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Do we know where dark matter is? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, or hey, I think I may have lost something.
0:11
Speaker 1: Oh did you lose your mind again? I happened a
0:14
Speaker 1: long time ago. This is something new? Is it important?
0:17
Speaker 1: It's actually kind of a big deal. Is it like
0:21
Speaker 1: really small and easy to lose, like your keys or
0:23
Speaker 1: your wedding band. No, it's much more embarrassing because it's
0:27
Speaker 1: really enormous, like cosmically large, like most of the stuff
0:31
Speaker 1: in the universe, and you can't find it. I've been
0:34
Speaker 1: looking everywhere for it. Well, I guess you know what
0:37
Speaker 1: to do. Oh yeah, what's that? You know? Give it
0:39
Speaker 1: a cool sounding name, and then ask the federal government
0:42
Speaker 1: to help you find it. Do you think anybody would
0:44
Speaker 1: actually fall for that? I think they already have. Hi
1:03
Speaker 1: am jorhammy cartoonists and the creator of PhD comics. Hi.
1:06
Speaker 1: I'm Daniel. I'm a particle physicist and a professor you
1:09
Speaker 1: see Irvine, and I am actually paid to hunt for
1:12
Speaker 1: missing things. M interesting, You're like the Lost and Found
1:15
Speaker 1: of the universe department. That sounds very unglamorous. I like
1:19
Speaker 1: to think of myself more of the Sherlock Holmes of
1:21
Speaker 1: the universe. I see you're not in like in the
1:25
Speaker 1: basement office with a little window that people go there
1:29
Speaker 1: and claim lost things. That's right. I'm not sitting here
1:31
Speaker 1: surrounded by people's lost purses or socks. Where do all
1:35
Speaker 1: the socks and the dryers go? That's what I want
1:37
Speaker 1: to know. We should have a whole episode about that.
1:39
Speaker 1: They're orbiting the Earth in the hose zone layer. Well,
1:41
Speaker 1: that socks made. They're all connected by wormholes. You think dryers, Well,
1:45
Speaker 1: that's spinning and that he could that create a warmhole.
1:48
Speaker 1: Keep running your dryer let us know. But welcome to
1:50
Speaker 1: our podcast Daniel and Jorge Explain the Universe, a production
1:53
Speaker 1: of I Heart Radio, which we operated as a sort
1:56
Speaker 1: of lost and found of ideas about the universe. Everything
1:59
Speaker 1: we have found about the universe and all the ideas
2:02
Speaker 1: we have lost. Everything we do know about what's going
2:05
Speaker 1: on out there, the size of the universe, the number
2:08
Speaker 1: of dimensions of space, whether or not they're alien creators
2:11
Speaker 1: on other planets looking through their telescopes at us. Every
2:15
Speaker 1: deep and enormous question about the universe you might consider
2:18
Speaker 1: we talk about here on the podcast. Yeah, because it
2:21
Speaker 1: is a huge universe full of amazing and wonderful things
2:23
Speaker 1: that seem to be screaming at us for us to
2:25
Speaker 1: find them. We get light from distant stars all the
2:28
Speaker 1: time at all times, radiation, cosmic rays, and it's all
2:32
Speaker 1: coming to us sort of one thing for us to
2:34
Speaker 1: find out and what's out there and to discover and
2:38
Speaker 1: learn about how it all works. You make the universe
2:41
Speaker 1: sounds sort of cooperative, like it wants to play a
2:43
Speaker 1: role in our science and be helpful. Universe, right, I mean,
2:47
Speaker 1: it's constantly shouting at us, isn't it. It is? But
2:49
Speaker 1: sometimes these clues are very, very subtle and frustratingly difficult
2:53
Speaker 1: to grasp. I feel like the universe is sort of
2:55
Speaker 1: playing cat and mouse with us, sometimes hiding all the
2:58
Speaker 1: best bits. It's like coy universe, mean, like it wants
3:02
Speaker 1: us to find it, but it's not telling us everything
3:04
Speaker 1: that it can about itself. Yeah, it doesn't just come
3:07
Speaker 1: right out and tell us it's deep nature. It sometimes
3:10
Speaker 1: seems to make sense, and then you dig deeper and
3:11
Speaker 1: discover oh my gosh, it's bonkers underneath. But there is
3:15
Speaker 1: a lot of fascinating information in those little hints that
3:18
Speaker 1: do arrive here on Earth. Yeah, and it's pretty amazing that,
3:22
Speaker 1: you know, if you think about it, that we're sitting
3:23
Speaker 1: on this little rock floating through space in a corner
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Speaker 1: of the galaxy, in a little corner of the universe.
3:27
Speaker 1: And somehow, from this light that we're getting from distant stars,
3:31
Speaker 1: we can somehow piece together the whole structure almost of
3:34
Speaker 1: the universe. I like how you just say somehow, you know,
3:37
Speaker 1: It's like the information comes and dot dot dot, we
3:40
Speaker 1: know these things. You just like, somehow my entire career
3:43
Speaker 1: right there. Yeah, somehow it goes to the lost and
3:45
Speaker 1: found department in the basement and we get a little
3:48
Speaker 1: memo about what happened. That's right. Somehow, because the government
3:53
Speaker 1: decides to fund basic science research, we have uncovered some
3:56
Speaker 1: truths about the universe. I guess what I mean by
3:59
Speaker 1: somehow thought dot dot is you know, mostly engineering and
4:02
Speaker 1: then some science, some happy collaboration between scientists and engineers.
4:07
Speaker 1: But yeah, we are learning more and more about the universe.
4:10
Speaker 1: And there's a big question, especially a big question about
4:13
Speaker 1: the universe, about what it's made out of and how
4:16
Speaker 1: it's what structured it has out there in the galaxies
4:19
Speaker 1: and also in between galaxies. What does it all look like,
4:22
Speaker 1: how's it all put together? It's really one of the
4:24
Speaker 1: biggest questions in the universe, and one of the first
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Speaker 1: questions you might ask, which is what is in the universe?
4:31
Speaker 1: What's the universe made out of? Where is all the
4:34
Speaker 1: stuff out there? And what kind of stuff is it? Really,
4:37
Speaker 1: it's sort of shocking and amazing that we don't know
4:40
Speaker 1: the answer to that very basic question about the nature
4:44
Speaker 1: of our own reality. Yeah, because the universe sort of
4:47
Speaker 1: pulled a fast one on us, right Like it did
4:48
Speaker 1: this big reveal twist somewhere a few decades ago where
4:52
Speaker 1: we thought we knew what the universe was made out of.
4:54
Speaker 1: It was stars and matter and atoms and things like that,
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Speaker 1: But suddenly we learned that that was only like five
5:00
Speaker 1: percent of it. You know. It's kind of like when
5:02
Speaker 1: you're watching a show and Sony you realize that you
5:05
Speaker 1: still have like ninety five episodes ago. It's sort of
5:08
Speaker 1: like the universe was wearing a mask and then somebody
5:11
Speaker 1: pulled it off and we discovered, oh my gosh, there's
5:13
Speaker 1: a lot more to it. So much for the universe
5:16
Speaker 1: being helpful and revealing, right, well, I think it just
5:18
Speaker 1: wants you to keep watching, I guess, trying to parse
5:21
Speaker 1: out the information, trying to keep it interesting. Maybe the
5:24
Speaker 1: show runners of the Universe are just trying to dole
5:26
Speaker 1: out the reveals a little bit at a time. Show runners, Huh,
5:30
Speaker 1: so you're a multitheist. I think it's got to be
5:34
Speaker 1: a committee. I mean, who, there's so much to do.
5:38
Speaker 1: That's not a flattering I guess description of it looks
5:41
Speaker 1: like it was made by a committee. The universe looks
5:43
Speaker 1: like it had too many writers. Well, it does sometimes
5:46
Speaker 1: seem inconsistent, you know. Well, it is sort of a
5:48
Speaker 1: mysterious question. What is the universe made out of? And
5:51
Speaker 1: as we've learned in the last few decades, it's not
5:54
Speaker 1: made out of just stars and atoms and elements. It's
5:57
Speaker 1: mostly made out of other things, namely dark matter and
6:00
Speaker 1: dark energy. The kind of stuff that makes up you
6:03
Speaker 1: and me and hamsters and ice cream and bologna sandwiches
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Speaker 1: is actually quite unusual in the universe. Most of the
6:11
Speaker 1: stuff that's out there is not stars and gas and
6:14
Speaker 1: dust and all the visible stuff. It's something else, something dark,
6:18
Speaker 1: something we've only recently discovered exists out there. Yeah, and
6:23
Speaker 1: so dark energy is this sort of the phenomenon where
6:25
Speaker 1: the universe's expanding faster and faster every second, but dark
6:29
Speaker 1: matter is especially sort of weird and concerning because it's matter,
6:32
Speaker 1: it's stuff. It's like exerting gravity. You can feel it,
6:35
Speaker 1: but we can't see it, which means we don't know
6:38
Speaker 1: where it is exactly. We know that dark matter is
6:41
Speaker 1: some kind of stuff. There's something out there exerting gravity
6:45
Speaker 1: on the rest of the universe, and we can sort
6:47
Speaker 1: of tell that it exists, but we don't know what
6:50
Speaker 1: it is. And we've done a lot of podcasts digging
6:53
Speaker 1: into what it might be. Is it axons? Is it
6:55
Speaker 1: black holes? Is it whimps? Is this something else? Something
6:59
Speaker 1: weird in there? Even more crazy ideas we haven't yet covered.
