WEBVTT - Are there galaxies without dark matter?

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<v Speaker 1>Hey, Daniel, most of the stuff in the universe is

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<v Speaker 1>dark matter, isn't that right?

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<v Speaker 2>Yeah, it's about eighty percent of the matter in the universe.

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<v Speaker 1>And it's all around us, like right here with us

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<v Speaker 1>immersed in dark matter.

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<v Speaker 2>Yeah, we are swimming in it.

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<v Speaker 1>And it's also inside of us, is it?

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<v Speaker 2>Yeah? It passes through us, it doesn't bounce off your skin.

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<v Speaker 1>Does that mean that we're partly made out of dark matter? Oh?

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<v Speaker 2>I guess. So that's kind of dark to think about.

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<v Speaker 1>So if I discover myself, I'm discovering dark matter.

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<v Speaker 2>Know thyself. Win a Nobel Prize.

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<v Speaker 1>Or at least a dark Nobel Prize. Technically all Nobel

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<v Speaker 1>prizes have dark matter inside of them too.

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<v Speaker 2>Maybe they'll give you the cash in dark money.

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<v Speaker 1>Ooh, that got dark quick.

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<v Speaker 3>Hi.

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<v Speaker 1>I'm Hoeham and cartoonists and the author of Oliver's Great

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<v Speaker 1>Big Universe.

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<v Speaker 2>Hi. I'm Daniel. I'm a particle physicist at CERN and

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<v Speaker 2>a professor at UC Irvine. And I'll take your money

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<v Speaker 2>dark or light.

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<v Speaker 1>Really, you'll take dark money. Isn't that dangerous for a

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<v Speaker 1>tenured professor?

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<v Speaker 2>No, that's what ten years for man. I can take money.

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<v Speaker 1>From us visuals. I guess technically, the university takes the

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<v Speaker 1>money and you get a cut of it.

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<v Speaker 2>I never really had to worry about that. Like, nobody

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<v Speaker 2>who got wealthy with dubious techniques ever offered me a

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<v Speaker 2>slush fund, So I've never had to really grapple with

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<v Speaker 2>that question.

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<v Speaker 1>Wait, how do you know? I mean, you've taken money.

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<v Speaker 1>Have you done your due diligence? How deep did you go?

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<v Speaker 2>You know, almost all my research money comes from the government,

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<v Speaker 2>and we all know the various crimes that the government

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<v Speaker 2>has committed.

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<v Speaker 1>Exactly all government money is dark money.

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<v Speaker 2>M do you believe taxes are theft? Is that where

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<v Speaker 2>this is going?

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<v Speaker 1>Well, technically, or funding comes from taxpayers and there must

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<v Speaker 1>be some sketchy tax payers out there, so your work

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<v Speaker 1>is painted.

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<v Speaker 2>So you're saying drug dealers who are paying taxes and

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<v Speaker 2>indirectly funding my research have made me complicit in their crimes.

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<v Speaker 1>That's right. Yes, assuming drug dealers pay taxes, which I

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<v Speaker 1>guess they do. If they're laundering money, I don't know.

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<v Speaker 1>I feel like we need a whole podcast episode just

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<v Speaker 1>to cover dark money.

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<v Speaker 2>Yeah, and then I suppose everybody's guilty.

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<v Speaker 1>That's right, We're all guilty of paying taxes, I guess.

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<v Speaker 1>But anyways, Welcome to our podcast Daniel and Jorge Explain

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<v Speaker 1>the Universe, a production of iHeartRadio.

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<v Speaker 2>In which we try not to overtax your brain while

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<v Speaker 2>explaining all of the mysteries of the universe. We want

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<v Speaker 2>to reveal the light universe and the dark universe, the

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<v Speaker 2>visible and the invisible. We want to show you how

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<v Speaker 2>the universe is not just what you see around you

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<v Speaker 2>and experience day to day, but so much more, so

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<v Speaker 2>much deeper, so many more mysteries waiting to be solved.

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<v Speaker 1>That's why we explore the universe, the dark corners of it,

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<v Speaker 1>the light corners, and the sketchy corners of it, because

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<v Speaker 1>our understanding of the universe is still a little bit sketchy, like.

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<v Speaker 2>The Mafia corners of the universe.

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<v Speaker 1>No, like, not clearly drawn.

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<v Speaker 2>Oh I see, I thought we're gonna be talking about

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<v Speaker 2>like the physics of New Jersey or something.

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<v Speaker 1>Yeah, there's the only some dark matters going on out there.

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<v Speaker 2>How do you eat all those rich cookies and not

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<v Speaker 2>gain weight? Really, it's amazing.

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<v Speaker 1>Yeah, Well, I think the secret is dark chocolate. It's

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<v Speaker 1>lower and fat.

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<v Speaker 2>Isn't it?

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<v Speaker 1>Is it?

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<v Speaker 2>Really? I don't know.

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<v Speaker 1>You're like, what, what, Why aren't I eating more dark chocolate.

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<v Speaker 2>I'm not really stopping myself from eating dark chocolate. But

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<v Speaker 2>that wasn't the reason I don't see dark chocolate is

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<v Speaker 2>like diet chocolate.

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<v Speaker 1>It's chocolate light. Oh no, wait, it's chocolate dark.

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<v Speaker 2>It's dark chocolate exactly. But there are lots of fun

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<v Speaker 2>questions out there in the universe, not just about how

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<v Speaker 2>waste management organizations in New Jersey are getting their money

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<v Speaker 2>and the calories that Daniel's eating. We're wondering about the

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<v Speaker 2>biggest questions in the universe, like where is all the stuff?

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<v Speaker 2>What is most of the universe made out of? But

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<v Speaker 2>how has it shaped the night sky that we see

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<v Speaker 2>today in the galaxy that we live in.

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<v Speaker 1>That's right, because what the universe is made out of

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<v Speaker 1>is maybe one of the biggest questions we can ask

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<v Speaker 1>about the universe. What is this whole place made out of?

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<v Speaker 1>What are we made out of? Are we made out

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<v Speaker 1>of dark matter? Can we exist without dark matter?

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<v Speaker 2>And we talk on this podcast a lot about dark matter.

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<v Speaker 2>We have lots of episodes about what it is and

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<v Speaker 2>where it is and how it works. And one thing

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<v Speaker 2>we often stress is that dark matter is part of

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<v Speaker 2>our galaxy, that most of our galaxy is actually dark

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<v Speaker 2>matter that has played a big role in the formation

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<v Speaker 2>of our galaxy. We wouldn't be here without it.

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<v Speaker 1>Yeah, we do talk a lot about dark matter. Daniel,

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<v Speaker 1>What percentage of our episodes would you say we talk

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<v Speaker 1>about dark matter? Is it representative of the amount of

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<v Speaker 1>dark matter in the universe? Are we ignoring dark matter

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<v Speaker 1>in a way?

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<v Speaker 2>Is dark matter underrepresented matter on podcasts? That's a good question.

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<v Speaker 1>Yeah, well, especially our podcast. If we're trying to explain

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<v Speaker 1>the universe and the universe is twenty seven percent dark matter.

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<v Speaker 2>You know, we might reach twenty seven percent of our

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<v Speaker 2>podcast being about dark matter. It's been a lot of them,

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<v Speaker 2>and I'm pretty sure we don't have two thirds of

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<v Speaker 2>them about dark energy. Maybe we should.

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<v Speaker 1>Yeah, yeah, that's what everyone is asking for more dark

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<v Speaker 1>energy episodes. But it is a pretty important part of

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<v Speaker 1>the universe. It's a pretty important part of our existence

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<v Speaker 1>because without dark matter, maybe galaxies would not have formed

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<v Speaker 1>the same way that they formed. The Milky Way would

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<v Speaker 1>might not be the same way it is now.

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<v Speaker 2>But we're always tempted to over general life to say

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<v Speaker 2>that the way we live in our certain situation over

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<v Speaker 2>here is the way the whole universe works. It's important

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<v Speaker 2>to take a step back and to ask whether our

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<v Speaker 2>way of life and our way of galaxying is the

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<v Speaker 2>only way that it can be.

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<v Speaker 1>And so today on the podcast, we'll be asking the question,

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<v Speaker 1>are there galaxies without dark matter? Would you call these

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<v Speaker 1>light galaxies then? Or galaxies light diet galaxies?

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<v Speaker 2>I call them tragic galaxies because they're probably galaxies where

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<v Speaker 2>everybody only eats white chocolate.

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<v Speaker 1>Or milk chocolate, some chocolate snobs. I also call that

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<v Speaker 1>a tragedy.

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<v Speaker 2>Also known as crimes against chocolate.

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<v Speaker 1>Are you saying Hershey's is dark money as well?

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<v Speaker 2>Her She's his garbage man?

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<v Speaker 1>Oh my gosh. And there goes our sponsorship deal with Hershe's.

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<v Speaker 1>We were so close to funding this thing for the

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<v Speaker 1>next twenty years, and you have to go and insult them.

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<v Speaker 2>Maybe you could hear my hesitation there. I'm being torn

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<v Speaker 2>between being honest and truthful on a hard science podcast

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<v Speaker 2>and pandering to our sponsors, and I just had to

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<v Speaker 2>be honest about it. I passed on hershe'es. I'd rather

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<v Speaker 2>have no chocolate than Hershe's.

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<v Speaker 1>That was pretty harsh to garbage. Oh my gosh. Well,

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<v Speaker 1>here here's the thing, Like when you eat it. Let's

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<v Speaker 1>say you eat like a chocolate cake at a restaurant.

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<v Speaker 1>How do you know they didn't use Hershey's chocolate.

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<v Speaker 2>You can taste it. Man, that's sour affront to chocolate.

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<v Speaker 2>It's turned so many people off of chocolate when chocolate

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<v Speaker 2>is this wonderful, amazing thing. Even milk chocolate can be

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<v Speaker 2>high quality, can be amazing. What they may in Pennsylvania. O, man,

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<v Speaker 2>it's a crime.

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<v Speaker 1>Now you're insulting the whole state of Pennsylvania.

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<v Speaker 2>Let's keep going. See how many people I can offend.

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<v Speaker 1>Yeah, yeah, let's moves and let's insult the whole universe.

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<v Speaker 2>Why don't you No, Fortunately, most of the universe is

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<v Speaker 2>dark matter and therefore appreciates dark chocolate, and so we're good.

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<v Speaker 2>We have the majority firmly on our side.

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<v Speaker 1>But doesn't Hershey's own some fancy brands like Scharfenberger or

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<v Speaker 1>something like that.

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<v Speaker 2>I don't know, I hope not.

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<v Speaker 1>Well, maybe you've been eating Hershey' chocolates all this time.

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<v Speaker 1>A plot twist. Anyways, we're talking about dark matter and

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<v Speaker 1>galaxies and could there exist galaxies without dark matter. These

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<v Speaker 1>would be like galaxies that don't have any dark matter

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<v Speaker 1>in them or around them.

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<v Speaker 2>Yeah, exactly, just stars and gas and dust and black holes,

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<v Speaker 2>no dark matter, all.

