Latest / Daniel and Kelly’s Extraordinary Universe / What's the matter with matter in the Universe?

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What's the matter with matter in the Universe? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, what's your favorite kind of pie?
• 0:10
  Speaker 2: Oh? Boy, what's not my favorite? I mean, they're all
• 0:13
  Speaker 2: so good.
• 0:14
  Speaker 1: Are you a fan of chocolate pie?
• 0:15
  Speaker 3: You know?
• 0:15
  Speaker 2: I am? I mean who isn't?
• 0:17
  Speaker 1: What about white chocolate pie?
• 0:19
  Speaker 2: Hold on? Is that a thing? I mean, who would
• 0:22
  Speaker 2: do that to a pie?
• 0:23
  Speaker 1: I'm sure there's a whole universe of pie out there.
• 0:26
  Speaker 1: I imagine there's a you know, someone in this infinite
• 0:28
  Speaker 1: universe that thinks white chocolate pie is the best.
• 0:30
  Speaker 2: If so, then that's definitely proof that aliens exist.
• 0:33
  Speaker 1: But I thought your favorite might be the number pie
• 0:36
  Speaker 1: pie are squared? Give you the area of the pie,
• 0:39
  Speaker 1: the actual pie you want to eat.
• 0:43
  Speaker 3: I guess pie are around pie for everyone.
• 1:01
  Speaker 1: Hi am Jorge Made, cartoonist and the author of Oliver's
• 1:03
  Speaker 1: Great Big Universe.
• 1:05
  Speaker 2: Hi. I'm Daniel. I'm a particle physicist and a professor
• 1:07
  Speaker 2: at UC Irvine, and I'll try almost any kind.
• 1:10
  Speaker 1: Of pie, really, any kind of pie. What about Durian pie?
• 1:14
  Speaker 1: Have you tried that one?
• 1:15
  Speaker 2: I might have to hold my nose, but I hear
• 1:17
  Speaker 2: Durian's delicious.
• 1:18
  Speaker 1: Yeah, I've come across Durian pie, and you kind of
• 1:22
  Speaker 1: do have to hold your nose. If it's not something
• 1:24
  Speaker 1: you're used to, it's pretty powerful stuff.
• 1:26
  Speaker 2: Sounds like an exotic adventure.
• 1:27
  Speaker 1: But anyways, welcome to our podcast, Daniel and Jorge Explain
• 1:30
  Speaker 1: the Universe, a production of iHeartRadio.
• 1:32
  Speaker 2: In which we take you on an exotic tour through
• 1:35
  Speaker 2: the craziness and mysteries of our universe. We hope to
• 1:38
  Speaker 2: explain to you everything that makes sense to us and
• 1:41
  Speaker 2: everything that doesn't make sense to us, about the way
• 1:43
  Speaker 2: this amazing, beautiful, sometimes tasty, and sometimes stinky universe works.
• 1:48
  Speaker 1: That's right. It is a delicious universe and fragrant as well,
• 1:51
  Speaker 1: full of amazing facts and incredible processes going on, all
• 1:54
  Speaker 1: of which we take together, mix it up, put it
• 1:57
  Speaker 1: inside of a crust, and bake it for you here
• 1:59
  Speaker 1: on the podcast.
• 2:00
  Speaker 2: One of the deepest but most concrete mysteries of the
• 2:02
  Speaker 2: universe is what's it all made out of? Anyway? What's
• 2:06
  Speaker 2: out there in the universe? How much of it is
• 2:08
  Speaker 2: this stuff? How much of it is that stuff? And
• 2:11
  Speaker 2: why Daniel, would you eat mystery pie?
• 2:13
  Speaker 1: Like, if somebody gave you mystery pie, would you still
• 2:16
  Speaker 1: eat it?
• 2:17
  Speaker 2: Depends on the somebody.
• 2:18
  Speaker 1: I gotta say, Yeah, I guess if it's like a
• 2:21
  Speaker 1: ten year old, you don't want to try a mystery pie.
• 2:24
  Speaker 1: But if it's like a famous chef, maybe.
• 2:25
  Speaker 2: Yeahah, sure, absolutely, yeah, I'll eat a mystery pie and
• 2:28
  Speaker 2: no problem.
• 2:29
  Speaker 1: What if it's like a world famous ten year old chef.
• 2:32
  Speaker 2: Is this some reality prank show? Then probably no.
• 2:38
  Speaker 1: That sounds like a great idea for the next YouTube
• 2:41
  Speaker 1: pit mystery pie.
• 2:43
  Speaker 2: But one reason you might take a bite of mystery
• 2:45
  Speaker 2: pie is just your curiosity. You want to know what
• 2:48
  Speaker 2: kind of pies are out there in the universe, what's
• 2:50
  Speaker 2: possible to make out of the basic building blocks? What
• 2:53
  Speaker 2: kinds of stuff are there in the universe? In the end,
• 2:56
  Speaker 2: that provides some crucial context to the nature of our existence.
• 3:00
  Speaker 2: Are we made out of the most common stuff? What
• 3:02
  Speaker 2: other kinds of stuff is out there? How does it
• 3:04
  Speaker 2: all come together to make this glorious cosmos?
• 3:07
  Speaker 1: I guess the universe is kind of like a deep
• 3:09
  Speaker 1: dish pie of mystery, isn't it, Because there's so much
• 3:12
  Speaker 1: we don't know about it. Ninety five percent of the
• 3:15
  Speaker 1: universe is a total mystery.
• 3:16
  Speaker 2: To us exactly. And we can take this pie metaphor
• 3:18
  Speaker 2: one step further and describe what the universe is made
• 3:21
  Speaker 2: out of In terms of a pie chart, Basically, some
• 3:24
  Speaker 2: fraction of the universe is made out of our kind
• 3:26
  Speaker 2: of matter, another fraction is made out of dark matter,
• 3:29
  Speaker 2: A much bigger fraction is dark energy, mysterious stuff we
• 3:33
  Speaker 2: don't even know. So understanding the universe pie is a
• 3:36
  Speaker 2: basic goal of physics.
• 3:38
  Speaker 1: Yeah, that's a very basic and deep question about the
• 3:40
  Speaker 1: universe that you can ask, what is the universe made
• 3:43
  Speaker 1: out of? What is it all about? And so we've
• 3:46
  Speaker 1: discovered some pretty eye opening facts about the universe, and
• 3:49
  Speaker 1: one of those facts is the idea that the kind
• 3:51
  Speaker 1: of stuff we're made out of, You and I, apples
• 3:54
  Speaker 1: and chocolate and white chocolate. Also, they're all made out
• 3:57
  Speaker 1: of kind of stuff that only represents five percent of
• 4:00
  Speaker 1: the entire universe.
• 4:01
  Speaker 2: Some of the most delicious moments in physics are those
• 4:04
  Speaker 2: times when you realize that everything you've understood is just
• 4:07
  Speaker 2: a tiny fraction of reality, that you've been looking at
• 4:10
  Speaker 2: it like an unrepresentative sample of everything, and that the
• 4:14
  Speaker 2: wider context is very different from what you imagined. It's
• 4:17
  Speaker 2: like you're opening a third eye to understanding what the
• 4:20
  Speaker 2: universe is really like. And in the last few decades.
• 4:23
  Speaker 2: That's been our experience discovering that most of the universe
• 4:26
  Speaker 2: is not the kind of stuff that makes up me
• 4:28
  Speaker 2: and you and hamsters and lamas and pies.
• 4:31
  Speaker 1: And stars and galaxies. It must have been pretty incredible
• 4:34
  Speaker 1: to think that all of these thousands of years of
• 4:37
  Speaker 1: science and knowledge seeking as humans have all been just
• 4:40
  Speaker 1: for the five percent of the universe that we now
• 4:43
  Speaker 1: know we represent.
• 4:44
  Speaker 2: Yeah, and it's sort of a special moment in science
• 4:46
  Speaker 2: when we know very well how little we know. We're
• 4:49
  Speaker 2: always prepared for the fact that there's stuff we don't
• 4:51
  Speaker 2: know out there. But now we can measure very precisely
• 4:54
  Speaker 2: what fraction of the universe is the stuff we know
• 4:57
  Speaker 2: anything about, which tells us very accurately how little we
• 5:01
  Speaker 2: know about the universe.
• 5:02
  Speaker 1: Yeah, And so today on the podcast, we'll be asking
• 5:04
  Speaker 1: the question, what's the matter with the matter in the universe?
• 5:13
  Speaker 1: Something wrong with the universe? Or is it more like,
• 5:15
  Speaker 1: what's the matter with you? Like it's doing something it
• 5:18
  Speaker 1: shouldn't be doing.
• 5:19
  Speaker 2: Well, there's lots of really interesting questions you can ask
• 5:21
  Speaker 2: about this universe. Pie the fractions of the universe that
• 5:24
  Speaker 2: are made up of the various components. One of them
• 5:26
  Speaker 2: is like why is it this way? Another another way?