7:02
Speaker 1: It's sort of like we know there's an elephant in
7:04
Speaker 1: the room or in the in the galaxy or the universe,
7:07
Speaker 1: but we don't know what it's. What this elephant is doing,
7:09
Speaker 1: or you know, what is it like striking a post?
7:12
Speaker 1: Is it jogging? Is it sleeping. It's kind of a
7:15
Speaker 1: big mystery, like we know it's there, but we sort
7:17
Speaker 1: of don't know what it's doing or what structure it has. Yeah,
7:21
Speaker 1: you can ask so many fascinating questions. So far, we've
7:24
Speaker 1: mostly focused on what is dark matter? Is it this
7:27
Speaker 1: kind of particle. Is it that kind of particle but
7:29
Speaker 1: equally interesting without even knowing what it's made out of,
7:32
Speaker 1: is just wondering, like where is the dark matter? Is
7:36
Speaker 1: it here with us in this room? Is it out
7:38
Speaker 1: there in deep space? Does it form planets and other
7:41
Speaker 1: kinds of structure? Is it smoothly spread out throughout the universe?
7:44
Speaker 1: Where in the universe is all of this stuff? So
7:48
Speaker 1: today on the podcast, we'll be asking the question, how
7:55
Speaker 1: do we know where dark matter is? It sounds like
7:59
Speaker 1: he lost you lost it again? Then I feel like
8:02
Speaker 1: we said a new word. It was, but now we
8:03
Speaker 1: don't know where it is. It is a really important
8:05
Speaker 1: question because knowing where the dark matter is can help
8:09
Speaker 1: us get clues about what it might be, because certain
8:12
Speaker 1: kinds of dark matter might clump up in different ways,
8:15
Speaker 1: and other kinds of dark matter might not. Yeah, is
8:18
Speaker 1: it is it chunky or smooth? I guess it's a
8:20
Speaker 1: big question about the peanut butter dark matter of the universe. Well,
8:24
Speaker 1: I hope the showrunners don't disagree about that, because then
8:26
Speaker 1: you might get chunky parts of the universe and creamy
8:29
Speaker 1: parts of the universe. You're not a creamy peanut butter fan.
8:33
Speaker 1: It's all about the chunks, man, It's all about the chunks. Well,
8:36
Speaker 1: to each their own may there's a different flavor of
8:38
Speaker 1: dark matter for every tape. New Tella there you go,
8:40
Speaker 1: New Tella is the best flavor of dark rutter and
8:45
Speaker 1: everyone gets a heart attack from being surrounded by all
8:48
Speaker 1: this saturated fat. But yeah, it's a big question. Where
8:51
Speaker 1: is dark matter and sort of like what structure it
8:54
Speaker 1: has in the universe. Is it sort of smoothed out
8:57
Speaker 1: there like a big cloud or is it is it chunky?
9:00
Speaker 1: Is it in like strands? Is it in clumps? What's
9:03
Speaker 1: it doing out there? And this is something that scientists
9:05
Speaker 1: are eager to figure out because they just want to
9:08
Speaker 1: know where all of this stuff is. They're trying to
9:11
Speaker 1: develop a picture of the universe both visible and invisible,
9:15
Speaker 1: and of course the invisible stuff much harder to see,
9:18
Speaker 1: but since there's much more of it than there is
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Speaker 1: the visible stuff, it's a very important question. So, as usual,
9:24
Speaker 1: we were wondering how many people out there had thought
9:26
Speaker 1: about this question of the location and structure of dark matter.
9:30
Speaker 1: So Daniel went out there to ask people on the internet,
9:32
Speaker 1: how do we know where dark matter is? So if
9:35
Speaker 1: you'd like to put your brain in the test and
9:37
Speaker 1: answer questions that leading physicists don't know the answer to,
9:41
Speaker 1: please write to me two questions at Daniel and Jorge
9:45
Speaker 1: dot com. Here's what people had to say. Well, doc
9:48
Speaker 1: matter reacts with gravity, so by looking out into the universe, Um,
9:53
Speaker 1: we can sort of detect where the doc matter is
9:55
Speaker 1: because of its effect on gravity. I know that it's
9:58
Speaker 1: by gravity. Is the gravitational lensing that makes it possible
10:02
Speaker 1: to have like some kind of ideas where it could
10:05
Speaker 1: be dark matter in the universe. The distortion of light
10:10
Speaker 1: suggests that there are more matter out there that we
10:13
Speaker 1: can actually say whenever it's like bands. Well, once again,
10:17
Speaker 1: I assume this has to do with the pot with
10:21
Speaker 1: the examples and instance, as you were talking of these
10:24
Speaker 1: massive tubs of argone if I'm not mistaken, that are
10:28
Speaker 1: deep underground. We cannot see what dark matter itself is.
10:32
Speaker 1: Light travels straight through it, but we can see other
10:36
Speaker 1: planets and we can see light being distorted by the
10:40
Speaker 1: gravitational effects of what dark matter is itself. I'd say
10:43
Speaker 1: we know where the dark matter is because we can
10:45
Speaker 1: see the gravitational force it exerts on the visible matter
10:49
Speaker 1: around it. Next to that, it also bends light from
10:54
Speaker 1: distant galaxies when it comes towards us. So I'd say
10:59
Speaker 1: gravity is the usual suspect for us knowing where dark
11:03
Speaker 1: matter is. Who is d M? I think I've heard
11:06
Speaker 1: that dark matter is everywhere, that it just kind of
11:10
Speaker 1: permeates the universe, so all over the place. So my
11:15
Speaker 1: guess is for what we know dark matter is, or
11:18
Speaker 1: how we know it is, if it gives off gravitational
11:21
Speaker 1: waves because it interacts with stuff through gravity, then we
11:26
Speaker 1: would use something that detects gravitational waves to see either
11:30
Speaker 1: how far away it is or where it is in
11:34
Speaker 1: respect to something else. I think we know where dark
11:37
Speaker 1: matter is by looking for localized gravitational effects like lensing
11:41
Speaker 1: or relative velocities that aren't completely explained by matter we
11:46
Speaker 1: can observe in other ways. Right, A pretty interesting answer
11:50
Speaker 1: is a lot of people went with gravity, which is true, right, Like,
11:54
Speaker 1: like that's how we initially discovered dark matter is through gravity. Yeah,
11:58
Speaker 1: that's basically the only way we can sense dark matter,
12:02
Speaker 1: and so gravity is basically the answered. Gravity is the
12:05
Speaker 1: reason we know dark matter exists, and dark matter is
12:07
Speaker 1: basically an explanation for otherwise unexplainable gravity. So yeah, gravity
12:13
Speaker 1: is basically our portal into the dark universe. And I
12:16
Speaker 1: like this person who said who is dark matter? It's
12:20
Speaker 1: like who it this new phone? I think that's because
12:24
Speaker 1: in the question I wrote d M instead of dark matter,
12:26
Speaker 1: assuming that everybody would know what DM meant, right, who
12:31
Speaker 1: doesn't know what DM is? I was sliding into some
12:34
Speaker 1: of these d M s with that one. I guess
12:37
Speaker 1: you're being a physicism using acronyms on people who had
12:40
Speaker 1: no idea what those acronyms are. But yeah, a lot
12:43
Speaker 1: of people seem to set and have a basic idea
12:46
Speaker 1: that it's through gravity. But the picture is a little
12:48
Speaker 1: bit more complicated than that, right, I mean, we sort
12:50
Speaker 1: of know that it's there because of gravity, but sort
12:54
Speaker 1: of finding out exactly where it is or whether it
12:56
Speaker 1: clumps or strands or smooth, that's much harder because we
13:00
Speaker 1: can't see it right exactly. We can't see it in
13:02
Speaker 1: the way that we can see the other kinds of matter.
