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Are there galaxies without dark matter? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, most of the stuff in the universe is
• 0:11
  Speaker 1: dark matter, isn't that right?
• 0:12
  Speaker 2: Yeah, it's about eighty percent of the matter in the universe.
• 0:16
  Speaker 1: And it's all around us, like right here with us
• 0:18
  Speaker 1: immersed in dark matter.
• 0:20
  Speaker 2: Yeah, we are swimming in it.
• 0:22
  Speaker 1: And it's also inside of us, is it?
• 0:24
  Speaker 2: Yeah? It passes through us, it doesn't bounce off your skin.
• 0:27
  Speaker 1: Does that mean that we're partly made out of dark matter? Oh?
• 0:30
  Speaker 2: I guess. So that's kind of dark to think about.
• 0:33
  Speaker 1: So if I discover myself, I'm discovering dark matter.
• 0:36
  Speaker 2: Know thyself. Win a Nobel Prize.
• 0:38
  Speaker 1: Or at least a dark Nobel Prize. Technically all Nobel
• 0:41
  Speaker 1: prizes have dark matter inside of them too.
• 0:43
  Speaker 2: Maybe they'll give you the cash in dark money.
• 0:45
  Speaker 1: Ooh, that got dark quick.
• 1:02
  Speaker 3: Hi.
• 1:02
  Speaker 1: I'm Hoeham and cartoonists and the author of Oliver's Great
• 1:05
  Speaker 1: Big Universe.
• 1:06
  Speaker 2: Hi. I'm Daniel. I'm a particle physicist at CERN and
• 1:09
  Speaker 2: a professor at UC Irvine. And I'll take your money
• 1:12
  Speaker 2: dark or light.
• 1:14
  Speaker 1: Really, you'll take dark money. Isn't that dangerous for a
• 1:16
  Speaker 1: tenured professor?
• 1:17
  Speaker 2: No, that's what ten years for man. I can take money.
• 1:19
  Speaker 1: From us visuals. I guess technically, the university takes the
• 1:26
  Speaker 1: money and you get a cut of it.
• 1:27
  Speaker 2: I never really had to worry about that. Like, nobody
• 1:30
  Speaker 2: who got wealthy with dubious techniques ever offered me a
• 1:33
  Speaker 2: slush fund, So I've never had to really grapple with
• 1:36
  Speaker 2: that question.
• 1:37
  Speaker 1: Wait, how do you know? I mean, you've taken money.
• 1:39
  Speaker 1: Have you done your due diligence? How deep did you go?
• 1:44
  Speaker 2: You know, almost all my research money comes from the government,
• 1:46
  Speaker 2: and we all know the various crimes that the government
• 1:49
  Speaker 2: has committed.
• 1:50
  Speaker 1: Exactly all government money is dark money.
• 1:53
  Speaker 2: M do you believe taxes are theft? Is that where
• 1:55
  Speaker 2: this is going?
• 1:57
  Speaker 1: Well, technically, or funding comes from taxpayers and there must
• 2:00
  Speaker 1: be some sketchy tax payers out there, so your work
• 2:03
  Speaker 1: is painted.
• 2:04
  Speaker 2: So you're saying drug dealers who are paying taxes and
• 2:07
  Speaker 2: indirectly funding my research have made me complicit in their crimes.
• 2:11
  Speaker 1: That's right. Yes, assuming drug dealers pay taxes, which I
• 2:15
  Speaker 1: guess they do. If they're laundering money, I don't know.
• 2:17
  Speaker 1: I feel like we need a whole podcast episode just
• 2:19
  Speaker 1: to cover dark money.
• 2:21
  Speaker 2: Yeah, and then I suppose everybody's guilty.
• 2:23
  Speaker 1: That's right, We're all guilty of paying taxes, I guess.
• 2:27
  Speaker 1: But anyways, Welcome to our podcast Daniel and Jorge Explain
• 2:30
  Speaker 1: the Universe, a production of iHeartRadio.
• 2:32
  Speaker 2: In which we try not to overtax your brain while
• 2:35
  Speaker 2: explaining all of the mysteries of the universe. We want
• 2:38
  Speaker 2: to reveal the light universe and the dark universe, the
• 2:41
  Speaker 2: visible and the invisible. We want to show you how
• 2:44
  Speaker 2: the universe is not just what you see around you
• 2:47
  Speaker 2: and experience day to day, but so much more, so
• 2:50
  Speaker 2: much deeper, so many more mysteries waiting to be solved.
• 2:54
  Speaker 1: That's why we explore the universe, the dark corners of it,
• 2:57
  Speaker 1: the light corners, and the sketchy corners of it, because
• 3:00
  Speaker 1: our understanding of the universe is still a little bit sketchy, like.
• 3:04
  Speaker 2: The Mafia corners of the universe.
• 3:06
  Speaker 1: No, like, not clearly drawn.
• 3:08
  Speaker 2: Oh I see, I thought we're gonna be talking about
• 3:10
  Speaker 2: like the physics of New Jersey or something.
• 3:12
  Speaker 1: Yeah, there's the only some dark matters going on out there.
• 3:17
  Speaker 2: How do you eat all those rich cookies and not
• 3:19
  Speaker 2: gain weight? Really, it's amazing.
• 3:20
  Speaker 1: Yeah, Well, I think the secret is dark chocolate. It's
• 3:24
  Speaker 1: lower and fat.
• 3:25
  Speaker 2: Isn't it?
• 3:26
  Speaker 1: Is it?
• 3:26
  Speaker 2: Really? I don't know.
• 3:27
  Speaker 1: You're like, what, what, Why aren't I eating more dark chocolate.
• 3:33
  Speaker 2: I'm not really stopping myself from eating dark chocolate. But
• 3:35
  Speaker 2: that wasn't the reason I don't see dark chocolate is
• 3:38
  Speaker 2: like diet chocolate.
• 3:40
  Speaker 1: It's chocolate light. Oh no, wait, it's chocolate dark.
• 3:43
  Speaker 2: It's dark chocolate exactly. But there are lots of fun
• 3:47
  Speaker 2: questions out there in the universe, not just about how
• 3:49
  Speaker 2: waste management organizations in New Jersey are getting their money
• 3:53
  Speaker 2: and the calories that Daniel's eating. We're wondering about the
• 3:56
  Speaker 2: biggest questions in the universe, like where is all the stuff?
• 3:59
  Speaker 2: What is most of the universe made out of? But
• 4:01
  Speaker 2: how has it shaped the night sky that we see
• 4:03
  Speaker 2: today in the galaxy that we live in.
• 4:05
  Speaker 1: That's right, because what the universe is made out of
• 4:08
  Speaker 1: is maybe one of the biggest questions we can ask
• 4:10
  Speaker 1: about the universe. What is this whole place made out of?
• 4:13
  Speaker 1: What are we made out of? Are we made out
• 4:15
  Speaker 1: of dark matter? Can we exist without dark matter?
• 4:18
  Speaker 2: And we talk on this podcast a lot about dark matter.
• 4:20
  Speaker 2: We have lots of episodes about what it is and
• 4:22
  Speaker 2: where it is and how it works. And one thing
• 4:25
  Speaker 2: we often stress is that dark matter is part of
• 4:28
  Speaker 2: our galaxy, that most of our galaxy is actually dark
• 4:31
  Speaker 2: matter that has played a big role in the formation
• 4:33
  Speaker 2: of our galaxy. We wouldn't be here without it.
• 4:35
  Speaker 1: Yeah, we do talk a lot about dark matter. Daniel,
• 4:38
  Speaker 1: What percentage of our episodes would you say we talk
• 4:42
  Speaker 1: about dark matter? Is it representative of the amount of
• 4:45
  Speaker 1: dark matter in the universe? Are we ignoring dark matter
• 4:47
  Speaker 1: in a way?
• 4:48
  Speaker 2: Is dark matter underrepresented matter on podcasts? That's a good question.
• 4:52
  Speaker 1: Yeah, well, especially our podcast. If we're trying to explain
• 4:55
  Speaker 1: the universe and the universe is twenty seven percent dark matter.
• 4:58
  Speaker 2: You know, we might reach twenty seven percent of our
• 5:00
  Speaker 2: podcast being about dark matter. It's been a lot of them,
• 5:03
  Speaker 2: and I'm pretty sure we don't have two thirds of
• 5:05
  Speaker 2: them about dark energy. Maybe we should.
• 5:07
  Speaker 1: Yeah, yeah, that's what everyone is asking for more dark
• 5:10
  Speaker 1: energy episodes. But it is a pretty important part of
• 5:13
  Speaker 1: the universe. It's a pretty important part of our existence
• 5:16
  Speaker 1: because without dark matter, maybe galaxies would not have formed
• 5:19
  Speaker 1: the same way that they formed. The Milky Way would
• 5:22
  Speaker 1: might not be the same way it is now.