• 5:28
  Speaker 2: But you can also ask questions like has it always
• 5:31
  Speaker 2: been this way? Has the universe had different distributions of
• 5:34
  Speaker 2: matter and dark matter and dark energy at other points
• 5:37
  Speaker 2: in its history? And what does the future hold?
• 5:41
  Speaker 1: Like how long has this pie been sitting out there?
• 5:43
  Speaker 1: And has it changed or decayed or rotted or changed
• 5:48
  Speaker 1: to another kind of pie In the meantime?
• 5:50
  Speaker 2: Does it matter when you get a slice of the
• 5:52
  Speaker 2: universe pie, You might get a very different serving. It
• 5:55
  Speaker 2: might be more chocolatey or less chocolity, depending on when
• 5:58
  Speaker 2: you got your slice.
• 6:00
  Speaker 1: Oh, like not just a mystery pie, but like an
• 6:02
  Speaker 1: ever changing mystery pie.
• 6:04
  Speaker 2: Exactly because the universe is a dynamic place. It's not static.
• 6:08
  Speaker 2: It's expanding and it's cooling, and it's flashing around and frothing,
• 6:12
  Speaker 2: it's making galaxies and black holes. All sorts of stuff
• 6:15
  Speaker 2: is happening, and so we have no reason to believe
• 6:17
  Speaker 2: that the Universe pie today is the Universe pie that's
• 6:20
  Speaker 2: always been.
• 6:21
  Speaker 1: Do you think the universe pie is getting more delicious
• 6:23
  Speaker 1: or less delicious?
• 6:25
  Speaker 2: I'm an optimist, so I like to think that the
• 6:27
  Speaker 2: Universe pies most delicious days are ahead of us.
• 6:30
  Speaker 1: Oh wait, so that means you're never going to eat
• 6:32
  Speaker 1: some are you until the day before you die or something?
• 6:36
  Speaker 2: No, it means I'll continuously eat it and every bite
• 6:38
  Speaker 2: will be more tasty than the last.
• 6:40
  Speaker 1: Oh but what if it's not infinite? What if it's finite?
• 6:43
  Speaker 1: Then you got to play some sort of like you know,
• 6:45
  Speaker 1: optimization game here? How much of the pie should you
• 6:48
  Speaker 1: eat every day?
• 6:49
  Speaker 2: You think there's some scenario where I eat the last
• 6:51
  Speaker 2: bite of the universe and use it up.
• 6:55
  Speaker 1: I don't know how hungry are you for knowledge about
• 6:58
  Speaker 1: the universe?
• 6:59
  Speaker 2: Famished, But you know what they say that last bite
• 7:01
  Speaker 2: is always the most delicious, like the last cookie in
• 7:04
  Speaker 2: the bag.
• 7:04
  Speaker 1: Yeah yeah, but although if you eat it right before
• 7:06
  Speaker 1: you talk, you're not going to have a very long
• 7:09
  Speaker 1: lasting memory of it.
• 7:10
  Speaker 2: But I guess if I eat the entire universe, then
• 7:12
  Speaker 2: I'm the last bite of the universe. I'd have to
• 7:14
  Speaker 2: like eat myself.
• 7:15
  Speaker 1: Wait, are you saying you're the most delicious thing in
• 7:17
  Speaker 1: the universe?
• 7:20
  Speaker 2: If I eat everything else, then I'm the only thing
• 7:22
  Speaker 2: left in the universe, and I'm both the most and
• 7:24
  Speaker 2: least delicious thing in this very realistic example.
• 7:29
  Speaker 1: Right right, Yeah, yeah, I don't know that. I think
• 7:32
  Speaker 1: our whole analogy just just broke down.
• 7:34
  Speaker 2: Yeah, Daniel contains super massive black holes.
• 7:37
  Speaker 1: But anyways, we have found out that the pie of
• 7:39
  Speaker 1: the universe is made out of only five percent of
• 7:42
  Speaker 1: the stuff that we're familiar with. The rest of the
• 7:44
  Speaker 1: stuff is a big mystery. And as you said, Daniel,
• 7:47
  Speaker 1: these percentages have been changing exactly.
• 7:49
  Speaker 2: You might think that five percent is a minuscule fraction
• 7:52
  Speaker 2: of the universe, but wait till you hear about the
• 7:55
  Speaker 2: fraction that it's been in the past.
• 7:57
  Speaker 1: Well, it's usually what we were wondering how many people
• 7:59
  Speaker 1: out there had thought about the pighness of the universe
• 8:02
  Speaker 1: and its composition, And so Daniel went out there and
• 8:04
  Speaker 1: asked folks, has normal matter always been five percent of
• 8:08
  Speaker 1: the universe?
• 8:09
  Speaker 2: Thanks very much to everybody who answers these questions. I
• 8:12
  Speaker 2: really appreciate it, and we love hearing your voices on
• 8:15
  Speaker 2: the podcast. Gives us the sense that the podcast is
• 8:17
  Speaker 2: not one directional. It's an interactive learning experience. So if
• 8:21
  Speaker 2: you'd like to participate in the future, please write to
• 8:23
  Speaker 2: me two questions at Danielandjorge dot com.
• 8:26
  Speaker 1: So think about it for a second. Do you think
• 8:29
  Speaker 1: normal matter has always been five percent of the universe.
• 8:34
  Speaker 1: Here's what people had to say.
• 8:36
  Speaker 4: Well, I guess you've got normal matter, and you've got
• 8:39
  Speaker 4: dark matter, and you've got dark energy. I know that
• 8:42
  Speaker 4: dark energy is getting bigger as the universe expands, so
• 8:47
  Speaker 4: I would guess that when the universe was really small,
• 8:50
  Speaker 4: just after the Big Bang, there wasn't any dark energy,
• 8:53
  Speaker 4: so therefore normal matter would have been a much greater proportion.
• 9:00
  Speaker 5: I don't think that there is any reason for that
• 9:03
  Speaker 5: percentage to be constant, but to assume that is also
• 9:07
  Speaker 5: to assume that regular matter, dark matter, or dark energy
• 9:10
  Speaker 5: to be created spontaneously.
• 9:13
  Speaker 6: No, norm matter has not always been five percent of
• 9:16
  Speaker 6: the universe dark or the Big Bang. As far as
• 9:18
  Speaker 6: I know, normal matter an inche matter almost out of
• 9:20
  Speaker 6: one ratio, but no matter one out is there for
• 9:23
  Speaker 6: a part of our own universe. As for dark round
• 9:25
  Speaker 6: and dark energy, I don't know about that part.
• 9:26
  Speaker 1: All right, awesome answers here. Most people just assume that
• 9:30
  Speaker 1: it hasn't always been five percent.
• 9:31
  Speaker 2: Yeah, I think people are ready to be surprised.
• 9:34
  Speaker 1: I guess maybe people are used to having a universe
• 9:36
  Speaker 1: be always changing, so like, why would it be the same.
• 9:39
  Speaker 2: Yeah, that's true, although the universe is fourteen billion years old,
• 9:42
  Speaker 2: so you might speculate that it's kind of grown up
• 9:45
  Speaker 2: already and not going to change that much anymore.
• 9:47
  Speaker 1: M all right, well let's dig into it, Daniel. Let's
• 9:50
  Speaker 1: first of all talk about this pie and where it
• 9:52
  Speaker 1: came from, and what we know about the pie of
• 9:53
  Speaker 1: the universe, how much normal matter is there now, and
• 9:57
  Speaker 1: is it really that normal?
• 10:00
  Speaker 2: Normal is really a very inappropriate label for our kind
• 10:03
  Speaker 2: of matter, because we're not the biggest chunk of the
• 10:05
  Speaker 2: pie at all. Really, we should call that like familiar matter,
• 10:09
  Speaker 2: or just our kind of matter, or.
• 10:10
  Speaker 1: Something delicious matter. Apparently we are the most delicious things
• 10:13
  Speaker 1: in the universe.
• 10:14
  Speaker 2: And so on the podcast, we often talk about the
• 10:16
  Speaker 2: universe pie in three different categories normal matter, dark matter,
• 10:21
  Speaker 2: and dark energy. Physicists usually break this down into four
• 10:25
  Speaker 2: different categories, where they distinguish normal matter and radiation into
• 10:30
  Speaker 2: two different categories. So let's break that down. Normal matter
• 10:34
  Speaker 2: is the kind of stuff that you and I are
• 10:36
  Speaker 2: made out of, things built out of quarks and electrons,
• 10:39
  Speaker 2: which usually go to making protons and neutrons, and then
• 10:41
  Speaker 2: electrons make atoms, etc. So all the atomic matter, but
• 10:45
  Speaker 2: it also includes things made out of exotic quarks, top
• 10:48
  Speaker 2: quarks and down corks and bottom quarks, stuff that doesn't
• 10:52
  Speaker 2: make up the proton and the neutron but can be
• 10:54
  Speaker 2: created in the universe. All this kind of stuff we
• 10:56
  Speaker 2: call baryonic matter or normal matter.