13:05
Speaker 1: It doesn't give off light, it doesn't reflect light. Remember
13:08
Speaker 1: that dark matter is a bit of a confusing name
13:10
Speaker 1: because dark matter is not actually dark. It's transparent, it's invisible.
13:15
Speaker 1: It's not like a cloud of dark matter between you
13:17
Speaker 1: and another star would block your view of that star.
13:21
Speaker 1: If that were tue would be much easier to see
13:23
Speaker 1: dark batter than it is today. Instead, light passes right
13:26
Speaker 1: through dark matter. So give us, maybe start us off
13:29
Speaker 1: with a refresher of dark matter or d M as
13:32
Speaker 1: the physicist call it. You know, what do we know
13:34
Speaker 1: about it? Well, we know that dark matter is about
13:36
Speaker 1: twenty five of the energy budget of the universe. That
13:40
Speaker 1: means if you take like a cubic light year of space,
13:42
Speaker 1: or any volume of space, then of the energy in
13:46
Speaker 1: that space is devoted to the mass of dark matter,
13:50
Speaker 1: whereas five percent of the energy budget of any chunk
13:53
Speaker 1: of space on average, you know, averaging over big distances,
13:57
Speaker 1: is devoted to making things like stars and gala seas
14:00
Speaker 1: and dust and giraffes and all of that kind of
14:02
Speaker 1: normal matter made out of atoms. And that means that
14:05
Speaker 1: there's a huge amount of dark matter. That a galaxy,
14:08
Speaker 1: for example, is mostly dark matter. That the universe, the
14:12
Speaker 1: stuff in the universe, the matter, you know, the physical
14:15
Speaker 1: form of the universe is mostly dark matter. So we've
14:18
Speaker 1: been studying the universe for thousands of years looking up
14:20
Speaker 1: at the sky wondering how things work, and only recently
14:24
Speaker 1: have we discovered that we've been missing most of it.
14:27
Speaker 1: So that's pretty exciting. And we know that dark matter
14:29
Speaker 1: is not made out of atoms, not made out of
14:32
Speaker 1: the kind of stuff that you and I are made
14:34
Speaker 1: out of. If it were, then it would probably interact
14:37
Speaker 1: with light. And we can also do some careful accounting
14:39
Speaker 1: from the very beginning of the universe, where we know
14:42
Speaker 1: something about how much material there was to make atoms,
14:45
Speaker 1: we can kind of account for where all of that went.
14:47
Speaker 1: So we're pretty sure. We're almost certain that dark matter
14:49
Speaker 1: is some kind of matter that doesn't give off light
14:52
Speaker 1: or reflect light. It must be made out of something else.
14:55
Speaker 1: And we know that it doesn't move very fast. We
14:57
Speaker 1: call it cold dark matter, the as if it did,
15:00
Speaker 1: it would spread out much more throughout the universe. Yeah,
15:03
Speaker 1: and it's kind of interesting because I think I almost
15:05
Speaker 1: feel like like, in a way, physicists called this thing
15:08
Speaker 1: or named it a little bit too early, you know
15:10
Speaker 1: what I mean, Like we gave it a name like
15:12
Speaker 1: a dark matter, maybe a little too early, Like maybe
15:14
Speaker 1: you should have kept going and say that you know
15:17
Speaker 1: of the universe is just something that we don't understand
15:20
Speaker 1: or something that is not like the rest of the
15:22
Speaker 1: stuff in the universe. Well, that doesn't work as well
15:25
Speaker 1: in grand proposals as a nice zingy phrase like dark matter.
15:31
Speaker 1: But yeah, but I know what, I guess the point
15:33
Speaker 1: is it really we don't know that much about it.
15:35
Speaker 1: I mean, we sort of know it's presence, or at
15:37
Speaker 1: least we know it's the effect on the rest of
15:39
Speaker 1: the universe, but we don't even know if it's matter. Right, Well,
15:41
Speaker 1: we know that it generates gravity, which suggests that it's
15:44
Speaker 1: curves space according to general relativity. And so either our
15:49
Speaker 1: understanding of how space curves is wrong, or it's some
15:53
Speaker 1: new kind of energy and matter that does curve space.
15:57
Speaker 1: And it's possible that we don't understand gravity. It's certain
16:00
Speaker 1: that we don't have complete understanding of gravity. There are
16:03
Speaker 1: alternative ideas to explain dark matter using variations on gravitational theory,
16:08
Speaker 1: but none of them can really explain everything that we see.
16:12
Speaker 1: And so you're right that we're not certain that it's matter.
16:15
Speaker 1: But it's the simplest explanation that fits all of the data.
16:18
Speaker 1: A new kind of particle that only interacts gravitationally, explains
16:21
Speaker 1: basically everything that we see out there in the universe,
16:25
Speaker 1: from the ripples in the earliest light to the structure
16:28
Speaker 1: of the universe today to the rotations of galaxies. So
16:31
Speaker 1: we're not certain, but it's the best candidate. Maybe should
16:34
Speaker 1: have called it dark probably matter or dark most likely
16:38
Speaker 1: matter d M M L D d M l M.
16:43
Speaker 1: But we, as you said, so far, we only know
16:45
Speaker 1: about it because of its gravitational effects, right, But we
16:48
Speaker 1: sort of know quite a few things about sort of
16:50
Speaker 1: generally where it is. We do have a good idea
16:53
Speaker 1: of where it might be because of its gravitational effects. Right,
16:57
Speaker 1: it is invisible. It doesn't give off light or interact
17:00
Speaker 1: with any other kind of force. But gravity is local. Right,
17:03
Speaker 1: if you are close to something, it tugs on you
17:06
Speaker 1: more strongly than if you're far from something. So if
17:08
Speaker 1: you're measuring the gravity of an object, if you can
17:10
Speaker 1: only tell if something is there because of its gravity,
17:13
Speaker 1: you can get an idea for where it is based
17:16
Speaker 1: on what it tugs on. For example, we can see
17:18
Speaker 1: the black holes are there because of the way the
17:21
Speaker 1: stars move around them without actually directly seeing black holes,
17:25
Speaker 1: and so gravity definitely can give you a picture as
17:28
Speaker 1: to where things are in the universe, and we have
17:30
Speaker 1: a rough idea for where dark matter is. We think
17:33
Speaker 1: that dark matter is mostly lined up with the normal matter.
17:37
Speaker 1: That where you see a galaxy is where there's a
17:40
Speaker 1: huge clump of dark matter. So every galaxy, we think,
17:43
Speaker 1: for example, is embedded in a huge cloud. We call
17:46
Speaker 1: it a dark matter halo for every galaxy. Yeah, it's
17:50
Speaker 1: like where you see regular stars and planets, you see
17:54
Speaker 1: dark matter. Or it's almost like the opposite, right, it's
17:56
Speaker 1: like where you see dark matter is where all the
17:59
Speaker 1: stars and planets formed. In a way, yes, stars are
18:01
Speaker 1: more like the tracers for the rest of stuff, right,
18:04
Speaker 1: dark matter leads the way. There's more dark matter than
18:07
Speaker 1: everything else. And so it's actually like where the dark
18:10
Speaker 1: matter started clumping is where the normal matter fell into
18:14
Speaker 1: it because of its gravity and then formed galaxies and
18:17
Speaker 1: stars and all kinds of stuff that we can see.
18:20
Speaker 1: So you know how when the military is fighting at night,
18:22
Speaker 1: they shoot bullets and then occasionally, like one out of
18:24
Speaker 1: every thousand bullets is a tracer. It's like glows, so
18:27
Speaker 1: they can see where they're shooting. Stars are sort of
18:29
Speaker 1: like that. They follow the dark matter, and they give
18:32
Speaker 1: us a clue as to where that dark matter is.
18:35
Speaker 1: And that's why we think that most galaxies are embedded
18:38
Speaker 1: in this cloud, this halo of sort of spherical, sort
18:42
Speaker 1: of a little bit elliptical dark matter that goes well
18:46
Speaker 1: beyond actually where the stars are. Yeah, I've heard this
18:49
Speaker 1: sort of analogy that regular matter like planets and stars.
