:26:33,720 --> 00:26:36,840
Speaker 1: it has an effect on how the stars spin. Right,

550
00:26:36,880 --> 00:26:39,240
Speaker 1: Like the old familiar story is that we know that

551
00:26:39,359 --> 00:26:42,080
Speaker 1: dark matter is there because we see the speed of

552
00:26:42,160 --> 00:26:45,359
Speaker 1: stars is way too high. If dark matter wasn't there,

553
00:26:45,600 --> 00:26:48,000
Speaker 1: then if the galaxy was spinning this quickly, it should

554
00:26:48,000 --> 00:26:51,639
Speaker 1: be throwing its stars out into interstellar space. It needs

555
00:26:51,720 --> 00:26:55,679
Speaker 1: more gravity something out there to hold those stars in place.

556
00:26:56,000 --> 00:26:58,800
Speaker 1: For the galaxy to spin this fast. That's the old

557
00:26:58,840 --> 00:27:01,200
Speaker 1: story that just tells us that dark matter is there.

558
00:27:01,280 --> 00:27:04,320
Speaker 1: But we can get much more fine grained information. We

559
00:27:04,359 --> 00:27:07,399
Speaker 1: can tell where in the galaxy that dark matter is

560
00:27:07,680 --> 00:27:10,720
Speaker 1: by measuring the velocity of stars at different points as

561
00:27:10,760 --> 00:27:13,600
Speaker 1: you move closer in or further out from the center

562
00:27:13,720 --> 00:27:14,520
Speaker 1: of the galaxy.

563
00:27:14,720 --> 00:27:16,720
Speaker 3: Right, Because I guess what you're saying is that if

564
00:27:16,760 --> 00:27:19,240
Speaker 3: the dark matter was all clumped together in the very

565
00:27:19,320 --> 00:27:22,880
Speaker 3: very center of the galaxy, the stars will move differently

566
00:27:22,920 --> 00:27:25,680
Speaker 3: than if it was more spread out throughout the whole galaxy. Right.

567
00:27:25,760 --> 00:27:28,760
Speaker 1: Yeah, If you are a star moving through the galaxy,

568
00:27:28,920 --> 00:27:31,280
Speaker 1: then the thing that determines to her speed is how

569
00:27:31,359 --> 00:27:34,359
Speaker 1: much stuff there is closer to the center of the

570
00:27:34,359 --> 00:27:37,200
Speaker 1: galaxy than you are. You're not sensitive to anything that's

571
00:27:37,240 --> 00:27:39,280
Speaker 1: further out than you. It's sort of like if you

572
00:27:39,520 --> 00:27:42,679
Speaker 1: dig into the Earth, then everything that's further out from you,

573
00:27:42,760 --> 00:27:46,200
Speaker 1: that's above you doesn't affect your gravity at all because

574
00:27:46,200 --> 00:27:48,560
Speaker 1: it all cancels out. It's only stuff that's closer to

575
00:27:48,600 --> 00:27:50,640
Speaker 1: the center of the Earth than you are. So it's

576
00:27:50,680 --> 00:27:53,760
Speaker 1: the same for a star. The star's speed basically tells

577
00:27:53,800 --> 00:27:56,800
Speaker 1: you how much stuff there is between it and the

578
00:27:56,840 --> 00:27:58,920
Speaker 1: center of the galaxy. So as you look at the

579
00:27:59,000 --> 00:28:01,479
Speaker 1: velocity of the star as you move further out from

580
00:28:01,520 --> 00:28:03,719
Speaker 1: the center, it gives you a picture for where that

581
00:28:03,800 --> 00:28:07,080
Speaker 1: dark matter has to be to explain that velocity. If

582
00:28:07,160 --> 00:28:09,440
Speaker 1: dark matter was all clumped together at the very center

583
00:28:09,440 --> 00:28:12,239
Speaker 1: of the galaxy, then stars, you know, halfway out from

584
00:28:12,320 --> 00:28:14,600
Speaker 1: the disc would be moving faster because there'd be a

585
00:28:14,680 --> 00:28:18,320
Speaker 1: stronger force from all that gravity. Instead, if it's spread

586
00:28:18,359 --> 00:28:20,800
Speaker 1: out really really far, then some of that stuff is

587
00:28:20,880 --> 00:28:23,520
Speaker 1: outside those stars and it doesn't affect them. It doesn't

588
00:28:23,560 --> 00:28:25,840
Speaker 1: pull them towards the center, it doesn't speed them up

589
00:28:25,880 --> 00:28:26,440
Speaker 1: as much.

590
00:28:26,880 --> 00:28:29,040
Speaker 3: Yeah, I guess it's sort of like, if you're in

591
00:28:29,080 --> 00:28:31,399
Speaker 3: the middle of a cloud of dark matter, you're not

592
00:28:31,440 --> 00:28:34,120
Speaker 3: going to feel its gravitation effects so much because it's

593
00:28:34,200 --> 00:28:37,280
Speaker 3: pulling you in all directions, Whereas if you're really really

594
00:28:37,320 --> 00:28:39,680
Speaker 3: far away from it, the whole blob, then you are

595
00:28:39,680 --> 00:28:42,000
Speaker 3: going to feel sort of its entire gravity. Yeah, And

596
00:28:42,040 --> 00:28:44,120
Speaker 3: so that's how we know that it's more dense in

597
00:28:44,160 --> 00:28:46,400
Speaker 3: the middle. And that's kind of important, right, It.

598
00:28:46,320 --> 00:28:49,560
Speaker 1: Is important, and it's not something that we really fully understand,

599
00:28:50,000 --> 00:28:52,040
Speaker 1: Like if you do simulations and you say we think

600
00:28:52,040 --> 00:28:54,640
Speaker 1: we understand how galaxies might have formed, and how this

601
00:28:55,000 --> 00:28:58,040
Speaker 1: halo formed and all the dark matter swirls together to

602
00:28:58,080 --> 00:29:00,280
Speaker 1: make this well that forms the galaxy, And then we

603
00:29:00,360 --> 00:29:03,240
Speaker 1: predict a certain shape for that dark matter density. We

604
00:29:03,320 --> 00:29:05,880
Speaker 1: predict it to be sort of like peaky near the center,

605
00:29:06,080 --> 00:29:08,560
Speaker 1: that like most of the dark matter should be right

606
00:29:08,640 --> 00:29:10,480
Speaker 1: at the center, and then it should fall off kind

607
00:29:10,480 --> 00:29:12,440
Speaker 1: of quickly. But what we observe when we go out

608
00:29:12,440 --> 00:29:14,400
Speaker 1: there and we look at the dark matter see where

609
00:29:14,440 --> 00:29:18,000
Speaker 1: it actually is based on these rotation curves, is that

610
00:29:18,040 --> 00:29:20,640
Speaker 1: it's not as peaky near the center. It's more like

611
00:29:20,680 --> 00:29:24,240
Speaker 1: a broad flat core, like a big blob of dark matter.

612
00:29:24,440 --> 00:29:27,080
Speaker 1: It's not as like pointed at the center. So that's

613
00:29:27,120 --> 00:29:30,920
Speaker 1: a current mystery we don't really understand. Dark matter doesn't

614
00:29:30,960 --> 00:29:33,880
Speaker 1: seem to be as clumped towards the center as we thought.

615
00:29:34,080 --> 00:29:35,360
Speaker 3: Now, I guess part of it is that, you know,

616
00:29:35,400 --> 00:29:37,200
Speaker 3: a lot of people sort of wonder like, if there

617
00:29:37,240 --> 00:29:39,280
Speaker 3: is that much dark matter out there, why doesn't it

618
00:29:39,360 --> 00:29:42,240
Speaker 3: just collapse into a dark matter black hole. But we've

619
00:29:42,280 --> 00:29:45,880
Speaker 3: talked about before how dark matter basically is not sticky

620
00:29:46,080 --> 00:29:49,440
Speaker 3: with itself, like it doesn't feel besides gravity, it doesn't

621
00:29:49,440 --> 00:29:51,760
Speaker 3: feel any other force. That would make it stick together,

622
00:29:51,920 --> 00:29:55,640
Speaker 3: like our items have the electromagnetic force to make them stick,

623
00:29:55,640 --> 00:29:58,160
Speaker 3: but dark matter it doesn't appear to have something like that.