• 11:00
  Speaker 1: Guys, all that stuff is stuff that has mass. Like,
• 11:03
  Speaker 1: those are all particles with mass.
• 11:05
  Speaker 2: Those are all particles with mass, but not all particles
• 11:08
  Speaker 2: with mass are in that category. Dark matter, for example,
• 11:11
  Speaker 2: we think has mass, but is not in that.
• 11:13
  Speaker 1: Category assuming it's a particle though.
• 11:15
  Speaker 2: Assuming it's a particle or some kind of matter with mass.
• 11:18
  Speaker 2: So yeah, all the normal matter has mass. That's one
• 11:20
  Speaker 2: of the distinctions, is this draw between matter and radiation.
• 11:24
  Speaker 2: Radiation is things that are effectively massless, things moving at
• 11:27
  Speaker 2: or very near the speed of light. So photons, for example,
• 11:31
  Speaker 2: are radiation. That's obvious. They have always been radiation, they
• 11:34
  Speaker 2: always will be radiation. Neutrinos are very very low mass
• 11:38
  Speaker 2: particles that move at almost to the speed of light,
• 11:40
  Speaker 2: so we count them as radiation even though technically they're
• 11:44
  Speaker 2: made out of stuff. They have mass to them.
• 11:46
  Speaker 1: Wait, neutrinas we don't count as normal matter. Why not.
• 11:49
  Speaker 2: We count them in the sort of normal slice, but
• 11:51
  Speaker 2: we don't call them matter. We call them radiation if
• 11:54
  Speaker 2: they're moving near the speed of light. And you'll see
• 11:56
  Speaker 2: why this distinction is important when we talk about how
• 11:58
  Speaker 2: the universe expand, because the expansion of the universe affects
• 12:02
  Speaker 2: radiation and matter differently.
• 12:04
  Speaker 1: All right, then what are the other slices of the pie.
• 12:07
  Speaker 2: One more caveat on the matter radiation distinction is that
• 12:10
  Speaker 2: the same particle can be in a different category depending
• 12:13
  Speaker 2: on its speed. If you take an electron, for example,
• 12:16
  Speaker 2: and it's just sitting there, it's not moving, it has mass,
• 12:19
  Speaker 2: it's not moving near the speed of light, you call
• 12:21
  Speaker 2: it matter. Speed up that electron to nearly the speed
• 12:24
  Speaker 2: of light, physicists now call that radiation because it's effectively massless,
• 12:29
  Speaker 2: its energy is so much bigger than its mass. It's
• 12:32
  Speaker 2: going near the speed of light, now you'd call that radiation.
• 12:35
  Speaker 2: So the same kind of particles can move from one
• 12:37
  Speaker 2: category to the other as the universe cools or heats, So,
• 12:41
  Speaker 2: as usual, physicists have given things kind of confusing.
• 12:44
  Speaker 1: Names, also kind of arbitrary. Is it's like when it
• 12:48
  Speaker 1: goes kind of fast, it's totally categorized it as one thing,
• 12:53
  Speaker 1: and when it goes kind of slow, it's totally something
• 12:56
  Speaker 1: totally different. Doesn't seem very scientific, Daniel.
• 12:59
  Speaker 2: Yeah, well, there are differences in the way the universe
• 13:02
  Speaker 2: treats things moving near the speed of light or at
• 13:04
  Speaker 2: the speed of light, and things moving slower. It's not
• 13:07
  Speaker 2: a bright line between them. There's a transition there. But
• 13:10
  Speaker 2: that's true for basically all of science. You know, it
• 13:13
  Speaker 2: was it different between water vapor and water liquid. There
• 13:16
  Speaker 2: is a difference, right, It's not a bright line, there's
• 13:18
  Speaker 2: a transition there between them. But we notice these trends
• 13:21
  Speaker 2: and we draw a dotted line and we treat them
• 13:23
  Speaker 2: as different stuff. So here physicists notice that as the
• 13:26
  Speaker 2: universe expands, matter and radiation get treated differently, so they've
• 13:30
  Speaker 2: drawn this line between them. Though you're right, it's really
• 13:33
  Speaker 2: part of a continuum.
• 13:34
  Speaker 1: All right. So then what are some of the other
• 13:36
  Speaker 1: pieces of the pie?
• 13:37
  Speaker 2: So the really big pieces of the pie are dark matter?
• 13:40
  Speaker 3: Right.
• 13:40
  Speaker 2: Dark matter is something that's out there in the universe.
• 13:42
  Speaker 2: We know. It's not made out of quarks and electrons
• 13:45
  Speaker 2: or photons or neutrinos or any kind of particle that
• 13:48
  Speaker 2: we know about. It's some kind of matter that's out there.
• 13:51
  Speaker 2: We see its effect gravitationally, so we know that it
• 13:54
  Speaker 2: has some kind of mass because it bends space in
• 13:57
  Speaker 2: the universe. It causes things to move, it hold together,
• 14:00
  Speaker 2: galaxies that spind changes the structure of the universe, all
• 14:04
  Speaker 2: this kind of stuff. So we know that it's out there,
• 14:06
  Speaker 2: but we don't know what it.
• 14:07
  Speaker 1: Is, right, And so far we only categorize it as
• 14:09
  Speaker 1: something different because we don't know what it is. It
• 14:11
  Speaker 1: could be like, if we know what it is, we
• 14:13
  Speaker 1: would maybe categorize it as normal matter too, right, Or
• 14:17
  Speaker 1: if a physicist looks at it and squints, maybe it'll
• 14:19
  Speaker 1: say it's radiation too well.
• 14:21
  Speaker 2: There could be dark matter and dark radiation, right. If
• 14:24
  Speaker 2: you have dark matter particles moving near the speed of light,
• 14:26
  Speaker 2: there would be dark radiation. We are very confident, though
• 14:29
  Speaker 2: nobody can ever be certain, that dark matter is not
• 14:32
  Speaker 2: made out of normal matter. It's not like a novel
• 14:35
  Speaker 2: rearrangement of quarks that hides in dark blobs, or some
• 14:38
  Speaker 2: weird combination of neutrinos, or just zillions and zillions of neutrinos.
• 14:43
  Speaker 2: And if you're curious about more details about that, check
• 14:45
  Speaker 2: out all of our episodes on dark matter. Briefly, we
• 14:48
  Speaker 2: are pretty sure that dark matter is not made of quarks,
• 14:51
  Speaker 2: because we know a lot about the number of quarks
• 14:53
  Speaker 2: in their universe that controls the amount of helium and
• 14:56
  Speaker 2: hygien made very early in the universe. So we're pretty
• 14:59
  Speaker 2: sure we can account for the number of quarks and
• 15:01
  Speaker 2: there's not enough to explain the dark matter. We know
• 15:04
  Speaker 2: that dark matter is not neutrinos because neutrinos move at
• 15:07
  Speaker 2: nearly the speed of light, and for dark matter to
• 15:09
  Speaker 2: have affected the shape and the structure of the universe
• 15:12
  Speaker 2: that we see today, the galaxies and their distributions, it
• 15:15
  Speaker 2: can't be moving very fast. It's low speed, and so
• 15:18
  Speaker 2: we're pretty sure dark matter is not made of neutrinos.
• 15:20
  Speaker 2: All of which to say, we're pretty sure that dark
• 15:22
  Speaker 2: matter is a different kind of stuff. It's not made
• 15:25
  Speaker 2: out of normal matter. It really is a different slice
• 15:27
  Speaker 2: of the pie.
• 15:27
  Speaker 1: All right. Well, let's get into maybe the deeper parts
• 15:30
  Speaker 1: of our mystery pie, which is dark energy. So let's
• 15:34
  Speaker 1: dig into that. But first let's take a quick break.
• 15:48
  Speaker 1: All right, we're eating up the pie of the universe
• 15:50
  Speaker 1: here today. We're serving folks a big slice or a
• 15:53
  Speaker 1: little slide.
• 15:54
  Speaker 2: We are serving up a big fat slice of universe pie,
• 15:57
  Speaker 2: and it's mostly a dark pie. I'm hoping it's chocolate.
• 16:01
  Speaker 1: You're hoping. What else could something dark in a pie case?
• 16:06
  Speaker 2: Yeah, mud pie is the most innocent example.
• 16:10
  Speaker 1: You go, boison berry.
• 16:12
  Speaker 5: Go.