18:52
Speaker 1: It's sort of like the sprinkling on the icing of
18:54
Speaker 1: a cupcake. Like, not just like in terms of our
18:57
Speaker 1: relative importance to that and this the eye of the universe,
19:00
Speaker 1: but also kind of like you know, sprinkles stick to
19:02
Speaker 1: the icing in a cupcake. You know, you can't sort
19:05
Speaker 1: of have sprinkles anywhere else. They're like, you know, the
19:08
Speaker 1: sprinkles sort of tell you where the icing is. Yeah,
19:12
Speaker 1: the sprinkles are sort of the stars, and the icing
19:15
Speaker 1: is sort of dark matter, and the cupcake itself is
19:17
Speaker 1: dark energy. That gives you sort of a sense of
19:20
Speaker 1: the relative fractions of the energy budget of the universe. Yeah,
19:24
Speaker 1: so I guess we are just the hangar ons of
19:26
Speaker 1: the universe. We're just hanging onto dark matter. And so
19:30
Speaker 1: let's get into a little bit more detail about what
19:32
Speaker 1: we know about this halo around galaxies and also how
19:35
Speaker 1: we know where it is, but first let's take a
19:38
Speaker 1: quick break. All right, we're talking about dark matter and
19:54
Speaker 1: where exactly it is, because I guess we know it's there,
19:57
Speaker 1: but physicists can't find it. It's pretty tricky to nail
20:00
Speaker 1: down an individual piece of dark matter, for example, because
20:04
Speaker 1: it only interacts gravitationally, imagine trying to find like an
20:07
Speaker 1: invisible piece of sand in your room. How would you
20:10
Speaker 1: detect it? It's gravity is essentially nothing because gravity is
20:14
Speaker 1: a really really weak force. All of the other forces
20:17
Speaker 1: of electromagnetism, even the weak force, is much stronger than gravity.
20:22
Speaker 1: So in order to detect something through gravity, you need
20:24
Speaker 1: to have a huge force. You need to have like
20:26
Speaker 1: a planet sized force or a solar system sized force
20:30
Speaker 1: because gravity is so weak. So when we use gravity
20:33
Speaker 1: to look for dark matter, we can only sort of
20:36
Speaker 1: tell the large scale structure. It's very difficult to get
20:39
Speaker 1: a fine grained picture of where things are. But even
20:42
Speaker 1: though it's very weak gravitation, and we do have a
20:45
Speaker 1: kind of a pretty good idea of what shape it
20:47
Speaker 1: has in the galaxy, right like this halo, it's not
20:50
Speaker 1: just like a blob. It has some sort of shaped
20:53
Speaker 1: to it. That's right. It tends to be denser at
20:55
Speaker 1: the core and thinner further out, much like the visible
20:59
Speaker 1: matter in the galaxy. See, and we can tell where
21:01
Speaker 1: it is because it has an effect on how the
21:03
Speaker 1: stars spin. Right, Like, the old familiar story is that
21:07
Speaker 1: we know that dark matter is there because we see
21:09
Speaker 1: the speed of stars is way too high. If dark
21:13
Speaker 1: matter wasn't there, then if the galaxy was spinning this quickly,
21:16
Speaker 1: you should be throwing its stars out into interstellar space.
21:19
Speaker 1: It needs more gravity, something out there to hold those
21:23
Speaker 1: stars in place for the galaxy to spin this fast.
21:26
Speaker 1: That's the old story that just tells us that dark
21:28
Speaker 1: matter is there. But we can get much more fine
21:31
Speaker 1: grained information. We can tell where in the galaxy that
21:35
Speaker 1: dark matter is by measuring the velocity of stars at
21:38
Speaker 1: different points as you move closer in or further out
21:41
Speaker 1: from the center of the galaxy. Right, Because I guess
21:44
Speaker 1: what you're saying is that if the dark matter was
21:46
Speaker 1: all clumped together in the very very center of the galaxy,
21:50
Speaker 1: the stars will move differently than if it was more
21:52
Speaker 1: spread out throughout the whole galaxy. Right. Yeah, If you
21:55
Speaker 1: are a star moving through the galaxy, then the thing
21:57
Speaker 1: that determines your speed is how much stuff there is
22:01
Speaker 1: closer to the center of the galaxy than you are.
22:04
Speaker 1: You're not sensitive to anything that's further out than you.
22:06
Speaker 1: It's sort of like if you dig into the Earth,
22:09
Speaker 1: then everything that's further out from you, that's above you
22:12
Speaker 1: doesn't affect your gravity at all because it all cancels out.
22:15
Speaker 1: It's only stuff that's closer to the center of the
22:17
Speaker 1: Earth than you are. So it's the same for a star.
22:20
Speaker 1: The star is speed basically tells you how much stuff
22:23
Speaker 1: there is between it and the center of the galaxy.
22:26
Speaker 1: So as you look at the velocity of the star
22:28
Speaker 1: as you move further out from the center, it gives
22:31
Speaker 1: you a picture for where that dark matter has to
22:33
Speaker 1: be to explain that velocity. The dark matter was all
22:36
Speaker 1: clumped together at the very center of the galaxy, then stars,
22:39
Speaker 1: you know, halfway out from the disk would be moving
22:41
Speaker 1: faster because it would be a stronger force from all
22:44
Speaker 1: of that gravity. Instead, if it's spread out really, really far,
22:48
Speaker 1: then some of that stuff is outside those stars and
22:50
Speaker 1: it doesn't affect them. It doesn't pull them towards the center,
22:53
Speaker 1: it doesn't speed them up as much. Yeah, I guess
22:56
Speaker 1: it's sort of like, if you're in the middle of
22:58
Speaker 1: a cloud of dark matter, you're not to feel its
23:00
Speaker 1: gravitation effects a much because it's pulling you in all directions,
23:04
Speaker 1: Whereas if you're really really far away from it, the
23:06
Speaker 1: whole blob, then you are going to feel sort of
23:08
Speaker 1: its entire gravity. Yeah, and so that's how we know
23:11
Speaker 1: that it's more dense in the middle. And that's kind
23:13
Speaker 1: of important, right, It is important, and it's not something
23:16
Speaker 1: that we really fully understand. Like if you do simulations
23:19
Speaker 1: and you say we think we understand how galaxies might
23:22
Speaker 1: have formed, and how this halo formed and all the
23:24
Speaker 1: dark matter swirls together to make this well that forms
23:27
Speaker 1: the galaxy, then we predict a certain shape for that
23:30
Speaker 1: dark matter density. We predicted to be sort of like
23:32
Speaker 1: peaky near the center, that like most of the dark
23:35
Speaker 1: matter should be right at the center and then should
23:38
Speaker 1: fall off kind of quickly. But what we observe when
23:40
Speaker 1: we go out there and we look at the dark
23:42
Speaker 1: matter see where it actually is based on these rotation curves,
23:46
Speaker 1: is that it's not as peaky near the center. It's
23:48
Speaker 1: more like a broad, flat core, like a big blob
23:52
Speaker 1: of dark matter, it's not as like pointed at the center.
23:55
Speaker 1: So that's a current mystery we don't really understand. Dark
23:58
Speaker 1: matter doesn't seem to be as clumped towards the center
24:01
Speaker 1: as we thought. And I guess part of it is that,
24:03
Speaker 1: you know, a lot of people sort of wonder like,
24:05
Speaker 1: if there is that much dark matter out there, why
24:07
Speaker 1: doesn't it just collapse into a dark matter black hole?
24:10
Speaker 1: But we we've talked about before how dark matter basically
24:13
Speaker 1: is not sticky with itself, like it doesn't feel besides gravity,
24:17
Speaker 1: It doesn't feel any other force that would make it
24:19
Speaker 1: stick together, Like our adoms have the electromagnetic force to
24:23
Speaker 1: make them stick, but dark matter doesn't appear to have
24:26
Speaker 1: something like that. And that force is important if you're
24:28
Speaker 1: going to fall into the black hole, because you have
24:30
Speaker 1: to have some way to lose your angular momentum. Dark matter,
24:33
Speaker 1: like everything else, is spinning and swirling, and the reason
24:36
Speaker 1: that things don't fall into a black hole is because
24:38
Speaker 1: they are swirling around it, the way the Earth is
24:41
Speaker 1: orbiting the Sun and not falling into it. For the
24:44
Speaker 1: Earth to fall into the Sun, it would have to
24:46
Speaker 1: somehow lose its velocity. It would have to bump into
24:49
Speaker 1: something we have to get slowed down. That only happens
24:52
Speaker 1: if there's some sort of like sticky force that can
24:55
Speaker 1: do that. So for a dark matter, that's really hard
24:58
Speaker 1: because it passes right through itself, it passes right through
25:01
Speaker 1: normal matter. It's very hard for it to lose its
25:03
Speaker 1: speed or its angular momentum. So that's why this halo
25:06
Speaker 1: of dark matter is bigger than the visible galaxy because
25:10
Speaker 1: dark matter actually finds it harder to collapse, harder to
25:13
Speaker 1: fall into black holes, right, you know, And so it's
25:16
Speaker 1: been this big diffusion. It has this interesting shape. So
25:18
Speaker 1: then how else can we sort of know about its
25:20
Speaker 1: structure besides it's sort of general blobby shape. Well, one
25:23
Speaker 1: of the listeners got it right thinking about how dark
25:26
Speaker 1: matter distorts the path of light. We can tell when
25:29
Speaker 1: there's a big blob of dark matter between us and
25:32
Speaker 1: something else because it acts like a lens in the sky.