624
00:29:58,280 --> 00:30:00,520
Speaker 1: And that force is important if you're going to fall

625
00:30:00,560 --> 00:30:02,320
Speaker 1: into the black hole, because you have to have some

626
00:30:02,480 --> 00:30:05,640
Speaker 1: way to lose your angular momentum. Dark matter, like everything else,

627
00:30:05,720 --> 00:30:08,680
Speaker 1: is spinning and swirling. And the reason that things don't

628
00:30:08,720 --> 00:30:11,320
Speaker 1: fall into a black hole is because they are swirling

629
00:30:11,360 --> 00:30:13,840
Speaker 1: around it, the way the Earth is orbiting the Sun

630
00:30:13,920 --> 00:30:16,560
Speaker 1: and not falling into it. For the Earth to fall

631
00:30:16,600 --> 00:30:19,760
Speaker 1: into the Sun, it would have to somehow lose its velocity,

632
00:30:19,920 --> 00:30:22,160
Speaker 1: it would have to bump into something, it'd have to

633
00:30:22,200 --> 00:30:25,160
Speaker 1: get slowed down. That only happens if there's some sort

634
00:30:25,160 --> 00:30:28,000
Speaker 1: of like sticky force that can do that. So for

635
00:30:28,120 --> 00:30:31,560
Speaker 1: dark matter, that's really hard because it passes right through itself,

636
00:30:31,560 --> 00:30:34,320
Speaker 1: it passes right through normal matter. It's very hard for

637
00:30:34,400 --> 00:30:36,880
Speaker 1: it to lose its speed or its angular momentum. So

638
00:30:36,920 --> 00:30:39,880
Speaker 1: that's why this halo of dark matter is bigger than

639
00:30:39,920 --> 00:30:43,240
Speaker 1: the visible galaxy because dark matter actually finds it harder

640
00:30:43,280 --> 00:30:46,200
Speaker 1: to collapse, harder to fall into black holes.

641
00:30:46,480 --> 00:30:48,720
Speaker 3: Right, Yeah, And so it's been this big diffuse and

642
00:30:48,760 --> 00:30:51,080
Speaker 3: it has this interesting shape. So then how else can

643
00:30:51,120 --> 00:30:53,760
Speaker 3: we sort of know about its structure besides its sort

644
00:30:53,800 --> 00:30:55,040
Speaker 3: of general blobby shape.

645
00:30:55,080 --> 00:30:57,440
Speaker 1: Well, one of the listeners got it right, thinking about

646
00:30:57,480 --> 00:31:00,680
Speaker 1: how dark matter distorts the path of light. We can

647
00:31:00,720 --> 00:31:03,360
Speaker 1: tell when there's a big blob of dark matter between

648
00:31:03,560 --> 00:31:06,400
Speaker 1: us and something else because it acts like a lens

649
00:31:06,480 --> 00:31:09,600
Speaker 1: in the sky. Remember that dark matter, even though it's

650
00:31:09,640 --> 00:31:12,800
Speaker 1: invisible and light can pass through it, it does change

651
00:31:12,840 --> 00:31:15,880
Speaker 1: the shape of space. And that means a space can

652
00:31:15,920 --> 00:31:18,480
Speaker 1: act like a lens, and so light will pass through it,

653
00:31:18,520 --> 00:31:20,680
Speaker 1: but it will get bent on the way. And so

654
00:31:20,760 --> 00:31:22,960
Speaker 1: if there's a big blob of dark matter between us

655
00:31:23,000 --> 00:31:25,600
Speaker 1: and a distant galaxy, for example, it will change the

656
00:31:25,680 --> 00:31:28,560
Speaker 1: shape of that galaxy distorted, just as if there was

657
00:31:28,600 --> 00:31:30,800
Speaker 1: a lens there. So we can use that to try

658
00:31:30,880 --> 00:31:34,200
Speaker 1: to get ideas for where dark matter might be interesting.

659
00:31:34,280 --> 00:31:37,600
Speaker 3: So dark matter does seem to clump within our galaxy,

660
00:31:38,120 --> 00:31:40,040
Speaker 3: Is that what you're saying? Like, maybe within our galaxy

661
00:31:40,080 --> 00:31:44,120
Speaker 3: there are spots where dark matter seems to be denser

662
00:31:44,160 --> 00:31:44,640
Speaker 3: than others.

663
00:31:44,760 --> 00:31:47,680
Speaker 1: That's hard to tell because this kind of gravitational effect

664
00:31:47,720 --> 00:31:51,360
Speaker 1: is kind of rare. Like you need a clear background galaxy,

665
00:31:51,600 --> 00:31:53,680
Speaker 1: and then you need a blob of dark matter right

666
00:31:53,760 --> 00:31:56,080
Speaker 1: in front of it has to be like perfectly lined up,

667
00:31:56,440 --> 00:31:58,800
Speaker 1: so it's tough to use this. They call this strong

668
00:31:58,920 --> 00:32:02,200
Speaker 1: gravitational lens to get a clear picture for where the

669
00:32:02,280 --> 00:32:05,240
Speaker 1: dark matter is, because we don't have really enough examples,

670
00:32:05,400 --> 00:32:07,360
Speaker 1: so it's not a great way to tell where the

671
00:32:07,440 --> 00:32:10,120
Speaker 1: dark matter is. A better way to tell if dark

672
00:32:10,200 --> 00:32:13,680
Speaker 1: matter clumps up is to look for its effect on stars.

673
00:32:14,120 --> 00:32:16,800
Speaker 1: So not just like the velocity of stars as they

674
00:32:16,840 --> 00:32:19,320
Speaker 1: weave around the center of the galaxy, but their motion

675
00:32:19,440 --> 00:32:22,320
Speaker 1: in other directions, like if there is a big clump

676
00:32:22,400 --> 00:32:25,280
Speaker 1: of dark matter, and especially dense blob of dark matter,

677
00:32:25,640 --> 00:32:28,240
Speaker 1: it will affect how stars are moving around it. It

678
00:32:28,240 --> 00:32:31,239
Speaker 1: will change the motion of those stars. It'll attract them,

679
00:32:31,280 --> 00:32:33,000
Speaker 1: it will deflect them.

680
00:32:33,040 --> 00:32:34,920
Speaker 3: I see. So if you look sort of look at

681
00:32:34,960 --> 00:32:37,200
Speaker 3: the overall motion of all the stars in the galaxy,

682
00:32:37,240 --> 00:32:38,840
Speaker 3: if you see that there are you know, sort of

683
00:32:38,880 --> 00:32:41,880
Speaker 3: wiggles here and there, or little you know, eddies or

684
00:32:41,920 --> 00:32:44,720
Speaker 3: little clumps of stars forming, then you know that there's

685
00:32:44,760 --> 00:32:47,360
Speaker 3: something else there and that it's not the dark matter

686
00:32:47,440 --> 00:32:48,560
Speaker 3: is not perfectly smooth.

687
00:32:48,800 --> 00:32:52,120
Speaker 1: Yes, And we recently launched a satellite called Gaya, which

688
00:32:52,160 --> 00:32:56,120
Speaker 1: is mapping the galaxy in four dimensions. It measures the position,

689
00:32:56,240 --> 00:33:00,040
Speaker 1: the location of all these stars and their velocity. I

690
00:33:00,040 --> 00:33:03,280
Speaker 1: think this incredible map. It has like a billion stars

691
00:33:03,600 --> 00:33:06,080
Speaker 1: with their position and their velocity map, and we can

692
00:33:06,200 --> 00:33:08,920
Speaker 1: use this to look for deviations where like, well, if

693
00:33:09,000 --> 00:33:11,920
Speaker 1: dark matter was perfectly smooth, what would we expect all

694
00:33:11,960 --> 00:33:14,920
Speaker 1: these stars to be doing. And are any stars doing

695
00:33:15,000 --> 00:33:17,240
Speaker 1: anything weird? And if they are, we can sort of

696
00:33:17,320 --> 00:33:19,960
Speaker 1: back that out and figure out where dark matter has

697
00:33:20,000 --> 00:33:23,280
Speaker 1: to be to explain any weird patterns of the star motion.

698
00:33:23,600 --> 00:33:26,120
Speaker 3: Right, Because I guess if dark matter was perfectly smooth

699
00:33:26,280 --> 00:33:30,040
Speaker 3: peanut butter, like this kind of smooth a cloud, then

700
00:33:30,280 --> 00:33:33,000
Speaker 3: you would expect all the stars to be basically moving

701
00:33:33,040 --> 00:33:35,640
Speaker 3: along as if it was in a lazy river, right,

702
00:33:35,720 --> 00:33:38,400
Speaker 3: Like everyone sort of moving of the same. You know,

703
00:33:38,560 --> 00:33:41,000
Speaker 3: nobody would be going much faster than or slower than

704
00:33:41,040 --> 00:33:41,960
Speaker 3: any of the other stars.