• 5:23
  Speaker 2: But we're always tempted to over general life to say
• 5:26
  Speaker 2: that the way we live in our certain situation over
• 5:28
  Speaker 2: here is the way the whole universe works. It's important
• 5:31
  Speaker 2: to take a step back and to ask whether our
• 5:33
  Speaker 2: way of life and our way of galaxying is the
• 5:36
  Speaker 2: only way that it can be.
• 5:38
  Speaker 1: And so today on the podcast, we'll be asking the question,
• 5:45
  Speaker 1: are there galaxies without dark matter? Would you call these
• 5:50
  Speaker 1: light galaxies then? Or galaxies light diet galaxies?
• 5:54
  Speaker 2: I call them tragic galaxies because they're probably galaxies where
• 5:56
  Speaker 2: everybody only eats white chocolate.
• 5:59
  Speaker 1: Or milk chocolate, some chocolate snobs. I also call that
• 6:03
  Speaker 1: a tragedy.
• 6:04
  Speaker 2: Also known as crimes against chocolate.
• 6:06
  Speaker 1: Are you saying Hershey's is dark money as well?
• 6:10
  Speaker 2: Her She's his garbage man?
• 6:12
  Speaker 1: Oh my gosh. And there goes our sponsorship deal with Hershe's.
• 6:20
  Speaker 1: We were so close to funding this thing for the
• 6:23
  Speaker 1: next twenty years, and you have to go and insult them.
• 6:25
  Speaker 2: Maybe you could hear my hesitation there. I'm being torn
• 6:28
  Speaker 2: between being honest and truthful on a hard science podcast
• 6:32
  Speaker 2: and pandering to our sponsors, and I just had to
• 6:34
  Speaker 2: be honest about it. I passed on hershe'es. I'd rather
• 6:37
  Speaker 2: have no chocolate than Hershe's.
• 6:38
  Speaker 1: That was pretty harsh to garbage. Oh my gosh. Well,
• 6:41
  Speaker 1: here here's the thing, Like when you eat it. Let's
• 6:43
  Speaker 1: say you eat like a chocolate cake at a restaurant.
• 6:46
  Speaker 1: How do you know they didn't use Hershey's chocolate.
• 6:48
  Speaker 2: You can taste it. Man, that's sour affront to chocolate.
• 6:51
  Speaker 2: It's turned so many people off of chocolate when chocolate
• 6:54
  Speaker 2: is this wonderful, amazing thing. Even milk chocolate can be
• 6:57
  Speaker 2: high quality, can be amazing. What they may in Pennsylvania. O, man,
• 7:01
  Speaker 2: it's a crime.
• 7:03
  Speaker 1: Now you're insulting the whole state of Pennsylvania.
• 7:07
  Speaker 2: Let's keep going. See how many people I can offend.
• 7:09
  Speaker 1: Yeah, yeah, let's moves and let's insult the whole universe.
• 7:13
  Speaker 2: Why don't you No, Fortunately, most of the universe is
• 7:15
  Speaker 2: dark matter and therefore appreciates dark chocolate, and so we're good.
• 7:19
  Speaker 2: We have the majority firmly on our side.
• 7:21
  Speaker 1: But doesn't Hershey's own some fancy brands like Scharfenberger or
• 7:24
  Speaker 1: something like that.
• 7:25
  Speaker 2: I don't know, I hope not.
• 7:26
  Speaker 1: Well, maybe you've been eating Hershey' chocolates all this time.
• 7:29
  Speaker 1: A plot twist. Anyways, we're talking about dark matter and
• 7:35
  Speaker 1: galaxies and could there exist galaxies without dark matter. These
• 7:39
  Speaker 1: would be like galaxies that don't have any dark matter
• 7:42
  Speaker 1: in them or around them.
• 7:43
  Speaker 2: Yeah, exactly, just stars and gas and dust and black holes,
• 7:47
  Speaker 2: no dark matter, all.
• 7:49
  Speaker 1: Right, Well, whether they are tragic or not. We were
• 7:51
  Speaker 1: wondering how many people out there had wondered about this
• 7:53
  Speaker 1: question and if they have any ideas about the answer.
• 7:56
  Speaker 2: Thanks very much to everybody out there who answers these questions,
• 8:00
  Speaker 2: they or not there supporters of Hershey's Crimes against Chocolate
• 8:03
  Speaker 2: would really appreciate everything you do. If you would like
• 8:06
  Speaker 2: to join this group, just write to me two questions
• 8:08
  Speaker 2: at Danielandjorge dot com.
• 8:10
  Speaker 1: I feel like maybe there's a Hurshi's employee out there
• 8:12
  Speaker 1: who listens to our podcast and it's now very very sad.
• 8:16
  Speaker 2: There's an easy fix for that, find a new job
• 8:19
  Speaker 2: or a new podcast. No, you've got to go to
• 8:21
  Speaker 2: the root of the problem.
• 8:22
  Speaker 1: That's right anyway. So think about it for a second.
• 8:24
  Speaker 1: Do you think there can be galaxies without dark matter?
• 8:28
  Speaker 1: Here's what people had to say.
• 8:30
  Speaker 4: I feel like there probably aren't, only because I know
• 8:34
  Speaker 4: that dark matter, as far as I know, is distributed
• 8:36
  Speaker 4: pretty much evenly throughout the universe. I think it tends
• 8:38
  Speaker 4: to be clustered in galaxies, but I think it tends
• 8:41
  Speaker 4: to be pretty uniform. So I would be surprised if
• 8:42
  Speaker 4: there were galaxies without dark matter.
• 8:44
  Speaker 5: I'm going to say why not, because, like going back
• 8:47
  Speaker 5: to an episode about uranium on Uranus, there could be
• 8:51
  Speaker 5: a tiny bit here and there in a galaxy. So
• 8:55
  Speaker 5: I'm going to say I don't know why there wouldn't
• 8:58
  Speaker 5: be dark matter in a galaxy. I'm also going to
• 9:01
  Speaker 5: say I don't know why there would.
• 9:03
  Speaker 3: I don't believe there are galaxies without dark matter because
• 9:06
  Speaker 3: dark matter is, in my understanding, a general term for
• 9:11
  Speaker 3: unknown matter, which is this and makes up the overwhelming
• 9:15
  Speaker 3: majority of the universe. Therefore, I think it is not
• 9:19
  Speaker 3: possible for galaxies not to have at least some dark
• 9:22
  Speaker 3: matter in them.
• 9:23
  Speaker 6: I'm not sure that we've observed any galaxies without dark matter,
• 9:25
  Speaker 6: but I suppose anything could be possible in this crazy universe.
• 9:29
  Speaker 6: It's also possible that all galaxies are without dark matter
• 9:31
  Speaker 6: and we just don't understand gravity all right.
• 9:33
  Speaker 1: Most people are skeptical about this question.
• 9:35
  Speaker 2: Yeah, people have the idea that dark matter is everywhere.
• 9:38
  Speaker 1: It's inescapable, you can't get away from it. It seems
• 9:41
  Speaker 1: in our question, although one person has kind of said,
• 9:43
  Speaker 1: why not, that's a good attitude to have.
• 9:45
  Speaker 2: Yeah, that's the whole attitude about physics, Like, well, maybe
• 9:48
  Speaker 2: everything is different from what we've boughter. Maybe there's something
• 9:51
  Speaker 2: really weird out there that could teach us something new
• 9:53
  Speaker 2: about the universe.
• 9:54
  Speaker 1: I do feel like that is a guiding question in
• 9:56
  Speaker 1: theoretical physics at least. Why not? Yes? Why not?
• 10:00
  Speaker 2: Sure? Maybe everything is just tiny cats at the quantum scale.
• 10:04
  Speaker 1: That's right, Maybe everything's just made out of Hersy's charga.
• 10:06
  Speaker 2: No, I got to hop somewhere else in the multiverse.
• 10:11
  Speaker 1: If that's the case, all right, Well let's get going
• 10:13
  Speaker 1: before this podcast gets too dark. Daniel, give us the basics.
• 10:17
  Speaker 1: What is dark matter? For those of us who haven't
• 10:20
  Speaker 1: listened to the twenty seven percent of our podcast.