25:36
Speaker 1: Remember that dark matter, even though it's invisible and light
25:39
Speaker 1: can pass through it, it does change the shape of space.
25:43
Speaker 1: And that means the space can act like a lens,
25:45
Speaker 1: and so light will pass through it, but it will
25:47
Speaker 1: get bent on the way. And so if there's a
25:49
Speaker 1: big blob of dark matter between us and a distant galaxy,
25:52
Speaker 1: for example, it will change the shape of that galaxy distorted,
25:56
Speaker 1: just as if there was a lens there. So we
25:58
Speaker 1: can use that to try to get idea is for
26:00
Speaker 1: where dark matter might be interesting. So dark matter does
26:04
Speaker 1: seem to clump within our galaxy? Is that what you're saying? Like,
26:07
Speaker 1: maybe within our galaxy there are spots where dark matter
26:10
Speaker 1: seems to be denser than others. That's hard to tell
26:14
Speaker 1: because this kind of gravitational effect is kind of rare,
26:17
Speaker 1: Like you need a clear background galaxy and then you
26:20
Speaker 1: need a blob of dark matter right in front of
26:22
Speaker 1: it has to be like perfectly lined up, so it's
26:25
Speaker 1: tough to use this. They call this strong gravitational lensing
26:29
Speaker 1: to get a clear picture for where the dark matter is,
26:31
Speaker 1: because we don't have really enough examples, so it's not
26:34
Speaker 1: a great way to tell where the dark matter is.
26:36
Speaker 1: A better way to tell if dark matter clumps up
26:39
Speaker 1: is to look for its effect on stars. So not
26:43
Speaker 1: just like the velocity of stars as they weave around
26:45
Speaker 1: the center of the galaxy, but their motion in other directions,
26:48
Speaker 1: like if there is a big clump of dark matter
26:51
Speaker 1: and especially dense blob of dark matter. It will affect
26:54
Speaker 1: how stars are moving around it. It will change the
26:57
Speaker 1: motion of those stars. It will attract them, it'll reflect them.
27:01
Speaker 1: I see. So if you look sort of look at
27:03
Speaker 1: the overall motion of all the stars in the galaxy,
27:05
Speaker 1: if you see that there are you know, sort of
27:07
Speaker 1: wiggles here and there or little you know, eddies or
27:10
Speaker 1: little clumps of stars forming, then you know that there's
27:13
Speaker 1: something else there and that it's not the dark matter
27:15
Speaker 1: is not perfectly smooth. Yes, And we recently launched a
27:19
Speaker 1: satellite called Gaya which is mapping the galaxy in four dimensions.
27:23
Speaker 1: It measures the position, the location of all these stars
27:26
Speaker 1: and their velocity. So we're getting this incredible map. It
27:30
Speaker 1: has like a billion stars with their position and their
27:33
Speaker 1: velocity map. Then we can use this to look for deviations.
27:36
Speaker 1: Were like, well, if dark matter was perfectly smooth, what
27:39
Speaker 1: would we expect all these stars to be doing. And
27:42
Speaker 1: are any stars doing anything weird? And if they are,
27:45
Speaker 1: we can sort of back that out and figure out
27:47
Speaker 1: where dark matter has to be to explain any weird
27:50
Speaker 1: patterns of the star motion. Right, Because I guess if
27:53
Speaker 1: dark matter was perfectly smooth, Peanut butter like this kind
27:56
Speaker 1: of smooth cloud. Then you would expect all the stars
28:00
Speaker 1: us to be basically moving along as if it was
28:03
Speaker 1: in a lazy river, right, like everyone sort of moving
28:05
Speaker 1: at the same You know, nobody would be going much
28:08
Speaker 1: faster than or slower than any of the other stars. Yeah,
28:10
Speaker 1: at the same radius, right, we expect as you go out,
28:13
Speaker 1: as you change your radius relative to the center of
28:15
Speaker 1: the galaxy, these things will change, just like they do
28:18
Speaker 1: in our solar system. Pluto's not moving around the Sun
28:21
Speaker 1: as fast as Jupiter, which is not moving as fast
28:23
Speaker 1: as Mercury, because as you go further out the gravitational
28:26
Speaker 1: force is weaker. But you'd expect things at the same
28:29
Speaker 1: radius to basically be having the same motion. And so
28:32
Speaker 1: if you see deviations, then you know dark matter is there,
28:35
Speaker 1: and we do sort of expect there to be clumps.
28:38
Speaker 1: We expect that the galaxy, for example, has lots of
28:40
Speaker 1: other galaxies inside of it that it has absorbed. We
28:44
Speaker 1: think that the history of our galaxy includes lots of
28:46
Speaker 1: collisions to form the Milky Way, and you would suspect
28:50
Speaker 1: that those galaxies might still have like their dark matter
28:53
Speaker 1: halos embedded within hours because I guess you would expect
28:57
Speaker 1: our matter to be clumpy because regg or matter is clumpy,
29:01
Speaker 1: so in a way, like wouldn't our regular matter also
29:04
Speaker 1: sort of catalyze or trigger dark matter to clump. Our
29:08
Speaker 1: matter is clumpy, But that's because it's sticky, right, It
29:10
Speaker 1: can form these blobs, so when gravity pulls it together,
29:14
Speaker 1: it sticks together and then that accumulates forms this runaway effect,
29:18
Speaker 1: whereas dark matter is not sticky, and so dark matter
29:21
Speaker 1: halos can pass right through each other without very much distortion.
29:24
Speaker 1: The other question is a cool one, like do stars
29:27
Speaker 1: form clumps of dark matter? And this is something people
29:30
Speaker 1: have studied. They've tried to look to see if there's
29:32
Speaker 1: like an intense blob of dark matter inside the sun,
29:35
Speaker 1: for example. But remember that there's much more dark matter
29:38
Speaker 1: than there is normal matter, and so dark matter sort
29:40
Speaker 1: of wins the gravitational battles. You would expect it to
29:43
Speaker 1: mostly go the other direction, that dark matter would influence
29:46
Speaker 1: the pattern of normal matter rather than vice versa. But yeah,
29:50
Speaker 1: it is a tug of war. Interesting you're saying dark
29:52
Speaker 1: matter is ignoring us. Basically it's ghosting us. It mostly can.
29:56
Speaker 1: But back to this question of like following the stars.
29:59
Speaker 1: There are some really cool things that we do see
30:02
Speaker 1: inside our galaxy. We can see the remnants of other
30:05
Speaker 1: galaxies that the Milky Way has eaten, little mini galaxies
30:09
Speaker 1: we call these dwarf galaxies, and some of these are
30:12
Speaker 1: really really interesting because they're super high in dark matter.
30:16
Speaker 1: Like our galaxy has a lot of dark matter, but
30:19
Speaker 1: some of these dwarf galaxies are almost entirely dark matter,
30:23
Speaker 1: and we can tell that they're there because we see
30:25
Speaker 1: stars orbiting these invisible dark matter halos. So there's sort
30:29
Speaker 1: of like many clumps of dark matter within our dark
30:33
Speaker 1: matter halo. Interesting, so it is clumpy, but maybe because
30:36
Speaker 1: we've we've added the clumps kind of yeah, because we've
30:39
Speaker 1: formed our big halo from a bunch of clumps. So
30:43
Speaker 1: we think these clumps formed initially each one of these
30:46
Speaker 1: its own galaxy, and then galaxies eventually do merge and
30:49
Speaker 1: collide and form bigger galaxies. And sometimes those dark matter
30:53
Speaker 1: halos don't necessarily spread out and just join like the
30:57
Speaker 1: original creamy blob of the Milky Way. They sort of
30:59
Speaker 1: stay there as chunks, and you can tell of they're
31:02
Speaker 1: because the stars swirling around those little dwarf galaxies. Well,
31:06
Speaker 1: here's the question. Do you think the dark matter in
31:08
Speaker 1: our galaxy is spinning also with the rest of the
31:11
Speaker 1: stars and galaxies, or is it just standing still. It's
31:14
Speaker 1: almost certainly spinning. That's the reason that it doesn't collapse
31:17
Speaker 1: into the black hole in the center. If it was
31:19
Speaker 1: standing still, then that gravity from that black hole would
31:22
Speaker 1: just suck it up. So it's almost certain that it's rotating.