705
00:33:42,120 --> 00:33:43,240
Speaker 1: Yeah, at the same radius.

706
00:33:43,320 --> 00:33:43,440
Speaker 13: Right.

707
00:33:43,440 --> 00:33:45,840
Speaker 1: We expect as you go out, as you change your

708
00:33:45,920 --> 00:33:48,520
Speaker 1: radius relative to the center of the galaxy, these things

709
00:33:48,560 --> 00:33:50,640
Speaker 1: will change, just like they do in our solar system.

710
00:33:50,720 --> 00:33:53,720
Speaker 1: Pluto is not moving around the Sun as fast as Jupiter,

711
00:33:54,040 --> 00:33:56,680
Speaker 1: which is not moving as fast as Mercury, because as

712
00:33:56,680 --> 00:33:59,240
Speaker 1: you go further out, the gravitational force is weaker, but

713
00:33:59,320 --> 00:34:02,240
Speaker 1: you'd expect at the same radius to basically be having

714
00:34:02,320 --> 00:34:05,400
Speaker 1: the same motion. And so if you see deviations, then

715
00:34:05,440 --> 00:34:07,720
Speaker 1: you know dark matter is there, and we do sort

716
00:34:07,760 --> 00:34:10,640
Speaker 1: of expect there to be clumps. We expect that the galaxy,

717
00:34:10,680 --> 00:34:14,160
Speaker 1: for example, has lots of other galaxies inside of it

718
00:34:14,200 --> 00:34:16,560
Speaker 1: that it has absorbed. We think that the history of

719
00:34:16,560 --> 00:34:20,120
Speaker 1: our galaxy includes lots of collisions to form the Milky Way,

720
00:34:20,640 --> 00:34:23,319
Speaker 1: and you would suspect that those galaxies might still have

721
00:34:23,520 --> 00:34:26,680
Speaker 1: like their dark matter halos embedded within.

722
00:34:26,560 --> 00:34:29,719
Speaker 3: Ours, because I guess you would expect darmatter to be

723
00:34:29,960 --> 00:34:33,040
Speaker 3: clumpy because regular matter is clumpy, so in a way

724
00:34:33,160 --> 00:34:37,120
Speaker 3: like would in our regular matter also sort of catalyze

725
00:34:37,200 --> 00:34:39,160
Speaker 3: or trigger dark matter to clump.

726
00:34:39,320 --> 00:34:42,000
Speaker 1: Our matter is clumpy, but that's because it's sticky, right,

727
00:34:42,040 --> 00:34:45,680
Speaker 1: It can form these blobs, so when gravity pulls it together,

728
00:34:46,040 --> 00:34:49,759
Speaker 1: it sticks together and then that accumulates forms this runaway effect,

729
00:34:49,880 --> 00:34:52,640
Speaker 1: whereas dark matter is not sticky, and so dark matter

730
00:34:52,719 --> 00:34:56,240
Speaker 1: halos can pass right through each other without very much distortion.

731
00:34:56,400 --> 00:34:59,000
Speaker 1: The other question is a cool one, like do stars

732
00:34:59,160 --> 00:35:02,000
Speaker 1: form clumps of dark matter? And this is something people

733
00:35:02,080 --> 00:35:03,919
Speaker 1: have studied. They've tried to look to see if there's

734
00:35:04,000 --> 00:35:07,080
Speaker 1: like an intense blob of dark matter inside the Sun,

735
00:35:07,200 --> 00:35:10,000
Speaker 1: for example. But remember that there's much more dark matter

736
00:35:10,080 --> 00:35:12,160
Speaker 1: than there is normal matter, and so dark matter sort

737
00:35:12,160 --> 00:35:15,239
Speaker 1: of wins the gravitational battles. You would expect it to

738
00:35:15,320 --> 00:35:18,040
Speaker 1: mostly go the other direction, that dark matter would influence

739
00:35:18,280 --> 00:35:21,120
Speaker 1: the pattern of normal matter rather than vice versa. But

740
00:35:21,200 --> 00:35:22,760
Speaker 1: yet it is a tug of warm.

741
00:35:23,000 --> 00:35:26,239
Speaker 3: Interesting, you're saying dark matter is ignoring us. Basically, it's

742
00:35:26,280 --> 00:35:26,879
Speaker 3: ghosting us.

743
00:35:27,280 --> 00:35:29,600
Speaker 1: It mostly can. But back to this question of like

744
00:35:29,680 --> 00:35:32,640
Speaker 1: following the stars, there are some really cool things that

745
00:35:32,680 --> 00:35:35,680
Speaker 1: we do see inside our galaxy. We can see the

746
00:35:35,760 --> 00:35:39,200
Speaker 1: remnants of other galaxies that the Milky Way has eaten,

747
00:35:39,520 --> 00:35:43,440
Speaker 1: little mini galaxies. We call these dwarf galaxies, and some

748
00:35:43,520 --> 00:35:46,840
Speaker 1: of these are really really interesting because they're super high

749
00:35:46,880 --> 00:35:49,840
Speaker 1: in dark matter. Like our galaxy has a lot of

750
00:35:49,920 --> 00:35:52,600
Speaker 1: dark matter, but some of these dwarf galaxies are almost

751
00:35:52,800 --> 00:35:55,839
Speaker 1: entirely dark matter, and we can tell that they're there

752
00:35:55,920 --> 00:36:00,000
Speaker 1: because we see stars orbiting these invisible dark matter hates.

753
00:36:00,600 --> 00:36:03,240
Speaker 1: So there's sort of like many clumps of dark matter

754
00:36:03,320 --> 00:36:05,320
Speaker 1: within our dark matter halo.

755
00:36:05,600 --> 00:36:09,520
Speaker 3: Interesting so it is clumpy, but maybe because we've added

756
00:36:09,560 --> 00:36:10,000
Speaker 3: the clumps.

757
00:36:10,080 --> 00:36:13,759
Speaker 1: Kind of yeah, because we've formed our big halo from

758
00:36:13,800 --> 00:36:16,760
Speaker 1: a bunch of clumps. So we think these clumps formed initially,

759
00:36:16,920 --> 00:36:19,600
Speaker 1: each one of these its own galaxy, and then galaxies

760
00:36:19,640 --> 00:36:23,480
Speaker 1: eventually do merge and collide and form bigger galaxies. And

761
00:36:23,600 --> 00:36:27,480
Speaker 1: sometimes those dark matter halos don't necessarily spread out and

762
00:36:27,640 --> 00:36:30,640
Speaker 1: just join like the original creamy blob of the Milky Way.

763
00:36:30,760 --> 00:36:33,040
Speaker 1: They sort of stay there as chunks, and you can

764
00:36:33,040 --> 00:36:37,400
Speaker 1: tell they're because the stars swirling around those little dwarf galaxies.

765
00:36:38,120 --> 00:36:40,120
Speaker 3: Well, here's the question. Do you think that dark matter

766
00:36:40,200 --> 00:36:42,640
Speaker 3: in our galaxy is spinning also with the rest of

767
00:36:42,680 --> 00:36:45,400
Speaker 3: the stars and galaxies, or does it just standing still.

768
00:36:45,480 --> 00:36:48,640
Speaker 1: It's almost certainly spinning. That's the reason that it doesn't

769
00:36:48,680 --> 00:36:51,239
Speaker 1: collapse into the black hole in the center. If it

770
00:36:51,320 --> 00:36:53,600
Speaker 1: was standing still, then that gravity from that black hole

771
00:36:53,600 --> 00:36:55,920
Speaker 1: would just suck it up. So it's almost certain that

772
00:36:56,000 --> 00:36:59,080
Speaker 1: it's rotating. That we can't measure that directly, right, we

773
00:36:59,160 --> 00:37:01,960
Speaker 1: haven't seen that, but we're fairly certain that it has

774
00:37:02,000 --> 00:37:03,560
Speaker 1: to be otherwise it would have collapsed.

775
00:37:03,680 --> 00:37:07,319
Speaker 3: So through strong gravitational lensing, we can tell that there

776
00:37:07,360 --> 00:37:10,560
Speaker 3: are some clumps out there, and through some of these

777
00:37:10,600 --> 00:37:13,120
Speaker 3: absorbed galaxies, we know there are clumps out there, but

778
00:37:13,160 --> 00:37:14,759
Speaker 3: what else do we know about this clumpiness.

