Latest / Daniel and Kelly’s Extraordinary Universe / Could the Universe be shaped like a donut?

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Could the Universe be shaped like a donut? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Or Hey, if you were designing a universe, would you
• 0:12
  Speaker 1: make it the shape of a snack food? M M.
• 0:15
  Speaker 1: It's a very specific question, Daniel. Can I make it
• 0:18
  Speaker 1: like a cheeto shaped or dorito shaped? Hey? If you're
• 0:21
  Speaker 1: making a universe, there are no rules. Oh, I can
• 0:24
  Speaker 1: go nuts or bananas? Hey? Can I make the universe
• 0:27
  Speaker 1: banana shape? How about you? I think instead of going
• 0:30
  Speaker 1: for fruity inspiration, I might draw my inspiration from chocolate.
• 0:35
  Speaker 1: What shape is chocolate? Well, you know, I like my
• 0:37
  Speaker 1: universe the way I like my chocolate, flat, dark, infinite.
• 0:43
  Speaker 1: You make it sound a little inappropriate. There isn't the
• 0:47
  Speaker 1: universe expanding? Also? Is that due to dark chocolate energy?
• 0:50
  Speaker 1: That's also the reason that I'm expanding? Are you going
• 0:52
  Speaker 1: to be shaped like a banana? Soon? I'm going to
• 0:54
  Speaker 1: be the size of the universe Eventually. We all have
• 0:56
  Speaker 1: a dreamy. Hi am or Hammon, cartoonists and the creator
• 1:13
  Speaker 1: of PhD comics. Hi, I'm Daniel. I'm a particle physicist
• 1:17
  Speaker 1: and a professor a UC Irvine, and I really do
• 1:19
  Speaker 1: feel like I have an infinite appetite for dark chocolate infinite. Wow,
• 1:23
  Speaker 1: that's a that's a lot. You know, if and it's
• 1:25
  Speaker 1: no joke. The music can eat it forever all the time. Yeah,
• 1:29
  Speaker 1: I feel like I've never reached the limit. You know,
• 1:31
  Speaker 1: somebody puts a plate of chocolate there, I just keep
• 1:34
  Speaker 1: snacking on it and eventually I gotta take it away.
• 1:36
  Speaker 1: So I've never reached the port where I'm like stop.
• 1:39
  Speaker 1: It sounds like you haven't tried hard enough, Daniel. I
• 1:43
  Speaker 1: need to do more experiments, that's what you're saying. That's right.
• 1:46
  Speaker 1: You need to explore the whole range of possibility, the
• 1:48
  Speaker 1: upper limits and the lower limits. There's got to be
• 1:51
  Speaker 1: a boundary, and I am devoted to finding it wherever
• 1:54
  Speaker 1: the cost, even if it causes your belt line to
• 1:58
  Speaker 1: go to be on bounded But anyways, welcome to our
• 2:01
  Speaker 1: podcast Daniel and Jorge Explain the Universe, a production of
• 2:04
  Speaker 1: I Heart Radio in which we snack our way to
• 2:07
  Speaker 1: an understanding of the craziest questions of the universe. We
• 2:11
  Speaker 1: try to take bite sized pieces out of some of
• 2:13
  Speaker 1: the biggest questions in the universe. How big is it,
• 2:16
  Speaker 1: what shape is it, where does it come from, where
• 2:19
  Speaker 1: is it going to end, what is it made out of?
• 2:21
  Speaker 1: And fundamentally, how does it all work? We talked about
• 2:24
  Speaker 1: all of these questions, and we try to explain all
• 2:26
  Speaker 1: of them to you while you sample your favorite snack food. Yes,
• 2:30
  Speaker 1: it is a very tasty or, as Daniel says, crazy universe,
• 2:34
  Speaker 1: crazy tasty universe for us to sample and enjoy and
• 2:36
  Speaker 1: savor because it is out there. It's very dark and
• 2:39
  Speaker 1: interesting and full of amazing things that we have yet
• 2:42
  Speaker 1: to discover. Speaking of snack foods, I know that people
• 2:44
  Speaker 1: like to munch on something while they watch TV. Do
• 2:46
  Speaker 1: you think people munch on something while they listen to podcasts? Wait?
• 2:50
  Speaker 1: I thought people fell asleep while listening to our podcasts.
• 2:53
  Speaker 1: Are they eating and sleeping at the same time? Isn't
• 2:55
  Speaker 1: it dangerous? I don't know. Do they fall asleep with
• 2:57
  Speaker 1: their hand in the Cheeto's bowl or something. I'm sure
• 3:01
  Speaker 1: there are people who snack while listening to us. I
• 3:03
  Speaker 1: mean I watched this year Is while listening to podcasts. Well,
• 3:06
  Speaker 1: I wondered if the crunching with interviewer they're listening. Well, anyway,
• 3:09
  Speaker 1: let us know if you have a favorite Daniel and
• 3:11
  Speaker 1: Jorhey snack food, maybe we can get them to sponsor
• 3:14
  Speaker 1: an episode. Yeah, it's great. I mean, you're feeding your
• 3:17
  Speaker 1: stomach and your mind at the same time, you're expanding
• 3:20
  Speaker 1: in all directions everywhere. Let's get episodes on dark energy
• 3:23
  Speaker 1: funded by dark chocolate makers. That's no joke, Daniel, we
• 3:27
  Speaker 1: could do it. Yeah, exactly happened to some of those
• 3:30
  Speaker 1: Swiss chocolate funds. Yeah, it happens a big chocolate moundy.
• 3:34
  Speaker 1: But anyways, it is a pretty interesting universe out there
• 3:37
  Speaker 1: that we can see from our little perch out here
• 3:39
  Speaker 1: and then sitting on top of a round rock in
• 3:41
  Speaker 1: the corner of the Milk Way gaxy in a small
• 3:44
  Speaker 1: corner of the gigantic universe that we are surrounded by.