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<v Speaker 1>Right, Well, whether they are tragic or not. We were

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<v Speaker 1>wondering how many people out there had wondered about this

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<v Speaker 1>question and if they have any ideas about the answer.

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<v Speaker 2>Thanks very much to everybody out there who answers these questions,

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<v Speaker 2>they or not there supporters of Hershey's Crimes against Chocolate

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<v Speaker 2>would really appreciate everything you do. If you would like

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<v Speaker 2>to join this group, just write to me two questions

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<v Speaker 2>at Danielandjorge dot com.

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<v Speaker 1>I feel like maybe there's a Hurshi's employee out there

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<v Speaker 1>who listens to our podcast and it's now very very sad.

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<v Speaker 2>There's an easy fix for that, find a new job

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<v Speaker 2>or a new podcast. No, you've got to go to

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<v Speaker 2>the root of the problem.

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<v Speaker 1>That's right anyway. So think about it for a second.

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<v Speaker 1>Do you think there can be galaxies without dark matter?

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<v Speaker 1>Here's what people had to say.

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<v Speaker 4>I feel like there probably aren't, only because I know

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<v Speaker 4>that dark matter, as far as I know, is distributed

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<v Speaker 4>pretty much evenly throughout the universe. I think it tends

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<v Speaker 4>to be clustered in galaxies, but I think it tends

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<v Speaker 4>to be pretty uniform. So I would be surprised if

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<v Speaker 4>there were galaxies without dark matter.

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<v Speaker 5>I'm going to say why not, because, like going back

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<v Speaker 5>to an episode about uranium on Uranus, there could be

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<v Speaker 5>a tiny bit here and there in a galaxy. So

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<v Speaker 5>I'm going to say I don't know why there wouldn't

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<v Speaker 5>be dark matter in a galaxy. I'm also going to

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<v Speaker 5>say I don't know why there would.

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<v Speaker 3>I don't believe there are galaxies without dark matter because

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<v Speaker 3>dark matter is, in my understanding, a general term for

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<v Speaker 3>unknown matter, which is this and makes up the overwhelming

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<v Speaker 3>majority of the universe. Therefore, I think it is not

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<v Speaker 3>possible for galaxies not to have at least some dark

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<v Speaker 3>matter in them.

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<v Speaker 6>I'm not sure that we've observed any galaxies without dark matter,

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<v Speaker 6>but I suppose anything could be possible in this crazy universe.

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<v Speaker 6>It's also possible that all galaxies are without dark matter

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<v Speaker 6>and we just don't understand gravity all right.

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<v Speaker 1>Most people are skeptical about this question.

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<v Speaker 2>Yeah, people have the idea that dark matter is everywhere.

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<v Speaker 1>It's inescapable, you can't get away from it. It seems

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<v Speaker 1>in our question, although one person has kind of said,

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<v Speaker 1>why not, that's a good attitude to have.

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<v Speaker 2>Yeah, that's the whole attitude about physics, Like, well, maybe

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<v Speaker 2>everything is different from what we've boughter. Maybe there's something

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<v Speaker 2>really weird out there that could teach us something new

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<v Speaker 2>about the universe.

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<v Speaker 1>I do feel like that is a guiding question in

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<v Speaker 1>theoretical physics at least. Why not? Yes? Why not?

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<v Speaker 2>Sure? Maybe everything is just tiny cats at the quantum scale.

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<v Speaker 1>That's right, Maybe everything's just made out of Hersy's charga.

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<v Speaker 2>No, I got to hop somewhere else in the multiverse.

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<v Speaker 1>If that's the case, all right, Well let's get going

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<v Speaker 1>before this podcast gets too dark. Daniel, give us the basics.

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<v Speaker 1>What is dark matter? For those of us who haven't

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<v Speaker 1>listened to the twenty seven percent of our podcast.

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<v Speaker 2>Episodes, dark matter is fascinating because we simultaneously know a

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<v Speaker 2>lot about it and very little. Like we know that

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<v Speaker 2>there's a lot of dark matter in the universe, and

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<v Speaker 2>we know that it's matter. We know there's something out

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<v Speaker 2>there that's creating gravity or curvature of space time, but

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<v Speaker 2>that it's invisible. It doesn't glow, it doesn't give off light.

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<v Speaker 2>It doesn't reflect light. We sense it only because we

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<v Speaker 2>see its gravitational effects on stuff. It's curving space, which

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<v Speaker 2>changes how things are moving through that space. We see

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<v Speaker 2>galaxies rotating much faster than their gravity would be able

0:10:56.280 --> 0:10:59.000
<v Speaker 2>to hold them together if there wasn't also dark matter

0:10:59.080 --> 0:11:01.280
<v Speaker 2>in them holding them together. On the other hand, we

0:11:01.320 --> 0:11:03.720
<v Speaker 2>don't know what stuff it is. We know that it

0:11:03.760 --> 0:11:06.160
<v Speaker 2>has gravity, so it's matter, but we don't know it's

0:11:06.200 --> 0:11:08.760
<v Speaker 2>some weird new kind of neutrino or a totally different

0:11:08.840 --> 0:11:11.520
<v Speaker 2>kind of particle we've never seen before, or a thousand

0:11:11.559 --> 0:11:14.880
<v Speaker 2>new kinds of particles, or something that's not even a particle.

0:11:15.360 --> 0:11:17.000
<v Speaker 2>So we know a lot about it on the sort

0:11:17.000 --> 0:11:20.520
<v Speaker 2>of cosmological scale, but very very little or almost nothing

0:11:20.559 --> 0:11:22.440
<v Speaker 2>about it at the particle level. Right.

0:11:22.559 --> 0:11:24.440
<v Speaker 1>Dark matter is this kind of mysterious stuff in the

0:11:24.520 --> 0:11:27.240
<v Speaker 1>universe that we kind of feel its presence. We can

0:11:27.280 --> 0:11:30.240
<v Speaker 1>see its presence through gravity, but as you said, you

0:11:30.280 --> 0:11:33.760
<v Speaker 1>can't see it because it doesn't interact with elechormagnetic light

0:11:33.920 --> 0:11:36.120
<v Speaker 1>or maybe any of the other forces in the universe,

0:11:36.160 --> 0:11:37.319
<v Speaker 1>and so you can't see it. And that's why you

0:11:37.360 --> 0:11:38.640
<v Speaker 1>call it dark exactly.

0:11:38.800 --> 0:11:40.640
<v Speaker 2>And a bunch of listeners write in with the idea

0:11:40.679 --> 0:11:43.959
<v Speaker 2>that maybe dark matter is matter in another universe that's

0:11:43.960 --> 0:11:47.440
<v Speaker 2>somehow leaking in to ours. Remember that dark matter is

0:11:47.440 --> 0:11:50.880
<v Speaker 2>creating gravity in our universe, or changing the curvature of

0:11:50.920 --> 0:11:54.400
<v Speaker 2>space in our universe, which means that it's in that space. Right,

0:11:54.400 --> 0:11:57.360
<v Speaker 2>it shares that space with us, which kind of means

0:11:57.400 --> 0:12:00.680
<v Speaker 2>that it's in our universe. We really do know that

0:12:00.760 --> 0:12:03.920
<v Speaker 2>dark matter is something in our universe that's changing the

0:12:03.960 --> 0:12:06.760
<v Speaker 2>shape of space. We can only see sort of indirectly

0:12:06.840 --> 0:12:09.800
<v Speaker 2>through gravity, which is really frustrating because gravity is the

0:12:09.840 --> 0:12:12.720
<v Speaker 2>worst way to see things. It's so weak that it

0:12:12.760 --> 0:12:14.040
<v Speaker 2>makes it very, very difficult.

0:12:14.720 --> 0:12:17.760
<v Speaker 1>But wait, couldn't it bend our space and not be

0:12:17.880 --> 0:12:18.640
<v Speaker 1>part of our space.

0:12:18.920 --> 0:12:21.440
<v Speaker 2>That's possible if you overthrow general relativity in our entire

0:12:21.520 --> 0:12:25.240
<v Speaker 2>understanding of space time. General relativity says that matter tells

0:12:25.320 --> 0:12:28.120
<v Speaker 2>space how to bend, and space tells matter how to move,

0:12:28.640 --> 0:12:31.920
<v Speaker 2>and that means matter in our space. Like in general relativity,

0:12:32.120 --> 0:12:34.800
<v Speaker 2>the curvature or space comes from the energy density in

0:12:35.000 --> 0:12:38.240
<v Speaker 2>that space. So if you have some like parallel space

0:12:38.400 --> 0:12:42.160
<v Speaker 2>overlaid on top of it, which can also bend that space,

0:12:42.280 --> 0:12:43.680
<v Speaker 2>then it seems to me like it would be part

0:12:43.679 --> 0:12:46.160
<v Speaker 2>of our space, you know, sort of like the by definition,

0:12:46.720 --> 0:12:49.240
<v Speaker 2>but yet you could augment or throw out general relativity

0:12:49.320 --> 0:12:51.839
<v Speaker 2>replace it with something totally different. But the simpler idea

0:12:52.000 --> 0:12:53.679
<v Speaker 2>is that it's just some kind of mass we can't

0:12:53.720 --> 0:12:56.240
<v Speaker 2>see that explains almost everything we see out there in

0:12:56.240 --> 0:12:58.959
<v Speaker 2>the universe. So it's sort of the best going explanation.

0:12:59.000 --> 0:13:02.120
<v Speaker 2>You can always make more or baroque complicated explanations if

0:13:02.120 --> 0:13:02.480
<v Speaker 2>you like.

0:13:02.640 --> 0:13:05.520
<v Speaker 1>Well, it kind of might as well be in another universe, right,

0:13:05.559 --> 0:13:07.720
<v Speaker 1>because if it doesn't feel a lot of the same

0:13:07.760 --> 0:13:10.120
<v Speaker 1>force as we don't, it's just kind of like ghostly

0:13:10.160 --> 0:13:12.679
<v Speaker 1>matter that's kind of living on top of us. There

0:13:12.760 --> 0:13:14.880
<v Speaker 1>might be beings made out of dark matter, right.

0:13:14.960 --> 0:13:17.360
<v Speaker 2>Physicists call the different sectors of the universe. If you

0:13:17.400 --> 0:13:19.880
<v Speaker 2>have like two different sets of particles that don't interact

0:13:19.880 --> 0:13:22.520
<v Speaker 2>at all except for through gravity, then we call those

0:13:22.559 --> 0:13:24.640
<v Speaker 2>like the lights of the visible sector and the dark

0:13:24.679 --> 0:13:27.440
<v Speaker 2>sector of the universe. And that's totally possible that you

0:13:27.480 --> 0:13:30.280
<v Speaker 2>could have a whole complicated physics happening in the dark

0:13:30.280 --> 0:13:33.480
<v Speaker 2>sector that we can't see. Now. Mostly we know that

0:13:33.600 --> 0:13:36.600
<v Speaker 2>dark matter can't interact with itself. If it did interact

0:13:36.600 --> 0:13:39.160
<v Speaker 2>with itself, it would form all sorts of complicated structure

0:13:39.240 --> 0:13:41.839
<v Speaker 2>and do all sorts of interesting things. We think that

0:13:41.960 --> 0:13:45.319
<v Speaker 2>dark matter is pretty spread out. However, there could be

0:13:45.360 --> 0:13:47.680
<v Speaker 2>a little component dark dark matter. Dark matter may be

0:13:47.720 --> 0:13:50.720
<v Speaker 2>lots of different kinds of things, and one little component

0:13:50.760 --> 0:13:53.319
<v Speaker 2>of it might be more complicated and do complicated things

0:13:53.360 --> 0:13:56.839
<v Speaker 2>like form life or ice cream, cones or cats or

0:13:56.960 --> 0:13:59.880
<v Speaker 2>good chocolate without violating what we've seen dark matter do,

0:14:00.040 --> 0:14:01.680
<v Speaker 2>which is mostly spread out smoothly.