• 10:22
  Speaker 2: Episodes, dark matter is fascinating because we simultaneously know a
• 10:26
  Speaker 2: lot about it and very little. Like we know that
• 10:30
  Speaker 2: there's a lot of dark matter in the universe, and
• 10:32
  Speaker 2: we know that it's matter. We know there's something out
• 10:35
  Speaker 2: there that's creating gravity or curvature of space time, but
• 10:39
  Speaker 2: that it's invisible. It doesn't glow, it doesn't give off light.
• 10:41
  Speaker 2: It doesn't reflect light. We sense it only because we
• 10:45
  Speaker 2: see its gravitational effects on stuff. It's curving space, which
• 10:49
  Speaker 2: changes how things are moving through that space. We see
• 10:53
  Speaker 2: galaxies rotating much faster than their gravity would be able
• 10:56
  Speaker 2: to hold them together if there wasn't also dark matter
• 10:59
  Speaker 2: in them holding them together. On the other hand, we
• 11:01
  Speaker 2: don't know what stuff it is. We know that it
• 11:03
  Speaker 2: has gravity, so it's matter, but we don't know it's
• 11:06
  Speaker 2: some weird new kind of neutrino or a totally different
• 11:08
  Speaker 2: kind of particle we've never seen before, or a thousand
• 11:11
  Speaker 2: new kinds of particles, or something that's not even a particle.
• 11:15
  Speaker 2: So we know a lot about it on the sort
• 11:17
  Speaker 2: of cosmological scale, but very very little or almost nothing
• 11:20
  Speaker 2: about it at the particle level. Right.
• 11:22
  Speaker 1: Dark matter is this kind of mysterious stuff in the
• 11:24
  Speaker 1: universe that we kind of feel its presence. We can
• 11:27
  Speaker 1: see its presence through gravity, but as you said, you
• 11:30
  Speaker 1: can't see it because it doesn't interact with elechormagnetic light
• 11:33
  Speaker 1: or maybe any of the other forces in the universe,
• 11:36
  Speaker 1: and so you can't see it. And that's why you
• 11:37
  Speaker 1: call it dark exactly.
• 11:38
  Speaker 2: And a bunch of listeners write in with the idea
• 11:40
  Speaker 2: that maybe dark matter is matter in another universe that's
• 11:43
  Speaker 2: somehow leaking in to ours. Remember that dark matter is
• 11:47
  Speaker 2: creating gravity in our universe, or changing the curvature of
• 11:50
  Speaker 2: space in our universe, which means that it's in that space. Right,
• 11:54
  Speaker 2: it shares that space with us, which kind of means
• 11:57
  Speaker 2: that it's in our universe. We really do know that
• 12:00
  Speaker 2: dark matter is something in our universe that's changing the
• 12:03
  Speaker 2: shape of space. We can only see sort of indirectly
• 12:06
  Speaker 2: through gravity, which is really frustrating because gravity is the
• 12:09
  Speaker 2: worst way to see things. It's so weak that it
• 12:12
  Speaker 2: makes it very, very difficult.
• 12:14
  Speaker 1: But wait, couldn't it bend our space and not be
• 12:17
  Speaker 1: part of our space.
• 12:18
  Speaker 2: That's possible if you overthrow general relativity in our entire
• 12:21
  Speaker 2: understanding of space time. General relativity says that matter tells
• 12:25
  Speaker 2: space how to bend, and space tells matter how to move,
• 12:28
  Speaker 2: and that means matter in our space. Like in general relativity,
• 12:32
  Speaker 2: the curvature or space comes from the energy density in
• 12:35
  Speaker 2: that space. So if you have some like parallel space
• 12:38
  Speaker 2: overlaid on top of it, which can also bend that space,
• 12:42
  Speaker 2: then it seems to me like it would be part
• 12:43
  Speaker 2: of our space, you know, sort of like the by definition,
• 12:46
  Speaker 2: but yet you could augment or throw out general relativity
• 12:49
  Speaker 2: replace it with something totally different. But the simpler idea
• 12:52
  Speaker 2: is that it's just some kind of mass we can't
• 12:53
  Speaker 2: see that explains almost everything we see out there in
• 12:56
  Speaker 2: the universe. So it's sort of the best going explanation.
• 12:59
  Speaker 2: You can always make more or baroque complicated explanations if
• 13:02
  Speaker 2: you like.
• 13:02
  Speaker 1: Well, it kind of might as well be in another universe, right,
• 13:05
  Speaker 1: because if it doesn't feel a lot of the same
• 13:07
  Speaker 1: force as we don't, it's just kind of like ghostly
• 13:10
  Speaker 1: matter that's kind of living on top of us. There
• 13:12
  Speaker 1: might be beings made out of dark matter, right.
• 13:14
  Speaker 2: Physicists call the different sectors of the universe. If you
• 13:17
  Speaker 2: have like two different sets of particles that don't interact
• 13:19
  Speaker 2: at all except for through gravity, then we call those
• 13:22
  Speaker 2: like the lights of the visible sector and the dark
• 13:24
  Speaker 2: sector of the universe. And that's totally possible that you
• 13:27
  Speaker 2: could have a whole complicated physics happening in the dark
• 13:30
  Speaker 2: sector that we can't see. Now. Mostly we know that
• 13:33
  Speaker 2: dark matter can't interact with itself. If it did interact
• 13:36
  Speaker 2: with itself, it would form all sorts of complicated structure
• 13:39
  Speaker 2: and do all sorts of interesting things. We think that
• 13:41
  Speaker 2: dark matter is pretty spread out. However, there could be
• 13:45
  Speaker 2: a little component dark dark matter. Dark matter may be
• 13:47
  Speaker 2: lots of different kinds of things, and one little component
• 13:50
  Speaker 2: of it might be more complicated and do complicated things
• 13:53
  Speaker 2: like form life or ice cream, cones or cats or
• 13:56
  Speaker 2: good chocolate without violating what we've seen dark matter do,
• 14:00
  Speaker 2: which is mostly spread out smoothly.
• 14:01
  Speaker 1: All right, Well, the question here today is can there
• 14:04
  Speaker 1: be galaxies without dark better? Why is this even a question? Like,
• 14:08
  Speaker 1: are most of the galaxies that we see out there?
• 14:10
  Speaker 1: Do they all have dark matter?
• 14:12
  Speaker 2: Yeah, the galaxies that you see out there in the
• 14:14
  Speaker 2: sky are like tracers. They're basically telling you where the
• 14:17
  Speaker 2: dark matter is in the universe. Remember that dark matter
• 14:20
  Speaker 2: is not something we can see, but it also dominates
• 14:23
  Speaker 2: the universe. Our estimates are that four fifths of the
• 14:26
  Speaker 2: matter in the universe is dark matter. So if you
• 14:29
  Speaker 2: like spin the wheel and pick a random object in
• 14:32
  Speaker 2: the universe, most likely you're going to get dark matter.
• 14:34
  Speaker 2: It's like overwhelmingly dark matter. So when the universe is
• 14:38
  Speaker 2: forming its structure and the gravity that determines like where
• 14:41
  Speaker 2: things are going to be. It's mostly the gravity of
• 14:43
  Speaker 2: dark matter that decides where things are going to clump
• 14:46
  Speaker 2: together and where things are not going to clump together. Remember,
• 14:49
  Speaker 2: the very early universe is mostly smooth, with a few
• 14:52
  Speaker 2: little blobs that are denser than others. The gravity of
• 14:56
  Speaker 2: those over dense pieces pull things together to form structure,
• 14:59
  Speaker 2: and that's where you get like galaxies in one part
• 15:01
  Speaker 2: of space and not galaxies in another part of space.
• 15:03
  Speaker 2: Where you have galaxies is where you had more dark
• 15:06
  Speaker 2: matter to pull that stuff together to form those galaxies.
• 15:10
  Speaker 1: Yeah, that's pretty wild to think that something we can't see,
• 15:13
  Speaker 1: that is invisible to us basically kind of dictates the
• 15:17
  Speaker 1: entire structure of the universe, right at least at the
• 15:19
  Speaker 1: galaxy level. Does dark matter also dictate things like superclusters
• 15:24
  Speaker 1: and beyond exactly?
• 15:26
  Speaker 2: And so you can imagine like these invisible wells, like
• 15:29
  Speaker 2: dark matter's curving space with Shepherd's the other kind of
• 15:32
  Speaker 2: matter together. So every time you look up at the
• 15:34
  Speaker 2: night sky and you see a galaxy, you should imagine
• 15:37
  Speaker 2: there's an invisible blob of dark matter surrounding that galaxy.
• 15:40
  Speaker 2: There's a whole halo that's created the conditions to form
• 15:44
  Speaker 2: that galaxy. We run simulations, for example, of a universe
• 15:46
  Speaker 2: without dark matter, and it doesn't form galaxies after fourteen
• 15:50
  Speaker 2: billion years. So quite literally, we would not be alive
• 15:53
  Speaker 2: without dark matter.