• 3:47
  Speaker 1: And even though we are stuck on this rock so far,
• 3:51
  Speaker 1: we can still ask some of the biggest, craziest questions
• 3:54
  Speaker 1: about the whole universe, about what's going on super duper
• 3:58
  Speaker 1: far away, because of course, we could look out into
• 4:01
  Speaker 1: the universe sort of like we're trapped in a tiny
• 4:03
  Speaker 1: lighthouse and we're looking out at the horizon and wondering
• 4:06
  Speaker 1: what's going on over there. We can't go visit it,
• 4:09
  Speaker 1: but it's sending us messages that let us find answers
• 4:12
  Speaker 1: to some of our biggest questions. Yeah, it to be
• 4:14
  Speaker 1: amazing that from our little point of view here in
• 4:16
  Speaker 1: the little rock floating out in space, we've managed to
• 4:19
  Speaker 1: know so much about the universe and what's out there,
• 4:21
  Speaker 1: But you've got to kind of wonder what else is
• 4:23
  Speaker 1: out there or what happens if you keep going out
• 4:25
  Speaker 1: there in space, what are you going to encounter? And
• 4:28
  Speaker 1: what is the overall shape of the universe. I think
• 4:31
  Speaker 1: it's a great metaphor for our curiosity. You know, we
• 4:35
  Speaker 1: think about what's going on on our planet, what's going
• 4:37
  Speaker 1: on in our solar system, what's going on in the galaxy.
• 4:40
  Speaker 1: No matter how far you explain, people will always wonder
• 4:44
  Speaker 1: what's going on further than that. Right, there's a no
• 4:48
  Speaker 1: limit to our curiosity, just like there's no limit to
• 4:50
  Speaker 1: my capacity to eat dark chocolate. No matter how far
• 4:53
  Speaker 1: you explained, people want to know what's past the edge
• 4:56
  Speaker 1: of the universe. One of those habits seems healthier than
• 4:59
  Speaker 1: the other, then I think they're tied together. I think
• 5:02
  Speaker 1: dark chocolate fuels my curiosity. What did your doctor say
• 5:06
  Speaker 1: about this type of curiosity? This is why I stop
• 5:08
  Speaker 1: going to doctors. Really, sorry, you are a doctor, so
• 5:12
  Speaker 1: really you don't need another doctor. That's right, I tell
• 5:15
  Speaker 1: myself to take off my pants because I no longer
• 5:17
  Speaker 1: fit into them. But I think the two questions are connected.
• 5:20
  Speaker 1: You know, could there be an infinite amount of dark
• 5:22
  Speaker 1: chocolate in the universe for me to eat? Only if
• 5:25
  Speaker 1: the universe is literally infinite? I see you're saying we're
• 5:28
  Speaker 1: sort of like humans are kind of like the eternal
• 5:32
  Speaker 1: busy buddies. You know, we're always wondering what's going on
• 5:34
  Speaker 1: out there beyond the horizon. Yeah, exactly. We want to
• 5:37
  Speaker 1: know what's just beyond our ability to understand. Will never
• 5:40
  Speaker 1: be satisfiment go you know what, a hundred billion light
• 5:43
  Speaker 1: years that's enough for me. There's always going to be
• 5:45
  Speaker 1: somebody who wonders. But what's past that? What if you
• 5:48
  Speaker 1: kept going? And I think the reason for that is
• 5:50
  Speaker 1: that the universe and history of science is filled with
• 5:53
  Speaker 1: crazy surprises. Every time we've looked further than we imagined,
• 5:57
  Speaker 1: we found crazy stuff. Think all the way back to
• 6:00
  Speaker 1: Edwin Hubble discovering that there are other galaxies out there.
• 6:03
  Speaker 1: What a mind blowing realization to think it's not just
• 6:06
  Speaker 1: our galaxy sitting in space, but that there are hundreds, thousands,
• 6:10
  Speaker 1: trillions of other galaxies. That's the kind of realization we're
• 6:13
  Speaker 1: going for. Yeah, it's pretty mind blowing. And so far
• 6:16
  Speaker 1: we can see pretty far out there into the universe.
• 6:19
  Speaker 1: We can see up to about forty five forty six
• 6:22
  Speaker 1: billion light years. That's as far as we can see, right,
• 6:25
  Speaker 1: that's as far as humans are able and can see, right,
• 6:28
  Speaker 1: because of the limitations of the speed of light. Yeah,
• 6:31
  Speaker 1: it's a bit of a subtle question how far we
• 6:33
  Speaker 1: can see in the universe. You might imagine it would
• 6:35
  Speaker 1: be the age of the universe times the speed of
• 6:37
  Speaker 1: light that photons would be racing through space to us,
• 6:40
  Speaker 1: and then we couldn't see anything past thirteen or fourteen
• 6:44
  Speaker 1: billion light years away. But because the universe is expanding,
• 6:48
  Speaker 1: some of the stuff that sent us photons a long
• 6:50
  Speaker 1: time ago is now much much further away, and so
• 6:53
  Speaker 1: we can see light from things that are now very
• 6:56
  Speaker 1: very far away, out to about forty five billion light years,
• 6:59
  Speaker 1: and it actually we'll even see after about sixty two
• 7:03
  Speaker 1: billion light years. Yeah, so that's kind of as far
• 7:05
  Speaker 1: as we can see, Like, that's the range of our
• 7:08
  Speaker 1: vision as human beings in this universe. But I guess,
• 7:11
  Speaker 1: like you said, there are people who wonder what's beyond that.