• 16:13
  Speaker 1: Well, the pie of the universe, as we talked about,
• 16:15
  Speaker 1: is about five percent the normal stuff that you and
• 16:18
  Speaker 1: I are made out of, that hamsters and stars and
• 16:20
  Speaker 1: planets are made out of. About twenty seven percent is
• 16:23
  Speaker 1: dark matter, at least matter. We so far that we
• 16:25
  Speaker 1: don't know whether where to categorize it or we don't
• 16:28
  Speaker 1: know much about it. But there is the big piece
• 16:30
  Speaker 1: of the pie. Basically, the filling of the pie is
• 16:32
  Speaker 1: something totally different.
• 16:34
  Speaker 2: All the stuff we talked about today, dark matter, normal matter,
• 16:38
  Speaker 2: radiation only adds up to be like thirty two percent
• 16:42
  Speaker 2: of the stuff in the universe. Normal matter is five percent,
• 16:45
  Speaker 2: dark matter is twenty seven percent. Radiation is almost zero.
• 16:49
  Speaker 2: It's like ten to the minus four fraction. Dark energy
• 16:53
  Speaker 2: is sixty eight percent of the energy of the universe,
• 16:56
  Speaker 2: and we don't know what it is. All we know
• 16:58
  Speaker 2: is that dark energy is the mysterious stuff that's making
• 17:01
  Speaker 2: the universe expand faster and faster.
• 17:04
  Speaker 1: Yeah, I guess. One day we notice that the universe
• 17:06
  Speaker 1: is expanding, it's accelerating in its expansion, and so we
• 17:10
  Speaker 1: gave that acceleration or whatever it might be causing that
• 17:12
  Speaker 1: acceleration a name. That's the name dark energy.
• 17:15
  Speaker 2: Yeah, exactly. And we measure that expansion and we think
• 17:18
  Speaker 2: how much energy does it take to make that accelerating
• 17:20
  Speaker 2: and expansion happen. And that's what we call dark energy.
• 17:23
  Speaker 2: And we have a couple different ways to measure dark energy.
• 17:25
  Speaker 2: One is just by measuring the expansion of the universe,
• 17:28
  Speaker 2: and the other is by looking at the overall curvature
• 17:31
  Speaker 2: of the universe. All these pieces of the pie have energy,
• 17:35
  Speaker 2: so they all contribute energy density to the universe. And
• 17:38
  Speaker 2: Einstein tells us that energy density, not just mass, is
• 17:41
  Speaker 2: what curves space and causes the effect we call gravity.
• 17:46
  Speaker 2: So all of these things contribute to the overall energy
• 17:48
  Speaker 2: density of the universe, which affects its curvature. As we
• 17:52
  Speaker 2: recently talked about on the podcast, they all add up
• 17:54
  Speaker 2: to make space nice and flat. So they all add
• 17:57
  Speaker 2: up to what we call the critical density. We can
• 18:00
  Speaker 2: measure the sum of all of these components by measuring
• 18:02
  Speaker 2: the overall curvature of the universe.
• 18:05
  Speaker 1: It's a flat pie universe. It's more like a pizza pie.
• 18:08
  Speaker 2: You're saying, it's a thin and crispy pie, not a
• 18:11
  Speaker 2: deep dish.
• 18:12
  Speaker 1: Well, I guess I have a question, which is, like,
• 18:13
  Speaker 1: what form is dark energy present in the universe? Like
• 18:18
  Speaker 1: is it actually there or is it kind of like
• 18:20
  Speaker 1: hidden behind the scenes that it only comes out when
• 18:23
  Speaker 1: to expand the universe.
• 18:24
  Speaker 2: We don't know what dark energy is like. We don't
• 18:26
  Speaker 2: have a microscopic picture of what it looks like, or
• 18:30
  Speaker 2: what it's doing, or how it works. We know really
• 18:33
  Speaker 2: very little about it. It fits into our model of
• 18:36
  Speaker 2: the expansion of the universe and the accelerating expansion of
• 18:38
  Speaker 2: the universe if it does a couple of things. If
• 18:41
  Speaker 2: it's some kind of field the way that light is
• 18:44
  Speaker 2: a wiggle in the electromagnetic field, we imagine some other
• 18:47
  Speaker 2: new kind of field, some kind of field that has
• 18:49
  Speaker 2: high potential energy the way, for example, the Higgs field
• 18:53
  Speaker 2: has high potential energy. It's stuck in its little local minimum,
• 18:56
  Speaker 2: which is why it has so much energy stored in it.
• 18:58
  Speaker 2: So if you have some field with a bunch of
• 19:00
  Speaker 2: potential energy stored. It has this accelerating expansion effect according
• 19:04
  Speaker 2: to general relativity. So that's basically all we know about it.
• 19:07
  Speaker 2: We don't know what field it is. We don't know
• 19:09
  Speaker 2: why it exists. It's a big mystery. If you try
• 19:12
  Speaker 2: to ask, like, well, do the fields we know about
• 19:14
  Speaker 2: can they provide that potential energy? Do to calculation, that
• 19:17
  Speaker 2: turns out the answer is know and the fields we
• 19:19
  Speaker 2: know about are off by about a factor of ten
• 19:22
  Speaker 2: to the one hundred. So we really very clueless about
• 19:25
  Speaker 2: what it is that's creating this accelerating expansion. And when
• 19:29
  Speaker 2: you hear dark energy, you should really just think about
• 19:31
  Speaker 2: it as a description of our observation of the expansion,
• 19:35
  Speaker 2: not any sort of understanding of what's causing it.
• 19:38
  Speaker 1: I wonder if it's like if you take two bowling
• 19:39
  Speaker 1: balls and you connect them with an invisible spring, and
• 19:43
  Speaker 1: then you bring the bowling balls together, you're creating like
• 19:46
  Speaker 1: some stored potential energy between the two boiling balls. It's
• 19:49
  Speaker 1: like you can't see it, but it's there. There's that
• 19:51
  Speaker 1: potential energy hitting there is and dark energy sort of
• 19:54
  Speaker 1: like that, like there's something about space that has this
• 19:56
  Speaker 1: potential energy stored in it. That's making everything accelerate then
• 20:00
  Speaker 1: get bigger.
• 20:00
  Speaker 2: Yeah, that's a great description. Potential energy can be sort
• 20:03
  Speaker 2: of invisible, right, it's just the configuration of that field.
• 20:06
  Speaker 2: That's what potential energy is. Like you put that bowling
• 20:09
  Speaker 2: ball up on a shelf, It's got energy stored in
• 20:12
  Speaker 2: its configuration, the fact that it's up on the shelf
• 20:14
  Speaker 2: and not on the floor. Now you take a field
• 20:16
  Speaker 2: and you displace it from zero, you say, okay, the
• 20:18
  Speaker 2: field has some value. And fields can have all sorts
• 20:20
  Speaker 2: of different potential energies depending on their values. And so
• 20:23
  Speaker 2: this particular field, whatever it is, seems to have a
• 20:26
  Speaker 2: lot of potential energy stored in it, in whatever configuration
• 20:29
  Speaker 2: it happens to be in. It's like an infinite number
• 20:32
  Speaker 2: of invisible bowling balls stored on the shelves everywhere.
• 20:35
  Speaker 1: In space, or like in an infinite number of invisible
• 20:38
  Speaker 1: springs tied between different everything together. Right.
• 20:41
  Speaker 2: Yeah, that are precompressed exactly. And there's a seeming contradiction
• 20:45
  Speaker 2: here in dark energy. Like on one hand, we say
• 20:48
  Speaker 2: dark energy is sixty eight percent of the energy in
• 20:50
  Speaker 2: the universe. You might think, wow, it's overwhelming, it's dominant,
• 20:53
  Speaker 2: it controls everything, But in our lives and in our
• 20:56
  Speaker 2: experience and in our Solar system. Dark energy is basically negligible,
• 21:00
  Speaker 2: like we didn't discover it until we looked at the
• 21:02
  Speaker 2: expansion of the universe on large scales. You can't see
• 21:05
  Speaker 2: it in the Solar System. You can't feel it with
• 21:07
  Speaker 2: gravitational experiences here on Earth. Because dark energy gets stronger
• 21:11
  Speaker 2: over greater distances. It's the opposite of gravity. Gravity gets
• 21:15
  Speaker 2: weaker over greater distances. Like we have two bowling balls
• 21:18
  Speaker 2: next to each other, they feel each other's gravity. You
• 21:20
  Speaker 2: put one on Jupiter, you can basically ignore the gravity
• 21:23
  Speaker 2: the other bowling ball. But dark energy because it's a
• 21:26
  Speaker 2: feature of space itself. As you get more space between stuff,
• 21:29
  Speaker 2: it gets stronger and stronger. So you don't see dark
• 21:32
  Speaker 2: energy unless you're looking over really really large distance scales,
• 21:35
  Speaker 2: And so it has no effect on my life, or
• 21:37
  Speaker 2: door life, or our life in the Solar System or
• 21:40
  Speaker 2: super high precision measurements of the orbit of Jupiter can't
• 21:43
  Speaker 2: detect any dark energy. But it is the dominant fraction
• 21:47
  Speaker 2: of energy in the universe.