0:14:01.880 --> 0:14:04.840
<v Speaker 1>All right, Well, the question here today is can there

0:14:04.880 --> 0:14:08.800
<v Speaker 1>be galaxies without dark better? Why is this even a question? Like,

0:14:08.840 --> 0:14:10.800
<v Speaker 1>are most of the galaxies that we see out there?

0:14:10.840 --> 0:14:12.080
<v Speaker 1>Do they all have dark matter?

0:14:12.200 --> 0:14:14.040
<v Speaker 2>Yeah, the galaxies that you see out there in the

0:14:14.080 --> 0:14:17.280
<v Speaker 2>sky are like tracers. They're basically telling you where the

0:14:17.400 --> 0:14:20.760
<v Speaker 2>dark matter is in the universe. Remember that dark matter

0:14:20.840 --> 0:14:23.360
<v Speaker 2>is not something we can see, but it also dominates

0:14:23.360 --> 0:14:26.600
<v Speaker 2>the universe. Our estimates are that four fifths of the

0:14:26.640 --> 0:14:29.760
<v Speaker 2>matter in the universe is dark matter. So if you

0:14:29.920 --> 0:14:32.120
<v Speaker 2>like spin the wheel and pick a random object in

0:14:32.120 --> 0:14:34.320
<v Speaker 2>the universe, most likely you're going to get dark matter.

0:14:34.440 --> 0:14:38.000
<v Speaker 2>It's like overwhelmingly dark matter. So when the universe is

0:14:38.040 --> 0:14:41.000
<v Speaker 2>forming its structure and the gravity that determines like where

0:14:41.000 --> 0:14:43.600
<v Speaker 2>things are going to be. It's mostly the gravity of

0:14:43.680 --> 0:14:46.920
<v Speaker 2>dark matter that decides where things are going to clump

0:14:46.920 --> 0:14:49.640
<v Speaker 2>together and where things are not going to clump together. Remember,

0:14:49.640 --> 0:14:52.480
<v Speaker 2>the very early universe is mostly smooth, with a few

0:14:52.680 --> 0:14:56.160
<v Speaker 2>little blobs that are denser than others. The gravity of

0:14:56.160 --> 0:14:59.240
<v Speaker 2>those over dense pieces pull things together to form structure,

0:14:59.240 --> 0:15:00.960
<v Speaker 2>and that's where you get like galaxies in one part

0:15:01.000 --> 0:15:03.200
<v Speaker 2>of space and not galaxies in another part of space.

0:15:03.560 --> 0:15:06.440
<v Speaker 2>Where you have galaxies is where you had more dark

0:15:06.480 --> 0:15:09.920
<v Speaker 2>matter to pull that stuff together to form those galaxies.

0:15:10.120 --> 0:15:13.280
<v Speaker 1>Yeah, that's pretty wild to think that something we can't see,

0:15:13.320 --> 0:15:17.000
<v Speaker 1>that is invisible to us basically kind of dictates the

0:15:17.160 --> 0:15:19.720
<v Speaker 1>entire structure of the universe, right at least at the

0:15:19.760 --> 0:15:24.400
<v Speaker 1>galaxy level. Does dark matter also dictate things like superclusters

0:15:24.440 --> 0:15:26.320
<v Speaker 1>and beyond exactly?

0:15:26.720 --> 0:15:29.400
<v Speaker 2>And so you can imagine like these invisible wells, like

0:15:29.480 --> 0:15:32.880
<v Speaker 2>dark matter's curving space with Shepherd's the other kind of

0:15:32.920 --> 0:15:34.680
<v Speaker 2>matter together. So every time you look up at the

0:15:34.760 --> 0:15:37.280
<v Speaker 2>night sky and you see a galaxy, you should imagine

0:15:37.280 --> 0:15:40.840
<v Speaker 2>there's an invisible blob of dark matter surrounding that galaxy.

0:15:40.880 --> 0:15:44.000
<v Speaker 2>There's a whole halo that's created the conditions to form

0:15:44.080 --> 0:15:46.920
<v Speaker 2>that galaxy. We run simulations, for example, of a universe

0:15:46.920 --> 0:15:50.440
<v Speaker 2>without dark matter, and it doesn't form galaxies after fourteen

0:15:50.440 --> 0:15:53.360
<v Speaker 2>billion years. So quite literally, we would not be alive

0:15:53.400 --> 0:15:54.280
<v Speaker 2>without dark matter.

0:15:54.680 --> 0:15:57.320
<v Speaker 1>And now is the same true for things like superclusters

0:15:57.640 --> 0:16:01.080
<v Speaker 1>and those giant bubbles of galaxies they're in the universe.

0:16:01.320 --> 0:16:03.920
<v Speaker 1>Is that dictated by dark matter as well? Or is

0:16:03.960 --> 0:16:07.040
<v Speaker 1>that more of the quantum fluctuations of the universe.

0:16:07.160 --> 0:16:10.360
<v Speaker 2>Well, both, because there's this cosmic web that tells us

0:16:10.360 --> 0:16:12.800
<v Speaker 2>where dark matter will be denser and where dark matter

0:16:12.840 --> 0:16:15.200
<v Speaker 2>will not be dense. These filaments in some places they

0:16:15.240 --> 0:16:18.480
<v Speaker 2>overlap and you get these wells where things pool together.

0:16:18.600 --> 0:16:21.600
<v Speaker 2>And so the whole cosmic web is dictated by dark

0:16:21.640 --> 0:16:24.240
<v Speaker 2>matter and the light matter. The normal matter, baryons and

0:16:24.280 --> 0:16:27.640
<v Speaker 2>quarks and electrons just follows that. And so it's not

0:16:27.680 --> 0:16:30.920
<v Speaker 2>like a supercluster has a super halo. A supercluster is

0:16:30.960 --> 0:16:35.000
<v Speaker 2>made of galaxies connected together by these filaments.

0:16:34.600 --> 0:16:36.760
<v Speaker 1>So it also has filaments of dark matter.

0:16:37.000 --> 0:16:40.400
<v Speaker 2>Absolutely, yes, there are filaments of dark matter connecting these

0:16:40.440 --> 0:16:42.840
<v Speaker 2>halos of around each galaxy.

0:16:43.080 --> 0:16:43.320
<v Speaker 3>Hmmm.

0:16:43.840 --> 0:16:46.280
<v Speaker 1>Interesting. All right, Well, let's dig into the question of

0:16:46.320 --> 0:16:50.840
<v Speaker 1>whether a galaxy can be clean of dark matter, or

0:16:50.880 --> 0:16:53.800
<v Speaker 1>whether it's kind of a requirement for a galaxy to form.

0:16:53.960 --> 0:16:55.760
<v Speaker 1>So let's dig into that, but first let's take a

0:16:55.800 --> 0:17:10.840
<v Speaker 1>quick break. All Right, we're asking the question, can you

0:17:10.880 --> 0:17:13.639
<v Speaker 1>have a galaxy without dark matter? Could you maybe have

0:17:13.720 --> 0:17:15.760
<v Speaker 1>a scripy little galaxy out there? That was like, no,

0:17:15.840 --> 0:17:17.920
<v Speaker 1>I don't care about dark matter. I'm just going to

0:17:18.240 --> 0:17:21.880
<v Speaker 1>gather all these all this gas and dust and on

0:17:21.920 --> 0:17:24.760
<v Speaker 1>my own without any help. That's kind of what we're asking.

0:17:24.440 --> 0:17:26.320
<v Speaker 2>Today, right, Yeah, exactly.

0:17:26.720 --> 0:17:29.560
<v Speaker 1>And so you talked about how most of the galaxies

0:17:30.119 --> 0:17:33.400
<v Speaker 1>that we see out there probably have dark matter, right,

0:17:33.480 --> 0:17:35.399
<v Speaker 1>we think they have dark matter, right, because they couldn't

0:17:35.400 --> 0:17:37.000
<v Speaker 1>be holding together without dark matter.

0:17:37.080 --> 0:17:39.880
<v Speaker 2>That's right. And it's even more than just most galaxies

0:17:39.920 --> 0:17:42.520
<v Speaker 2>have dark matter. It turns out galaxies are sort of

0:17:42.560 --> 0:17:46.479
<v Speaker 2>like extra rich in dark matter, Like most galaxies have

0:17:46.760 --> 0:17:49.639
<v Speaker 2>more dark matter than the average dark matter density in

0:17:49.680 --> 0:17:50.320
<v Speaker 2>the universe.

0:17:50.640 --> 0:17:52.479
<v Speaker 1>Wait, what what do you mean? Like, what are some numbers?

0:17:52.680 --> 0:17:54.960
<v Speaker 2>So if you average over the whole universe, like what

0:17:55.119 --> 0:17:58.120
<v Speaker 2>fraction of matter in the universe is dark matter? That's

0:17:58.160 --> 0:18:01.520
<v Speaker 2>eighty four percent by counting. We don't know how many

0:18:01.600 --> 0:18:04.479
<v Speaker 2>dark matter particles there are that's by mass, Like, what

0:18:04.560 --> 0:18:07.280
<v Speaker 2>fraction of the mass of stuff in the universe is

0:18:07.359 --> 0:18:09.800
<v Speaker 2>dark matter? That's about eighty four percent. But when you

0:18:09.840 --> 0:18:12.520
<v Speaker 2>look at galaxies and you ask, like, what fraction of

0:18:12.520 --> 0:18:15.640
<v Speaker 2>the mass in a galaxy is dark matter, that's more

0:18:15.760 --> 0:18:19.879
<v Speaker 2>like ninety one percent. So galaxies have like about half

0:18:19.960 --> 0:18:23.680
<v Speaker 2>as much normal matter as the average normal matter density

0:18:23.680 --> 0:18:27.280
<v Speaker 2>in the universe. Galaxies are like concentrated blobs of dark matter.

0:18:27.520 --> 0:18:31.040
<v Speaker 1>And we get these numbers by measuring how fast the

0:18:31.040 --> 0:18:33.920
<v Speaker 1>galaxies are rotating and kind of guessing how much dark

0:18:33.920 --> 0:18:35.520
<v Speaker 1>matter you need to hold it all together.

0:18:36.359 --> 0:18:38.840
<v Speaker 2>Yeah, not so much guessing, measuring right, But you're right.