• 7:14
  Speaker 1: You know, if I keep going past sixty five billion
• 7:17
  Speaker 1: light years, what am I going to find a Am
• 7:19
  Speaker 1: I gonna hit a wall? Is this the universe gonna
• 7:21
  Speaker 1: go on forever? Or is it going to do something
• 7:24
  Speaker 1: even crazier? Yeah, we'll put me in that category. I'm
• 7:26
  Speaker 1: not satisfied to only see sixty two billion light years away.
• 7:30
  Speaker 1: I want to know all of it. I want to
• 7:31
  Speaker 1: know what's beyond that. I want to know if it
• 7:33
  Speaker 1: goes on forever, if it curves on back on itself,
• 7:35
  Speaker 1: if there's some huge cosmic store of dark chocolate just
• 7:38
  Speaker 1: past the edge of our vision. That's your carrot, Daniel, like,
• 7:43
  Speaker 1: you know, the the character that's driving you to do science.
• 7:46
  Speaker 1: You just want to discover dark chocolate everywhere, even among
• 7:49
  Speaker 1: the fundamental particles. When I know there's a dark chocko Latino, Yeah,
• 7:52
  Speaker 1: there could be a secret dark chocolate reservoir past the
• 7:55
  Speaker 1: edge of the observable universe. You know what in my
• 7:57
  Speaker 1: next grant, I'm gonna put funds for dark chocolate, and
• 8:00
  Speaker 1: I'm gonna argue that it's essential for doing science, at
• 8:04
  Speaker 1: least for Daniel. For Daniel to do science, you need
• 8:06
  Speaker 1: a little motivation in the morning. Well, you know, there
• 8:08
  Speaker 1: is a clear correlation between chocolate consumption and Nobel Prize winning.
• 8:12
  Speaker 1: I'm not saying it's causal, but there's a correlation. So
• 8:14
  Speaker 1: you know, when I do apply to the Nestly Foundation
• 8:18
  Speaker 1: for Chocolate Research, then I think I'm gonna put that in.
• 8:21
  Speaker 1: There is a causation, I think, you know, I think
• 8:23
  Speaker 1: winning Nobel Prices makes you hungrier for chocolate, right, there's
• 8:26
  Speaker 1: something about that, you know, Northern European, you know flair
• 8:29
  Speaker 1: that makes you create the chocolate. I don't know. I
• 8:31
  Speaker 1: think it works either way. If you win the Nobel Prize,
• 8:33
  Speaker 1: you celebrate byating chocolate. If you don't win the Nobel Prize,
• 8:36
  Speaker 1: you console yourself by eating chocolate. I see. So the
• 8:40
  Speaker 1: correlation doesn't make sense to your physics, at least the
• 8:43
  Speaker 1: way you do it, not the way that I eat chocolate.
• 8:45
  Speaker 1: But it is an interesting question. What's out there beyond
• 8:48
  Speaker 1: the universe, beyond the sixty five billion light years that
• 8:50
  Speaker 1: we might be able to see some day or you're
• 8:52
  Speaker 1: gonna hit a wall, is it going to come around?
• 8:54
  Speaker 1: What is the size and shape of the universe? And
• 8:57
  Speaker 1: I love thinking about this question, the shape of the universe,
• 9:01
  Speaker 1: because it feels like something really deep and fundamental. It's
• 9:04
  Speaker 1: something that must be like written into the real source
• 9:07
  Speaker 1: code of the universe. It's not just like, oh, where
• 9:10
  Speaker 1: is the planet or where are there aliens? Things that
• 9:13
  Speaker 1: are affected by randomness. Is something which is a deep
• 9:15
  Speaker 1: truth about the universe. It's something which reveals its nature. Yeah,
• 9:20
  Speaker 1: you're basically asking the biggest question you can ask, right,
• 9:22
  Speaker 1: Like when you're asking about the size and shape of
• 9:24
  Speaker 1: the universe, you're asking what is the cion shape of
• 9:27
  Speaker 1: the entire universe? Like it has to encompass all of it,
• 9:30
  Speaker 1: not just like a part of it, all of it. Yeah,
• 9:32
  Speaker 1: I guess you could ask what is the shape of
• 9:34
  Speaker 1: the multiverse? That might be a bigger question. Oh what,
• 9:39
  Speaker 1: you just blew my mind? What is the shape of
• 9:41
  Speaker 1: the multiverse? Shape like a Marble movie? It's shaped like
• 9:45
  Speaker 1: a dollar bill, I'm pretty sure, shaped like the money
• 9:49
  Speaker 1: they're they're breaking in. I hope they're spending some of
• 9:51
  Speaker 1: that money on chocolate to celebrate. But it is the
• 9:53
  Speaker 1: biggest question we can ask, and it's a question we
• 9:55
  Speaker 1: don't know the answer to, right. I mean, first of all,
• 9:57
  Speaker 1: we can't see out to the edge of the universe, right,
• 10:01
  Speaker 1: at least we haven't seen an edge to it, and
• 10:03
  Speaker 1: so it's kind of a weird question to ask what
• 10:05
  Speaker 1: is the size and shape of the universe if we
• 10:06
  Speaker 1: can't see all of it. It's like sitting in the
• 10:08
  Speaker 1: middle of the United States and asking what is the
• 10:10
  Speaker 1: shape of the United States if you've never been to
• 10:13
  Speaker 1: or can ever get to the coast? That's right. But
• 10:16
  Speaker 1: although it seems impossible, science always finds a way to
• 10:19
  Speaker 1: like take the first nibble off of the question, to think, well, well,
• 10:23
  Speaker 1: what can we do if we can't answer the entire
• 10:26
  Speaker 1: question directly? Then? You know, can we find some way
• 10:29
  Speaker 1: to limit the possible answers? What can we do with
• 10:31
  Speaker 1: our limited data and our limited view of the universe?