• 15:54
  Speaker 1: And now is the same true for things like superclusters
• 15:57
  Speaker 1: and those giant bubbles of galaxies they're in the universe.
• 16:01
  Speaker 1: Is that dictated by dark matter as well? Or is
• 16:03
  Speaker 1: that more of the quantum fluctuations of the universe.
• 16:07
  Speaker 2: Well, both, because there's this cosmic web that tells us
• 16:10
  Speaker 2: where dark matter will be denser and where dark matter
• 16:12
  Speaker 2: will not be dense. These filaments in some places they
• 16:15
  Speaker 2: overlap and you get these wells where things pool together.
• 16:18
  Speaker 2: And so the whole cosmic web is dictated by dark
• 16:21
  Speaker 2: matter and the light matter. The normal matter, baryons and
• 16:24
  Speaker 2: quarks and electrons just follows that. And so it's not
• 16:27
  Speaker 2: like a supercluster has a super halo. A supercluster is
• 16:30
  Speaker 2: made of galaxies connected together by these filaments.
• 16:34
  Speaker 1: So it also has filaments of dark matter.
• 16:37
  Speaker 2: Absolutely, yes, there are filaments of dark matter connecting these
• 16:40
  Speaker 2: halos of around each galaxy.
• 16:43
  Speaker 3: Hmmm.
• 16:43
  Speaker 1: Interesting. All right, Well, let's dig into the question of
• 16:46
  Speaker 1: whether a galaxy can be clean of dark matter, or
• 16:50
  Speaker 1: whether it's kind of a requirement for a galaxy to form.
• 16:53
  Speaker 1: So let's dig into that, but first let's take a
• 16:55
  Speaker 1: quick break. All Right, we're asking the question, can you
• 17:10
  Speaker 1: have a galaxy without dark matter? Could you maybe have
• 17:13
  Speaker 1: a scripy little galaxy out there? That was like, no,
• 17:15
  Speaker 1: I don't care about dark matter. I'm just going to
• 17:18
  Speaker 1: gather all these all this gas and dust and on
• 17:21
  Speaker 1: my own without any help. That's kind of what we're asking.
• 17:24
  Speaker 2: Today, right, Yeah, exactly.
• 17:26
  Speaker 1: And so you talked about how most of the galaxies
• 17:30
  Speaker 1: that we see out there probably have dark matter, right,
• 17:33
  Speaker 1: we think they have dark matter, right, because they couldn't
• 17:35
  Speaker 1: be holding together without dark matter.
• 17:37
  Speaker 2: That's right. And it's even more than just most galaxies
• 17:39
  Speaker 2: have dark matter. It turns out galaxies are sort of
• 17:42
  Speaker 2: like extra rich in dark matter, Like most galaxies have
• 17:46
  Speaker 2: more dark matter than the average dark matter density in
• 17:49
  Speaker 2: the universe.
• 17:50
  Speaker 1: Wait, what what do you mean? Like, what are some numbers?
• 17:52
  Speaker 2: So if you average over the whole universe, like what
• 17:55
  Speaker 2: fraction of matter in the universe is dark matter? That's
• 17:58
  Speaker 2: eighty four percent by counting. We don't know how many
• 18:01
  Speaker 2: dark matter particles there are that's by mass, Like, what
• 18:04
  Speaker 2: fraction of the mass of stuff in the universe is
• 18:07
  Speaker 2: dark matter? That's about eighty four percent. But when you
• 18:09
  Speaker 2: look at galaxies and you ask, like, what fraction of
• 18:12
  Speaker 2: the mass in a galaxy is dark matter, that's more
• 18:15
  Speaker 2: like ninety one percent. So galaxies have like about half
• 18:19
  Speaker 2: as much normal matter as the average normal matter density
• 18:23
  Speaker 2: in the universe. Galaxies are like concentrated blobs of dark matter.
• 18:27
  Speaker 1: And we get these numbers by measuring how fast the
• 18:31
  Speaker 1: galaxies are rotating and kind of guessing how much dark
• 18:33
  Speaker 1: matter you need to hold it all together.
• 18:36
  Speaker 2: Yeah, not so much guessing, measuring right, But you're right.
• 18:38
  Speaker 2: It's looking at how the galaxy rotates. We can measure
• 18:41
  Speaker 2: the speed of those stars in the galaxy as they
• 18:44
  Speaker 2: whizz around the center by looking at their light and
• 18:47
  Speaker 2: seeing how it's red shifted or blue shifted. You're looking
• 18:50
  Speaker 2: at a galaxy, some of the stars will be moving
• 18:52
  Speaker 2: away from you and some moving towards you, so to
• 18:54
  Speaker 2: be red or blue shifted their light from the Doppler shift,
• 18:57
  Speaker 2: so you can measure their velocities. So you can look
• 19:00
  Speaker 2: at the velocity of stars as they get further and
• 19:03
  Speaker 2: further away from the center, and in order to hold
• 19:05
  Speaker 2: a star at a certain velocity a certain radius, you
• 19:08
  Speaker 2: need a gravitational force there, so you can calculate exactly
• 19:11
  Speaker 2: how much gravity is needed to hold a star at
• 19:13
  Speaker 2: a certain radius. They have all these stars at different
• 19:16
  Speaker 2: distances from the center, telling you exactly how much gravity
• 19:19
  Speaker 2: you need to keep those stars going at that speed,
• 19:23
  Speaker 2: and then you can add up how much you can see,
• 19:25
  Speaker 2: like count all the visible stars and the rest you
• 19:28
  Speaker 2: suppose is dark matter. I guess that's what you mean
• 19:31
  Speaker 2: by guessing.
• 19:31
  Speaker 1: Yeah, inferring guessing.
• 19:34
  Speaker 2: There's a whole field of statistical inference that we should
• 19:37
  Speaker 2: just call guessing.
• 19:37
  Speaker 1: Yeah, Well, I mean you don't actually know how many
• 19:41
  Speaker 1: stars there are in that galaxy right, so far away
• 19:43
  Speaker 1: you can't see the individual stars, so you're also sort
• 19:47
  Speaker 1: of inferring how many stars there are there. You're guessing
• 19:50
  Speaker 1: a little bit, aren't you.
• 19:51
  Speaker 2: There's always uncertainty in these measurements. Absolutely, and you're right
• 19:53
  Speaker 2: that we cannot resolve individual stars, especially near the center
• 19:57
  Speaker 2: where things get very dense. But we can see this streams, right,
• 20:00
  Speaker 2: we can see streams of stars. We have models for
• 20:02
  Speaker 2: how these galaxies work. But absolutely there's always uncertainty, but
• 20:06
  Speaker 2: the uncertainty in these calculations is tiny compared to the
• 20:09
  Speaker 2: size of dark matter. So there's no uncertainty that there's
• 20:12
  Speaker 2: a lot of dark matter in these galaxies because remember
• 20:15
  Speaker 2: the fractions we're talking about here, like ninety percent, which
• 20:18
  Speaker 2: means you're looking at a star, you're measuring its velocity.
• 20:21
  Speaker 2: You figure out how much gravity is needed to hold
• 20:24
  Speaker 2: it there so it doesn't fly out into intergalactic space.
• 20:27
  Speaker 2: And when you add up all the stars, you get
• 20:28
  Speaker 2: like ten percent of the gravity you need. So there's
• 20:31
  Speaker 2: a huge missing chunk.
• 20:34
  Speaker 1: And you're sure it's not just you know, a lot
• 20:36
  Speaker 1: of asteroids or rocks that don't close.
• 20:38
  Speaker 2: Yeah, that's a great question. Could dark matter just be
• 20:41
  Speaker 2: normal matter that we're not seeing right, just like dark
• 20:44
  Speaker 2: chunks of matter. So people have looked for that directly.
• 20:47
  Speaker 2: Those are called MACHOs massive compact halo objects, and we
• 20:52
  Speaker 2: think we would see those occasionally, like they would pass
• 20:55
  Speaker 2: in front of stars if there was a lot of them.
• 20:57
  Speaker 2: If they were really big, we would have spotted them.
• 20:58
  Speaker 2: So people have looked for that kind of stuff and
• 21:01
  Speaker 2: not seen it. Plus we know something about how much
• 21:03
  Speaker 2: normal matter there was in the very early universe because
• 21:07
  Speaker 2: it dictates the fraction of elements that were produced, like
• 21:10
  Speaker 2: the hydrogen and helium and lithium very sensitive to the
• 21:13
  Speaker 2: density of quarks and electrons in the early universe. We
• 21:17
  Speaker 2: talked about that once Big Bang nucleosynthesis. So we have
• 21:20
  Speaker 2: a pretty good handle on how much normal matter there
• 21:23
  Speaker 2: was around, and we can explain where most of that
• 21:26
  Speaker 2: is now and the rest of it's got to be
• 21:27
  Speaker 2: dark matter.