• 21:48
  Speaker 1: I guess it's the reminder of how big the universe is.
• 21:51
  Speaker 1: Right like places like Earth where you have a lot
• 21:53
  Speaker 1: of normal matter clumped together, or even our Solar System,
• 21:57
  Speaker 1: they're pretty rare in the universe, right Like, outside the Earth,
• 22:00
  Speaker 1: it's mostly empty space. Outside of the Solar System, it's
• 22:03
  Speaker 1: even more empty space.
• 22:04
  Speaker 2: In the same way that dark matter overwhelms the kind
• 22:07
  Speaker 2: of matter in the universe that's also not in our experience, right, Like,
• 22:10
  Speaker 2: you can't detect dark matter in our Solar System by
• 22:13
  Speaker 2: looking at like how the voyager probe moves in its gravity,
• 22:16
  Speaker 2: and that's because it's spread out through all that empty space.
• 22:19
  Speaker 2: Dark matter is just not nearly as clumped as normal matter.
• 22:22
  Speaker 2: So all the space between us and other stars, where
• 22:24
  Speaker 2: there's almost no normal matter, is smoothly filled with dark matter. So,
• 22:27
  Speaker 2: as you say, the bigness of the universe means that
• 22:30
  Speaker 2: that adds up to a big chunk of the pie.
• 22:32
  Speaker 1: All right, Well, let's summarize the pie. Then we know
• 22:34
  Speaker 1: that the pie of the universe is about five percent
• 22:36
  Speaker 1: normal matter, twenty seven percent dark matter, almost zero radiation,
• 22:42
  Speaker 1: and about sixty eight percent dark energy. That's the pie
• 22:45
  Speaker 1: of the universe as we know it today. And so
• 22:47
  Speaker 1: the big question of the episode today is has that
• 22:50
  Speaker 1: always been the case? Has it always been those percentages
• 22:53
  Speaker 1: and will it change in the future. So what do
• 22:55
  Speaker 1: we know of that new.
• 22:56
  Speaker 2: So we know that these fractions can change and have changed,
• 23:00
  Speaker 2: and we're radically different earlier in the history of the
• 23:02
  Speaker 2: universe and very likely will be radically different in the
• 23:06
  Speaker 2: future of the universe. And it turns out we live
• 23:09
  Speaker 2: in a very peculiar time in the history of the
• 23:11
  Speaker 2: universe when these fractions are at all similar to each other.
• 23:15
  Speaker 2: Most times in the universe, one of these fractions totally dominated,
• 23:19
  Speaker 2: and in the deep future of the universe, we expect
• 23:21
  Speaker 2: one of these fractions to totally dominate. Sort of weird
• 23:24
  Speaker 2: that things are kind of at all in balance right now.
• 23:28
  Speaker 1: Well, I guess there are different ways that the universe
• 23:31
  Speaker 1: can change its composition, right, Like maybe a normal matter
• 23:36
  Speaker 1: turns into dark matter, or dark matter turns into dark energy,
• 23:39
  Speaker 1: or something like pies is there, but the ingredients change
• 23:43
  Speaker 1: from one to the other. Or you could have like
• 23:45
  Speaker 1: the pies growing and just like every day there's more
• 23:47
  Speaker 1: dark energy, there's more dark matter, and so the percentages
• 23:50
  Speaker 1: change in that way, right, So in which way is
• 23:53
  Speaker 1: the universe changing?
• 23:54
  Speaker 2: So in all of those ways? Number one, a certain
• 23:57
  Speaker 2: kind of stuff can turn into other kinds of stuff.
• 23:59
  Speaker 2: For example, matter can emit radiation. An electron hanging out
• 24:03
  Speaker 2: in the universe can give off a photon that increases
• 24:05
  Speaker 2: the radiation fraction of the universe. Photons can turn into matter,
• 24:09
  Speaker 2: a photon can convert into an electron, and a positron
• 24:12
  Speaker 2: that's converting radiation into matter. Very straightforward, and these kind
• 24:15
  Speaker 2: of processes happen. Also, we think between normal matter and
• 24:18
  Speaker 2: dark matter that we can dig into that in a minute. Also,
• 24:22
  Speaker 2: the universe is expanding, and as it expands, the various
• 24:26
  Speaker 2: fractions get treated differently. They dilute differently as the universe
• 24:30
  Speaker 2: gets bigger. Finally, the universe is cooling. It's lowering in temperature,
• 24:35
  Speaker 2: and as the temperature gets colder, some of these processes
• 24:38
  Speaker 2: that convert one thing to another turn on or off
• 24:41
  Speaker 2: at various times. So all of these things change through
• 24:44
  Speaker 2: the history of the universe because of all of those reasons.
• 24:47
  Speaker 1: Interesting. So there are many ways in which the universe
• 24:50
  Speaker 1: is changing. Although I feel like you forgot the third
• 24:52
  Speaker 1: way in which the universe can change.
• 24:53
  Speaker 2: What's that puberty?
• 24:56
  Speaker 1: No, when a physicist you know, looks at it differently
• 24:58
  Speaker 1: or wakes up differently, I think that's radiation. No, you
• 25:02
  Speaker 1: know today that looks like normal matter to it.
• 25:06
  Speaker 2: There's always an arbitrariness to how we call these things,
• 25:09
  Speaker 2: and you know, I look forward to arguing with alien
• 25:11
  Speaker 2: physicists about the meaning of radiation.
• 25:14
  Speaker 1: But as you said, the universe has gone through a
• 25:15
  Speaker 1: lot of changes in its body. I guess it's gone
• 25:20
  Speaker 1: through puberty, or is going through puberty. And so Daniel
• 25:22
  Speaker 1: maybe steps through what some of these changes have been
• 25:25
  Speaker 1: and what are they like, what happened in them.
• 25:26
  Speaker 2: So let's start at the very beginning and zoom back
• 25:29
  Speaker 2: to the earliest point we know, which is when the
• 25:31
  Speaker 2: universe was very dense and very hot, right, filled with
• 25:35
  Speaker 2: some kind of plasma. We don't know what happened before this.
• 25:37
  Speaker 2: We don't know what came before this to create this hot,
• 25:40
  Speaker 2: dense plasma. Their theories about inflation and infulton particles and
• 25:45
  Speaker 2: all that is very very speculative. What we are very
• 25:47
  Speaker 2: certain about is that about thirteen point eight billion years ago,
• 25:50
  Speaker 2: the universe was very hot and very dense and everything
• 25:54
  Speaker 2: was sloshing around. And from that point on we can
• 25:57
  Speaker 2: model very precisely how that expands and how it cools,
• 26:00
  Speaker 2: and everything changes and that's about as far back as
• 26:02
  Speaker 2: we can go and be confident. We can speculate deeper
• 26:05
  Speaker 2: and talk about like crazy theories about before that. But
• 26:07
  Speaker 2: what we know very well precision cosmology takes us back
• 26:10
  Speaker 2: to that moment when the universe was very hot and
• 26:12
  Speaker 2: very dense, and at that moment we think the universe
• 26:15
  Speaker 2: was ninety nine point ninety nine a bunch more nine
• 26:19
  Speaker 2: percent radiation, that it was essentially all radiation and everything
• 26:23
  Speaker 2: else was a tiny fraction.
• 26:25
  Speaker 1: Like it was all basically light in the Trino's right,
• 26:27
  Speaker 1: because that's the only thing you count as radiation is
• 26:30
  Speaker 1: light in theatrinas.
• 26:31
  Speaker 2: Oh, I'm going to disappoint you. Actually, if you had
• 26:34
  Speaker 2: electrons back then, they counted as radiation because the universe
• 26:37
  Speaker 2: was so hot, electrons were moving at the speed of light,
• 26:41
  Speaker 2: and basically everything was moving it nearly the speed of light.
• 26:44
  Speaker 2: Because the universe was so hot, everything was so fast
• 26:47
  Speaker 2: that it didn't really matter how much mass it had,
• 26:50
  Speaker 2: even like a top cork if it existed back then.
• 26:52
  Speaker 2: The top cork is the most massive fundamental particle we
• 26:55
  Speaker 2: know about. But back then, in the very early universe,
• 26:58
  Speaker 2: it was moving it basically the speed of light because
• 27:00
  Speaker 2: the universe was that hot, and so everything gets counted
• 27:03
  Speaker 2: as radiation.
• 27:08
  Speaker 1: I feel like you're basically just making stuff up that.
• 27:13
  Speaker 2: I tried to warn you.
• 27:15
  Speaker 1: It's been taken to it for a second here, like,
• 27:17
  Speaker 1: why is it important that if it's moving it closest
• 27:19
  Speaker 1: to the speed of flight you call electron radiation. Isn't
• 27:22
  Speaker 1: it still an electron?