31:25
Speaker 1: That we can't measure that directly, right, we haven't seen that,
31:28
Speaker 1: but we're fairly certain that it has to be otherwise
31:31
Speaker 1: it would have collapsed. So through strong gravitational lensing we
31:34
Speaker 1: can tell that there are some clumps out there, and
31:38
Speaker 1: through some of these absorbed galaxies we know there are
31:40
Speaker 1: clumps out there, but what else do we know about
31:42
Speaker 1: this clumpiness. We can also try to measure the clumpiness
31:45
Speaker 1: by looking at the effect of our gravity on things
31:48
Speaker 1: near the galaxy. So sometimes these mini galaxies or these
31:52
Speaker 1: globular clusters get sucked inside the galaxy. Sometimes they're in
31:55
Speaker 1: orbit around the galaxy. So for example, the large Magellanic
31:59
Speaker 1: Cloud is a ab of stuff that's sort of like
32:01
Speaker 1: a satellite galaxy of the Milky Way, and often these
32:04
Speaker 1: galaxies get torn apart, they don't hold themselves together. They
32:08
Speaker 1: turn into these streams. So around the galaxy there are
32:11
Speaker 1: these things called stellar streams, which are these like lines
32:15
Speaker 1: of stars moving in a loop sort of around the galaxies.
32:19
Speaker 1: It's sort of like the galaxy has rings of stars
32:22
Speaker 1: interesting like accidents almost, yeah, and they're sort of swooping
32:28
Speaker 1: around the galaxy. And those are very sensitive to the
32:31
Speaker 1: distribution of dark matter. So if, for example, the dark
32:35
Speaker 1: matter halo is clumpy, it will affect how those stars
32:38
Speaker 1: get pulled apart and whether they're like gaps in those streams.
32:42
Speaker 1: So there are people right now studying these stellar streams.
32:44
Speaker 1: They're like probes of that dark matter halo to look
32:47
Speaker 1: to see if there are clumps in the dark matter
32:49
Speaker 1: halo or to see if it's like perfectly spherical or
32:52
Speaker 1: kind of elliptical. So these are very nice ways to
32:55
Speaker 1: tell how much dark matter they're passing through and how
32:58
Speaker 1: clumpy it is. Interesting, and so that's one way to
33:01
Speaker 1: sort of know the clumpiness of dark matter. And what
33:04
Speaker 1: have we learned from all of these different ways. We
33:06
Speaker 1: don't have a great picture of where dark matter is
33:09
Speaker 1: in the galaxy. People often write in and ask like
33:12
Speaker 1: where is the dark matter? Can we see like planets
33:14
Speaker 1: of dark matter or that's kind of stuff. Really, we're
33:17
Speaker 1: not very sensitive to the details. We know that the
33:19
Speaker 1: Milky Way has a big blob of dark matter that
33:23
Speaker 1: it's probably elliptical, you know, it's not totally spherical. We
33:26
Speaker 1: can see some clumps where these faint dwarf galaxies were absorbed,
33:31
Speaker 1: but we don't have a great sense for the structure
33:33
Speaker 1: of the dark matter. It's mostly smooth, but we can't
33:36
Speaker 1: see things smaller than like, you know, tend to the
33:39
Speaker 1: six stars. I see, like the smallest clump we can
33:42
Speaker 1: sort of tell right now is is tend to the
33:45
Speaker 1: six billions of kilometers maybe tend to the six solar
33:48
Speaker 1: masses equivalents of dark matter? Is like the smallest chunk
33:52
Speaker 1: of things we can tell. Well, that's like our our
33:55
Speaker 1: best resolution of our picture of dark matter in the galaxy.
33:58
Speaker 1: It's like a pixel this side a million sons. So
34:02
Speaker 1: we're not very sensitive and that's just because it's mostly smooth.
34:05
Speaker 1: There aren't a lot of features to see, who we think,
34:08
Speaker 1: and because we're not very sensitive to it. Again, gravity
34:11
Speaker 1: is very weak and it's our only way of interacting
34:13
Speaker 1: with it. Which makes it kind of frustrating. All right, well,
34:16
Speaker 1: it sounds like it's still yet to be discovered. Who knows,
34:19
Speaker 1: Maybe it's forming giant dark matter squirrels or bananas or
34:23
Speaker 1: grass out there, but we just can't see it with
34:25
Speaker 1: our current resolution. And so let's get a little bit
34:28
Speaker 1: into what the overall picture of dark matter then is
34:30
Speaker 1: in the universe, and also what's happening between galaxies. But
34:34
Speaker 1: first let's take another quick break. All right, Daniel has
34:49
Speaker 1: lost his dark matter and his mind apparently. Did you
34:53
Speaker 1: lose your mind looking for the dark matter? I did.
34:55
Speaker 1: It's driving me crazy. Where are you? It's avoiding you,
34:59
Speaker 1: It's else see you. It is a little bit ghosting
35:02
Speaker 1: us as humanity because we know it's there, but it
35:05
Speaker 1: doesn't seem to want to make expressence known to us.
35:08
Speaker 1: In detail, we sort of have a big picture of
35:10
Speaker 1: it that it's in a big clump around the galaxy,
35:13
Speaker 1: mostly concentrated in the middle. We see some clumps out there,
35:15
Speaker 1: but we don't know the exact structure of dark matter.
35:18
Speaker 1: But we do sort of know it's density out there, right,
35:20
Speaker 1: We have some figures for its general density. Yeah, And
35:23
Speaker 1: it's quite interesting because we think that on average over
35:26
Speaker 1: the universe, dark matters like eight percent of the matter
35:30
Speaker 1: of the universe, but in our neighborhood it is actually
35:33
Speaker 1: a little bit different. The Milky Way, for example, we
35:35
Speaker 1: think is nine five dark matter. So our whole galaxy
35:39
Speaker 1: is kind of badly named. It should be called like
35:42
Speaker 1: the Dark Way or something, the Chocolate Milk Galaxy, the
35:45
Speaker 1: darklan Milky Way, the Dark Chocolate Way or something. So
35:49
Speaker 1: we're like nine percent dark matter, which means if we
35:52
Speaker 1: have like you know, the equivalent of ninety billion times
35:56
Speaker 1: the mass of the Sun in terms of stars and
35:59
Speaker 1: gas and all that kind of stuff, that means that
36:01
Speaker 1: there's like two trillion times the mass of the Sun
36:04
Speaker 1: in dark matter. It's just so much more. And it's
36:07
Speaker 1: like twenty times as much dark matter in our galaxy
36:11
Speaker 1: as normal matter, which is a bigger ratio than the
36:13
Speaker 1: rest of the universe. Well, that's true for regular matter.
36:16
Speaker 1: To write like our Milky Way has a higher density
36:19
Speaker 1: of regular matter than the rest of the universe or
36:22
Speaker 1: some other parts of the universe. Right, Yeah, that's true.
36:24
Speaker 1: But on average galaxies have about eighty percent dark matter,
36:28
Speaker 1: and our galaxy is so there's a big variation. Galaxy
36:32
Speaker 1: the galaxy and how much dark matter there is. Oh,
36:35
Speaker 1: I mean our galaxy has more than other galaxies. Absolutely, Yeah,
36:39
Speaker 1: we are a darker galaxy than most interesting we're more mysterious,
36:42
Speaker 1: I guess. Yeah. And there's some galaxies out there that
36:45
Speaker 1: are overwhelmingly dark matter, like nine points something per cent.
36:48
Speaker 1: And then there are some galaxies that have very little
36:50
Speaker 1: dark matter. We think that might be evidence of collisions
36:52
Speaker 1: where dark matter gets separated from the normal matter. All
36:55
Speaker 1: sorts of crazy stuff. These things tell you the crazy
36:58
Speaker 1: cosmic history of all of these objects. Interesting. And again
37:01
Speaker 1: you can tell by when you look at these galaxies
37:03
Speaker 1: out there. You can tell that they're holding on together
37:07
Speaker 1: more than they should by the number of stars or
37:09
Speaker 1: the brightness of them. Right. Yeah, you can tell when
37:11
Speaker 1: a galaxy is overwhelmingly dark matter because it's stars are
37:14
Speaker 1: moving super duper fast compared to how bright they are,
37:17
Speaker 1: And so we can see these faint dwarf galaxies, for example,
37:20
Speaker 1: just have a handful of stars, but they're whizzing around
37:23
Speaker 1: in a circle and there's not nearly enough gravity to
37:26
Speaker 1: hold them in place. Just from the stuff that we
37:28
Speaker 1: can see, so it's pretty cool, but it's so far away.