• 10:35
  Speaker 1: And to me, it's incredible what we have learned where
• 10:38
  Speaker 1: we have been able to rule out about the universe
• 10:40
  Speaker 1: just from looking around. Wait, are you saying we can
• 10:43
  Speaker 1: just ask the universe what its size and shape is?
• 10:45
  Speaker 1: You know, I emailed the universe and invited to be
• 10:47
  Speaker 1: on the podcast, but it hasn't answered yet, and so
• 10:50
  Speaker 1: I'm gonna have to resort to the classic way, which
• 10:52
  Speaker 1: is doing physics experiment, which is basically asking the universe
• 10:56
  Speaker 1: question interesting. Yeah. I guess if you ask anyone by
• 10:58
  Speaker 1: email what their size and shape is that you probably
• 11:01
  Speaker 1: won't get an answer. I got a cease and desist
• 11:04
  Speaker 1: from the universe legal department. Yeah, stay away from me,
• 11:07
  Speaker 1: blocked blocked by the universe. Wow, that's harsh. But it
• 11:12
  Speaker 1: is an interesting question. And so today on the podcast
• 11:15
  Speaker 1: we'll be asking is the universe shaped like a doughnut?
• 11:24
  Speaker 1: And does it taste like a donut? To Daniel, this
• 11:26
  Speaker 1: this seems like a very specific question. It's a specific question,
• 11:29
  Speaker 1: an idea motivated by some hints we've seen in some
• 11:32
  Speaker 1: recent studies that suggested that maybe the universe is chocolate
• 11:36
  Speaker 1: filled after all, chocolate filled or shaped like a donut
• 11:40
  Speaker 1: or like a donut wood chocolate in it exactly, the
• 11:42
  Speaker 1: universe might be a chocolate field donut in the end.
• 11:45
  Speaker 1: I don't think I've ever heard of a chocolate field donut.
• 11:48
  Speaker 1: Usually chocolate is on top. You've never heard of a
• 11:51
  Speaker 1: chocolate filled donut? Are you serious? Well, if it has
• 11:53
  Speaker 1: something in the inside, then it's not a donut, is it. Man?
• 11:56
  Speaker 1: What's a jelly filled donut? Then? Is it a jelly
• 11:58
  Speaker 1: filled not donut? It's a jelly filled cake and p
• 12:02
  Speaker 1: have a hole in the middle. Right to be a donut,
• 12:04
  Speaker 1: doesn't it? Oh? Man, this is turned into a food
• 12:06
  Speaker 1: argument podcast. I hear those are more popular than the
• 12:09
  Speaker 1: science podcast. Let's roll with it. We've done the murder
• 12:13
  Speaker 1: mystery podcast recently and now we're doing the food argument podcast. Absolutely.
• 12:17
  Speaker 1: I think there's lots of different shapes donuts can be.
• 12:20
  Speaker 1: I think the classic shape, you know, it was basically
• 12:22
  Speaker 1: a tourist with a hole in the middle. But you
• 12:24
  Speaker 1: could still put chocolate inside that taurus right, Oh, you
• 12:28
  Speaker 1: mean along like the ring of it you can stuff
• 12:30
  Speaker 1: with with chocolate. Ring of chocolate? Why not? Sounds delicious
• 12:33
  Speaker 1: to me? It sounds like a heart attack. Well, we'll
• 12:36
  Speaker 1: see if the universe agrees with me or with you
• 12:38
  Speaker 1: by the end of the episode. But anyways, apparently it's
• 12:41
  Speaker 1: a possibility that the universe could be shaped like a doughnut,
• 12:45
  Speaker 1: which sounds both bulgeous and filling. And so that's the
• 12:48
  Speaker 1: question we'll be trying to answer today. And so, as usual,
• 12:51
  Speaker 1: we were wondering how many people out there had thought
• 12:52
  Speaker 1: about equating the universe with a treat like a donut. So,
• 12:56
  Speaker 1: thanks to our ever rotating group of volunteers who answer
• 12:59
  Speaker 1: the questions for us on the podcast. It's always fun
• 13:02
  Speaker 1: to hear your answers, and it gives us a sense
• 13:04
  Speaker 1: for what people are thinking. If you'd like to participate
• 13:07
  Speaker 1: for a future episode, please don't be shy. I know
• 13:09
  Speaker 1: you're out there waiting for an invitation. There's right to
• 13:12
  Speaker 1: me two questions at Daniel and Jorge dot com and
• 13:15
  Speaker 1: Dennie will send your donut if you answer the question right.
• 13:18
  Speaker 1: You're a digital donut, a little emoji donut. Well, think
• 13:22
  Speaker 1: about it for a second. Do you think the universe
• 13:24
  Speaker 1: is shaped like a donut? Here's what people have to say.
• 13:28
  Speaker 1: I think the universe could be shaped like a donut,
• 13:31
  Speaker 1: and it could be that we just don't see all
• 13:35
  Speaker 1: of the universe from our advantage point. But I doubt
• 13:40
  Speaker 1: that it is because I would think we would see
• 13:43
  Speaker 1: some kind of evidence of it in the microwave background.
• 13:46
  Speaker 1: So from that, it really appears that the universe, uh
• 13:52
  Speaker 1: does not have curvature to it. I don't think the
• 13:54
  Speaker 1: universe has shaped like a donut unless initial expansion was
• 13:57
  Speaker 1: not uniform in every direction. I would guess it's more
• 14:01
  Speaker 1: likely to be a hollow sphere. Yeah, I mean could
• 14:03
  Speaker 1: it be as an interesting question? I uh, yeah, I
• 14:06
  Speaker 1: mean I imagine it could be. I think I've heard
• 14:08
  Speaker 1: about this before. I remember at some point somebody, maybe
• 14:12
  Speaker 1: in a documentary or something, saying that if you look straightforward,
• 14:16
  Speaker 1: if you were able to like see into infinity, you
• 14:21
  Speaker 1: would look at the back of your head, which I
• 14:23
  Speaker 1: guess if you were on a donut you could probably
• 14:27
  Speaker 1: do that if space was like sort of bat like that.