779
00:37:14,840 --> 00:37:17,320
Speaker 1: We can also try to measure the clumpiness by looking

780
00:37:17,320 --> 00:37:21,400
Speaker 1: at the effect of our gravity on things near the galaxy.

781
00:37:21,560 --> 00:37:25,120
Speaker 1: So sometimes these mini galaxies or these globular clusters get

782
00:37:25,160 --> 00:37:29,000
Speaker 1: sucked inside the galaxy. Sometimes they're in orbit around the galaxy.

783
00:37:29,120 --> 00:37:31,680
Speaker 1: So for example, the large Madeleinic cloud is a blob

784
00:37:31,719 --> 00:37:34,480
Speaker 1: of stuff that's sort of like a satellite galaxy of

785
00:37:34,520 --> 00:37:37,880
Speaker 1: the Milky Way. And often these galaxies get torn apart,

786
00:37:38,200 --> 00:37:41,200
Speaker 1: they don't hold themselves together. They turn into these streams.

787
00:37:41,680 --> 00:37:45,120
Speaker 1: So around the galaxy there are these things called stellar streams,

788
00:37:45,160 --> 00:37:48,799
Speaker 1: which are these lines of stars moving in a loop

789
00:37:48,920 --> 00:37:51,040
Speaker 1: sort of around the galaxies. But it's sort of like

790
00:37:51,080 --> 00:37:53,680
Speaker 1: the galaxy has rings of stars.

791
00:37:53,680 --> 00:37:56,920
Speaker 3: Mm interesting like accidents almost.

792
00:37:58,600 --> 00:38:01,640
Speaker 1: Yeah, and they're sort of swooped around the galaxy. And

793
00:38:01,680 --> 00:38:04,640
Speaker 1: those are very sensitive to the distribution of dark matter.

794
00:38:05,040 --> 00:38:08,240
Speaker 1: So if, for example, the dark matter halo is clumpy,

795
00:38:08,440 --> 00:38:11,200
Speaker 1: it'll affect how those stars get pulled apart and whether

796
00:38:11,200 --> 00:38:14,160
Speaker 1: they're like gaps in those streams. So there are people

797
00:38:14,200 --> 00:38:17,480
Speaker 1: right now studying these stellar streams. They're like probes of

798
00:38:17,520 --> 00:38:20,000
Speaker 1: that dark matter halo to look to see if there

799
00:38:20,000 --> 00:38:22,080
Speaker 1: are clumps in the dark matter halo, or to see

800
00:38:22,080 --> 00:38:25,839
Speaker 1: if it's like perfectly spherical or kind of elliptical. So

801
00:38:25,880 --> 00:38:28,279
Speaker 1: these are very nice ways to tell how much dark

802
00:38:28,280 --> 00:38:30,480
Speaker 1: matter they're passing through and how clumpy it is.

803
00:38:31,520 --> 00:38:34,000
Speaker 3: Interesting, and so that's one way to sort of know

804
00:38:34,360 --> 00:38:36,879
Speaker 3: the clumpiness of dark matter, and what have we learned

805
00:38:36,880 --> 00:38:38,240
Speaker 3: from all of these different ways.

806
00:38:38,320 --> 00:38:40,520
Speaker 1: We don't have a great picture of where dark matter

807
00:38:40,600 --> 00:38:42,960
Speaker 1: is in the galaxy. People often write in and ask,

808
00:38:43,120 --> 00:38:45,640
Speaker 1: like where is the dark matter? Can we see like

809
00:38:45,800 --> 00:38:48,560
Speaker 1: planets of dark matter or that kind of stuff. Really,

810
00:38:48,600 --> 00:38:51,319
Speaker 1: we're not very sensitive to the details. We know that

811
00:38:51,360 --> 00:38:54,040
Speaker 1: the Milky Way has a big blob of dark matter

812
00:38:54,320 --> 00:38:58,000
Speaker 1: that it's probably elliptical, you know, it's not totally spherical.

813
00:38:58,200 --> 00:39:01,600
Speaker 1: We can see some clumps where these faint dwarf galaxies

814
00:39:01,640 --> 00:39:04,160
Speaker 1: were absorbed, but we don't have a great sense for

815
00:39:04,239 --> 00:39:07,840
Speaker 1: the structure of the dark matter. It's mostly smooth, but

816
00:39:07,920 --> 00:39:10,439
Speaker 1: we can't see things smaller than like, you know, ten

817
00:39:10,520 --> 00:39:11,600
Speaker 1: to the six stars.

818
00:39:12,239 --> 00:39:15,400
Speaker 3: I see like the smallest clump. We can sort of tell,

819
00:39:15,520 --> 00:39:19,399
Speaker 3: right now is ten to the six billions of kilometers.

820
00:39:18,800 --> 00:39:21,919
Speaker 1: Maybe ten to the six solar masses equivalents of dark

821
00:39:21,960 --> 00:39:25,000
Speaker 1: matter is like the smallest chunk of thing we can tell.

822
00:39:25,239 --> 00:39:28,960
Speaker 3: WHOA, that's like our best resolution of our picture of

823
00:39:29,040 --> 00:39:31,080
Speaker 3: dark matter in the galaxy. It's like a pixel to

824
00:39:31,160 --> 00:39:33,440
Speaker 3: side a million sons.

825
00:39:33,600 --> 00:39:36,000
Speaker 1: Yeah, so we're not very sensitive, and that's just because

826
00:39:36,040 --> 00:39:39,120
Speaker 1: it's mostly smooth. There aren't a lot of features to see,

827
00:39:39,239 --> 00:39:42,280
Speaker 1: we think, and because we're not very sensitive to it. Again,

828
00:39:42,320 --> 00:39:44,719
Speaker 1: gravity is very weak and it's our only way of

829
00:39:44,760 --> 00:39:46,880
Speaker 1: interacting with it, which makes it kind of frustrating.

830
00:39:47,160 --> 00:39:49,560
Speaker 3: All right, well, it sounds like it's still yet to

831
00:39:49,600 --> 00:39:53,279
Speaker 3: be discovered. Who knows. Maybe it's forming giant dark matter

832
00:39:53,360 --> 00:39:56,040
Speaker 3: squirrels or bananas or girasses out there, but we just

833
00:39:56,080 --> 00:39:58,759
Speaker 3: can't see it with our current resolution. And so let's

834
00:39:58,800 --> 00:40:01,200
Speaker 3: get a little bit into what the overall picture of

835
00:40:01,280 --> 00:40:04,160
Speaker 3: dark matter then is in the universe, and also what's

836
00:40:04,160 --> 00:40:07,440
Speaker 3: happening between galaxies. But first, let's take another quick break.

837
00:40:11,840 --> 00:40:13,640
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838
00:40:13,760 --> 00:40:16,880
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839
00:40:16,920 --> 00:40:20,960
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844
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847
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848
00:40:46,719 --> 00:40:50,480
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871
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872
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874
00:42:07,480 --> 00:42:10,359
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875
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Speaker 3: All right, Daniel has lost his dark matter and his mind. Apparently,

887
00:42:52,920 --> 00:42:54,759
Speaker 3: did you lose your mind looking for the dark matter?

888
00:42:54,880 --> 00:42:57,160
Speaker 1: I did. It's driving me crazy. Where are you?

889
00:42:59,080 --> 00:43:01,520
Speaker 3: It's avoiding you, to ghosting you. It is a little

890
00:43:01,520 --> 00:43:04,600
Speaker 3: bit ghosting us as humanity because we know it's there,

891
00:43:04,680 --> 00:43:07,240
Speaker 3: but it doesn't seem to want to make its presence

892
00:43:07,520 --> 00:43:09,960
Speaker 3: known to us. In detail, we sort of have a

893
00:43:10,000 --> 00:43:12,120
Speaker 3: big picture of it that it's in a big clump

894
00:43:12,120 --> 00:43:14,919
Speaker 3: around the galaxy, mostly concentrated in the middle. We see

895
00:43:14,960 --> 00:43:17,040
Speaker 3: some clumps out there, but we don't know the exact

896
00:43:17,120 --> 00:43:19,399
Speaker 3: structure of dark matter, but we sort of do sort

897
00:43:19,440 --> 00:43:21,239
Speaker 3: of know it's density out there, right, We have some

898
00:43:21,280 --> 00:43:22,920
Speaker 3: figures for its general density.