37:30
Speaker 1: How do you know it's not just like filled with
37:32
Speaker 1: the black holes or something or rocks dark rocks, because
37:36
Speaker 1: we can see light passing through it, right, we can
37:38
Speaker 1: see through it to something else behind it. If there
37:41
Speaker 1: was a black hole there, it would absorb the light.
37:43
Speaker 1: If it was just like a huge death star or
37:45
Speaker 1: something cloaked, then it would absorb that light. So we
37:48
Speaker 1: see it as invisible, not as dark all right, So then, um,
37:52
Speaker 1: it's sort of danser in our galaxy. What about in
37:55
Speaker 1: in our more immediate neighborhood or like a star solar
37:57
Speaker 1: system also extra dark mattery, it's and in our neighborhood.
38:01
Speaker 1: Remember that while the Milky Way is dark matter that
38:05
Speaker 1: is spread out throughout the stars, we think, so normal
38:08
Speaker 1: matter clumps up a lot more than dark matter, which
38:11
Speaker 1: means that there isn't that much dark matter in any
38:14
Speaker 1: like cubic light year of space. So in a cubic
38:17
Speaker 1: light year of space, there's less than a one quarter
38:20
Speaker 1: of one one thousands of the mass of the Sun.
38:23
Speaker 1: In a cubic light year of space, that's like a
38:26
Speaker 1: quarter of the mass of Jupiter in a cubic light
38:29
Speaker 1: year of space. That's how much dark matter there is.
38:31
Speaker 1: M t gives you took Jupiter and spread it over
38:34
Speaker 1: billions of miles, right, it wouldn't be very much. And
38:37
Speaker 1: you know, if you zoom in, for example, into like
38:39
Speaker 1: a cubic meter, that's like ten the minus twenty two
38:44
Speaker 1: grams of dark matter in a cubic meter. So you know,
38:47
Speaker 1: if you look at the space around you in your office,
38:50
Speaker 1: for example, then there's just like a super tiny amount
38:53
Speaker 1: of dark matter, almost hard to measure, but some of
38:56
Speaker 1: it's there are a few particles. And if you zoom
38:58
Speaker 1: out to like the whole volume of the Earth, there's
39:00
Speaker 1: less than a kilogram of dark matter in the volume
39:04
Speaker 1: of the Earth. Again, these are sort of approximations, right,
39:06
Speaker 1: because he told me earlier that our ability to resolve
39:09
Speaker 1: or a resolution of dark matter is pretty bad. So
39:12
Speaker 1: how do we know like that there isn't the sort
39:14
Speaker 1: of clump of dark matter just around us right now?
39:16
Speaker 1: We don't know absolutely, We do not know. We are
39:19
Speaker 1: not sensitive to these things, so it could be a
39:21
Speaker 1: lot clumpier than we think these numbers are, assuming that
39:24
Speaker 1: dark matter is mostly smoothly spread out throughout the galaxy
39:28
Speaker 1: according to the distribution that we've seen from the radius.
39:31
Speaker 1: But we absolutely cannot tell if there's like a huge
39:33
Speaker 1: blob of dark matter that we're sitting in, or if
39:36
Speaker 1: there's almost no dark matter in our neighborhood. And remember
39:39
Speaker 1: that we have experiments underground looking for dark matter particles.
39:42
Speaker 1: They're basically trying to measure how the Earth is moving
39:45
Speaker 1: through this dark matter wind, and they haven't seen anything.
39:49
Speaker 1: And one of my favorite explanations is like, well, maybe
39:51
Speaker 1: we're just happy to be sitting in a bubble that
39:53
Speaker 1: has almost no dark matter in it, which would make
39:56
Speaker 1: it impossible for us to detect that dark matter wind.
39:59
Speaker 1: We just don't know it's ghosting us and avoiding is
40:02
Speaker 1: physically at the same time. But it's kind of interesting
40:05
Speaker 1: because I think what you talked about earlier, how like
40:07
Speaker 1: in the volume of the Earth, there's about one squirrels
40:11
Speaker 1: worth full of dark matter. Like that's not a lot, right,
40:13
Speaker 1: and the whole Earth is pretty big, but you only
40:15
Speaker 1: have one squirrel full of dark matter, And that's why
40:18
Speaker 1: we can't ever detected gravitationally. You know, we're literally looking
40:21
Speaker 1: for a squirrel that's hiding inside the Earth, and that's
40:25
Speaker 1: pretty hard to tell the difference. We can't measure the
40:27
Speaker 1: number of squirrels on Earth. Using grabic, you'd go nuts.
40:33
Speaker 1: But yeah, so let's talk about then, now, sort of
40:35
Speaker 1: dark matter between galaxies, because you know, there's a lot
40:38
Speaker 1: of space between galaxies and we sort of have a
40:40
Speaker 1: pretty good idea of the structure of the universe, you know,
40:43
Speaker 1: the galaxy clusters and superclusters. Is does dark matter also
40:47
Speaker 1: follow these clusters? We think that mostly does, and again
40:50
Speaker 1: we think it's sort of the opposite that normal matter
40:53
Speaker 1: follows the path of dark matter. But it's much harder
40:56
Speaker 1: to see the things between galaxies because there's much less
41:01
Speaker 1: light there and there's much less visible objects. Like mostly
41:04
Speaker 1: we have seen where dark matter is within our galaxy
41:07
Speaker 1: by following the path of stars, their rotation, their wiggles,
41:10
Speaker 1: their distortions, all that kind of stuff where there are
41:13
Speaker 1: in stars like tracers to tell us where things are
41:16
Speaker 1: between galaxies. So it's much trickier. Yeah, I guess you
41:20
Speaker 1: you can sort of extrapolate, right what we can see
41:22
Speaker 1: a little bit around this, then you sort of assume
41:25
Speaker 1: that that's what's happening maybe in the rest of the
41:27
Speaker 1: universe sort of, But we know that the galaxies are
41:29
Speaker 1: very different from the rest of the universe. Like, we
41:31
Speaker 1: know that there's a huge gravitational well that we are sitting,
41:35
Speaker 1: and that's why there's a galaxy right here, there's a
41:37
Speaker 1: big blob of dark matter. What does it look like
41:40
Speaker 1: between our galaxy and Andromeda? You know, are there strands
41:43
Speaker 1: of dark matter? How quickly does it pete route? Are
41:46
Speaker 1: there blobs of dark matter out there without any stars
41:49
Speaker 1: in them at all? And so one way we can
41:51
Speaker 1: try to figure that out is to look at how
41:53
Speaker 1: light from distant galaxies is distorted as it passes through
41:57
Speaker 1: that space. I see, based can do the gravitational lensing
42:01
Speaker 1: but with galaxies and look for blobs in between galaxy.
42:04
Speaker 1: But then these blobs we got to be humongous, right,
42:06
Speaker 1: those blobs would have to be humongous. And in this
42:09
Speaker 1: case we use a slightly different technique than we do
42:12
Speaker 1: for looking at like one specific blob before. What we
42:15
Speaker 1: were doing is called strong lensing, And that's like, I
42:18
Speaker 1: want to have a blob of dark matter right between
42:20
Speaker 1: me and another galaxy, so I can see like a
42:22
Speaker 1: massive distortion. You can see one galaxy how it's distorted,
42:25
Speaker 1: and you can use that to measure the massive stuff
42:28
Speaker 1: between you and other galaxies. If instead you think the
42:30
Speaker 1: dark matter is sort of spread out, so it doesn't
42:33
Speaker 1: really distort any individual photon that much. You can do
42:37
Speaker 1: something called weak gravitational lensing where you look at lots
42:41
Speaker 1: of galaxies and you look for lots of very small
42:44
Speaker 1: distortions and you sort of add them up statistically to
42:47
Speaker 1: get a map for where the dark matter might be
42:49
Speaker 1: and where it might not be. So you see sort
42:52
Speaker 1: of like fewer generalized distortions over here and more generalized
42:55
Speaker 1: distortions over there. It can tell you sort of where
42:58
Speaker 1: the dark matter is dens or and where it's less dense. Interesting,
43:02
Speaker 1: you started looking for sort of wiggles in the overall picture.
43:05
Speaker 1: But how would you know that is dark matter? Would
43:07
Speaker 1: that be changing? Are you assuming that it changes as
43:10
Speaker 1: our view of the universe changes. Well, we think we
43:12
Speaker 1: know what galaxies should look like when they're not distorted,
43:15
Speaker 1: and so we compare how galaxies look too ideas of
43:19
Speaker 1: how a galaxy should look when it's not distorted, and
43:21
Speaker 1: how it should look when it's slightly distorted by dark matter.