0:18:38.840 --> 0:18:41.879
<v Speaker 2>It's looking at how the galaxy rotates. We can measure

0:18:41.960 --> 0:18:44.560
<v Speaker 2>the speed of those stars in the galaxy as they

0:18:44.560 --> 0:18:47.560
<v Speaker 2>whizz around the center by looking at their light and

0:18:47.600 --> 0:18:50.160
<v Speaker 2>seeing how it's red shifted or blue shifted. You're looking

0:18:50.160 --> 0:18:52.120
<v Speaker 2>at a galaxy, some of the stars will be moving

0:18:52.160 --> 0:18:54.600
<v Speaker 2>away from you and some moving towards you, so to

0:18:54.720 --> 0:18:57.800
<v Speaker 2>be red or blue shifted their light from the Doppler shift,

0:18:57.840 --> 0:19:00.440
<v Speaker 2>so you can measure their velocities. So you can look

0:19:00.440 --> 0:19:03.080
<v Speaker 2>at the velocity of stars as they get further and

0:19:03.320 --> 0:19:05.560
<v Speaker 2>further away from the center, and in order to hold

0:19:05.560 --> 0:19:08.159
<v Speaker 2>a star at a certain velocity a certain radius, you

0:19:08.280 --> 0:19:11.120
<v Speaker 2>need a gravitational force there, so you can calculate exactly

0:19:11.119 --> 0:19:13.919
<v Speaker 2>how much gravity is needed to hold a star at

0:19:13.960 --> 0:19:16.240
<v Speaker 2>a certain radius. They have all these stars at different

0:19:16.280 --> 0:19:19.920
<v Speaker 2>distances from the center, telling you exactly how much gravity

0:19:19.960 --> 0:19:22.600
<v Speaker 2>you need to keep those stars going at that speed,

0:19:23.080 --> 0:19:25.520
<v Speaker 2>and then you can add up how much you can see,

0:19:25.720 --> 0:19:28.280
<v Speaker 2>like count all the visible stars and the rest you

0:19:28.359 --> 0:19:31.159
<v Speaker 2>suppose is dark matter. I guess that's what you mean

0:19:31.240 --> 0:19:32.120
<v Speaker 2>by guessing.

0:19:31.840 --> 0:19:33.440
<v Speaker 1>Yeah, inferring guessing.

0:19:34.480 --> 0:19:37.000
<v Speaker 2>There's a whole field of statistical inference that we should

0:19:37.040 --> 0:19:37.719
<v Speaker 2>just call guessing.

0:19:37.840 --> 0:19:41.440
<v Speaker 1>Yeah, Well, I mean you don't actually know how many

0:19:41.480 --> 0:19:43.840
<v Speaker 1>stars there are in that galaxy right, so far away

0:19:43.880 --> 0:19:46.960
<v Speaker 1>you can't see the individual stars, so you're also sort

0:19:47.000 --> 0:19:50.119
<v Speaker 1>of inferring how many stars there are there. You're guessing

0:19:50.160 --> 0:19:50.919
<v Speaker 1>a little bit, aren't you.

0:19:51.160 --> 0:19:53.960
<v Speaker 2>There's always uncertainty in these measurements. Absolutely, and you're right

0:19:53.960 --> 0:19:57.600
<v Speaker 2>that we cannot resolve individual stars, especially near the center

0:19:57.640 --> 0:20:00.800
<v Speaker 2>where things get very dense. But we can see this streams, right,

0:20:00.840 --> 0:20:02.879
<v Speaker 2>we can see streams of stars. We have models for

0:20:02.920 --> 0:20:06.600
<v Speaker 2>how these galaxies work. But absolutely there's always uncertainty, but

0:20:06.680 --> 0:20:09.919
<v Speaker 2>the uncertainty in these calculations is tiny compared to the

0:20:09.960 --> 0:20:12.760
<v Speaker 2>size of dark matter. So there's no uncertainty that there's

0:20:12.800 --> 0:20:15.600
<v Speaker 2>a lot of dark matter in these galaxies because remember

0:20:15.680 --> 0:20:18.560
<v Speaker 2>the fractions we're talking about here, like ninety percent, which

0:20:18.600 --> 0:20:21.400
<v Speaker 2>means you're looking at a star, you're measuring its velocity.

0:20:21.400 --> 0:20:24.120
<v Speaker 2>You figure out how much gravity is needed to hold

0:20:24.119 --> 0:20:26.840
<v Speaker 2>it there so it doesn't fly out into intergalactic space.

0:20:27.240 --> 0:20:28.600
<v Speaker 2>And when you add up all the stars, you get

0:20:28.600 --> 0:20:31.280
<v Speaker 2>like ten percent of the gravity you need. So there's

0:20:31.280 --> 0:20:33.040
<v Speaker 2>a huge missing chunk.

0:20:34.200 --> 0:20:36.880
<v Speaker 1>And you're sure it's not just you know, a lot

0:20:36.880 --> 0:20:38.640
<v Speaker 1>of asteroids or rocks that don't close.

0:20:38.840 --> 0:20:41.119
<v Speaker 2>Yeah, that's a great question. Could dark matter just be

0:20:41.359 --> 0:20:44.080
<v Speaker 2>normal matter that we're not seeing right, just like dark

0:20:44.200 --> 0:20:47.200
<v Speaker 2>chunks of matter. So people have looked for that directly.

0:20:47.440 --> 0:20:52.600
<v Speaker 2>Those are called MACHOs massive compact halo objects, and we

0:20:52.640 --> 0:20:54.960
<v Speaker 2>think we would see those occasionally, like they would pass

0:20:55.040 --> 0:20:56.959
<v Speaker 2>in front of stars if there was a lot of them.

0:20:57.000 --> 0:20:58.919
<v Speaker 2>If they were really big, we would have spotted them.

0:20:58.960 --> 0:21:00.960
<v Speaker 2>So people have looked for that kind of stuff and

0:21:01.000 --> 0:21:03.680
<v Speaker 2>not seen it. Plus we know something about how much

0:21:03.840 --> 0:21:06.960
<v Speaker 2>normal matter there was in the very early universe because

0:21:07.000 --> 0:21:10.240
<v Speaker 2>it dictates the fraction of elements that were produced, like

0:21:10.280 --> 0:21:13.320
<v Speaker 2>the hydrogen and helium and lithium very sensitive to the

0:21:13.359 --> 0:21:17.240
<v Speaker 2>density of quarks and electrons in the early universe. We

0:21:17.320 --> 0:21:20.359
<v Speaker 2>talked about that once Big Bang nucleosynthesis. So we have

0:21:20.400 --> 0:21:23.400
<v Speaker 2>a pretty good handle on how much normal matter there

0:21:23.480 --> 0:21:26.000
<v Speaker 2>was around, and we can explain where most of that

0:21:26.200 --> 0:21:27.720
<v Speaker 2>is now and the rest of it's got to be

0:21:27.800 --> 0:21:28.280
<v Speaker 2>dark matter.

0:21:28.480 --> 0:21:30.880
<v Speaker 1>And so basically every galaxy out there that we've seen,

0:21:32.000 --> 0:21:35.440
<v Speaker 1>we see that it's spinning faster than it should, or

0:21:35.440 --> 0:21:37.880
<v Speaker 1>it's holding together more than it should, so we think

0:21:38.040 --> 0:21:39.560
<v Speaker 1>it has dark matter. And now is that true for

0:21:39.600 --> 0:21:40.920
<v Speaker 1>every galaxy we've seen out there?

0:21:40.960 --> 0:21:43.440
<v Speaker 2>It turns out there's a pretty wide variety. Like when

0:21:43.480 --> 0:21:45.879
<v Speaker 2>you look at galaxies out there, some of them have

0:21:46.040 --> 0:21:48.159
<v Speaker 2>a lot of dark matter, and some of them have

0:21:48.200 --> 0:21:51.080
<v Speaker 2>a huge amount of dark matter. There is a variety.

0:21:51.119 --> 0:21:54.399
<v Speaker 2>It turns out that smaller galaxies tend to have more

0:21:54.520 --> 0:21:57.879
<v Speaker 2>dark matter than really massive galaxies.

0:21:57.400 --> 0:22:00.480
<v Speaker 1>Not by absolute amounts, but just relative town these stars

0:22:00.480 --> 0:22:00.760
<v Speaker 1>they have.

0:22:00.880 --> 0:22:04.040
<v Speaker 2>Yeah, exactly higher dark matter fractions, I should say, And

0:22:04.080 --> 0:22:07.359
<v Speaker 2>that's because galaxies are better at holding onto their dark

0:22:07.440 --> 0:22:11.080
<v Speaker 2>matter than they are their normal matter. Galaxies are crazy places.

0:22:11.119 --> 0:22:13.560
<v Speaker 2>There's winds from all the stars, right every star is

0:22:13.600 --> 0:22:17.640
<v Speaker 2>a fusion furnace and pushing out protons and electrons. These

0:22:17.840 --> 0:22:21.600
<v Speaker 2>cellar winds are pushing gas out of the galaxies. Then

0:22:21.640 --> 0:22:24.040
<v Speaker 2>there are supernovas going off all the time, blowing things

0:22:24.119 --> 0:22:27.800
<v Speaker 2>up and pushing things out. There's radiation, really intense radiation

0:22:27.920 --> 0:22:30.880
<v Speaker 2>from the center of the galaxy that's pushing gas out.

0:22:31.040 --> 0:22:34.240
<v Speaker 2>So galaxies are basically exploding and they're pushing a lot

0:22:34.240 --> 0:22:37.400
<v Speaker 2>of their matter out. And so the smaller galaxy is,

0:22:37.720 --> 0:22:40.879
<v Speaker 2>the less it's capable of holding onto its normal matter,

0:22:41.160 --> 0:22:44.359
<v Speaker 2>the less it's capable of resisting these forces that push

0:22:44.480 --> 0:22:46.680
<v Speaker 2>gas out of the galaxies.

0:22:46.200 --> 0:22:48.119
<v Speaker 1>Because the bigger galaxies have more gravity.

0:22:48.160 --> 0:22:50.760
<v Speaker 2>Basically right exactly, the bigger galaxies are still doing this,

0:22:50.800 --> 0:22:52.680
<v Speaker 2>but they have more gravity so they can hold onto

0:22:52.680 --> 0:22:55.720
<v Speaker 2>their normal matter. So smaller galaxies, which have weaker gravity,

0:22:55.840 --> 0:22:58.440
<v Speaker 2>lose more of this normal matter. So you look out

0:22:58.480 --> 0:23:01.000
<v Speaker 2>there at dwarf galaxies, really tiny ones. They can be

0:23:01.080 --> 0:23:03.119
<v Speaker 2>like ninety nine percent dark matter.

0:23:03.320 --> 0:23:06.040
<v Speaker 1>But wouldn't the larger galaxies also be better at holding

0:23:06.040 --> 0:23:09.919
<v Speaker 1>onto their dark matter, Like wouldn't smaller galaxies lose some

0:23:10.000 --> 0:23:12.600
<v Speaker 1>dark matter eventually, like it might evaporate or something.