899
00:43:23,040 --> 00:43:25,759
Speaker 1: Yeah, And it's quite interesting because we think that on

900
00:43:25,880 --> 00:43:29,640
Speaker 1: average over the universe, dark matter is like eighty percent

901
00:43:29,760 --> 00:43:32,600
Speaker 1: of the matter of the universe, but in our neighborhood

902
00:43:32,600 --> 00:43:35,400
Speaker 1: it's actually a little bit different. The Milky Way, for example,

903
00:43:35,400 --> 00:43:39,120
Speaker 1: we think is ninety five percent dark matter. So our

904
00:43:39,120 --> 00:43:41,480
Speaker 1: whole galaxy is kind of badly named. It should be

905
00:43:41,520 --> 00:43:43,760
Speaker 1: called like the dark Way or something.

906
00:43:43,560 --> 00:43:47,240
Speaker 3: The chocolate milk Galaxy, the darkly Milky Way.

907
00:43:47,080 --> 00:43:50,080
Speaker 1: The dark Chocolate Way or something. So we're like ninety

908
00:43:50,160 --> 00:43:52,600
Speaker 1: five percent dark matter, which means if we have, like

909
00:43:53,000 --> 00:43:56,840
Speaker 1: you know, the equivalent of ninety billion times the mass

910
00:43:56,880 --> 00:43:59,520
Speaker 1: of the Sun in terms of stars and gas and

911
00:43:59,560 --> 00:44:01,440
Speaker 1: all that kind of stuff, that means that there's like

912
00:44:01,480 --> 00:44:05,080
Speaker 1: two trillion times the mass of the Sun in dark matter.

913
00:44:05,320 --> 00:44:07,800
Speaker 1: There's just so much more of it. It's like twenty

914
00:44:08,000 --> 00:44:12,080
Speaker 1: times as much dark matter in our galaxy as normal matter,

915
00:44:12,320 --> 00:44:14,319
Speaker 1: which is a bigger ratio than in the rest of

916
00:44:14,320 --> 00:44:14,880
Speaker 1: the universe.

917
00:44:15,120 --> 00:44:17,879
Speaker 3: Well, that's true for regular matter too, right, Like our

918
00:44:18,000 --> 00:44:20,640
Speaker 3: Milky Way has a higher density of regular matter than

919
00:44:20,680 --> 00:44:23,280
Speaker 3: the rest of the universe or other parts of the universe.

920
00:44:23,360 --> 00:44:27,320
Speaker 1: Right, Yeah, that's true. But on average galaxies have about

921
00:44:27,320 --> 00:44:30,480
Speaker 1: eighty percent dark matter, and our galaxy is ninety five percent,

922
00:44:30,800 --> 00:44:33,880
Speaker 1: So there's a big variation galaxy the galaxy and how

923
00:44:33,960 --> 00:44:35,080
Speaker 1: much dark matter there is.

924
00:44:35,480 --> 00:44:37,920
Speaker 3: Oh, you mean our galaxy has more than other galaxies?

925
00:44:38,640 --> 00:44:41,080
Speaker 1: Absolutely, Yeah, we are a darker galaxy than.

926
00:44:41,040 --> 00:44:43,319
Speaker 3: Most interesting we're more mysterious, I guess.

927
00:44:43,400 --> 00:44:46,200
Speaker 1: Yeah. And there's some galaxies out there that are overwhelmingly

928
00:44:46,280 --> 00:44:48,960
Speaker 1: dark matter, like ninety nine points something percent. And then

929
00:44:48,960 --> 00:44:51,279
Speaker 1: there are some galaxies that have very little dark matter.

930
00:44:51,320 --> 00:44:53,480
Speaker 1: We think that might be evidence of collisions where dark

931
00:44:53,520 --> 00:44:56,279
Speaker 1: matter gets separated from the normal matter. All sorts of

932
00:44:56,320 --> 00:44:59,560
Speaker 1: crazy stuff. These things tell you the crazy cosmic history

933
00:44:59,560 --> 00:45:00,880
Speaker 1: of all of these objects.

934
00:45:00,880 --> 00:45:03,120
Speaker 3: Interesting. And again you can tell by when you look

935
00:45:03,160 --> 00:45:05,440
Speaker 3: at these galaxies out there. You can tell that they're

936
00:45:05,480 --> 00:45:08,520
Speaker 3: holding on together more than they should by the number

937
00:45:08,560 --> 00:45:10,319
Speaker 3: of stars or the brightness of them. Right.

938
00:45:10,440 --> 00:45:12,920
Speaker 1: Yeah, you can tell when a galaxy is overwhelmingly dark

939
00:45:12,960 --> 00:45:16,280
Speaker 1: matter because its stars are moving super duper fast compared

940
00:45:16,280 --> 00:45:18,359
Speaker 1: to how bright they are. And so we can see

941
00:45:18,360 --> 00:45:21,600
Speaker 1: these faint dwarf galaxies, for example, just have a handful

942
00:45:21,600 --> 00:45:24,120
Speaker 1: of stars, but they're whizzing around in a circle and

943
00:45:24,120 --> 00:45:27,000
Speaker 1: there's not nearly enough gravity to hold them in place.

944
00:45:27,360 --> 00:45:29,440
Speaker 1: Just from the stuff that we can see, so it's

945
00:45:29,480 --> 00:45:30,240
Speaker 1: pretty cool.

946
00:45:30,040 --> 00:45:31,440
Speaker 3: But it's so far away. How do you know it's

947
00:45:31,440 --> 00:45:34,160
Speaker 3: not just like filled with the black holes or something,

948
00:45:34,400 --> 00:45:36,000
Speaker 3: or rocks dark rocks.

949
00:45:35,880 --> 00:45:38,160
Speaker 1: Because we can see light passing through it, right, we

950
00:45:38,160 --> 00:45:41,000
Speaker 1: can see through it to something else behind it. If

951
00:45:41,000 --> 00:45:43,000
Speaker 1: there was a black hole there, it would absorb the light.

952
00:45:43,040 --> 00:45:45,600
Speaker 1: If it was just like a huge death star or

953
00:45:45,600 --> 00:45:48,799
Speaker 1: something cloaked, then it would absorb that light. So we

954
00:45:48,880 --> 00:45:51,439
Speaker 1: see it as invisible, not as dark, all.

955
00:45:51,400 --> 00:45:54,200
Speaker 3: Right, So then it's sort of denser in our galaxy.

956
00:45:54,200 --> 00:45:57,080
Speaker 3: What about in our more immediate neighborhood or like a

957
00:45:57,120 --> 00:46:00,440
Speaker 3: star solar system also extra dark mattery and.

958
00:46:00,400 --> 00:46:02,680
Speaker 1: In our neighborhood. Remember that while the Milky Way is

959
00:46:02,800 --> 00:46:06,480
Speaker 1: ninety five percent dark matter that is spread out throughout

960
00:46:06,520 --> 00:46:09,400
Speaker 1: the stars, we think, so normal matter clumps up a

961
00:46:09,440 --> 00:46:12,480
Speaker 1: lot more than dark matter, which means that there isn't

962
00:46:12,520 --> 00:46:15,799
Speaker 1: that much dark matter in any like cubic light year

963
00:46:15,880 --> 00:46:18,720
Speaker 1: of space. So in a cubic light year of space,

964
00:46:18,920 --> 00:46:21,520
Speaker 1: there's less than a one quarter of one to one

965
00:46:21,560 --> 00:46:24,640
Speaker 1: thousands of the mass of the Sun in a cubic

966
00:46:24,719 --> 00:46:26,880
Speaker 1: light year of space. That's like a quarter of the

967
00:46:26,920 --> 00:46:29,640
Speaker 1: mass of Jupiter in a cubic light year of space.

968
00:46:29,680 --> 00:46:31,200
Speaker 1: That's how much dark matter there is.

969
00:46:32,280 --> 00:46:35,800
Speaker 3: If you took Jupiter and spread it over billions of miles, right.

970
00:46:36,040 --> 00:46:37,680
Speaker 1: It wouldn't be very much. And you know, if you

971
00:46:37,800 --> 00:46:41,120
Speaker 1: zoom in, for example, into like a cubic meter, that's

972
00:46:41,239 --> 00:46:45,439
Speaker 1: like ten to the minus twenty two grams of dark

973
00:46:45,480 --> 00:46:47,840
Speaker 1: matter in a cubic meter. So you know, if you

974
00:46:47,880 --> 00:46:51,000
Speaker 1: look at the space around you in your office, for example,

975
00:46:51,200 --> 00:46:54,200
Speaker 1: then there's just like a super tiny amount of dark matter,

976
00:46:54,440 --> 00:46:56,759
Speaker 1: almost hard to measure, but some of it's there are

977
00:46:56,800 --> 00:46:58,960
Speaker 1: few particles. And if you zoom out to like the

978
00:46:58,960 --> 00:47:02,279
Speaker 1: whole volume of the Earth, there's less than a kilogram

979
00:47:02,360 --> 00:47:04,760
Speaker 1: of dark matter in the volume of the Earth.