• 27:23
  Speaker 2: It's still an electron. Absolutely, but it's going to be
• 27:26
  Speaker 2: important as soon as the universe starts expanding and cooling.
• 27:29
  Speaker 2: I just want to add that there are other reasons
• 27:31
  Speaker 2: to think that the very early universe was filled with radiation,
• 27:34
  Speaker 2: not just because we call it radiation, but also because frankly,
• 27:37
  Speaker 2: it was very hot and filled with charge particles. And
• 27:40
  Speaker 2: what do hot charge particles do? They give off photons.
• 27:43
  Speaker 2: So there really were a lot of photons in the
• 27:46
  Speaker 2: early universe, many more than we have today. It's not
• 27:49
  Speaker 2: just a slippery naming.
• 27:50
  Speaker 1: Scheme, but it's partly a slippery naming scheme.
• 27:54
  Speaker 2: Yes, absolutely, it's partly a slippery naming scheme, but not entirely. Also,
• 27:59
  Speaker 2: in the early universe there was matter and antimatter, and
• 28:01
  Speaker 2: that annihilates and forms photons, So there really was a
• 28:05
  Speaker 2: lot of photons in the early universe. So it's partially
• 28:07
  Speaker 2: a slippery naming scheme calling stuff radiation that today we wouldn't.
• 28:11
  Speaker 2: But also there really was a lot of radiation stuff
• 28:14
  Speaker 2: that we would call radiation today.
• 28:16
  Speaker 1: Well, then let's maybe be helpful to people and break
• 28:19
  Speaker 1: it down out of this is what you call radiation,
• 28:22
  Speaker 1: how much of it was actually light and how much
• 28:24
  Speaker 1: of it was electrons or quarks or you know things
• 28:28
  Speaker 1: with that today we would call normal matter.
• 28:31
  Speaker 2: Thanks to the CMB and bearing on acoustic oscillation, we
• 28:34
  Speaker 2: can actually make measurements of like the photon to quark ratios.
• 28:39
  Speaker 2: So we do have those numbers. They change sort of
• 28:42
  Speaker 2: rapidly as the universe is cooling.
• 28:44
  Speaker 1: Okay, So then so then what was causing these changes
• 28:47
  Speaker 1: between like electrons and photons.
• 28:50
  Speaker 2: So what happens very early on is the universe starts
• 28:53
  Speaker 2: to expand, Right. What's happening is more space is being created,
• 28:56
  Speaker 2: but you're not creating more stuff, right, and so what
• 28:59
  Speaker 2: how if you have like ten particles in a tiny
• 29:02
  Speaker 2: box and then you make the box bigger, all now
• 29:05
  Speaker 2: the density decreases, right, and it gets more dilute, And
• 29:09
  Speaker 2: that makes sense. So the energy density is dropping, dropping, dropping,
• 29:12
  Speaker 2: So that's how it works for matter. Right, you increase
• 29:15
  Speaker 2: the box, you get a drop in energy density. For radiation,
• 29:18
  Speaker 2: the rules are a little bit different. For radiation. The
• 29:21
  Speaker 2: energy density actually drops faster as you increase the size
• 29:25
  Speaker 2: of the box because the particles get red shifted. The
• 29:28
  Speaker 2: expansion of space stretches the wavelengths of these particles.
• 29:32
  Speaker 1: Of the photons, but doesn't it stretch also the electron.
• 29:36
  Speaker 2: If they're moving very fast, if they're treated as radiation,
• 29:39
  Speaker 2: And that's the distinction. If you're considered radiation, it's because
• 29:42
  Speaker 2: effectively you have no mass, which makes your wavelength expand.
• 29:47
  Speaker 2: As space gets expanded, that means that your energy density
• 29:50
  Speaker 2: is dropping more quickly, because not only are you getting
• 29:52
  Speaker 2: more dilute, you're also getting red shifted. So as time
• 29:56
  Speaker 2: goes on, the energy density of radiation drops fast faster
• 30:00
  Speaker 2: than the energy density of matter, which does two things. One,
• 30:04
  Speaker 2: now some of that radiation slides over into the matter category,
• 30:07
  Speaker 2: and also the radiation slice of the pie starts to
• 30:10
  Speaker 2: decrease relative to the matter slice of the pie.
• 30:13
  Speaker 1: Okay, I think what you're maybe saying is that as
• 30:16
  Speaker 1: the universe expands, the amount of energy that's stored in
• 30:19
  Speaker 1: momentum is changing. That's really kind of what's happening, right,
• 30:24
  Speaker 1: And so at some point, for example, the electrons were
• 30:27
  Speaker 1: mostly momentum because we're going so fast, but at some
• 30:29
  Speaker 1: point they slowed down so much that they were mostly
• 30:33
  Speaker 1: just the mass of the electron.
• 30:34
  Speaker 2: Yeah, that's exactly right. But remember that there's something else
• 30:37
  Speaker 2: also happening for photons, right. Photons do get red shifted,
• 30:41
  Speaker 2: and people write and ask about this all the time,
• 30:43
  Speaker 2: like what happens to the energy of a red shifted photon?
• 30:46
  Speaker 2: Where did it go? Because we've had the concept of
• 30:48
  Speaker 2: conservation of energy banged into our head for so many years. Well,
• 30:52
  Speaker 2: expanding space does not respect the conservation of energy. If
• 30:55
  Speaker 2: you have a photon in a chunk of space and
• 30:56
  Speaker 2: then you expand that space, the wavelength of that phot
• 31:00
  Speaker 2: quoton also gets expanded because space itself is expanding, and
• 31:03
  Speaker 2: so now it has less energy. And that energy didn't
• 31:06
  Speaker 2: go anywhere. There's just is less of it, because the
• 31:09
  Speaker 2: universe doesn't respect conservation of energy unless space is fixed.
• 31:13
  Speaker 1: But I guess the question is, like, it sounds like
• 31:15
  Speaker 1: our classification of what you call radiation and matter was
• 31:18
  Speaker 1: changing very rapidly during those times. But were the actual
• 31:21
  Speaker 1: like number of electrons changing, Were there more electrons being
• 31:24
  Speaker 1: created or destroyed? Was there more light being created or destroyed?
• 31:28
  Speaker 1: Or did that mostly stay the same.
• 31:30
  Speaker 2: In the very beginning that mostly stayed the same, like
• 31:32
  Speaker 2: you had a bunch of processes. These things are sloshing
• 31:34
  Speaker 2: back and forth. Photons can turn into electron positron pairs.
• 31:38
  Speaker 2: Electron positron pairs can annihilate back into photons. The very
• 31:41
  Speaker 2: early universe, we think was in thermal equilibrium. They have
• 31:44
  Speaker 2: all these things going in both directions and they basically equalized.
• 31:47
  Speaker 2: That's what happens at a hot plasma if you give
• 31:49
  Speaker 2: it enough time. Then the universe is expanding and some
• 31:52
  Speaker 2: stuff gets slowed down, so it falls out of the
• 31:54
  Speaker 2: radiation category into matter, and stuff that got left in
• 31:57
  Speaker 2: the radiation category is losing energy density compared to the
• 32:01
  Speaker 2: matter category. So now the matter category is growing and
• 32:04
  Speaker 2: the radiation category is dropping, and so eventually we get
• 32:07
  Speaker 2: to a matter dominated universe. The universe started out dominated
• 32:11
  Speaker 2: by radiation, but as it expands and cools, it becomes
• 32:14
  Speaker 2: a matter dominated universe.
• 32:17
  Speaker 1: And so in that way, you would say that the
• 32:18
  Speaker 1: percentages of matter and radiation in the universe were changing.
• 32:22
  Speaker 2: That's exactly right, and that happened about fifty thousand years
• 32:26
  Speaker 2: after the Big Bang. At that point, the matter and
• 32:28
  Speaker 2: radiation sort of hit a crossover point. Radiation is dropping
• 32:31
  Speaker 2: more quickly, but it started out higher, and that's the
• 32:34
  Speaker 2: point where they cross each other. Meanwhile, dark energy is
• 32:37
  Speaker 2: humming along like a tiny fraction of the universe like
• 32:40
  Speaker 2: zeros or zerz are one this whole time, playing the
• 32:43
  Speaker 2: long game, waiting for its turn to dominate. But at
• 32:46
  Speaker 2: around fifty thousand years the universe became matter dominated. It
• 32:50
  Speaker 2: was cooled enough that a lot of stuff slowed down
• 32:52
  Speaker 2: and became essentially called matter, and the photon energies got
• 32:56
  Speaker 2: decreased because of this radiation stretching.