0:23:12.680 --> 0:23:15.439
<v Speaker 2>Yeah, that depends on what dark matter does. And in

0:23:15.480 --> 0:23:18.840
<v Speaker 2>this theory, dark matter does nothing but gravity, and so

0:23:18.880 --> 0:23:21.200
<v Speaker 2>you can't really lose your dark matter. Like to lose

0:23:21.200 --> 0:23:23.160
<v Speaker 2>your dark matter, you need some force that's pushing out

0:23:23.160 --> 0:23:25.680
<v Speaker 2>on it. But gravity is just attractive. So all these

0:23:25.720 --> 0:23:28.200
<v Speaker 2>forces like the solar winds and the radiation and the

0:23:28.240 --> 0:23:30.920
<v Speaker 2>supernova basically have no impact on the dark matter. Dark

0:23:30.920 --> 0:23:33.480
<v Speaker 2>matter just like brushes it right off, Like supernova could

0:23:33.480 --> 0:23:35.639
<v Speaker 2>happen right next to you, and a dark matter particle

0:23:35.680 --> 0:23:36.920
<v Speaker 2>would be like whatever, dude.

0:23:37.080 --> 0:23:39.480
<v Speaker 1>And so we haven't seen any galaxies without dark matter,

0:23:39.560 --> 0:23:41.520
<v Speaker 1>So then why are we asking the question are there

0:23:41.560 --> 0:23:44.199
<v Speaker 1>galaxies without dark matter? Is it more of like, is

0:23:44.240 --> 0:23:46.520
<v Speaker 1>it possible to have a galaxy without dark matter? Or

0:23:47.160 --> 0:23:49.439
<v Speaker 1>are we asking like could there be galaxies where we

0:23:49.520 --> 0:23:51.160
<v Speaker 1>haven't noticed it doesn't have dark matter?

0:23:51.240 --> 0:23:53.840
<v Speaker 2>Yeah, so great question. We're curious about this for lots

0:23:53.880 --> 0:23:56.520
<v Speaker 2>of reasons, Like number one, we have a theory about

0:23:56.600 --> 0:23:58.840
<v Speaker 2>how the structure of the universe came to be and

0:23:58.880 --> 0:24:01.680
<v Speaker 2>how it made galaxy and this nice story we told

0:24:01.720 --> 0:24:05.840
<v Speaker 2>you about over densities clumping together to form galaxies et cetera,

0:24:05.880 --> 0:24:07.720
<v Speaker 2>et cetera. But we'd like to test that. We'd like

0:24:07.800 --> 0:24:11.040
<v Speaker 2>to make sure that's correct. We're often surprised when we

0:24:11.040 --> 0:24:13.920
<v Speaker 2>look out in the universe and see how things actually work,

0:24:14.560 --> 0:24:16.440
<v Speaker 2>and so what we'd like to do is check our

0:24:16.440 --> 0:24:20.000
<v Speaker 2>predictions about like the dark matter fractions of galaxies against

0:24:20.040 --> 0:24:23.359
<v Speaker 2>reality and see is this really the way things work. Also,

0:24:23.400 --> 0:24:26.280
<v Speaker 2>this really helps us understand what dark matter is, because

0:24:26.320 --> 0:24:29.080
<v Speaker 2>seeing how dark matter varies across the universe can tell

0:24:29.119 --> 0:24:31.120
<v Speaker 2>us something about the nature of dark matter and help

0:24:31.200 --> 0:24:34.800
<v Speaker 2>us test various alternative theories about what dark matter might

0:24:35.000 --> 0:24:37.040
<v Speaker 2>or might not be. But it's not exactly true that

0:24:37.080 --> 0:24:39.639
<v Speaker 2>we've never seen a galaxy without dark matter. People are

0:24:39.680 --> 0:24:41.960
<v Speaker 2>out there looking for these and they found some pretty

0:24:41.960 --> 0:24:42.720
<v Speaker 2>weird cases.

0:24:43.680 --> 0:24:46.720
<v Speaker 1>Interesting, all right, what are some of these cases?

0:24:46.880 --> 0:24:49.800
<v Speaker 2>So this galaxy group kind of nearby on cosmic scales,

0:24:49.840 --> 0:24:52.560
<v Speaker 2>that's sixty three million light years away. It's called in

0:24:52.720 --> 0:24:57.320
<v Speaker 2>GC one zero five two, and basically it's an elliptical

0:24:57.359 --> 0:25:00.199
<v Speaker 2>galaxy in the Cetus constellation. We've known about if like

0:25:00.240 --> 0:25:02.399
<v Speaker 2>two hundred and fifty years or so, but there's actually

0:25:02.400 --> 0:25:05.119
<v Speaker 2>a little group of galaxies. It's like a major galaxy

0:25:05.440 --> 0:25:08.040
<v Speaker 2>with a bunch of little galaxies nearby. They call these

0:25:08.119 --> 0:25:10.479
<v Speaker 2>dwarf galaxies, so it's a whole group, so they call

0:25:10.560 --> 0:25:13.800
<v Speaker 2>the group of galaxies. And these little galaxies are actually

0:25:13.920 --> 0:25:17.359
<v Speaker 2>ultra diffuse galaxies. That means they're galaxies that are not

0:25:17.480 --> 0:25:20.080
<v Speaker 2>very bright, they have very few stars in them. And

0:25:20.160 --> 0:25:24.160
<v Speaker 2>these ultra diffuse galaxies near this NGC group they think

0:25:24.280 --> 0:25:26.720
<v Speaker 2>might have no dark matter in them at all.

0:25:27.040 --> 0:25:27.879
<v Speaker 1>Why do they think that?

0:25:28.160 --> 0:25:30.359
<v Speaker 2>So they look at the rotations of these galaxies and

0:25:30.359 --> 0:25:33.520
<v Speaker 2>they do that calculation and they estimate zero dark matter.

0:25:34.119 --> 0:25:35.960
<v Speaker 2>Like every time you're doing this, you're not assuming the

0:25:36.040 --> 0:25:39.160
<v Speaker 2>dark matter. You're measuring it. And sometimes it comes out

0:25:39.240 --> 0:25:42.520
<v Speaker 2>ninety percent, sometimes eighty four percent, sometimes ninety nine percent.

0:25:42.600 --> 0:25:44.800
<v Speaker 2>In this case, it comes out close to zero or

0:25:44.920 --> 0:25:48.560
<v Speaker 2>consistent with zero. So they think these are little galaxies

0:25:48.560 --> 0:25:50.400
<v Speaker 2>that have no dark matter in them at all.

0:25:50.480 --> 0:25:52.240
<v Speaker 1>So that's pretty wild. That means that you can have

0:25:52.280 --> 0:25:53.520
<v Speaker 1>a galaxy without dark matter.

0:25:53.720 --> 0:25:57.360
<v Speaker 2>Yeah, it's fascinating because remember, our theory of galaxy formation

0:25:57.600 --> 0:26:00.960
<v Speaker 2>is that basically every big galaxy is a merger of

0:26:01.000 --> 0:26:03.960
<v Speaker 2>a bunch of small galaxies. Big galaxies don't like form

0:26:04.000 --> 0:26:05.920
<v Speaker 2>all at once in a single collapse. You have a

0:26:05.960 --> 0:26:08.520
<v Speaker 2>bunch of baby galaxies then merge to make bigger and

0:26:08.520 --> 0:26:11.560
<v Speaker 2>bigger galaxies, so like a bottoms up approach. And so

0:26:11.600 --> 0:26:13.520
<v Speaker 2>if your big galaxy ends up with a lot of

0:26:13.600 --> 0:26:16.040
<v Speaker 2>dark matter in it, that means that the little galaxies

0:26:16.040 --> 0:26:17.960
<v Speaker 2>that made it should each have their own dark matter.

0:26:18.080 --> 0:26:19.920
<v Speaker 2>And we look out a dwarf galaxies and we mostly

0:26:19.920 --> 0:26:21.840
<v Speaker 2>see them having dark matter. In fact, some of them

0:26:21.880 --> 0:26:23.960
<v Speaker 2>have a lot. So it is really weird to see

0:26:23.960 --> 0:26:27.000
<v Speaker 2>these little galaxies without any dark matter at all. And

0:26:27.000 --> 0:26:29.240
<v Speaker 2>the question is like, did they form this way or

0:26:29.280 --> 0:26:32.639
<v Speaker 2>did something happen to strip them of their dark matter.

0:26:32.600 --> 0:26:34.600
<v Speaker 1>Or maybe they formed later in the universe.

0:26:34.840 --> 0:26:37.959
<v Speaker 2>Yeah, exactly, And so that's a fascinating question. And so

0:26:38.000 --> 0:26:39.720
<v Speaker 2>there's a group that's done a study of these and

0:26:39.720 --> 0:26:42.240
<v Speaker 2>they have a theory about how these little diffuse galaxies

0:26:42.600 --> 0:26:44.440
<v Speaker 2>ended up without any dark matter in them.

0:26:44.600 --> 0:26:45.240
<v Speaker 1>What's a theory.

0:26:45.359 --> 0:26:47.840
<v Speaker 2>So the theory is basically a mini version of the

0:26:47.840 --> 0:26:51.120
<v Speaker 2>Bullet cluster. You remember. The Bullet cluster is this famous

0:26:51.160 --> 0:26:53.440
<v Speaker 2>example that really convinced a lot of people that dark

0:26:53.480 --> 0:26:55.679
<v Speaker 2>matter was a real thing. It was a cluster of

0:26:55.720 --> 0:26:58.840
<v Speaker 2>galaxies that collided with another cluster of galaxies and we

0:26:58.920 --> 0:27:00.679
<v Speaker 2>saw that what happened to them the gas and the

0:27:00.760 --> 0:27:03.679
<v Speaker 2>dust and the dark matter was very different. So the

0:27:03.720 --> 0:27:06.680
<v Speaker 2>gas and the dust interacted and created collisions. The dark

0:27:06.720 --> 0:27:09.080
<v Speaker 2>matter passed right through because it doesn't they direct at

0:27:09.080 --> 0:27:13.080
<v Speaker 2>that level, gravity's not strong enough, so basically separated the

0:27:13.200 --> 0:27:16.160
<v Speaker 2>dark matter from the normal matter. So the bullet cluster,

0:27:16.240 --> 0:27:17.679
<v Speaker 2>and now you have a blob in the middle with

0:27:17.720 --> 0:27:19.199
<v Speaker 2>a bunch of normal matter in it, and then you

0:27:19.280 --> 0:27:21.840
<v Speaker 2>have dark matter on both sides, so we can see

0:27:21.840 --> 0:27:24.840
<v Speaker 2>through gravitational lensing. So they think that might be similar

0:27:24.880 --> 0:27:26.919
<v Speaker 2>to what happened in this case, that maybe there was

0:27:26.960 --> 0:27:30.360
<v Speaker 2>a big collision between two other objects, and these two

0:27:30.359 --> 0:27:32.360
<v Speaker 2>things that we're seeing now they call them DF two

0:27:32.560 --> 0:27:35.440
<v Speaker 2>and DF four are basically the results of that, like

0:27:35.840 --> 0:27:39.040
<v Speaker 2>chunks of stars and gas and dust that got stripped

0:27:39.080 --> 0:27:41.800
<v Speaker 2>of their dark matter and a collision and then tossed aside.