980
00:47:05,040 --> 00:47:07,439
Speaker 3: Again, these are sort of approximations, right, because you told

981
00:47:07,440 --> 00:47:10,280
Speaker 3: me earlier that our ability to resolve or a resolution

982
00:47:10,360 --> 00:47:12,440
Speaker 3: of dark matter is pretty bad. So how do we

983
00:47:12,520 --> 00:47:15,399
Speaker 3: know that there isn't sort of clump of dark matter

984
00:47:15,640 --> 00:47:16,719
Speaker 3: just around us right now?

985
00:47:16,960 --> 00:47:19,719
Speaker 1: We don't know. Absolutely, We do not know. We are

986
00:47:19,760 --> 00:47:21,600
Speaker 1: not sensitive to these things, so it could be a

987
00:47:21,600 --> 00:47:24,600
Speaker 1: lot clumpier than we think these numbers are, assuming that

988
00:47:24,719 --> 00:47:28,360
Speaker 1: dark matter is mostly smoothly spread out throughout the galaxy

989
00:47:28,600 --> 00:47:31,240
Speaker 1: according to the distribution that we've seen from the radius,

990
00:47:31,440 --> 00:47:34,000
Speaker 1: but we absolutely cannot tell if there's like a huge

991
00:47:34,040 --> 00:47:36,080
Speaker 1: blob of dark matter that we're sitting in, or if

992
00:47:36,120 --> 00:47:39,200
Speaker 1: there's almost no dark matter in our neighborhood. And remember

993
00:47:39,200 --> 00:47:42,600
Speaker 1: that we have experiments underground looking for dark matter particles.

994
00:47:42,640 --> 00:47:45,600
Speaker 1: They're basically trying to measure how the Earth is moving

995
00:47:45,640 --> 00:47:48,800
Speaker 1: through this dark matter wind, and they haven't seen anything.

996
00:47:49,160 --> 00:47:51,720
Speaker 1: And one of my favorite explanations is like, well, maybe

997
00:47:51,760 --> 00:47:53,920
Speaker 1: we're just happening to be sitting in a bubble that

998
00:47:54,000 --> 00:47:56,600
Speaker 1: has almost no dark matter in it, which would make

999
00:47:56,640 --> 00:47:59,239
Speaker 1: it impossible for us to detect that dark matter wind.

1000
00:47:59,520 --> 00:48:00,440
Speaker 1: We just don't know.

1001
00:48:00,800 --> 00:48:03,879
Speaker 3: It's ghosting us an avoiding us physically at the same time.

1002
00:48:04,080 --> 00:48:06,560
Speaker 3: But it's kind of interesting because I think we talked

1003
00:48:06,560 --> 00:48:09,160
Speaker 3: about earlier how like in the volume of the Earth,

1004
00:48:09,160 --> 00:48:12,640
Speaker 3: there's about one squirrels worth full of dark matter. Like

1005
00:48:12,760 --> 00:48:14,480
Speaker 3: that's not a lot, right, and the whole Earth is

1006
00:48:14,480 --> 00:48:16,799
Speaker 3: pretty big, but you only have one squirrel full of

1007
00:48:16,880 --> 00:48:17,640
Speaker 3: dark matter.

1008
00:48:17,560 --> 00:48:20,880
Speaker 1: And that's why we can't ever detect it gravitationally. You know,

1009
00:48:21,000 --> 00:48:24,360
Speaker 1: we're literally looking for a squirrel that's hiding inside the Earth,

1010
00:48:24,960 --> 00:48:27,560
Speaker 1: and that's pretty hard to tell the difference. We can't

1011
00:48:27,560 --> 00:48:31,000
Speaker 1: measure the number of squirrels on Earth using ground you'd

1012
00:48:31,000 --> 00:48:31,600
Speaker 1: go nuts.

1013
00:48:33,080 --> 00:48:35,239
Speaker 3: But yeah, so let's talk about then, now sort of

1014
00:48:35,320 --> 00:48:38,160
Speaker 3: dark matter between galaxies, because you know, there's a lot

1015
00:48:38,200 --> 00:48:40,600
Speaker 3: of space between galaxies and we sort of have a

1016
00:48:40,600 --> 00:48:43,600
Speaker 3: pretty good idea of the structure of the universe, you know,

1017
00:48:43,760 --> 00:48:47,320
Speaker 3: the galaxy clusters and superclusters. Is does dark matter also

1018
00:48:47,360 --> 00:48:48,400
Speaker 3: follow these clusters?

1019
00:48:48,560 --> 00:48:50,759
Speaker 1: We think that mostly does, And again we think it's

1020
00:48:50,800 --> 00:48:54,520
Speaker 1: sort of the opposite that normal matter follows the path

1021
00:48:54,719 --> 00:48:58,200
Speaker 1: of dark matter. But it's much harder to see the

1022
00:48:58,239 --> 00:49:01,520
Speaker 1: things between galaxies because there's much less light there and

1023
00:49:01,560 --> 00:49:04,919
Speaker 1: there's much less visible objects. Like mostly we have seen

1024
00:49:04,960 --> 00:49:07,799
Speaker 1: where dark matter is within our galaxy by following the

1025
00:49:07,840 --> 00:49:12,000
Speaker 1: path of stars their rotation, their wiggles, their distortions, all

1026
00:49:12,000 --> 00:49:14,440
Speaker 1: that kind of stuff. Well, there are in stars like

1027
00:49:14,560 --> 00:49:18,279
Speaker 1: tracers to tell us where things are between galaxies, so

1028
00:49:18,320 --> 00:49:19,480
Speaker 1: it's much trickier.

1029
00:49:19,719 --> 00:49:22,000
Speaker 3: Yeah, I guess you can sort of extrapolate right where

1030
00:49:22,040 --> 00:49:24,200
Speaker 3: we can see a little bit around this, then you

1031
00:49:24,280 --> 00:49:26,600
Speaker 3: sort of assume that that's what's happening maybe in the

1032
00:49:26,640 --> 00:49:28,160
Speaker 3: rest of the universe sort of.

1033
00:49:28,239 --> 00:49:30,480
Speaker 1: But we know that the galaxies are very different from

1034
00:49:30,480 --> 00:49:32,399
Speaker 1: the rest of the universe, Like we know that there's

1035
00:49:32,440 --> 00:49:35,359
Speaker 1: a huge gravitational well that we are sitting in. That's

1036
00:49:35,360 --> 00:49:38,240
Speaker 1: why there's a galaxy right here. There's a big blob

1037
00:49:38,280 --> 00:49:40,919
Speaker 1: of dark matter. What does it look like between our

1038
00:49:41,000 --> 00:49:44,600
Speaker 1: galaxy and Andromeda? You know, are there strands of dark matter?

1039
00:49:44,760 --> 00:49:47,560
Speaker 1: How quickly does it peede out? Are there blobs of

1040
00:49:47,640 --> 00:49:50,200
Speaker 1: dark matter out there without any stars in them at all?

1041
00:49:50,360 --> 00:49:52,480
Speaker 1: And so one way we can try to figure that

1042
00:49:52,600 --> 00:49:55,319
Speaker 1: out is to look at how light from distant galaxies

1043
00:49:55,520 --> 00:49:58,120
Speaker 1: is distorted as it passes through that space.

1044
00:49:59,280 --> 00:50:01,880
Speaker 3: I see. You can do the gravitational lensing, but with

1045
00:50:02,000 --> 00:50:05,040
Speaker 3: galaxies and look for blobs in between galaxy. But then

1046
00:50:05,080 --> 00:50:07,120
Speaker 3: these blobs would got to be humongous, right.