• 21:28
  Speaker 1: And so basically every galaxy out there that we've seen,
• 21:32
  Speaker 1: we see that it's spinning faster than it should, or
• 21:35
  Speaker 1: it's holding together more than it should, so we think
• 21:38
  Speaker 1: it has dark matter. And now is that true for
• 21:39
  Speaker 1: every galaxy we've seen out there?
• 21:40
  Speaker 2: It turns out there's a pretty wide variety. Like when
• 21:43
  Speaker 2: you look at galaxies out there, some of them have
• 21:46
  Speaker 2: a lot of dark matter, and some of them have
• 21:48
  Speaker 2: a huge amount of dark matter. There is a variety.
• 21:51
  Speaker 2: It turns out that smaller galaxies tend to have more
• 21:54
  Speaker 2: dark matter than really massive galaxies.
• 21:57
  Speaker 1: Not by absolute amounts, but just relative town these stars
• 22:00
  Speaker 1: they have.
• 22:00
  Speaker 2: Yeah, exactly higher dark matter fractions, I should say, And
• 22:04
  Speaker 2: that's because galaxies are better at holding onto their dark
• 22:07
  Speaker 2: matter than they are their normal matter. Galaxies are crazy places.
• 22:11
  Speaker 2: There's winds from all the stars, right every star is
• 22:13
  Speaker 2: a fusion furnace and pushing out protons and electrons. These
• 22:17
  Speaker 2: cellar winds are pushing gas out of the galaxies. Then
• 22:21
  Speaker 2: there are supernovas going off all the time, blowing things
• 22:24
  Speaker 2: up and pushing things out. There's radiation, really intense radiation
• 22:27
  Speaker 2: from the center of the galaxy that's pushing gas out.
• 22:31
  Speaker 2: So galaxies are basically exploding and they're pushing a lot
• 22:34
  Speaker 2: of their matter out. And so the smaller galaxy is,
• 22:37
  Speaker 2: the less it's capable of holding onto its normal matter,
• 22:41
  Speaker 2: the less it's capable of resisting these forces that push
• 22:44
  Speaker 2: gas out of the galaxies.
• 22:46
  Speaker 1: Because the bigger galaxies have more gravity.
• 22:48
  Speaker 2: Basically right exactly, the bigger galaxies are still doing this,
• 22:50
  Speaker 2: but they have more gravity so they can hold onto
• 22:52
  Speaker 2: their normal matter. So smaller galaxies, which have weaker gravity,
• 22:55
  Speaker 2: lose more of this normal matter. So you look out
• 22:58
  Speaker 2: there at dwarf galaxies, really tiny ones. They can be
• 23:01
  Speaker 2: like ninety nine percent dark matter.
• 23:03
  Speaker 1: But wouldn't the larger galaxies also be better at holding
• 23:06
  Speaker 1: onto their dark matter, Like wouldn't smaller galaxies lose some
• 23:10
  Speaker 1: dark matter eventually, like it might evaporate or something.
• 23:12
  Speaker 2: Yeah, that depends on what dark matter does. And in
• 23:15
  Speaker 2: this theory, dark matter does nothing but gravity, and so
• 23:18
  Speaker 2: you can't really lose your dark matter. Like to lose
• 23:21
  Speaker 2: your dark matter, you need some force that's pushing out
• 23:23
  Speaker 2: on it. But gravity is just attractive. So all these
• 23:25
  Speaker 2: forces like the solar winds and the radiation and the
• 23:28
  Speaker 2: supernova basically have no impact on the dark matter. Dark
• 23:30
  Speaker 2: matter just like brushes it right off, Like supernova could
• 23:33
  Speaker 2: happen right next to you, and a dark matter particle
• 23:35
  Speaker 2: would be like whatever, dude.
• 23:37
  Speaker 1: And so we haven't seen any galaxies without dark matter,
• 23:39
  Speaker 1: So then why are we asking the question are there
• 23:41
  Speaker 1: galaxies without dark matter? Is it more of like, is
• 23:44
  Speaker 1: it possible to have a galaxy without dark matter? Or
• 23:47
  Speaker 1: are we asking like could there be galaxies where we
• 23:49
  Speaker 1: haven't noticed it doesn't have dark matter?
• 23:51
  Speaker 2: Yeah, so great question. We're curious about this for lots
• 23:53
  Speaker 2: of reasons, Like number one, we have a theory about
• 23:56
  Speaker 2: how the structure of the universe came to be and
• 23:58
  Speaker 2: how it made galaxy and this nice story we told
• 24:01
  Speaker 2: you about over densities clumping together to form galaxies et cetera,
• 24:05
  Speaker 2: et cetera. But we'd like to test that. We'd like
• 24:07
  Speaker 2: to make sure that's correct. We're often surprised when we
• 24:11
  Speaker 2: look out in the universe and see how things actually work,
• 24:14
  Speaker 2: and so what we'd like to do is check our
• 24:16
  Speaker 2: predictions about like the dark matter fractions of galaxies against
• 24:20
  Speaker 2: reality and see is this really the way things work. Also,
• 24:23
  Speaker 2: this really helps us understand what dark matter is, because
• 24:26
  Speaker 2: seeing how dark matter varies across the universe can tell
• 24:29
  Speaker 2: us something about the nature of dark matter and help
• 24:31
  Speaker 2: us test various alternative theories about what dark matter might
• 24:35
  Speaker 2: or might not be. But it's not exactly true that
• 24:37
  Speaker 2: we've never seen a galaxy without dark matter. People are
• 24:39
  Speaker 2: out there looking for these and they found some pretty
• 24:41
  Speaker 2: weird cases.
• 24:43
  Speaker 1: Interesting, all right, what are some of these cases?
• 24:46
  Speaker 2: So this galaxy group kind of nearby on cosmic scales,
• 24:49
  Speaker 2: that's sixty three million light years away. It's called in
• 24:52
  Speaker 2: GC one zero five two, and basically it's an elliptical
• 24:57
  Speaker 2: galaxy in the Cetus constellation. We've known about if like
• 25:00
  Speaker 2: two hundred and fifty years or so, but there's actually
• 25:02
  Speaker 2: a little group of galaxies. It's like a major galaxy
• 25:05
  Speaker 2: with a bunch of little galaxies nearby. They call these
• 25:08
  Speaker 2: dwarf galaxies, so it's a whole group, so they call
• 25:10
  Speaker 2: the group of galaxies. And these little galaxies are actually
• 25:13
  Speaker 2: ultra diffuse galaxies. That means they're galaxies that are not
• 25:17
  Speaker 2: very bright, they have very few stars in them. And
• 25:20
  Speaker 2: these ultra diffuse galaxies near this NGC group they think
• 25:24
  Speaker 2: might have no dark matter in them at all.
• 25:27
  Speaker 1: Why do they think that?
• 25:28
  Speaker 2: So they look at the rotations of these galaxies and
• 25:30
  Speaker 2: they do that calculation and they estimate zero dark matter.
• 25:34
  Speaker 2: Like every time you're doing this, you're not assuming the
• 25:36
  Speaker 2: dark matter. You're measuring it. And sometimes it comes out
• 25:39
  Speaker 2: ninety percent, sometimes eighty four percent, sometimes ninety nine percent.
• 25:42
  Speaker 2: In this case, it comes out close to zero or
• 25:44
  Speaker 2: consistent with zero. So they think these are little galaxies
• 25:48
  Speaker 2: that have no dark matter in them at all.
• 25:50
  Speaker 1: So that's pretty wild. That means that you can have
• 25:52
  Speaker 1: a galaxy without dark matter.
• 25:53
  Speaker 2: Yeah, it's fascinating because remember, our theory of galaxy formation
• 25:57
  Speaker 2: is that basically every big galaxy is a merger of
• 26:01
  Speaker 2: a bunch of small galaxies. Big galaxies don't like form
• 26:04
  Speaker 2: all at once in a single collapse. You have a
• 26:05
  Speaker 2: bunch of baby galaxies then merge to make bigger and
• 26:08
  Speaker 2: bigger galaxies, so like a bottoms up approach. And so
• 26:11
  Speaker 2: if your big galaxy ends up with a lot of
• 26:13
  Speaker 2: dark matter in it, that means that the little galaxies
• 26:16
  Speaker 2: that made it should each have their own dark matter.