• 14:29
  Speaker 1: I don't think the universe has shaped like a donut.
• 14:32
  Speaker 1: I think if it was, we would have noticed by now,
• 14:34
  Speaker 1: because there'll be a lot less potions and activity coming
• 14:38
  Speaker 1: from the center of the donut. I think so long
• 14:40
  Speaker 1: as information is consistently available across the entire universe, it
• 14:48
  Speaker 1: really doesn't matter what the shape is. And as long
• 14:51
  Speaker 1: as time and space are concentric, I think the universe
• 14:58
  Speaker 1: can be any shape, and there are probably universes of
• 15:03
  Speaker 1: all ships and sizes out there. Frankly, I think that
• 15:07
  Speaker 1: the universe could be shaped like anything, and it wouldn't
• 15:09
  Speaker 1: surprise me. There's this idea that you know, you start
• 15:13
  Speaker 1: in one place, and you go in one direction, and
• 15:16
  Speaker 1: then eventually you're just back at that place, and yeah,
• 15:19
  Speaker 1: that fits. Donut shaped to me. It's me, all right,
• 15:22
  Speaker 1: a pretty wide range of answers. Some people seem skeptical,
• 15:24
  Speaker 1: some people were like, sure, why not? The universe is
• 15:27
  Speaker 1: is weird and surprising. It could be shaped like anything exactly,
• 15:31
  Speaker 1: and some people even commenting that donuts have jam inside
• 15:34
  Speaker 1: of them, and so they're arguing, I think they agree
• 15:36
  Speaker 1: with you. You know that the shape we're talking about,
• 15:38
  Speaker 1: it's more like a bagel than a donut. Interesting, whoa
• 15:41
  Speaker 1: should we rename the episode? Then? Could the universe be
• 15:44
  Speaker 1: shaped like a bagel? Well, the question then is can
• 15:46
  Speaker 1: you put chocolate inside of a bagel? Obviously, yes, Daniel,
• 15:50
  Speaker 1: you can put chocolate inside of anything, all right, then
• 15:53
  Speaker 1: I'm fine with it, what I call it whatever, as
• 15:54
  Speaker 1: long as you with chocolate inside of it. Man, you
• 15:58
  Speaker 1: really hungry here to day, Daniel? Have you had your
• 16:00
  Speaker 1: daily dose of chocolate yet? I'll reward myself after the podcast.
• 16:03
  Speaker 1: I mean, this podcast is not infinite, I guess. So. Yeah,
• 16:06
  Speaker 1: So let's get into this idea of the shape of
• 16:08
  Speaker 1: the universe, because it's kind of weird to think about
• 16:11
  Speaker 1: the universe having a shape, right, because it could the
• 16:13
  Speaker 1: universe could be infinite, and even if it's not infinite.
• 16:16
  Speaker 1: Are you saying, like the walls of the universe are
• 16:18
  Speaker 1: shaped like a donut? What exactly do you mean by
• 16:20
  Speaker 1: the shape of space? So the shape of space refers
• 16:22
  Speaker 1: to basically how the universe is connected on a really
• 16:26
  Speaker 1: really big scale. You know, we are used to thinking
• 16:29
  Speaker 1: about space is like something we are floating in, and
• 16:32
  Speaker 1: we're also used to thinking about space is like maybe
• 16:35
  Speaker 1: being curved. General relativity tells us that mass and energy
• 16:39
  Speaker 1: and all this kind of stuff can curve space. That's
• 16:42
  Speaker 1: sort of a local curvature. But you know, if you
• 16:44
  Speaker 1: have curvature in space, if space bends in some way,
• 16:48
  Speaker 1: and you can imagine putting it together into some big shape.
• 16:51
  Speaker 1: Like if space doesn't curve, if it's flat, you can
• 16:54
  Speaker 1: make an infinite sheet. That would be the shape of space.
• 16:56
  Speaker 1: Where space does curve, you can imagine it being like
• 16:59
  Speaker 1: a sphere or a donut or some other weird shape.
• 17:02
  Speaker 1: So the curvature of space is connected to this other
• 17:05
  Speaker 1: question of the shape of space. Curvature sort of local,
• 17:09
  Speaker 1: the shape of space is sort of global. It's asking like,
• 17:12
  Speaker 1: how is all of space put together? Think in two
• 17:15
  Speaker 1: dimensions for a moment, because it's easier imagine you have
• 17:18
  Speaker 1: a bunch of rings and you have to connect them
• 17:20
  Speaker 1: together into a two D surface. You could loop them
• 17:24
  Speaker 1: together into a flat sheet, or you could make it
• 17:26
  Speaker 1: into a sphere or something else. These are different shapes
• 17:30
  Speaker 1: because they have fundamentally different properties, And mathematically speaking, two
• 17:36
  Speaker 1: shapes are fundamentally the same if you can smoothly morph
• 17:40
  Speaker 1: one into another. So two different shaped spheres, or even
• 17:44
  Speaker 1: a sphere and ellipsoid are basically the same shape. But
• 17:48
  Speaker 1: a sphere and an infinite flat sheet are not the same.
• 17:52
  Speaker 1: One is infinite, the other is finite. There are other shapes,
• 17:56
  Speaker 1: like ones with a hole in it, like a donut,
• 17:58
  Speaker 1: that are not the same as a sphere or a
• 18:00
  Speaker 1: flat sheet. That's the meaning of shape in a global sense,
• 18:05
  Speaker 1: which is related to, but different from curvature in a
• 18:08
  Speaker 1: local sense. Mm hmm, I see, And I guess just
• 18:11
  Speaker 1: to be clear, you actually mean space time, right, because
• 18:14
  Speaker 1: when you're talking about gravity kind of bending or giving
• 18:17
  Speaker 1: space curvature, you're actually talking about the curvature of space time. Right.