1047
00:50:06,920 --> 00:50:09,440
Speaker 1: Those blobs would have to be humongous. And in this

1048
00:50:09,560 --> 00:50:12,239
Speaker 1: case we use a slightly different technique than we do

1049
00:50:12,400 --> 00:50:15,319
Speaker 1: for looking at like one specific blob before. What we

1050
00:50:15,320 --> 00:50:18,040
Speaker 1: were doing is called strong lensing. And that's like, I

1051
00:50:18,120 --> 00:50:20,480
Speaker 1: want to have a blob of dark matter right between

1052
00:50:20,480 --> 00:50:22,560
Speaker 1: me and another galaxy, so I can see like a

1053
00:50:22,600 --> 00:50:25,960
Speaker 1: massive distortion. You can see one galaxy how it's distorted,

1054
00:50:26,000 --> 00:50:27,920
Speaker 1: and you can use that to measure the mass of

1055
00:50:27,960 --> 00:50:30,640
Speaker 1: stuff between you and other galaxies. If instead you think

1056
00:50:30,680 --> 00:50:33,200
Speaker 1: that dark matter's sort of spread out so it doesn't

1057
00:50:33,239 --> 00:50:37,239
Speaker 1: really distort any individual photon that much. You can do

1058
00:50:37,320 --> 00:50:41,080
Speaker 1: something called weak gravitational lensing where you look at lots

1059
00:50:41,120 --> 00:50:44,000
Speaker 1: of galaxies and you look for lots of very small

1060
00:50:44,080 --> 00:50:47,200
Speaker 1: distortions and you sort of add them up statistically to

1061
00:50:47,200 --> 00:50:49,440
Speaker 1: get a map for where the dark matter might be

1062
00:50:49,600 --> 00:50:52,080
Speaker 1: and where it might not be. So you see sort

1063
00:50:52,080 --> 00:50:56,000
Speaker 1: of like fewer generalized distortions over here and more generalized

1064
00:50:56,040 --> 00:50:58,800
Speaker 1: distortions over there. It can tell you sort of where

1065
00:50:58,840 --> 00:51:01,320
Speaker 1: the dark matter is denser and where it's less dense.

1066
00:51:01,760 --> 00:51:04,239
Speaker 3: Interesting, you're sort of looking for sort of wiggles in

1067
00:51:04,320 --> 00:51:06,759
Speaker 3: the overall picture. But how would you know that is

1068
00:51:06,840 --> 00:51:09,080
Speaker 3: dark matter? Would that be changing or are you assuming

1069
00:51:09,120 --> 00:51:11,520
Speaker 3: that it changes as our view of the universe changes.

1070
00:51:11,800 --> 00:51:13,920
Speaker 1: Well, we think we know what galaxies should look like

1071
00:51:13,960 --> 00:51:17,480
Speaker 1: when they're not distorted, and so we compare how galaxies

1072
00:51:17,520 --> 00:51:20,239
Speaker 1: look to ideas of how a galaxy should look when

1073
00:51:20,280 --> 00:51:22,279
Speaker 1: it's not distorted and how it should look when it's

1074
00:51:22,320 --> 00:51:25,839
Speaker 1: slightly distorted by dark matter. And so we can use

1075
00:51:25,880 --> 00:51:29,600
Speaker 1: that to estimate like how much distortion galaxies have. But

1076
00:51:29,640 --> 00:51:31,520
Speaker 1: it's very very weak, you know, it's hard to tell

1077
00:51:31,560 --> 00:51:34,960
Speaker 1: the difference in a galaxy that's undistorted and slightly distorted.

1078
00:51:35,080 --> 00:51:37,480
Speaker 1: And that's why we need like thousands of galaxies to

1079
00:51:37,480 --> 00:51:39,799
Speaker 1: add this up statistically to get a sense for where

1080
00:51:39,800 --> 00:51:41,360
Speaker 1: the dark matter is interesting.

1081
00:51:41,400 --> 00:51:43,359
Speaker 3: And this is like an ongoing thing, right, Like there's

1082
00:51:43,400 --> 00:51:46,800
Speaker 3: people looking for these ripples in our view of the universe.

1083
00:51:46,920 --> 00:51:49,480
Speaker 1: Yeah, this is recent. Actually there's a program using a

1084
00:51:49,560 --> 00:51:53,759
Speaker 1: huge telescope with a massive camera five hundred and seventy megapixels.

1085
00:51:53,800 --> 00:51:56,719
Speaker 1: It's called the Dark Energy Survey, and this camera is

1086
00:51:56,760 --> 00:51:59,600
Speaker 1: basically built just to do this, just to look at

1087
00:51:59,640 --> 00:52:02,479
Speaker 1: all the galaxies and build a huge map. And they've

1088
00:52:02,520 --> 00:52:06,280
Speaker 1: studied one hundred million galaxies out there, like think about

1089
00:52:06,320 --> 00:52:09,400
Speaker 1: all the crazy stars and planets and everything that's out there,

1090
00:52:09,600 --> 00:52:12,360
Speaker 1: one hundred million of those. And they have built a

1091
00:52:12,480 --> 00:52:15,600
Speaker 1: map of where they think the dark matter is between

1092
00:52:15,840 --> 00:52:18,320
Speaker 1: galaxies using this weak lensing idea.

1093
00:52:18,400 --> 00:52:20,279
Speaker 3: Because I guess we can tell how old they are

1094
00:52:20,360 --> 00:52:23,080
Speaker 3: the galaxies, right, and how far away they are from us,

1095
00:52:23,080 --> 00:52:24,799
Speaker 3: not just in the night sky, and so we can

1096
00:52:24,800 --> 00:52:26,760
Speaker 3: build this three D map right exactly.

1097
00:52:26,800 --> 00:52:29,080
Speaker 1: We know where galaxies are because we can look at

1098
00:52:29,120 --> 00:52:31,960
Speaker 1: like Type one A supernova within them, we can measure

1099
00:52:32,000 --> 00:52:34,239
Speaker 1: their brightness, and we can tell how far away they

1100
00:52:34,280 --> 00:52:37,040
Speaker 1: are based on how bright they appear to be here

1101
00:52:37,080 --> 00:52:39,959
Speaker 1: on Earth. So we have this incredible three D map

1102
00:52:40,000 --> 00:52:42,200
Speaker 1: of all the galaxies, and then this thing is taking

1103
00:52:42,239 --> 00:52:44,960
Speaker 1: careful pictures of them to try to estimate how much

1104
00:52:45,040 --> 00:52:48,120
Speaker 1: each one is distorted, and then it's comparing that to

1105
00:52:48,200 --> 00:52:51,000
Speaker 1: our idea for where we think the dark matter should be.

1106
00:52:51,200 --> 00:52:53,120
Speaker 1: We have an idea for where we think dark matter

1107
00:52:53,120 --> 00:52:55,279
Speaker 1: should be based on where all the galaxies are. We

1108
00:52:55,360 --> 00:52:58,000
Speaker 1: can sort of back that up to the early universe

1109
00:52:58,360 --> 00:53:00,880
Speaker 1: and say where would the dark mind have to have

1110
00:53:01,080 --> 00:53:03,879
Speaker 1: been in order to make these galaxies end up here

1111
00:53:03,920 --> 00:53:06,480
Speaker 1: and that galaxy end up there to sort of create

1112
00:53:06,520 --> 00:53:09,359
Speaker 1: the large scale structure that we see, because we think

1113
00:53:09,400 --> 00:53:12,919
Speaker 1: that mostly where galaxies ended up depends on where dark

1114
00:53:12,960 --> 00:53:15,600
Speaker 1: matter was. So we have like a simulation for where

1115
00:53:15,600 --> 00:53:18,280
Speaker 1: we think the dark matter should be on our idea

1116
00:53:18,280 --> 00:53:20,120
Speaker 1: of how it all works, and then we go out

1117
00:53:20,160 --> 00:53:22,319
Speaker 1: and measure it and build a real map of where

1118
00:53:22,320 --> 00:53:24,759
Speaker 1: the dark matter is, and then we can compare the

1119
00:53:24,800 --> 00:53:26,400
Speaker 1: two and that's when the fund starts.

1120
00:53:26,600 --> 00:53:29,000
Speaker 3: WHOA, So, why have we phound? Do they match? Or

1121
00:53:29,080 --> 00:53:30,000
Speaker 3: are they very different?

1122
00:53:30,280 --> 00:53:33,000
Speaker 1: They mostly match? Like it mostly makes sense the dark

1123
00:53:33,040 --> 00:53:36,239
Speaker 1: matter is mostly where we expect, but there are some deviations.