0:27:42.280 --> 0:27:45.280
<v Speaker 1>Well, why wouldn't some little bit of dark matter go

0:27:45.400 --> 0:27:45.960
<v Speaker 1>with them.

0:27:45.800 --> 0:27:47.680
<v Speaker 2>Because the dark matter and the normal man have very

0:27:47.680 --> 0:27:51.000
<v Speaker 2>different experiences in a collision, Like dark matter basically passes

0:27:51.040 --> 0:27:53.159
<v Speaker 2>the right through. There's not really a collision when it

0:27:53.200 --> 0:27:55.640
<v Speaker 2>comes to dark matter. It's like two ghosts just phasing

0:27:55.680 --> 0:27:58.440
<v Speaker 2>through each other whereas two people bumping into a hallway

0:27:58.480 --> 0:28:01.119
<v Speaker 2>are going to change their direction. So imagine you have

0:28:01.240 --> 0:28:03.680
<v Speaker 2>like a ghost inside you and somebody else is a

0:28:03.720 --> 0:28:06.280
<v Speaker 2>ghost inside them, and you have a collision in the hallway.

0:28:06.440 --> 0:28:08.920
<v Speaker 2>The ghosts just keep on going and the living people

0:28:08.920 --> 0:28:11.560
<v Speaker 2>bounce off each other. Now you know you're separated from

0:28:11.600 --> 0:28:12.000
<v Speaker 2>your ghost.

0:28:12.160 --> 0:28:14.280
<v Speaker 1>But there's so many that they discovered. Did they all

0:28:14.320 --> 0:28:15.600
<v Speaker 1>get that way from the collision?

0:28:15.680 --> 0:28:18.080
<v Speaker 2>So they've only found these two, and they have this

0:28:18.160 --> 0:28:21.720
<v Speaker 2>reconstruction of the collision that suggests that these two things

0:28:21.760 --> 0:28:25.639
<v Speaker 2>happen somewhere near NGC and created its collision, and it

0:28:25.680 --> 0:28:28.480
<v Speaker 2>should have also created a bunch of other ultra diiffuse

0:28:28.560 --> 0:28:31.719
<v Speaker 2>galaxies that they should be able to spot. That there

0:28:31.720 --> 0:28:33.880
<v Speaker 2>should be like five or six of these that came

0:28:33.920 --> 0:28:36.720
<v Speaker 2>out of the collision that also have no dark matter.

0:28:36.960 --> 0:28:38.280
<v Speaker 2>So they're going to go and look for those.

0:28:39.000 --> 0:28:41.160
<v Speaker 1>So these are you said, these are dwarf galaxies.

0:28:41.440 --> 0:28:44.760
<v Speaker 2>Yeah, they're ultra diffuse galaxies. They're also dwarf galaxies, so

0:28:44.800 --> 0:28:46.080
<v Speaker 2>they're small and they're not.

0:28:46.200 --> 0:28:48.120
<v Speaker 1>Very bright, so in a way they kind of got

0:28:48.120 --> 0:28:50.840
<v Speaker 1>made later or not? Are they as old as the universe?

0:28:50.920 --> 0:28:54.640
<v Speaker 2>Well, they think this collision happened about eight billion years ago,

0:28:54.680 --> 0:28:57.160
<v Speaker 2>and so how you age these things, I guess depends,

0:28:57.280 --> 0:29:01.520
<v Speaker 2>like the way they are now started about billion years ago. Now,

0:29:01.560 --> 0:29:04.440
<v Speaker 2>of course, have some progenitor or something that they came from. Right,

0:29:04.480 --> 0:29:06.800
<v Speaker 2>there was a larger object they were a part of

0:29:07.120 --> 0:29:09.760
<v Speaker 2>which definitely had dark matter in it, and their dark

0:29:09.800 --> 0:29:12.360
<v Speaker 2>matter is now sprayed in some other direction. So they

0:29:12.360 --> 0:29:15.440
<v Speaker 2>become separated from their dark matter. And this must have

0:29:15.480 --> 0:29:18.080
<v Speaker 2>been a pretty mammoth event. I mean, they reconstruct this

0:29:18.120 --> 0:29:21.000
<v Speaker 2>thing and it's like a collision at three hundred kilometers

0:29:21.040 --> 0:29:23.880
<v Speaker 2>per second of these huge cosmic objects.

0:29:24.000 --> 0:29:27.840
<v Speaker 1>Well, so these are galaxies that had dark matter, but

0:29:27.880 --> 0:29:30.160
<v Speaker 1>then they got stripped away of their dark matter, and

0:29:30.200 --> 0:29:32.040
<v Speaker 1>so that shows that, hey, you can have a galaxy

0:29:32.640 --> 0:29:34.920
<v Speaker 1>without dark matter. But I guess maybe the larger question

0:29:35.160 --> 0:29:38.200
<v Speaker 1>is can you form a galaxy without dark matter?

0:29:38.440 --> 0:29:40.440
<v Speaker 2>Right? And so as you say, that's the deep question

0:29:40.520 --> 0:29:43.360
<v Speaker 2>about the nature of the formation of structure in the universe,

0:29:43.920 --> 0:29:46.000
<v Speaker 2>and so far the answer to that is no, we

0:29:46.080 --> 0:29:48.400
<v Speaker 2>do not think it's possible to form a galaxy without

0:29:48.480 --> 0:29:50.600
<v Speaker 2>dark matter. We think you need that dark matter around

0:29:50.600 --> 0:29:53.000
<v Speaker 2>to gather enough gas and dust to make stars and

0:29:53.120 --> 0:29:56.640
<v Speaker 2>to make a galaxy that without dark matter, normal matter

0:29:56.680 --> 0:29:59.600
<v Speaker 2>doesn't have enough gravity to have formed galaxies this early

0:29:59.600 --> 0:30:02.160
<v Speaker 2>in the UNI. If you had a universe without dark matter,

0:30:02.400 --> 0:30:04.600
<v Speaker 2>or big section of it without dark matter and just

0:30:04.680 --> 0:30:07.840
<v Speaker 2>normal matter, it would form galaxies eventually, but it would

0:30:07.840 --> 0:30:09.080
<v Speaker 2>take a lot longer to do so.

0:30:09.280 --> 0:30:11.280
<v Speaker 1>But you know, as I understand it, and during the

0:30:11.280 --> 0:30:14.520
<v Speaker 1>Big Bang, things were really hot and dense, and there

0:30:14.520 --> 0:30:17.320
<v Speaker 1>were pockets of things, and there were quantum fluctuations which

0:30:17.360 --> 0:30:21.560
<v Speaker 1>maybe created pockets of extra densities here and there. Couldn't

0:30:21.560 --> 0:30:24.440
<v Speaker 1>there have been a pocket of extra density of normal

0:30:24.480 --> 0:30:27.200
<v Speaker 1>stuff but not dark matter. That then when the universe

0:30:27.240 --> 0:30:30.400
<v Speaker 1>blew up, it became a galaxy without dark matter, like

0:30:30.480 --> 0:30:33.320
<v Speaker 1>during the Big Bang. Why does the normal matter have

0:30:33.440 --> 0:30:34.760
<v Speaker 1>to follow the dark matter?

0:30:34.880 --> 0:30:38.120
<v Speaker 2>I guess it does in general because dark matter dominates

0:30:38.160 --> 0:30:40.320
<v Speaker 2>because it's just so much more of it. So it

0:30:40.320 --> 0:30:42.840
<v Speaker 2>basically like sets the scene for everything. But you're right,

0:30:42.880 --> 0:30:46.120
<v Speaker 2>it's theoretically possible to have a downward fluctuation in the

0:30:46.200 --> 0:30:49.120
<v Speaker 2>dark matter and an upward fluctuation in the normal matter.

0:30:49.480 --> 0:30:51.480
<v Speaker 2>So you get some region of space where you have

0:30:51.600 --> 0:30:54.920
<v Speaker 2>like extra super dense normal matter and almost no dark matter.

0:30:55.280 --> 0:30:58.080
<v Speaker 2>That's possible. Yeah, and so in principle that could happen,

0:30:58.080 --> 0:31:00.520
<v Speaker 2>and if it had enough matter, then it would form

0:31:00.560 --> 0:31:03.640
<v Speaker 2>a galaxy on its own. So in principle that's not impossible,

0:31:03.840 --> 0:31:05.880
<v Speaker 2>but we've never seen that, and I don't know what

0:31:05.960 --> 0:31:08.640
<v Speaker 2>the chances are of that happening theoretically.

0:31:08.120 --> 0:31:10.200
<v Speaker 1>Like, how many galaxies have we've done this calculation to

0:31:10.200 --> 0:31:11.800
<v Speaker 1>make sure that it has dark matter in it.

0:31:12.000 --> 0:31:15.200
<v Speaker 2>Yeah, that's a great question. We've measured the rotation velocity

0:31:15.240 --> 0:31:18.600
<v Speaker 2>of thousands and thousands of galaxies, but that's a tiny

0:31:18.640 --> 0:31:21.120
<v Speaker 2>fraction of the number of galaxies that are out there.

0:31:21.120 --> 0:31:23.160
<v Speaker 2>In the number of galaxies we can see, most of

0:31:23.200 --> 0:31:25.320
<v Speaker 2>the galaxies we can see, we can't measure their rotation

0:31:25.440 --> 0:31:28.840
<v Speaker 2>velocity because you're looking at like one pixel or two pixels.

0:31:29.160 --> 0:31:30.720
<v Speaker 2>You need to be able to sort of resolve the

0:31:30.760 --> 0:31:33.520
<v Speaker 2>whole galaxies you can see like from one side versus

0:31:33.640 --> 0:31:36.200
<v Speaker 2>light from the other side. So it's tricky. But yeah,

0:31:36.200 --> 0:31:38.880
<v Speaker 2>we haven't looked at that many galaxies as possible. There

0:31:38.880 --> 0:31:41.920
<v Speaker 2>are galaxies out there that did really form without dark matter.

0:31:42.480 --> 0:31:44.200
<v Speaker 1>All right, Well, it seems like we kind of answered

0:31:44.200 --> 0:31:47.000
<v Speaker 1>the question of the episode, which is can you have

0:31:47.040 --> 0:31:49.760
<v Speaker 1>a galaxy without dark matter? The answer is yes, you

0:31:49.760 --> 0:31:52.840
<v Speaker 1>can have maybe galaxies that had dark matter, but then

0:31:52.840 --> 0:31:56.080
<v Speaker 1>they lose it or they get left behind by the

0:31:56.160 --> 0:31:59.760
<v Speaker 1>dark matter and so they're dark matter less. Or maybe

0:31:59.800 --> 0:32:01.720
<v Speaker 1>they could have forded at the beginning of the universe

0:32:01.800 --> 0:32:04.600
<v Speaker 1>in theory, but we haven't seen one yet. All right, Well,

0:32:04.680 --> 0:32:07.800
<v Speaker 1>let's dig into what this all means about our understanding

0:32:07.880 --> 0:32:10.840
<v Speaker 1>of dark matter and also gravity and whether or not

0:32:10.960 --> 0:32:14.520
<v Speaker 1>it needs to be overhauled. But first, let's take another

0:32:14.600 --> 0:32:29.920
<v Speaker 1>quick break. Orright, we're asking the question can there be

0:32:30.120 --> 0:32:35.600
<v Speaker 1>a galaxy with no dark chocolate? And the answer is probably,

0:32:35.640 --> 0:32:37.880
<v Speaker 1>But that's not a universe Daniel wants to live in.