• 26:18
  Speaker 2: And we look out a dwarf galaxies and we mostly
• 26:19
  Speaker 2: see them having dark matter. In fact, some of them
• 26:21
  Speaker 2: have a lot. So it is really weird to see
• 26:23
  Speaker 2: these little galaxies without any dark matter at all. And
• 26:27
  Speaker 2: the question is like, did they form this way or
• 26:29
  Speaker 2: did something happen to strip them of their dark matter.
• 26:32
  Speaker 1: Or maybe they formed later in the universe.
• 26:34
  Speaker 2: Yeah, exactly, And so that's a fascinating question. And so
• 26:38
  Speaker 2: there's a group that's done a study of these and
• 26:39
  Speaker 2: they have a theory about how these little diffuse galaxies
• 26:42
  Speaker 2: ended up without any dark matter in them.
• 26:44
  Speaker 1: What's a theory.
• 26:45
  Speaker 2: So the theory is basically a mini version of the
• 26:47
  Speaker 2: Bullet cluster. You remember. The Bullet cluster is this famous
• 26:51
  Speaker 2: example that really convinced a lot of people that dark
• 26:53
  Speaker 2: matter was a real thing. It was a cluster of
• 26:55
  Speaker 2: galaxies that collided with another cluster of galaxies and we
• 26:58
  Speaker 2: saw that what happened to them the gas and the
• 27:00
  Speaker 2: dust and the dark matter was very different. So the
• 27:03
  Speaker 2: gas and the dust interacted and created collisions. The dark
• 27:06
  Speaker 2: matter passed right through because it doesn't they direct at
• 27:09
  Speaker 2: that level, gravity's not strong enough, so basically separated the
• 27:13
  Speaker 2: dark matter from the normal matter. So the bullet cluster,
• 27:16
  Speaker 2: and now you have a blob in the middle with
• 27:17
  Speaker 2: a bunch of normal matter in it, and then you
• 27:19
  Speaker 2: have dark matter on both sides, so we can see
• 27:21
  Speaker 2: through gravitational lensing. So they think that might be similar
• 27:24
  Speaker 2: to what happened in this case, that maybe there was
• 27:26
  Speaker 2: a big collision between two other objects, and these two
• 27:30
  Speaker 2: things that we're seeing now they call them DF two
• 27:32
  Speaker 2: and DF four are basically the results of that, like
• 27:35
  Speaker 2: chunks of stars and gas and dust that got stripped
• 27:39
  Speaker 2: of their dark matter and a collision and then tossed aside.
• 27:42
  Speaker 1: Well, why wouldn't some little bit of dark matter go
• 27:45
  Speaker 1: with them.
• 27:45
  Speaker 2: Because the dark matter and the normal man have very
• 27:47
  Speaker 2: different experiences in a collision, Like dark matter basically passes
• 27:51
  Speaker 2: the right through. There's not really a collision when it
• 27:53
  Speaker 2: comes to dark matter. It's like two ghosts just phasing
• 27:55
  Speaker 2: through each other whereas two people bumping into a hallway
• 27:58
  Speaker 2: are going to change their direction. So imagine you have
• 28:01
  Speaker 2: like a ghost inside you and somebody else is a
• 28:03
  Speaker 2: ghost inside them, and you have a collision in the hallway.
• 28:06
  Speaker 2: The ghosts just keep on going and the living people
• 28:08
  Speaker 2: bounce off each other. Now you know you're separated from
• 28:11
  Speaker 2: your ghost.
• 28:12
  Speaker 1: But there's so many that they discovered. Did they all
• 28:14
  Speaker 1: get that way from the collision?
• 28:15
  Speaker 2: So they've only found these two, and they have this
• 28:18
  Speaker 2: reconstruction of the collision that suggests that these two things
• 28:21
  Speaker 2: happen somewhere near NGC and created its collision, and it
• 28:25
  Speaker 2: should have also created a bunch of other ultra diiffuse
• 28:28
  Speaker 2: galaxies that they should be able to spot. That there
• 28:31
  Speaker 2: should be like five or six of these that came
• 28:33
  Speaker 2: out of the collision that also have no dark matter.
• 28:36
  Speaker 2: So they're going to go and look for those.
• 28:39
  Speaker 1: So these are you said, these are dwarf galaxies.
• 28:41
  Speaker 2: Yeah, they're ultra diffuse galaxies. They're also dwarf galaxies, so
• 28:44
  Speaker 2: they're small and they're not.
• 28:46
  Speaker 1: Very bright, so in a way they kind of got
• 28:48
  Speaker 1: made later or not? Are they as old as the universe?
• 28:50
  Speaker 2: Well, they think this collision happened about eight billion years ago,
• 28:54
  Speaker 2: and so how you age these things, I guess depends,
• 28:57
  Speaker 2: like the way they are now started about billion years ago. Now,
• 29:01
  Speaker 2: of course, have some progenitor or something that they came from. Right,
• 29:04
  Speaker 2: there was a larger object they were a part of
• 29:07
  Speaker 2: which definitely had dark matter in it, and their dark
• 29:09
  Speaker 2: matter is now sprayed in some other direction. So they
• 29:12
  Speaker 2: become separated from their dark matter. And this must have
• 29:15
  Speaker 2: been a pretty mammoth event. I mean, they reconstruct this
• 29:18
  Speaker 2: thing and it's like a collision at three hundred kilometers
• 29:21
  Speaker 2: per second of these huge cosmic objects.
• 29:24
  Speaker 1: Well, so these are galaxies that had dark matter, but
• 29:27
  Speaker 1: then they got stripped away of their dark matter, and
• 29:30
  Speaker 1: so that shows that, hey, you can have a galaxy
• 29:32
  Speaker 1: without dark matter. But I guess maybe the larger question
• 29:35
  Speaker 1: is can you form a galaxy without dark matter?
• 29:38
  Speaker 2: Right? And so as you say, that's the deep question
• 29:40
  Speaker 2: about the nature of the formation of structure in the universe,
• 29:43
  Speaker 2: and so far the answer to that is no, we
• 29:46
  Speaker 2: do not think it's possible to form a galaxy without
• 29:48
  Speaker 2: dark matter. We think you need that dark matter around
• 29:50
  Speaker 2: to gather enough gas and dust to make stars and
• 29:53
  Speaker 2: to make a galaxy that without dark matter, normal matter
• 29:56
  Speaker 2: doesn't have enough gravity to have formed galaxies this early
• 29:59
  Speaker 2: in the UNI. If you had a universe without dark matter,
• 30:02
  Speaker 2: or big section of it without dark matter and just
• 30:04
  Speaker 2: normal matter, it would form galaxies eventually, but it would
• 30:07
  Speaker 2: take a lot longer to do so.
• 30:09
  Speaker 1: But you know, as I understand it, and during the
• 30:11
  Speaker 1: Big Bang, things were really hot and dense, and there
• 30:14
  Speaker 1: were pockets of things, and there were quantum fluctuations which
• 30:17
  Speaker 1: maybe created pockets of extra densities here and there. Couldn't
• 30:21
  Speaker 1: there have been a pocket of extra density of normal
• 30:24
  Speaker 1: stuff but not dark matter. That then when the universe
• 30:27
  Speaker 1: blew up, it became a galaxy without dark matter, like
• 30:30
  Speaker 1: during the Big Bang. Why does the normal matter have
• 30:33
  Speaker 1: to follow the dark matter?
• 30:34
  Speaker 2: I guess it does in general because dark matter dominates
• 30:38
  Speaker 2: because it's just so much more of it. So it
• 30:40
  Speaker 2: basically like sets the scene for everything. But you're right,
• 30:42
  Speaker 2: it's theoretically possible to have a downward fluctuation in the
• 30:46
  Speaker 2: dark matter and an upward fluctuation in the normal matter.
• 30:49
  Speaker 2: So you get some region of space where you have
• 30:51
  Speaker 2: like extra super dense normal matter and almost no dark matter.
• 30:55
  Speaker 2: That's possible. Yeah, and so in principle that could happen,
• 30:58
  Speaker 2: and if it had enough matter, then it would form
• 31:00
  Speaker 2: a galaxy on its own. So in principle that's not impossible,
• 31:03
  Speaker 2: but we've never seen that, and I don't know what
• 31:05
  Speaker 2: the chances are of that happening theoretically.
• 31:08
  Speaker 1: Like, how many galaxies have we've done this calculation to
• 31:10
  Speaker 1: make sure that it has dark matter in it.