• 33:00
  Speaker 1: And let's get into then the long game of dark
• 33:02
  Speaker 1: matter and the newcomer dark energy and how it took
• 33:05
  Speaker 1: over the entire pie of the universe. Let's dig into that,
• 33:08
  Speaker 1: but first let's take another quick break. All right, we're
• 33:23
  Speaker 1: talking about the universe pie, the chocolate pie, the white
• 33:27
  Speaker 1: chocolate pies, all the pies in the universe and what
• 33:30
  Speaker 1: they're made out of, and how that's been changing since
• 33:32
  Speaker 1: the birth of the universe and throughout all of its
• 33:34
  Speaker 1: history and maybe into the future. And so if we've
• 33:37
  Speaker 1: learned that at the beginning of the universe, physicists would
• 33:39
  Speaker 1: qualify most of the stuff in the universe as radiation
• 33:42
  Speaker 1: because it was going so fast. But as it cooled
• 33:44
  Speaker 1: down and expanded, things sort of slowed down enough that
• 33:46
  Speaker 1: they started to be called more a regular matter, which
• 33:49
  Speaker 1: is the kind of matter that we're made out of.
• 33:51
  Speaker 1: And so you were saying that dark matter was sort
• 33:52
  Speaker 1: of waiting in the wings to make an appearance here
• 33:55
  Speaker 1: in the history of the universe.
• 33:56
  Speaker 2: Well, it was more thinking about dark energy as waiting
• 33:58
  Speaker 2: in the wings. Playing the very very very long the
• 34:00
  Speaker 2: billion year game, the first few tens of thousands of
• 34:04
  Speaker 2: years were a battle between radiation and matter, including normal
• 34:08
  Speaker 2: matter and dark matter. Both of those contributions combined were
• 34:11
  Speaker 2: still smaller than radiation in the very early times. Than
• 34:14
  Speaker 2: after about fifty thousand years, matter together overwhelmed the radiation category,
• 34:19
  Speaker 2: and that includes dark matter and normal matter. But there's
• 34:22
  Speaker 2: an interesting mix there between the matter and the dark
• 34:25
  Speaker 2: matter fractions, Like there were a bunch of electrons and
• 34:27
  Speaker 2: there were a bunch of protons, but we also think
• 34:29
  Speaker 2: there was a lot of dark matter in the early universe,
• 34:32
  Speaker 2: and we suspect very strongly that in the early universe,
• 34:35
  Speaker 2: matter and dark matter were turning into each other, that
• 34:37
  Speaker 2: there was some kind of force that allowed one to
• 34:40
  Speaker 2: turn into the other or back.
• 34:42
  Speaker 1: Wait. And so in the point zero zero zero zero
• 34:44
  Speaker 1: zero zero one percent of stuff in the early universe
• 34:46
  Speaker 1: that you call matter, you're also including dark matter in there.
• 34:49
  Speaker 2: Also including dark matter in there exactly. And we think
• 34:52
  Speaker 2: that there was a higher percentage of that that was
• 34:55
  Speaker 2: dark matter than there is today. That at the matter
• 34:57
  Speaker 2: section of the pie, dark matter plus normal matter, that
• 35:00
  Speaker 2: there was more dark matter as a fraction than there
• 35:03
  Speaker 2: is today.
• 35:04
  Speaker 1: And you're saying that dark matter can turn and was
• 35:06
  Speaker 1: turning into regular matter and vice versa because it was
• 35:10
  Speaker 1: so hot.
• 35:10
  Speaker 2: Yes, And we don't understand how this works, because we
• 35:12
  Speaker 2: don't know what dark matter is and whether it's a
• 35:14
  Speaker 2: particle and what forces it feels. But in order to
• 35:17
  Speaker 2: tell the story and be consistent with everything we understand,
• 35:19
  Speaker 2: there needs to be some mechanism for dark matter and
• 35:22
  Speaker 2: normal matter to slosh into each other to explain how
• 35:24
  Speaker 2: we got from having more dark matter in the early
• 35:26
  Speaker 2: universe to having less of it. Today. The story goes
• 35:29
  Speaker 2: something like this dark matter we think is some massive particle,
• 35:33
  Speaker 2: some very heavy particle, And as the universe is cooling,
• 35:36
  Speaker 2: matter and dark matter are turning back and forth into
• 35:39
  Speaker 2: each other. But as things get colder and colder, matter
• 35:42
  Speaker 2: can no longer turn into dark matter because dark matter
• 35:45
  Speaker 2: is too heavy. No longer is there enough energy around
• 35:48
  Speaker 2: to smash together normal matter particles and turn it into
• 35:51
  Speaker 2: dark matter. So dark matter creation stops, but dark matter
• 35:54
  Speaker 2: annihilation doesn't. Dark matter is still turning into normal matter
• 35:58
  Speaker 2: right in that one direction, but the reverse this process
• 36:00
  Speaker 2: is no longer happening. So instead of being in balance,
• 36:03
  Speaker 2: now dark matter is turning into normal matter, but it's
• 36:06
  Speaker 2: not happening the reverse, and so the dark matter fraction shrinks.
• 36:10
  Speaker 1: Whoa. So like the regular matter was growing.
• 36:13
  Speaker 2: In the early universe, some dark matter got turned into
• 36:16
  Speaker 2: normal matter and then it got frozen out. It didn't
• 36:18
  Speaker 2: get turned back into dark matter like it did when
• 36:20
  Speaker 2: things were hot and slashing around freely.
• 36:23
  Speaker 1: Then to pie bake exactly.
• 36:26
  Speaker 2: Somebody baked the pie. And then as the universe expands,
• 36:29
  Speaker 2: it cools further and there's less and less dark matter.
• 36:32
  Speaker 2: Then there's not enough dark matter around for there to
• 36:35
  Speaker 2: be much annihilation, So dark matter stops turning into normal
• 36:38
  Speaker 2: matter and it gets freezes in or baked in. I
• 36:41
  Speaker 2: guess you could call it in our analogy, but in
• 36:43
  Speaker 2: physics terms they call it dark matter freeze out.
• 36:46
  Speaker 1: So then when when did this freeze out or baking happen?
• 36:48
  Speaker 2: It happened about ten to the miners eight seconds after
• 36:51
  Speaker 2: the Big Bang?
• 36:52
  Speaker 1: WHOA what? And since then it's been the same proportion
• 36:56
  Speaker 1: of regular matter and dark matter.
• 36:58
  Speaker 2: Yeah, the dark matter normal matter fraction got frozen in
• 37:01
  Speaker 2: very early on as the universe expanded and cooled, and
• 37:05
  Speaker 2: then the radiation normal matter fraction changes as things expand further.
• 37:09
  Speaker 2: And that turnover point was like ten thousand years. So
• 37:12
  Speaker 2: there's lots of really fascinating time scales here.
• 37:14
  Speaker 1: Okay, So then since ten to the minus second, since
• 37:17
  Speaker 1: the Big Bang, and we've had the same amount of
• 37:19
  Speaker 1: regular matter and dark matter. It sounds like what you're saying,
• 37:22
  Speaker 1: and so those proportions stayed the same, right, it would
• 37:25
  Speaker 1: be about five to one, right, m M. But then
• 37:28
  Speaker 1: waiting in the wings, you're saying there was dark energy.
• 37:31
  Speaker 1: So back then there was no dark energy.
• 37:32
  Speaker 2: There was not no dark energy, but there was less
• 37:35
  Speaker 2: universe than there is today, and dark energy is built
• 37:38
  Speaker 2: into space. Every chunk of space comes with dark energy.
• 37:41
  Speaker 2: So you have less universe, you have less dark energy,
• 37:43
  Speaker 2: but it's constant in density, right, more universe, less universe,
• 37:47
  Speaker 2: You don't change the density of dark energy. But as
• 37:50
  Speaker 2: you expand the universe, you do change the density of
• 37:52
  Speaker 2: matter and radiation. As we talked about, you expand the box,
• 37:55
  Speaker 2: you have a smaller density of matter, and radiation drops
• 37:58
  Speaker 2: even faster. As the universe is expanding, both matter and
• 38:02
  Speaker 2: radiation are having their energy density drop very very quickly.
• 38:06
  Speaker 2: But dark energy isn't. It's a constant density. You make
• 38:09
  Speaker 2: more space, you get more dark energy. That doesn't happen
• 38:12
  Speaker 2: for electrons. So as the universe expands, dark energy starts
• 38:15
  Speaker 2: to creep up in its fraction.
• 38:17
  Speaker 1: Although I wonder if you can make the case that
• 38:19
  Speaker 1: dark energy was always there, like if we don't know
• 38:22
  Speaker 1: what it is, and it's just like a hidden, invisible
• 38:24
  Speaker 1: potential energy that could may move things, wouldn't you say
• 38:28
  Speaker 1: it was already built into the universe from the beginning.