• 18:20
  Speaker 1: You kind of have to take time into consideration as well. Well.
• 18:23
  Speaker 1: We do think about how the shape of space evolves
• 18:26
  Speaker 1: with time, and the shape also contributes to how the
• 18:29
  Speaker 1: universe expands or contracts or doesn't. So, yes, this is
• 18:32
  Speaker 1: a dynamical thing. The shape of the universe, the curvature
• 18:35
  Speaker 1: of the universe can change, although it's fundamental shape cannot. Yes,
• 18:40
  Speaker 1: it is interesting to think about it as a function
• 18:41
  Speaker 1: of time, right, And it's kind of interesting that you
• 18:44
  Speaker 1: related to this idea because I know we've talked a
• 18:46
  Speaker 1: lot in this podcast about how gravity and energy kind
• 18:49
  Speaker 1: of bends space time in the sense that you know,
• 18:51
  Speaker 1: we're not going around the Sun because it's pulling on us.
• 18:54
  Speaker 1: We're going around the Sun because the space time around
• 18:56
  Speaker 1: it is sort of shaped like a bowl on so
• 18:58
  Speaker 1: we're kind of stuck in this loop around it. Is
• 19:01
  Speaker 1: that sort of what you're saying, Like, on a global
• 19:03
  Speaker 1: or universal scale, does the universe have some sort of
• 19:06
  Speaker 1: like curvature to it. It's a really fascinating idea that
• 19:09
  Speaker 1: you know, space is curved, that gravity is not like
• 19:12
  Speaker 1: a force the way we think about other forces, but
• 19:14
  Speaker 1: instead it's a fictitious force, the force that appears to
• 19:18
  Speaker 1: act because we can't see that space is curved. Like
• 19:20
  Speaker 1: if I look at a piece of space, I can't
• 19:23
  Speaker 1: tell you is it curved? But I could shine a
• 19:25
  Speaker 1: light through it, and if I see where that light goes,
• 19:27
  Speaker 1: then I can tell you whether space is curved. So
• 19:30
  Speaker 1: we can see the effect of curved space even though
• 19:32
  Speaker 1: we can't see the curvature directly. So now imagine you're
• 19:35
  Speaker 1: building a universe and I'm giving you pieces of it,
• 19:38
  Speaker 1: and each piece has curvature. So if I only give
• 19:41
  Speaker 1: you curved pieces, what kind of big universe shape can
• 19:45
  Speaker 1: you build? But you can't build like an infinitely flat space,
• 19:48
  Speaker 1: like a huge sheet, But you can build like a
• 19:50
  Speaker 1: sphere or a circle, or a donut or something else
• 19:53
  Speaker 1: that has a curved surface. So the building blocks of
• 19:56
  Speaker 1: the local curvature I'm giving you do constrain the kind
• 19:59
  Speaker 1: of overall shape that you can build for the universe.
• 20:02
  Speaker 1: M I think what you're saying is it's kind of like,
• 20:04
  Speaker 1: you know, if I measure the land around me, and
• 20:06
  Speaker 1: I see that the land occurs around me downwards or
• 20:10
  Speaker 1: towards the end of the Earth, then it kind of
• 20:13
  Speaker 1: tells you that me the whole Earth is round exactly.
• 20:16
  Speaker 1: If you can measure the curvature on the surface of
• 20:18
  Speaker 1: the Earth, then you know that the Earth can't go
• 20:20
  Speaker 1: on forever or can't be flat because you see that
• 20:22
  Speaker 1: it's curved. Right. If you imagine that the rest of
• 20:25
  Speaker 1: the Earth is curved the way the piece you're standing
• 20:27
  Speaker 1: on its curved, then that suggests that probably it's a
• 20:30
  Speaker 1: big sphere, although you know, it could also be a donut,
• 20:34
  Speaker 1: or I guess it could also be kind of like
• 20:35
  Speaker 1: a dome, right, a dome goes on forever, Like if
• 20:38
  Speaker 1: you're standing on a parable at the bottom of parable,
• 20:41
  Speaker 1: you think it's it's curved, but it could keep going forever. Right. Mathematically,
• 20:45
  Speaker 1: a problem has the opposite curvature of a sphere. But yeah,
• 20:48
  Speaker 1: parabola is an open construction, so you can have curvature
• 20:52
  Speaker 1: and still have an infinite shape, so that curvature is
• 20:55
  Speaker 1: negative instead of positive. But that's a detail, right. So
• 20:58
  Speaker 1: you're saying that, you know, somehow we can't see the
• 21:00
  Speaker 1: entire universe, but maybe by measuring the curvature around us,
• 21:04
  Speaker 1: we could maybe you know, get a sense or an
• 21:07
  Speaker 1: idea of what the overall shape to the universe is exactly.
• 21:10
  Speaker 1: And what we want is the curvature on the biggest scale.
• 21:13
  Speaker 1: We don't really care about how much the Sun is
• 21:16
  Speaker 1: bending the shape of space in our neighborhood that doesn't
• 21:19
  Speaker 1: affect the overall shape of the universe. We want to know,
• 21:22
  Speaker 1: like how much is all the stuff in the universe
• 21:24
  Speaker 1: bending the shape of space? What shape of spaces that
• 21:27
  Speaker 1: consistent with just in the same way, like if you
• 21:30
  Speaker 1: want to measure the surface of the Earth, you could
• 21:32
  Speaker 1: be standing locally on something flat or something point de
• 21:35
  Speaker 1: or you know, in a bowl or something, doesn't change
• 21:37
  Speaker 1: the overall structure. So when we measure the curvature universe,
• 21:41
  Speaker 1: we want to make measurements on as big a scale
• 21:43
  Speaker 1: as possible to avoid like the local fluctuations and deviations.