43:24
Speaker 1: And so we can use that to estimate like how
43:27
Speaker 1: much distortion galaxies have. But it's very very weak. You know,
43:30
Speaker 1: it's hard to tell the difference between a galaxy that's
43:32
Speaker 1: undistorted and slightly distorted. And that's why we need like
43:36
Speaker 1: thousands of galaxies to add this up statistically to get
43:39
Speaker 1: a sense for where the dark matter is interesting. And
43:41
Speaker 1: this is like an ongoing thing, right, Like there's people
43:44
Speaker 1: looking for these ripples in our view of the universe. Yeah,
43:47
Speaker 1: this is recent. Actually there's a program using a huge
43:49
Speaker 1: telescope with a massive camera five hundred and seventy megapixels.
43:53
Speaker 1: It's called the Dark Energy Survey, and this camera is
43:56
Speaker 1: basically build just to do this, just to look at
43:59
Speaker 1: all the galaxies and build a huge map. And they've
44:02
Speaker 1: studied a hundred million galaxies out there, like think about
44:06
Speaker 1: all the crazy stars and planets and everything that's out there,
44:09
Speaker 1: a hundred million of those. And they have built a
44:12
Speaker 1: map of where they think the dark matter is between
44:15
Speaker 1: galaxies using this weak lensing idea. Because I guess we
44:19
Speaker 1: can tell how old they are the galaxies, right, and
44:21
Speaker 1: how far away they are from from us, not just
44:23
Speaker 1: in the night sky, and so we can build the
44:25
Speaker 1: three D map right exactly. We know where galaxies are
44:28
Speaker 1: because we can look at like Type one A supernova
44:30
Speaker 1: within them. We can measure their brightness, and we can
44:33
Speaker 1: tell how far away they are based on how bright
44:35
Speaker 1: they appear to be here on Earth. So we have
44:38
Speaker 1: this incredible three D map of all the galaxies and
44:41
Speaker 1: then this thing is taking careful pictures of them to
44:43
Speaker 1: try to estimate how much each one is distorted, and
44:46
Speaker 1: then it's comparing that to our idea for where we
44:49
Speaker 1: think the dark matter should be. We have an idea
44:51
Speaker 1: for where we think dark matter should be based on
44:53
Speaker 1: where all the galaxies are. Sort of back that up
44:56
Speaker 1: to the early universe and say where were the dark
44:59
Speaker 1: matter are have to have been in order to make
45:02
Speaker 1: these galaxies end up here and that galaxy end up
45:05
Speaker 1: there to sort of create the large scale structure that
45:07
Speaker 1: we see, because we think that mostly where galaxies ended
45:11
Speaker 1: up depends on where dark matter was. So we have
45:14
Speaker 1: like a simulation for where we think the dark matter
45:16
Speaker 1: should be based on our idea of how it all works.
45:19
Speaker 1: And then we go out of measure and build a
45:21
Speaker 1: real map of where the dark matter is, and then
45:23
Speaker 1: we can compare the two and that's when the fund starts. WHOA,
45:26
Speaker 1: So what have we found? Do they match or are
45:29
Speaker 1: they very different? They mostly match, like it mostly makes sense.
45:32
Speaker 1: The dark matter is mostly where we expect, but there
45:35
Speaker 1: are some deviations. It looks like dark matter sort of
45:37
Speaker 1: more spread out than we expected. Instead of being in
45:40
Speaker 1: these like thin strands between galaxies, it tends to be
45:44
Speaker 1: sometimes in places where you don't expect. It's like spread
45:47
Speaker 1: out and globbed out more than we expected, more than
45:50
Speaker 1: our simulations predict. It's just a few percent compared to
45:54
Speaker 1: our predictions, but it's an important deviation. It means that
45:57
Speaker 1: like maybe something's going on with that dark matter, or
46:00
Speaker 1: maybe dark matter is made out of something weird we
46:01
Speaker 1: didn't understand, or maybe we've made a mistake in building
46:04
Speaker 1: our map of dark matter. But it's sort of like
46:06
Speaker 1: at the edge of science. These are very very recent
46:09
Speaker 1: results that dark matter just wants to be alone between
46:14
Speaker 1: the galaxy or maybe it got timed out. But I
46:16
Speaker 1: guess what's surprising is that there is dark matter between
46:18
Speaker 1: galaxies because there's not much matter out there, so why
46:22
Speaker 1: wouldn't this dark matter also accumulate regular matter. There is
46:26
Speaker 1: actually a good bit of regular matter out there is
46:28
Speaker 1: it's just not glowing like a huge amount of the
46:31
Speaker 1: atoms in the universe are actually between galaxies and this
46:35
Speaker 1: intergalactic matter, these streams of stuff, and so there's a
46:39
Speaker 1: good bit of stuff out there. There's just not enough
46:41
Speaker 1: density to form stars and planets and galaxies and all
46:45
Speaker 1: kinds of stuff. And so that's why it's not as
46:48
Speaker 1: visible because it doesn't glow interesting. But then what what
46:51
Speaker 1: make some dark matter have a lot of bright matter
46:54
Speaker 1: in it and some nut the denser blobs of dark
46:57
Speaker 1: batter did form enough stuff to create galaxies and stars.
47:00
Speaker 1: The kind of strands we're talking about between galaxies is
47:04
Speaker 1: a smaller fraction of the amount of dark matter. The
47:06
Speaker 1: density is not there in order to create the gravitational Well,
47:09
Speaker 1: you need to attract enough hydrogen to get stars to form.
47:13
Speaker 1: Pretty cool, so the galaxy still show you where like
47:16
Speaker 1: the densest blobs of dark matter are in the universe,
47:19
Speaker 1: Like the dark matter also various in density out there. Absolutely, yeah, alright,
47:23
Speaker 1: well it sort of sounds like do you sort of
47:25
Speaker 1: know where dark matter is but maybe not too super
47:27
Speaker 1: high resolution where you can tell if it's super clumpy
47:31
Speaker 1: or super smooth. And also we have a pretty good
47:34
Speaker 1: picture of where it is in the whole universe. Yeah,
47:36
Speaker 1: and scientists are working very hard to build more and
47:39
Speaker 1: more sensitive tools to try to use these little gravitational
47:43
Speaker 1: clues to build as accurate and as localized a map
47:46
Speaker 1: of dark matter as possible, because the better map of
47:49
Speaker 1: dark matter we get in our galaxy, the more we
47:51
Speaker 1: can study what it might be and how it came
47:54
Speaker 1: to be where it is. Yeah. I guess the big
47:57
Speaker 1: question now is what are you gonna do when you
47:58
Speaker 1: find it? Daniel retire. Yeah, like the dog that finally
48:03
Speaker 1: catches up the car and doesn't know what to do
48:05
Speaker 1: with it. Oh, we'll find some other mystery to focus on.
48:08
Speaker 1: You'll come up with another cool name darkest. We always
48:11
Speaker 1: have the seventy of the universe called dark energy that
48:14
Speaker 1: we haven't even gotten started on. Yeah, that's another huge mystery,
48:17
Speaker 1: huge hole in our view of the universe and them.
48:20
Speaker 1: Hopefully you won't lose your mind trying to find them.
48:22
Speaker 1: I lost eight years ago. All right, Well, another awesome
48:27
Speaker 1: reminder that there's still a lot of universe out there
48:29
Speaker 1: to be discoverned. You know, anyone listening to this could
48:31
Speaker 1: be the person that comes up with the next great
48:33
Speaker 1: idea or the next grade concept that makes sense of
48:37
Speaker 1: all this and helps us find these big mysteries in
48:39
Speaker 1: the universe. If you're in trance by the concept of
48:42
Speaker 1: building maps and discovering the way the world is and
48:45
Speaker 1: how everything looks. Remember, the biggest map of the most
48:48
Speaker 1: important stuff in the universe is still being drawn. You
48:51
Speaker 1: can really lose yourself in that search for lost things.
48:54
Speaker 1: Come join me in the asylum or the basement Lost
48:57
Speaker 1: and Found department. Well, if they for joining us, we
49:00
Speaker 1: hope you enjoyed that. See you next time. Thanks for listening,
49:11
Speaker 1: and remember that Daniel and Jorge explained. The Universe is
49:14
Speaker 1: a production of I Heart Radio. Or more podcast from
49:18
Speaker 1: my Heart Radio visit the I Heart Radio app, Apple Podcasts,
49:22
Speaker 1: or wherever you listen to your favorite shows.