0:32:38.400 --> 0:32:41.720
<v Speaker 2>No, that's right, Transport me somewhere else in the multiverse asap.

0:32:42.000 --> 0:32:43.560
<v Speaker 1>What if you end up in the universe where there's

0:32:43.640 --> 0:32:45.120
<v Speaker 1>only milk or white chocolate?

0:32:46.000 --> 0:32:48.240
<v Speaker 2>Just keep smashing that button until I get somewhere good.

0:32:48.640 --> 0:32:49.800
<v Speaker 1>No, No, you only get one trim.

0:32:49.880 --> 0:32:53.920
<v Speaker 2>Would you take the risk? I'm pretty happy with our universe,

0:32:53.960 --> 0:32:54.200
<v Speaker 2>you know.

0:32:54.240 --> 0:32:57.440
<v Speaker 1>It's a pretty good one. We're talking about whether galaxies

0:32:57.480 --> 0:33:00.560
<v Speaker 1>can exist without dark matter, and the answers yes, they

0:33:00.560 --> 0:33:03.160
<v Speaker 1>can be stripped away of their dark matter, or theoretically

0:33:03.280 --> 0:33:05.360
<v Speaker 1>they could form in the early universe, but we haven't

0:33:05.400 --> 0:33:08.400
<v Speaker 1>seen one yet, and so maybe probably not. What do

0:33:08.400 --> 0:33:10.840
<v Speaker 1>you think happened? Why haven't we seen any? If they

0:33:10.880 --> 0:33:13.080
<v Speaker 1>can form without dark matter, why haven't we seen any.

0:33:13.240 --> 0:33:15.280
<v Speaker 2>I think that'd be really unlikely. I mean, the kind

0:33:15.320 --> 0:33:18.360
<v Speaker 2>of fluctuations we're talking about are very, very large. In

0:33:18.440 --> 0:33:20.800
<v Speaker 2>the early universe, you had just sort of like energy,

0:33:20.960 --> 0:33:24.360
<v Speaker 2>and then it decays into matter. As the universe expands

0:33:24.360 --> 0:33:26.920
<v Speaker 2>in cools, and every kind of matter is sort of

0:33:26.960 --> 0:33:29.600
<v Speaker 2>made uniformly, so you get more dark matter made and

0:33:29.680 --> 0:33:32.120
<v Speaker 2>less normal matter. But in order to have no dark

0:33:32.120 --> 0:33:35.200
<v Speaker 2>matter made, you'd need a really big fluctuation. You expect

0:33:35.200 --> 0:33:37.400
<v Speaker 2>to get like eighty five percent and you get zero.

0:33:37.560 --> 0:33:39.400
<v Speaker 2>It's like flipping a coin one hundred times in a

0:33:39.480 --> 0:33:42.160
<v Speaker 2>row and getting only heads instead of half heads and

0:33:42.200 --> 0:33:45.440
<v Speaker 2>half tails. It's pretty unlikely. Now the universe is really big,

0:33:45.440 --> 0:33:47.600
<v Speaker 2>of course, so that means that eventually it's going to happen,

0:33:47.680 --> 0:33:51.360
<v Speaker 2>especially if the universe is infinite. But it's so unlikely

0:33:51.720 --> 0:33:53.080
<v Speaker 2>that it's not going to be the first kind of

0:33:53.080 --> 0:33:55.480
<v Speaker 2>galaxy we see out there, or even in the first tranch.

0:33:56.000 --> 0:33:57.440
<v Speaker 2>Eventually we might spot one.

0:33:57.520 --> 0:33:59.240
<v Speaker 1>Well, as you said, it is possible to have a

0:33:59.240 --> 0:34:02.720
<v Speaker 1>galaxy without dark matter. We've seen it in some ultra

0:34:02.760 --> 0:34:06.600
<v Speaker 1>diffuse galaxies and they've done a measurement on these galaxies, right,

0:34:06.920 --> 0:34:09.319
<v Speaker 1>They've measured how fast it's spinning, and they're pretty sure

0:34:09.320 --> 0:34:10.640
<v Speaker 1>there's no dark matter in them.

0:34:10.719 --> 0:34:13.480
<v Speaker 2>Yeah, there were a series of papers where people said, oh,

0:34:13.560 --> 0:34:15.840
<v Speaker 2>there's no dark matter, and another group did a different

0:34:15.840 --> 0:34:18.200
<v Speaker 2>measurement said no, there is some dark matter. And then

0:34:18.200 --> 0:34:20.840
<v Speaker 2>there were follow up papers arguing, and now they're pretty

0:34:20.840 --> 0:34:23.200
<v Speaker 2>sure there's no dark matter in these but there's always

0:34:23.200 --> 0:34:25.759
<v Speaker 2>somebody out there who disagrees. I mean, it's astronomy, after all.

0:34:25.800 --> 0:34:27.000
<v Speaker 1>That's right, they're all just guessing.

0:34:28.920 --> 0:34:31.719
<v Speaker 2>They're all just doing their best statistical inference.

0:34:33.160 --> 0:34:37.680
<v Speaker 1>That's right. That's a great word for guessing. I'm just kidding.

0:34:38.719 --> 0:34:41.640
<v Speaker 1>A best guessing. How about that best guessing. Nobody has

0:34:41.640 --> 0:34:42.280
<v Speaker 1>a better guess.

0:34:42.360 --> 0:34:44.400
<v Speaker 2>You know, those whole departments of people who do nothing

0:34:44.440 --> 0:34:48.759
<v Speaker 2>but statistics for a living. I'm trashing Hershey's, but you're

0:34:48.800 --> 0:34:50.080
<v Speaker 2>trashing statistics. Man.

0:34:50.800 --> 0:34:53.720
<v Speaker 1>No, there's nothing wrong with guessing wrong.

0:34:55.400 --> 0:34:56.880
<v Speaker 2>How do you know the universe is the way it is.

0:34:57.000 --> 0:34:57.800
<v Speaker 2>We're just guessing.

0:34:58.040 --> 0:35:00.760
<v Speaker 1>A best guess doesn't mean that you're making thing. So randomly.

0:35:00.840 --> 0:35:03.200
<v Speaker 1>You're just using the best information you have to make

0:35:03.320 --> 0:35:05.760
<v Speaker 1>a best estimate or inference, right.

0:35:05.640 --> 0:35:08.040
<v Speaker 2>I suppose so. I think if it's very well informed,

0:35:08.040 --> 0:35:09.719
<v Speaker 2>it's not really a guess, you know.

0:35:09.800 --> 0:35:12.960
<v Speaker 1>But you're one hundred percent sure it's also not a fact.

0:35:13.040 --> 0:35:15.840
<v Speaker 2>Yeah, that's true. That's why we use statistics to describe

0:35:15.920 --> 0:35:19.960
<v Speaker 2>our uncertainties. Anyway. One of the things we are uncertain

0:35:20.000 --> 0:35:22.799
<v Speaker 2>about is the nature of dark matter. Like, a lot

0:35:22.840 --> 0:35:24.600
<v Speaker 2>of the stuff we talk about for dark matter is

0:35:24.719 --> 0:35:27.560
<v Speaker 2>kind of unsatisfyingly indirect, and a lot of people out

0:35:27.600 --> 0:35:30.920
<v Speaker 2>there treat dark matter like it's some placeholder, not a

0:35:30.960 --> 0:35:34.960
<v Speaker 2>real theory of the universe because we never see it directly.

0:35:35.000 --> 0:35:37.600
<v Speaker 2>We don't can't really grapple with it and grasp.

0:35:37.280 --> 0:35:39.600
<v Speaker 1>It directly, right right. Well, I think it's interesting that

0:35:39.680 --> 0:35:44.120
<v Speaker 1>we have or that astronomers have found galaxies without dark matter,

0:35:44.160 --> 0:35:46.280
<v Speaker 1>because it almost gives you kind of like a test

0:35:46.360 --> 0:35:49.000
<v Speaker 1>case to confirm that the other galaxies that we have

0:35:49.040 --> 0:35:51.480
<v Speaker 1>seen with dark matter actually have dark matter and it's

0:35:51.560 --> 0:35:55.080
<v Speaker 1>not just some weird, you know, fluke or mistake in

0:35:55.120 --> 0:35:56.440
<v Speaker 1>our theory of gravity.

0:35:56.719 --> 0:35:59.200
<v Speaker 2>Mm hmm. Yeah, it's a cool test case. It's like

0:35:59.239 --> 0:36:02.839
<v Speaker 2>a control, right, would you see galaxies without dark matter

0:36:02.920 --> 0:36:04.719
<v Speaker 2>if they were there. So it's nice to have some

0:36:04.880 --> 0:36:07.920
<v Speaker 2>verification that we're seeing that. It's also, as you say,

0:36:07.960 --> 0:36:11.640
<v Speaker 2>a great test bed for comparing various theories of dark matter,

0:36:11.920 --> 0:36:14.800
<v Speaker 2>which make different predictions about what would happen in these scenarios.

0:36:14.960 --> 0:36:15.400
<v Speaker 3>Mm.

0:36:15.480 --> 0:36:17.520
<v Speaker 1>Yeah, different guesses about dark.

0:36:17.320 --> 0:36:21.239
<v Speaker 2>Matter, different ideas, different theoretical.

0:36:22.840 --> 0:36:25.520
<v Speaker 1>All right, well, how do these galaxies help us decide

0:36:25.600 --> 0:36:26.880
<v Speaker 1>what dark matter is made out of.

0:36:27.040 --> 0:36:29.520
<v Speaker 2>Well, one of the most popular alternatives to dark matter

0:36:29.640 --> 0:36:32.080
<v Speaker 2>as a theory of matter, some kind of stuff in

0:36:32.120 --> 0:36:35.920
<v Speaker 2>the universe is an alternative theory of gravity to say, well,

0:36:35.920 --> 0:36:38.240
<v Speaker 2>there's no other stuff in the universe. We're seeing everything

0:36:38.280 --> 0:36:41.520
<v Speaker 2>there is. It's just that gravity works differently from what

0:36:41.600 --> 0:36:44.480
<v Speaker 2>we expected. Because remember, the argument for dark matter is like,

0:36:44.560 --> 0:36:47.160
<v Speaker 2>we understand gravity, and there's a lot more gravity than

0:36:47.200 --> 0:36:49.319
<v Speaker 2>we can explain with the visible stuff, So there must

0:36:49.320 --> 0:36:52.480
<v Speaker 2>be more stuff. There must be invisible stuff creating that gravity.

0:36:53.000 --> 0:36:55.920
<v Speaker 2>But what if instead we just don't understand how gravity

0:36:55.960 --> 0:36:59.560
<v Speaker 2>works and it can be explained by all the visible

0:36:59.560 --> 0:37:02.080
<v Speaker 2>stuff if you tweak your theory of gravity.