• 38:31
  Speaker 2: Yeah, that's exactly the model, right. We think it's an
• 38:33
  Speaker 2: inherent part of space. As long as you had space,
• 38:36
  Speaker 2: you had dark energy, and it was there during this
• 38:38
  Speaker 2: first ten to the minus eight seconds, but the other
• 38:41
  Speaker 2: stuff had such a high energy density that it swamped
• 38:44
  Speaker 2: the whole pie. The dark energy was always there with
• 38:46
  Speaker 2: its same energy density, it was just small compared to
• 38:49
  Speaker 2: the energy density of the other components, which then faded
• 38:52
  Speaker 2: as the universe expanded.
• 38:54
  Speaker 1: I guess what I mean is like, if you're closing
• 38:56
  Speaker 1: the universe off at a certain point, and you're saying
• 38:58
  Speaker 1: that the energy of the universe is not conserve, there's
• 39:01
  Speaker 1: more energy being pumped into it. But what if you
• 39:04
  Speaker 1: count the pump and where this energy is coming from.
• 39:06
  Speaker 1: Then maybe I wonder if you could say that dark
• 39:09
  Speaker 1: energy was always there at the same percentage.
• 39:11
  Speaker 2: Yeah, perhaps when there's lots of the theories of dark
• 39:14
  Speaker 2: energy and what it might be is it's some kind
• 39:15
  Speaker 2: of weird field, is it some kind of other stuff?
• 39:18
  Speaker 2: And so those various theories would upset these fractions if
• 39:21
  Speaker 2: you included them.
• 39:22
  Speaker 1: Yeah, And so that's how the universe pie has been changing.
• 39:25
  Speaker 1: But it seems like it hasn't really changed much since
• 39:28
  Speaker 1: ten to the minus eight seconds, since the Big Bang,
• 39:30
  Speaker 1: except just that dark energy has been growing.
• 39:32
  Speaker 2: Yeah, dark energy has been growing, which makes for a
• 39:35
  Speaker 2: fascinating tug of war because dark energy is growing. But
• 39:38
  Speaker 2: in the early universe, like a billion years in, we're
• 39:41
  Speaker 2: still matter dominated, right, Radiation is faded away. We're in
• 39:44
  Speaker 2: a matter dominated universe. The universe is expanding, but that
• 39:47
  Speaker 2: expansion is now decelerating. It's slowing down because the universe
• 39:51
  Speaker 2: is dominated by matter, and what does matter tend to do?
• 39:54
  Speaker 2: Pulls things together, right, It slows down the expansion of
• 39:57
  Speaker 2: the universe. But because it was expanding still even though decelerating,
• 40:02
  Speaker 2: dark energy is creeping up and up and up, and
• 40:04
  Speaker 2: dark energy makes the universe expand faster, and eventually the
• 40:07
  Speaker 2: universe expanded enough so that dark energy just took over.
• 40:11
  Speaker 2: And around eight billion years after the beginning of the universe,
• 40:14
  Speaker 2: dark energy was the dominant fraction. And you'll see that
• 40:17
  Speaker 2: the universe start to accelerate its expansion because dark energy
• 40:20
  Speaker 2: takes over. And that's basically the future. Dark energy is
• 40:24
  Speaker 2: a runaway process. If nothing else changes, dark energy will
• 40:27
  Speaker 2: continue to grow as a fraction of the energy density
• 40:29
  Speaker 2: of the universe, making the universe accelerate faster and faster,
• 40:33
  Speaker 2: increasing the dark energy fraction faster and faster.
• 40:36
  Speaker 1: All right, So then in like a billion years from now,
• 40:38
  Speaker 1: what's going to be the percentage or a pie breakdown
• 40:40
  Speaker 1: of the universe if you had to get well.
• 40:43
  Speaker 2: It's like sixty eight percent dark energy. Now it's just
• 40:47
  Speaker 2: going to increase. It's going to go to ninety percent,
• 40:49
  Speaker 2: ninety five percent, ninety nine percent. But this is over
• 40:51
  Speaker 2: billions and billions and trillions of years into the future.
• 40:55
  Speaker 2: But it's only going to crank up. But you know,
• 40:56
  Speaker 2: we don't know that, right. Remember, we don't know what
• 40:58
  Speaker 2: dark energy is. We don't know for sure what its
• 41:01
  Speaker 2: behavior is going to be in the future. This model
• 41:03
  Speaker 2: describes very very well the history of these energy fractions
• 41:06
  Speaker 2: and how the universe pie has changed, with a couple
• 41:08
  Speaker 2: of caveats, like the measurements of the dark energy in
• 41:11
  Speaker 2: the early universe don't one hundred percent agree with our
• 41:14
  Speaker 2: measurements in the late universe. There is this hubble tension.
• 41:17
  Speaker 2: But mostly this picture of the sloshing pieces of the
• 41:20
  Speaker 2: pie holds together very well and matches all of our data.
• 41:23
  Speaker 1: All right. So then in the future, normal matter, dark matter,
• 41:26
  Speaker 1: and radiation, they're all going to stay the same amount,
• 41:29
  Speaker 1: but the percentage is going to go down because dark
• 41:31
  Speaker 1: energy is growing, and so in the far future it's
• 41:34
  Speaker 1: going to be like ninety nine nine nine nine percent
• 41:37
  Speaker 1: dark energy then the universe.
• 41:38
  Speaker 2: Yeah, exactly. And the universe started out dominated by one
• 41:41
  Speaker 2: fraction of the pie, it's going to end up dominated
• 41:43
  Speaker 2: by another fraction of the pie. And there's this little
• 41:45
  Speaker 2: window in the middle where multiple fractions are not zero.
• 41:49
  Speaker 2: It's a very unusual time to be in the universe
• 41:52
  Speaker 2: when you have a bunch of different kinds of stuff around.
• 41:54
  Speaker 2: You got dark matter, you've got normal matter, you've got
• 41:56
  Speaker 2: dark energy, all at the same time.
• 41:58
  Speaker 1: But according to you, we're still in the undelicious part
• 42:01
  Speaker 1: of the universe. Apparently, apparently, thanks, things can't get worse,
• 42:07
  Speaker 1: but they can only get better.
• 42:09
  Speaker 2: I wouldn't say undelicious, I'd say less delicious than the future.
• 42:12
  Speaker 2: That's the optimistic way to think about it.
• 42:14
  Speaker 1: Oh, there you go. And let's hope that dark energy
• 42:16
  Speaker 1: is the most delicious thing in the universe, because that
• 42:19
  Speaker 1: seems to be the only thing that's going to be
• 42:21
  Speaker 1: around in the future percentage wise.
• 42:22
  Speaker 2: And caveats for those of you who they like to
• 42:24
  Speaker 2: think a lot about dark matter. This assumes a fairly
• 42:27
  Speaker 2: simple model of dark matter. That it was in thermal
• 42:29
  Speaker 2: equilibrium with everything else early in the universe. That doesn't
• 42:32
  Speaker 2: have to be the case. You can have other theories
• 42:34
  Speaker 2: of dark matter axions, et cetera that are so weakly
• 42:37
  Speaker 2: coupled that hardly interacts, so they're not thermally mixed with
• 42:40
  Speaker 2: the stuff in the universe. There's lots of other ways
• 42:42
  Speaker 2: you could change this picture. This is like the simplest
• 42:44
  Speaker 2: model we can make that describes everything we see, and
• 42:47
  Speaker 2: it works really pretty well.
• 42:49
  Speaker 1: All right, Well, it's another big reminder that the universe
• 42:52
  Speaker 1: is a big, mysterious piece of pastry out there. There's
• 42:55
  Speaker 1: still a lot to learn, a lot to explore, a
• 42:57
  Speaker 1: lot to taste, and a lot to bake as well.
• 43:00
  Speaker 2: And as you take a big bite of the universe,
• 43:02
  Speaker 2: remember that not only is our kind of stuff unusual
• 43:05
  Speaker 2: in the universe, it's getting more and more unusual. And
• 43:08
  Speaker 2: we live in an unusual time in the universe when
• 43:11
  Speaker 2: our kind of stuff and dark matter is a really
• 43:14
  Speaker 2: significant fraction of the stuff out there in the universe,
• 43:17
  Speaker 2: alien civilizations in the year three trillion, that we'll have
• 43:19
  Speaker 2: a very different kind of physics to deal with.
• 43:21
  Speaker 1: All right, Well, we hope you go out there and
• 43:24
  Speaker 1: grab your piece of the pie of the curiosity and
• 43:26
  Speaker 1: mystery of the universe. You hope you enjoyed that. Thanks
• 43:30
  Speaker 1: for joining us, See you next time.
• 43:40
  Speaker 2: Thanks for listening, and remember that Daniel and Jorge Explain
• 43:42
  Speaker 2: the Universe is a production of iHeartRadio. For more podcasts
• 43:46
  Speaker 2: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
• 43:51
  Speaker 2: you listen to your favorite shows.