1124
00:53:36,320 --> 00:53:38,520
Speaker 1: It looks like dark matter is sort of more spread

1125
00:53:38,520 --> 00:53:41,400
Speaker 1: out than we expected. Instead of being in these like

1126
00:53:41,520 --> 00:53:45,400
Speaker 1: thin strands between galaxies, it tends to be sometimes in

1127
00:53:45,480 --> 00:53:48,080
Speaker 1: places where you don't expect. It's like spread out and

1128
00:53:48,440 --> 00:53:52,240
Speaker 1: globbed out more than we expected, more than our simulations predict.

1129
00:53:52,520 --> 00:53:55,480
Speaker 1: It's just a few percent compared to our predictions, but

1130
00:53:55,560 --> 00:53:58,560
Speaker 1: it's an important deviation. It means that like, maybe something

1131
00:53:58,600 --> 00:54:00,480
Speaker 1: is going on with that dark matter, or maybe dark

1132
00:54:00,520 --> 00:54:02,760
Speaker 1: matter is made out of something weird we didn't understand,

1133
00:54:02,840 --> 00:54:05,200
Speaker 1: or maybe we've made a mistake in building our map

1134
00:54:05,239 --> 00:54:07,160
Speaker 1: of dark matter. But it's sort of like at the

1135
00:54:07,320 --> 00:54:10,160
Speaker 1: edge of science. These are very very recent results.

1136
00:54:10,719 --> 00:54:14,239
Speaker 3: That dark matter just wants to be alone between the

1137
00:54:14,239 --> 00:54:16,319
Speaker 3: galaxy or maybe it got timed out. But I guess

1138
00:54:16,360 --> 00:54:19,520
Speaker 3: what's surprising is that there is dark matter between galaxies

1139
00:54:19,560 --> 00:54:22,920
Speaker 3: because there's not much matter out there, so why wouldn't

1140
00:54:22,920 --> 00:54:25,960
Speaker 3: this dark matter also accumulate regular matter.

1141
00:54:26,120 --> 00:54:28,560
Speaker 1: There is actually a good bit of regular matter out there,

1142
00:54:28,719 --> 00:54:31,480
Speaker 1: it's just not glowing, like a huge amount of the

1143
00:54:31,719 --> 00:54:35,000
Speaker 1: atoms in the universe are actually between galaxies, and this

1144
00:54:35,120 --> 00:54:39,600
Speaker 1: intergalactic matter these streams of stuff, and so there's a

1145
00:54:39,600 --> 00:54:41,800
Speaker 1: good bit of stuff out there. There's just not enough

1146
00:54:41,840 --> 00:54:45,839
Speaker 1: density to form stars and planets and galaxies and all

1147
00:54:45,920 --> 00:54:48,000
Speaker 1: kinds of stuff. And so that's why it's not as

1148
00:54:48,080 --> 00:54:50,760
Speaker 1: visible because it doesn't glow interesting.

1149
00:54:50,880 --> 00:54:53,279
Speaker 3: But then what makes some dark matter have a lot

1150
00:54:53,320 --> 00:54:54,919
Speaker 3: of bright matter in it, and.

1151
00:54:55,680 --> 00:54:58,800
Speaker 1: The denser blobs of dark matter did form enough stuff

1152
00:54:58,840 --> 00:55:01,960
Speaker 1: to create galaxies and stars. The kind of strands we're

1153
00:55:01,960 --> 00:55:05,360
Speaker 1: talking about between galaxies is a smaller fraction of the

1154
00:55:05,400 --> 00:55:07,799
Speaker 1: amount of dark matter. The density is not there in

1155
00:55:07,920 --> 00:55:11,040
Speaker 1: order to create the gravitational well, you need to attract

1156
00:55:11,160 --> 00:55:14,680
Speaker 1: enough hydrogen to get stars to form. Pretty cool, so

1157
00:55:14,719 --> 00:55:17,640
Speaker 1: the galaxies still show you where like the densest blobs

1158
00:55:17,640 --> 00:55:19,240
Speaker 1: of dark matter are in the universe.

1159
00:55:19,560 --> 00:55:22,359
Speaker 3: Like the dark matter also varies in density out there.

1160
00:55:22,440 --> 00:55:23,560
Speaker 1: Absolutely, Yeah, all.

1161
00:55:23,520 --> 00:55:25,239
Speaker 3: Right, well it sort of sounds like you sort of

1162
00:55:25,280 --> 00:55:27,839
Speaker 3: know where dark matter is, but maybe not to super

1163
00:55:27,960 --> 00:55:31,000
Speaker 3: high resolution where you can tell if it's super clumpy

1164
00:55:31,080 --> 00:55:34,359
Speaker 3: or super smooth. And also we have a pretty good

1165
00:55:34,360 --> 00:55:36,600
Speaker 3: picture of where it is in the whole universe.

1166
00:55:36,719 --> 00:55:39,479
Speaker 1: Yeah, And scientists are working very hard to build more

1167
00:55:39,520 --> 00:55:42,560
Speaker 1: and more sensitive tools to try to use these little

1168
00:55:42,600 --> 00:55:46,360
Speaker 1: gravitational clues to build as accurate and as localized a

1169
00:55:46,440 --> 00:55:49,399
Speaker 1: map of dark matter as possible, because the better map

1170
00:55:49,440 --> 00:55:51,680
Speaker 1: of dark matter we get in our galaxy, the more

1171
00:55:51,719 --> 00:55:54,120
Speaker 1: we can study what it might be and how it

1172
00:55:54,160 --> 00:55:55,680
Speaker 1: came to be where it is.

1173
00:55:56,120 --> 00:55:58,040
Speaker 3: Yeah. I guess the big question now is what are

1174
00:55:58,080 --> 00:56:01,920
Speaker 3: you going to do when you find it? Daniel retire. Yeah,

1175
00:56:02,360 --> 00:56:04,600
Speaker 3: like the dog that finally catches up the car and

1176
00:56:04,719 --> 00:56:05,759
Speaker 3: doesn't know what to do with it.

1177
00:56:05,840 --> 00:56:07,960
Speaker 1: Oh, we'll find some other mystery to focus on.

1178
00:56:08,160 --> 00:56:10,560
Speaker 3: You'll come up with another cool name darkest matter.

1179
00:56:10,760 --> 00:56:12,879
Speaker 1: We always have the seventy percent of the universe called

1180
00:56:12,960 --> 00:56:15,640
Speaker 1: dark energy that we haven't even gotten started on.

1181
00:56:15,920 --> 00:56:19,160
Speaker 3: Yeah, that's another huge mystery, huge hole in our view

1182
00:56:19,160 --> 00:56:21,680
Speaker 3: of the universe, and hopefully you won't lose your mind

1183
00:56:21,760 --> 00:56:22,360
Speaker 3: trying to find it.

1184
00:56:22,560 --> 00:56:26,080
Speaker 1: I lost it here, all right.

1185
00:56:26,160 --> 00:56:28,320
Speaker 3: Well, another awesome reminder that there's still a lot of

1186
00:56:28,400 --> 00:56:31,280
Speaker 3: universe out there to be discovered. You know, anyone listening

1187
00:56:31,320 --> 00:56:33,200
Speaker 3: to this could be the person that comes up with

1188
00:56:33,239 --> 00:56:36,799
Speaker 3: the next great idea or the next grade concept that

1189
00:56:36,960 --> 00:56:38,960
Speaker 3: makes sense of all this and helps us find these

1190
00:56:38,960 --> 00:56:40,360
Speaker 3: big mysteries in the universe.

1191
00:56:40,560 --> 00:56:43,240
Speaker 1: If you're in tranced by the concept of building maps

1192
00:56:43,239 --> 00:56:47,000
Speaker 1: and discovering the way the world is and how everything looks. Remember,

1193
00:56:47,280 --> 00:56:49,680
Speaker 1: the biggest map of the most important stuff in the

1194
00:56:49,760 --> 00:56:51,560
Speaker 1: universe is still being drawn.

1195
00:56:51,760 --> 00:56:54,760
Speaker 3: You can really lose yourself in that search for lost things.

1196
00:56:54,880 --> 00:56:56,560
Speaker 1: Come join me in the asylum.

1197
00:56:56,560 --> 00:57:00,719
Speaker 3: Or the basement Lost and Found Department. Thanks for joining us.

1198
00:57:00,760 --> 00:57:03,280
Speaker 3: We hope you enjoyed that. See you next time.

1199
00:57:11,160 --> 00:57:14,000
Speaker 1: Thanks for listening, and remember that. Daniel and Jorge Explain

1200
00:57:14,040 --> 00:57:18,040
Speaker 1: the Universe is a production of iHeartRadio. For more podcasts

1201
00:57:18,040 --> 00:57:22,720
Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

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