• 21:47
  Speaker 1: Are you saying that the universe might have a curvature
• 21:50
  Speaker 1: to it, or that space might have a curvature to
• 21:53
  Speaker 1: it without stuff in it, you know, like um or
• 21:57
  Speaker 1: are you saying space as a curvature to it because
• 21:59
  Speaker 1: there's stuff in it? This is a really important point
• 22:01
  Speaker 1: and a subtle one. Universe has some curvature and some shape, right,
• 22:06
  Speaker 1: Either the curvature is positive like a sphere, or it's
• 22:09
  Speaker 1: flat like an infinite surface, or it's negative like a
• 22:12
  Speaker 1: parabola as you described, And that just is part of
• 22:15
  Speaker 1: the nature of the universe. And you can put stuff
• 22:18
  Speaker 1: in the universe, and that can change the amount of curvature.
• 22:21
  Speaker 1: You can make the universe more or less curved by
• 22:24
  Speaker 1: putting stuff into it, but you can't change the fundamental
• 22:27
  Speaker 1: nature of the curvature or topology of the universe. You
• 22:30
  Speaker 1: can't like make a sphere universe into a flat universe
• 22:33
  Speaker 1: by putting stuff into it or the other way around.
• 22:36
  Speaker 1: M I see, because I think you know, there's sort
• 22:39
  Speaker 1: of two things, right, there's stuff and energy and then
• 22:41
  Speaker 1: there's space. Those are two kind of separate things, and
• 22:44
  Speaker 1: so you can imagine that an empty universe where it's
• 22:46
  Speaker 1: just space, or like if you took out all the
• 22:49
  Speaker 1: stuff and energy in our universe, we would just be
• 22:51
  Speaker 1: left with the space we have. And so you're saying, like,
• 22:54
  Speaker 1: without any stuff in it, what is the shape of
• 22:56
  Speaker 1: that space? Does that space have curvature or is it
• 22:59
  Speaker 1: totally like flat? Then even yeah, although we're interested in
• 23:02
  Speaker 1: our universe, which does have stuff in it, and the
• 23:04
  Speaker 1: stuff affects the amount of curvature. You know, for example,
• 23:07
  Speaker 1: one of the things in our universe is dark energy,
• 23:10
  Speaker 1: which we don't understand at all, but it's expanding the
• 23:13
  Speaker 1: universe that tends to decrease the curvature of the universe
• 23:15
  Speaker 1: like stretches everything out. Imagine you're like the little prints.
• 23:18
  Speaker 1: You're standing on a tiny little planet, and then that
• 23:20
  Speaker 1: planet expands to be huge. The curvature you experience decreases,
• 23:24
  Speaker 1: the amount that planet curves away under you decreases. So
• 23:27
  Speaker 1: the stuff that's in the universe can affect the amount
• 23:30
  Speaker 1: of curvature, but it can't affect the shape, right, Like
• 23:32
  Speaker 1: you can't take a curved universe and put stuff into
• 23:34
  Speaker 1: it and make it flat. You can contract it. Like
• 23:37
  Speaker 1: you put a bunch of mass into a universe that's
• 23:39
  Speaker 1: a sphere, it's going to make that collapse into a point. Interesting,
• 23:42
  Speaker 1: So you're saying that if I take out of all
• 23:44
  Speaker 1: the stuff and energy in the universe, I would be
• 23:46
  Speaker 1: left foot space and that space could be in the
• 23:48
  Speaker 1: shape of a sphere or a donut or something and
• 23:51
  Speaker 1: the stuff and it wouldn't be able to change that
• 23:53
  Speaker 1: wouldn't be able to change that. And it's connected to
• 23:55
  Speaker 1: this other idea that you know the shape of space.
• 23:57
  Speaker 1: It's apology. It's global curvature plus the amount of stuff,
• 24:01
  Speaker 1: the mount of the density, and the dark energy all
• 24:03
  Speaker 1: those together determine like whether the universe is expanding or contracting.
• 24:08
  Speaker 1: This is one element of that. We're talking about the
• 24:10
  Speaker 1: shape of space, which is not something that can be
• 24:12
  Speaker 1: changed by dark energy or dark matter or the arrangement
• 24:16
  Speaker 1: of stuff. It's just like it's something that's deep and
• 24:18
  Speaker 1: true about the universe itself. It's just baked in. I mean,
• 24:21
  Speaker 1: it's it's it's it's sape right, like you know nothing,
• 24:25
  Speaker 1: but the universe can change its safe yeah, and shape
• 24:27
  Speaker 1: in a very specific way, right. You know, we're not
• 24:29
  Speaker 1: talking about size. You can have a universe that's a
• 24:31
  Speaker 1: sphere and it can grow or it can collapse. In
• 24:33
  Speaker 1: that sense, the universe can change, but it can't change
• 24:36
  Speaker 1: from like a sphere to an infinitely flat universe or
• 24:39
  Speaker 1: to a parabolic universe. That kind of thing can't change, right,
• 24:43
  Speaker 1: And you know that sort of mathematically or how do
• 24:45
  Speaker 1: you know that it can change? Well, I guess we
• 24:47
  Speaker 1: know that mathematically, but you can also just think about
• 24:49
  Speaker 1: it intuitively, like how could you take a finite universe
• 24:53
  Speaker 1: which is a sphere and suddenly transform it into an
• 24:56
  Speaker 1: infinite flat plane. And you can imagine a sphere even
• 24:59
  Speaker 1: grow going you know, arbitrarily large, But then how do
• 25:02
  Speaker 1: you suddenly transform that smoothly into a flat universe. You know,
• 25:07
  Speaker 1: the universe can change, it can't evolve, but it has
• 25:09
  Speaker 1: to happen in a smooth way. You can't have like
• 25:12
  Speaker 1: a sudden jump where the universe is all of a
• 25:14
  Speaker 1: sudden completely different. This is like a basic element of
• 25:17
  Speaker 1: the nature of the universe. It doesn't seem like you
• 25:19
  Speaker 1: could change it from one moment to the next. All right, well,
• 25:21
  Speaker 1: let's get into what are the possible shapes and curvatures
• 25:25
  Speaker 1: of the universe and what it all means. But first
• 25:28
  Speaker 1: let's take a quick break. Right then, we're talking about
• 25:42
  Speaker 1: the size and shape of the universe, which seems a
• 25:45
  Speaker 1: little rude to be asking these questions about the universe.