0:37:01.800 --> 0:37:04.800
<v Speaker 1>Meaning like what if gravity just gets stronger, The bigger

0:37:04.840 --> 0:37:08.480
<v Speaker 1>the distances that might account for why galaxies are holding

0:37:08.520 --> 0:37:12.240
<v Speaker 1>on together without needing dark matter. That's kind of the idea, right.

0:37:12.120 --> 0:37:15.000
<v Speaker 2>That's kind of the idea. More specifically, there's this theory

0:37:15.040 --> 0:37:20.600
<v Speaker 2>called mond modified Newtonian dynamics that suggests that gravity's mostly

0:37:20.719 --> 0:37:23.720
<v Speaker 2>like Newton described, but there are some tweaks. It depends

0:37:23.760 --> 0:37:27.680
<v Speaker 2>on the acceleration of these objects, and for some accelerations,

0:37:27.719 --> 0:37:30.480
<v Speaker 2>gravity gets stronger or weaker, and you know, it's a

0:37:30.480 --> 0:37:33.880
<v Speaker 2>little baroquely like added these terms and these tweaks basically

0:37:33.920 --> 0:37:36.839
<v Speaker 2>to explain these rotation curves, to say like, oh, these

0:37:36.880 --> 0:37:39.279
<v Speaker 2>stars are accelerating more than those stars. So if we

0:37:39.680 --> 0:37:42.600
<v Speaker 2>change the way gravity works, can we describe the rotation

0:37:42.719 --> 0:37:45.719
<v Speaker 2>curves that we see. The answer is yes, you can

0:37:45.960 --> 0:37:49.720
<v Speaker 2>devise a theory to describe the rotation curves that explain

0:37:49.760 --> 0:37:53.000
<v Speaker 2>how these galaxies are rotating without needing dark matter. If

0:37:53.040 --> 0:37:55.719
<v Speaker 2>you tweak gravity, right, you have to tweak something, either

0:37:55.800 --> 0:37:57.719
<v Speaker 2>change the amount of matter that's there, or you change

0:37:57.719 --> 0:37:58.640
<v Speaker 2>the way gravity works.

0:37:58.800 --> 0:38:00.680
<v Speaker 1>Well, first of all, you just saw of the whole

0:38:00.680 --> 0:38:02.560
<v Speaker 1>period of human history, the Baroque period.

0:38:03.520 --> 0:38:06.600
<v Speaker 2>I meant that in a positive way, right right.

0:38:07.520 --> 0:38:09.760
<v Speaker 1>The second of lie, I know that we've talked about before.

0:38:09.840 --> 0:38:13.680
<v Speaker 1>How you know, maybe Mond modified Newtonian dynamics, it could

0:38:13.719 --> 0:38:16.799
<v Speaker 1>replace dark matter. But we confirm dark matter in other

0:38:16.840 --> 0:38:17.920
<v Speaker 1>ways right exactly.

0:38:17.960 --> 0:38:20.600
<v Speaker 2>So Mond is a success in describing the rotations of

0:38:20.640 --> 0:38:26.520
<v Speaker 2>galaxies by making these beautiful baroque extensions to Newton's theory.

0:38:26.640 --> 0:38:28.680
<v Speaker 2>But there's lots of other ways we've seen dark matter,

0:38:28.920 --> 0:38:31.680
<v Speaker 2>like in the ripples of the cosmic microwave background from

0:38:31.719 --> 0:38:34.359
<v Speaker 2>the very early universe. We can see how that early

0:38:34.440 --> 0:38:38.560
<v Speaker 2>universe plasma sloshing around, and that depends very sensitively on

0:38:38.600 --> 0:38:41.920
<v Speaker 2>the amount of dark matter, which slashes differently in that

0:38:42.040 --> 0:38:46.120
<v Speaker 2>plasma than normal matter did, and Mond cannot explain that. Also,

0:38:46.160 --> 0:38:48.520
<v Speaker 2>the Bullet cluster shows us that dark matter can be

0:38:48.640 --> 0:38:52.120
<v Speaker 2>separated from normal matter. It's not just a different way

0:38:52.120 --> 0:38:55.320
<v Speaker 2>that gravity works for normal matter. It really is something

0:38:55.360 --> 0:38:58.440
<v Speaker 2>else with its own gravity. So Mond really struggles to

0:38:58.560 --> 0:39:01.520
<v Speaker 2>explain everything that the theory of dark matter can explain,

0:39:01.640 --> 0:39:04.319
<v Speaker 2>but it's still a popular alternative, and this is another way.

0:39:04.360 --> 0:39:06.879
<v Speaker 2>These dark matter free galaxies are another way to draw

0:39:07.000 --> 0:39:10.200
<v Speaker 2>contrast between what Mond predicts and what dark matter predict

0:39:10.200 --> 0:39:12.400
<v Speaker 2>because according to Mond, there is no dark matter and

0:39:12.440 --> 0:39:15.239
<v Speaker 2>gravity only depends on the visible matter. And so these

0:39:15.239 --> 0:39:18.000
<v Speaker 2>altered diffuse galaxies with no dark matter should behave the

0:39:18.000 --> 0:39:20.480
<v Speaker 2>same way all the other galaxies do because there's no

0:39:20.560 --> 0:39:22.839
<v Speaker 2>dark matter in any of them. But we do see

0:39:22.840 --> 0:39:27.040
<v Speaker 2>a difference. We see that these guys are rotating more slowly, right,

0:39:27.080 --> 0:39:30.680
<v Speaker 2>and so Mond struggles to explain how slowly rotating these

0:39:30.719 --> 0:39:34.120
<v Speaker 2>galaxies are without any dark matter, whereas dark matter can

0:39:34.160 --> 0:39:35.839
<v Speaker 2>explain all of it. It's like, well, this has more

0:39:35.920 --> 0:39:38.399
<v Speaker 2>dark matter, that has less dark matter. So because dark

0:39:38.440 --> 0:39:41.759
<v Speaker 2>matter can be variable in the universe, some galaxies have

0:39:41.840 --> 0:39:44.800
<v Speaker 2>more and some have less. Whereas the rules of gravity

0:39:44.840 --> 0:39:47.359
<v Speaker 2>have to be the same, Mind is sort of hamstrung

0:39:47.640 --> 0:39:51.319
<v Speaker 2>and can't really explain the variation of all these galaxies.

0:39:51.560 --> 0:39:53.200
<v Speaker 1>Well, I feel like Mond was already kind of dead

0:39:53.200 --> 0:39:55.720
<v Speaker 1>in the water for all these other reasons for a while.

0:39:55.920 --> 0:39:58.440
<v Speaker 1>But it is kind of interesting that seeing a galaxy

0:39:58.480 --> 0:40:01.800
<v Speaker 1>without dark matter almost kind of helps prove that it exists.

0:40:01.880 --> 0:40:03.680
<v Speaker 2>Yeah, that is really interesting, and I agree with you

0:40:03.719 --> 0:40:06.280
<v Speaker 2>that Mond is not a theory we should take terribly seriously.

0:40:06.280 --> 0:40:10.160
<v Speaker 2>In dark matter is overwhelming the better guess for what's

0:40:10.200 --> 0:40:11.120
<v Speaker 2>going on in the universe.

0:40:11.200 --> 0:40:13.480
<v Speaker 1>Yeah, there you go. See, I brought you on board.

0:40:15.239 --> 0:40:16.120
<v Speaker 2>I'm loving that word.

0:40:16.200 --> 0:40:18.040
<v Speaker 1>Now you're like, I guess, I guess so.

0:40:18.320 --> 0:40:20.640
<v Speaker 2>But you know, full caveats. There are some things that

0:40:20.719 --> 0:40:22.920
<v Speaker 2>dark matter can't explain. There are a few galaxies out

0:40:22.920 --> 0:40:24.959
<v Speaker 2>there that don't make any sense that no dark matter

0:40:25.000 --> 0:40:27.839
<v Speaker 2>can really explain. Some people think that some hybrid like

0:40:28.160 --> 0:40:30.640
<v Speaker 2>mostly dark matter with a little bit of mind is

0:40:30.640 --> 0:40:32.680
<v Speaker 2>what we need to explain everything that's out there in

0:40:32.680 --> 0:40:35.200
<v Speaker 2>the universe, and so it's best to keep an open mind.

0:40:35.239 --> 0:40:37.400
<v Speaker 2>It's also always nice to find a new way to

0:40:37.480 --> 0:40:40.360
<v Speaker 2>test our understanding of dark matter and gravity in general,

0:40:40.760 --> 0:40:43.239
<v Speaker 2>and so these galaxies without dark matter are a nice

0:40:43.280 --> 0:40:43.920
<v Speaker 2>test bed.

0:40:43.719 --> 0:40:46.719
<v Speaker 1>For that interesting you could call the new theory darkmond

0:40:48.280 --> 0:40:51.720
<v Speaker 1>all right, well, another interesting example of how the universe

0:40:51.800 --> 0:40:54.319
<v Speaker 1>just always has surprises. Like you think that maybe you

0:40:54.520 --> 0:40:58.279
<v Speaker 1>need dark matter to have a galaxy, but only one

0:40:58.320 --> 0:41:00.600
<v Speaker 1>day you find galaxies without dark matter, and it makes

0:41:00.600 --> 0:41:03.319
<v Speaker 1>you think, and it actually maybe helps you confirm the

0:41:03.360 --> 0:41:05.960
<v Speaker 1>existence of something as mysterious as dark.

0:41:05.840 --> 0:41:07.920
<v Speaker 2>Matter, and it goes to show you that the universe

0:41:07.960 --> 0:41:10.719
<v Speaker 2>does all these experiments for us. We can just look

0:41:10.800 --> 0:41:13.080
<v Speaker 2>up in the night sky and find the examples of

0:41:13.120 --> 0:41:17.000
<v Speaker 2>galaxies smashing into other galaxies or black holes colliding. All

0:41:17.120 --> 0:41:20.600
<v Speaker 2>these things are wonderful experiments that help reveal the nature

0:41:20.640 --> 0:41:22.799
<v Speaker 2>of the universe, the rules that it follows, and how

0:41:22.840 --> 0:41:23.799
<v Speaker 2>it all works.

0:41:24.200 --> 0:41:25.719
<v Speaker 1>Yeah, I guess.

0:41:27.880 --> 0:41:31.879
<v Speaker 2>Or you guess in the end, aren't we all just guessing? Man?

0:41:32.400 --> 0:41:35.360
<v Speaker 1>All right, Well, we hope you enjoyed that. Thanks for

0:41:35.440 --> 0:41:37.399
<v Speaker 1>joining us, See you next time.

0:41:42.600 --> 0:41:45.759
<v Speaker 2>For more science and curiosity, come find us on social media,

0:41:45.880 --> 0:41:49.360
<v Speaker 2>where we answer questions and post video. We're on Twitter

0:41:49.440 --> 0:41:52.360
<v Speaker 2>at this word instant and now TikTok. And remember that

0:41:52.520 --> 0:41:56.320
<v Speaker 2>Daniel and Jorge Explain the Universe is a production of iHeartRadio.

0:41:56.840 --> 0:42:00.520
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0:42:09.120 --> 0:42:09.440
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