• 31:12
  Speaker 2: Yeah, that's a great question. We've measured the rotation velocity
• 31:15
  Speaker 2: of thousands and thousands of galaxies, but that's a tiny
• 31:18
  Speaker 2: fraction of the number of galaxies that are out there.
• 31:21
  Speaker 2: In the number of galaxies we can see, most of
• 31:23
  Speaker 2: the galaxies we can see, we can't measure their rotation
• 31:25
  Speaker 2: velocity because you're looking at like one pixel or two pixels.
• 31:29
  Speaker 2: You need to be able to sort of resolve the
• 31:30
  Speaker 2: whole galaxies you can see like from one side versus
• 31:33
  Speaker 2: light from the other side. So it's tricky. But yeah,
• 31:36
  Speaker 2: we haven't looked at that many galaxies as possible. There
• 31:38
  Speaker 2: are galaxies out there that did really form without dark matter.
• 31:42
  Speaker 1: All right, Well, it seems like we kind of answered
• 31:44
  Speaker 1: the question of the episode, which is can you have
• 31:47
  Speaker 1: a galaxy without dark matter? The answer is yes, you
• 31:49
  Speaker 1: can have maybe galaxies that had dark matter, but then
• 31:52
  Speaker 1: they lose it or they get left behind by the
• 31:56
  Speaker 1: dark matter and so they're dark matter less. Or maybe
• 31:59
  Speaker 1: they could have forded at the beginning of the universe
• 32:01
  Speaker 1: in theory, but we haven't seen one yet. All right, Well,
• 32:04
  Speaker 1: let's dig into what this all means about our understanding
• 32:07
  Speaker 1: of dark matter and also gravity and whether or not
• 32:10
  Speaker 1: it needs to be overhauled. But first, let's take another
• 32:14
  Speaker 1: quick break. Orright, we're asking the question can there be
• 32:30
  Speaker 1: a galaxy with no dark chocolate? And the answer is probably,
• 32:35
  Speaker 1: But that's not a universe Daniel wants to live in.
• 32:38
  Speaker 2: No, that's right, Transport me somewhere else in the multiverse asap.
• 32:42
  Speaker 1: What if you end up in the universe where there's
• 32:43
  Speaker 1: only milk or white chocolate?
• 32:46
  Speaker 2: Just keep smashing that button until I get somewhere good.
• 32:48
  Speaker 1: No, No, you only get one trim.
• 32:49
  Speaker 2: Would you take the risk? I'm pretty happy with our universe,
• 32:53
  Speaker 2: you know.
• 32:54
  Speaker 1: It's a pretty good one. We're talking about whether galaxies
• 32:57
  Speaker 1: can exist without dark matter, and the answers yes, they
• 33:00
  Speaker 1: can be stripped away of their dark matter, or theoretically
• 33:03
  Speaker 1: they could form in the early universe, but we haven't
• 33:05
  Speaker 1: seen one yet, and so maybe probably not. What do
• 33:08
  Speaker 1: you think happened? Why haven't we seen any? If they
• 33:10
  Speaker 1: can form without dark matter, why haven't we seen any.
• 33:13
  Speaker 2: I think that'd be really unlikely. I mean, the kind
• 33:15
  Speaker 2: of fluctuations we're talking about are very, very large. In
• 33:18
  Speaker 2: the early universe, you had just sort of like energy,
• 33:20
  Speaker 2: and then it decays into matter. As the universe expands
• 33:24
  Speaker 2: in cools, and every kind of matter is sort of
• 33:26
  Speaker 2: made uniformly, so you get more dark matter made and
• 33:29
  Speaker 2: less normal matter. But in order to have no dark
• 33:32
  Speaker 2: matter made, you'd need a really big fluctuation. You expect
• 33:35
  Speaker 2: to get like eighty five percent and you get zero.
• 33:37
  Speaker 2: It's like flipping a coin one hundred times in a
• 33:39
  Speaker 2: row and getting only heads instead of half heads and
• 33:42
  Speaker 2: half tails. It's pretty unlikely. Now the universe is really big,
• 33:45
  Speaker 2: of course, so that means that eventually it's going to happen,
• 33:47
  Speaker 2: especially if the universe is infinite. But it's so unlikely
• 33:51
  Speaker 2: that it's not going to be the first kind of
• 33:53
  Speaker 2: galaxy we see out there, or even in the first tranch.
• 33:56
  Speaker 2: Eventually we might spot one.
• 33:57
  Speaker 1: Well, as you said, it is possible to have a
• 33:59
  Speaker 1: galaxy without dark matter. We've seen it in some ultra
• 34:02
  Speaker 1: diffuse galaxies and they've done a measurement on these galaxies, right,
• 34:06
  Speaker 1: They've measured how fast it's spinning, and they're pretty sure
• 34:09
  Speaker 1: there's no dark matter in them.
• 34:10
  Speaker 2: Yeah, there were a series of papers where people said, oh,
• 34:13
  Speaker 2: there's no dark matter, and another group did a different
• 34:15
  Speaker 2: measurement said no, there is some dark matter. And then
• 34:18
  Speaker 2: there were follow up papers arguing, and now they're pretty
• 34:20
  Speaker 2: sure there's no dark matter in these but there's always
• 34:23
  Speaker 2: somebody out there who disagrees. I mean, it's astronomy, after all.
• 34:25
  Speaker 1: That's right, they're all just guessing.
• 34:28
  Speaker 2: They're all just doing their best statistical inference.
• 34:33
  Speaker 1: That's right. That's a great word for guessing. I'm just kidding.
• 34:38
  Speaker 1: A best guessing. How about that best guessing. Nobody has
• 34:41
  Speaker 1: a better guess.
• 34:42
  Speaker 2: You know, those whole departments of people who do nothing
• 34:44
  Speaker 2: but statistics for a living. I'm trashing Hershey's, but you're
• 34:48
  Speaker 2: trashing statistics. Man.
• 34:50
  Speaker 1: No, there's nothing wrong with guessing wrong.
• 34:55
  Speaker 2: How do you know the universe is the way it is.
• 34:57
  Speaker 2: We're just guessing.
• 34:58
  Speaker 1: A best guess doesn't mean that you're making thing. So randomly.
• 35:00
  Speaker 1: You're just using the best information you have to make
• 35:03
  Speaker 1: a best estimate or inference, right.
• 35:05
  Speaker 2: I suppose so. I think if it's very well informed,
• 35:08
  Speaker 2: it's not really a guess, you know.
• 35:09
  Speaker 1: But you're one hundred percent sure it's also not a fact.
• 35:13
  Speaker 2: Yeah, that's true. That's why we use statistics to describe
• 35:15
  Speaker 2: our uncertainties. Anyway. One of the things we are uncertain
• 35:20
  Speaker 2: about is the nature of dark matter. Like, a lot
• 35:22
  Speaker 2: of the stuff we talk about for dark matter is
• 35:24
  Speaker 2: kind of unsatisfyingly indirect, and a lot of people out
• 35:27
  Speaker 2: there treat dark matter like it's some placeholder, not a
• 35:30
  Speaker 2: real theory of the universe because we never see it directly.
• 35:35
  Speaker 2: We don't can't really grapple with it and grasp.
• 35:37
  Speaker 1: It directly, right right. Well, I think it's interesting that
• 35:39
  Speaker 1: we have or that astronomers have found galaxies without dark matter,
• 35:44
  Speaker 1: because it almost gives you kind of like a test
• 35:46
  Speaker 1: case to confirm that the other galaxies that we have
• 35:49
  Speaker 1: seen with dark matter actually have dark matter and it's
• 35:51
  Speaker 1: not just some weird, you know, fluke or mistake in
• 35:55
  Speaker 1: our theory of gravity.
• 35:56
  Speaker 2: Mm hmm. Yeah, it's a cool test case. It's like
• 35:59
  Speaker 2: a control, right, would you see galaxies without dark matter
• 36:02
  Speaker 2: if they were there. So it's nice to have some
• 36:04
  Speaker 2: verification that we're seeing that. It's also, as you say,
• 36:07
  Speaker 2: a great test bed for comparing various theories of dark matter,
• 36:11
  Speaker 2: which make different predictions about what would happen in these scenarios.
• 36:14
  Speaker 3: Mm.
• 36:15
  Speaker 1: Yeah, different guesses about dark.
• 36:17
  Speaker 2: Matter, different ideas, different theoretical.
• 36:22
  Speaker 1: All right, well, how do these galaxies help us decide
• 36:25
  Speaker 1: what dark matter is made out of.
• 36:27
  Speaker 2: Well, one of the most popular alternatives to dark matter
• 36:29
  Speaker 2: as a theory of matter, some kind of stuff in
• 36:32
  Speaker 2: the universe is an alternative theory of gravity to say, well,
• 36:35
  Speaker 2: there's no other stuff in the universe. We're seeing everything
• 36:38
  Speaker 2: there is. It's just that gravity works differently from what
• 36:41
  Speaker 2: we expected. Because remember, the argument for dark matter is like,
• 36:44
  Speaker 2: we understand gravity, and there's a lot more gravity than
• 36:47
  Speaker 2: we can explain with the visible stuff, So there must
• 36:49
  Speaker 2: be more stuff. There must be invisible stuff creating that gravity.
• 36:53
  Speaker 2: But what if instead we just don't understand how gravity
• 36:55
  Speaker 2: works and it can be explained by all the visible
• 36:59
  Speaker 2: stuff if you tweak your theory of gravity.
• 37:01
  Speaker 1: Meaning like what if gravity just gets stronger, The bigger
• 37:04
  Speaker 1: the distances that might account for why galaxies are holding
• 37:08
  Speaker 1: on together without needing dark matter. That's kind of the idea, right.
• 37:12
  Speaker 2: That's kind of the idea. More specifically, there's this theory
• 37:15
  Speaker 2: called mond modified Newtonian dynamics that suggests that gravity's mostly
• 37:20
  Speaker 2: like Newton described, but there are some tweaks. It depends
• 37:23
  Speaker 2: on the acceleration of these objects, and for some accelerations,
• 37:27
  Speaker 2: gravity gets stronger or weaker, and you know, it's a
• 37:30
  Speaker 2: little baroquely like added these terms and these tweaks basically
• 37:33
  Speaker 2: to explain these rotation curves, to say like, oh, these
• 37:36
  Speaker 2: stars are accelerating more than those stars. So if we
• 37:39
  Speaker 2: change the way gravity works, can we describe the rotation
• 37:42
  Speaker 2: curves that we see. The answer is yes, you can
• 37:45
  Speaker 2: devise a theory to describe the rotation curves that explain
• 37:49
  Speaker 2: how these galaxies are rotating without needing dark matter. If
• 37:53
  Speaker 2: you tweak gravity, right, you have to tweak something, either
• 37:55
  Speaker 2: change the amount of matter that's there, or you change
• 37:57
  Speaker 2: the way gravity works.
• 37:58
  Speaker 1: Well, first of all, you just saw of the whole
• 38:00
  Speaker 1: period of human history, the Baroque period.
• 38:03
  Speaker 2: I meant that in a positive way, right right.
• 38:07
  Speaker 1: The second of lie, I know that we've talked about before.
• 38:09
  Speaker 1: How you know, maybe Mond modified Newtonian dynamics, it could
• 38:13
  Speaker 1: replace dark matter. But we confirm dark matter in other
• 38:16
  Speaker 1: ways right exactly.
• 38:17
  Speaker 2: So Mond is a success in describing the rotations of
• 38:20
  Speaker 2: galaxies by making these beautiful baroque extensions to Newton's theory.
• 38:26
  Speaker 2: But there's lots of other ways we've seen dark matter,
• 38:28
  Speaker 2: like in the ripples of the cosmic microwave background from
• 38:31
  Speaker 2: the very early universe. We can see how that early
• 38:34
  Speaker 2: universe plasma sloshing around, and that depends very sensitively on
• 38:38
  Speaker 2: the amount of dark matter, which slashes differently in that
• 38:42
  Speaker 2: plasma than normal matter did, and Mond cannot explain that. Also,
• 38:46
  Speaker 2: the Bullet cluster shows us that dark matter can be
• 38:48
  Speaker 2: separated from normal matter. It's not just a different way
• 38:52
  Speaker 2: that gravity works for normal matter. It really is something
• 38:55
  Speaker 2: else with its own gravity. So Mond really struggles to
• 38:58
  Speaker 2: explain everything that the theory of dark matter can explain,
• 39:01
  Speaker 2: but it's still a popular alternative, and this is another way.
• 39:04
  Speaker 2: These dark matter free galaxies are another way to draw
• 39:07
  Speaker 2: contrast between what Mond predicts and what dark matter predict
• 39:10
  Speaker 2: because according to Mond, there is no dark matter and
• 39:12
  Speaker 2: gravity only depends on the visible matter. And so these
• 39:15
  Speaker 2: altered diffuse galaxies with no dark matter should behave the
• 39:18
  Speaker 2: same way all the other galaxies do because there's no
• 39:20
  Speaker 2: dark matter in any of them. But we do see
• 39:22
  Speaker 2: a difference. We see that these guys are rotating more slowly, right,
• 39:27
  Speaker 2: and so Mond struggles to explain how slowly rotating these
• 39:30
  Speaker 2: galaxies are without any dark matter, whereas dark matter can
• 39:34
  Speaker 2: explain all of it. It's like, well, this has more
• 39:35
  Speaker 2: dark matter, that has less dark matter. So because dark
• 39:38
  Speaker 2: matter can be variable in the universe, some galaxies have
• 39:41
  Speaker 2: more and some have less. Whereas the rules of gravity
• 39:44
  Speaker 2: have to be the same, Mind is sort of hamstrung
• 39:47
  Speaker 2: and can't really explain the variation of all these galaxies.
• 39:51
  Speaker 1: Well, I feel like Mond was already kind of dead
• 39:53
  Speaker 1: in the water for all these other reasons for a while.
• 39:55
  Speaker 1: But it is kind of interesting that seeing a galaxy
• 39:58
  Speaker 1: without dark matter almost kind of helps prove that it exists.
• 40:01
  Speaker 2: Yeah, that is really interesting, and I agree with you
• 40:03
  Speaker 2: that Mond is not a theory we should take terribly seriously.
• 40:06
  Speaker 2: In dark matter is overwhelming the better guess for what's
• 40:10
  Speaker 2: going on in the universe.
• 40:11
  Speaker 1: Yeah, there you go. See, I brought you on board.
• 40:15
  Speaker 2: I'm loving that word.
• 40:16
  Speaker 1: Now you're like, I guess, I guess so.
• 40:18
  Speaker 2: But you know, full caveats. There are some things that
• 40:20
  Speaker 2: dark matter can't explain. There are a few galaxies out
• 40:22
  Speaker 2: there that don't make any sense that no dark matter
• 40:25
  Speaker 2: can really explain. Some people think that some hybrid like
• 40:28
  Speaker 2: mostly dark matter with a little bit of mind is
• 40:30
  Speaker 2: what we need to explain everything that's out there in
• 40:32
  Speaker 2: the universe, and so it's best to keep an open mind.
• 40:35
  Speaker 2: It's also always nice to find a new way to
• 40:37
  Speaker 2: test our understanding of dark matter and gravity in general,
• 40:40
  Speaker 2: and so these galaxies without dark matter are a nice
• 40:43
  Speaker 2: test bed.
• 40:43
  Speaker 1: For that interesting you could call the new theory darkmond
• 40:48
  Speaker 1: all right, well, another interesting example of how the universe
• 40:51
  Speaker 1: just always has surprises. Like you think that maybe you
• 40:54
  Speaker 1: need dark matter to have a galaxy, but only one
• 40:58
  Speaker 1: day you find galaxies without dark matter, and it makes
• 41:00
  Speaker 1: you think, and it actually maybe helps you confirm the
• 41:03
  Speaker 1: existence of something as mysterious as dark.
• 41:05
  Speaker 2: Matter, and it goes to show you that the universe
• 41:07
  Speaker 2: does all these experiments for us. We can just look
• 41:10
  Speaker 2: up in the night sky and find the examples of
• 41:13
  Speaker 2: galaxies smashing into other galaxies or black holes colliding. All
• 41:17
  Speaker 2: these things are wonderful experiments that help reveal the nature
• 41:20
  Speaker 2: of the universe, the rules that it follows, and how
• 41:22
  Speaker 2: it all works.
• 41:24
  Speaker 1: Yeah, I guess.
• 41:27
  Speaker 2: Or you guess in the end, aren't we all just guessing? Man?
• 41:32
  Speaker 1: All right, Well, we hope you enjoyed that. Thanks for
• 41:35
  Speaker 1: joining us, See you next time.
• 41:42
  Speaker 2: For more science and curiosity, come find us on social media,
• 41:45
  Speaker 2: where we answer questions and post video. We're on Twitter
• 41:49
  Speaker 2: at this word instant and now TikTok. And remember that
• 41:52
  Speaker 2: Daniel and Jorge Explain the Universe is a production of iHeartRadio.
• 41:56
  Speaker 2: More podcasts from iHeart Radio visit the iHeartRadio you Apple
• 42:00
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• 42:09
  Speaker 2: M HM.