• 25:47
  Speaker 1: Maybe the universe is very bashful about these things. Physics
• 25:51
  Speaker 1: asking rude questions since fifteen four, being nosy about people's
• 25:56
  Speaker 1: size and shape exactly upsetting the apple carts. Since for ever,
• 26:00
  Speaker 1: that's our goal. You know, we don't care if the
• 26:02
  Speaker 1: truth is offensive. We just want to know. Discarding with
• 26:07
  Speaker 1: social conventions since the beginning of the ton of time,
• 26:11
  Speaker 1: as if we understood social conventions, right, that's the reason
• 26:13
  Speaker 1: we went into physics, because we just don't know how
• 26:16
  Speaker 1: to do the other stuff. Oh man, your meetings would
• 26:19
  Speaker 1: must be pretty interesting or not or not. You know,
• 26:23
  Speaker 1: a world without subtlety might be a little boring. You know,
• 26:26
  Speaker 1: that's a fun stereotype. But you know, physicists are social creatures.
• 26:29
  Speaker 1: I work in a collaboration of thousands of people, and
• 26:31
  Speaker 1: we have lots of meetings and politics and have to
• 26:33
  Speaker 1: deal with each other. And you know, people work together
• 26:36
  Speaker 1: and don't work together, and it's all sorts of stuff
• 26:39
  Speaker 1: exactly the same way you find in every other field
• 26:42
  Speaker 1: of human endeavors. Right right, it is a human endeavor.
• 26:45
  Speaker 1: And we were talking about the size and shape to
• 26:47
  Speaker 1: the universe, and you were telling me that, you know,
• 26:49
  Speaker 1: the the universe can have a shape even if I
• 26:51
  Speaker 1: take like if I take out all this stuff and
• 26:52
  Speaker 1: energy in it, put it somewhere out of nowhere, But
• 26:55
  Speaker 1: you put it somewhere, the universe that's left over would
• 26:57
  Speaker 1: have a shape to it. What exactly does mean a shape?
• 27:01
  Speaker 1: Does it mean it has walls and a boundary like
• 27:04
  Speaker 1: edges to it? It tells you how space is connected.
• 27:06
  Speaker 1: I think about space not it's just like where stuff happens.
• 27:11
  Speaker 1: But I think if it's sort of like a fabric
• 27:13
  Speaker 1: and it's woven together, and if I move to the left,
• 27:16
  Speaker 1: then I'm moving into a new bit of space. And
• 27:19
  Speaker 1: then you can ask, like, well, what happens if you
• 27:21
  Speaker 1: keep going you just encounter a new space or potentially
• 27:25
  Speaker 1: space could be connected in such a way that if
• 27:28
  Speaker 1: you keep going, you come back to where you started right,
• 27:31
  Speaker 1: sort of like a Packman universe. And so that's entirely possible,
• 27:34
  Speaker 1: and that tells you something about the shape of space.
• 27:37
  Speaker 1: So when we say, like, what if space was a sphere,
• 27:40
  Speaker 1: what do we mean by that? We don't mean that
• 27:42
  Speaker 1: space is literally a three D sphere. We're talking about
• 27:45
  Speaker 1: the surface of the sphere, the two D surface of
• 27:48
  Speaker 1: the sphere, as an analogy to our three dimensional space,
• 27:52
  Speaker 1: because it's hard to think about three D surfaces of
• 27:55
  Speaker 1: four D spheres. But if you're on the surface of
• 27:57
  Speaker 1: a sphere, that surface is finite. It doesn't go on forever,
• 28:01
  Speaker 1: but it also doesn't have an edge. That's an example
• 28:04
  Speaker 1: of how space could be connected. So really we're talking about, like,
• 28:07
  Speaker 1: how do we've bits of space together into a larger fabric,
• 28:11
  Speaker 1: and then what is the shape of that fabric? Right?
• 28:13
  Speaker 1: But I think it sounds to me like you're making
• 28:15
  Speaker 1: an assumption about the universe, which is that it doesn't
• 28:18
  Speaker 1: have an edge or a wall to it, right, Like,
• 28:21
  Speaker 1: it doesn't seem like you're considering that possibility that if
• 28:24
  Speaker 1: I just hop on a spaceship and I keep going
• 28:26
  Speaker 1: for you know, a ninety billion light years, eventually I'm
• 28:29
  Speaker 1: maybe you're gonna hit a wall. Like that's not a
• 28:30
  Speaker 1: possibility that physicists seemed to consider because it maybe it
• 28:33
  Speaker 1: doesn't make sense to you. That's a possibility I'm totally
• 28:36
  Speaker 1: open to considering. You're right that it doesn't make sense
• 28:38
  Speaker 1: to me. It would break a lot of things and
• 28:41
  Speaker 1: be very very odd. That would make it super fun
• 28:44
  Speaker 1: and interesting to discover. I wouldn't think it's something that's
• 28:47
  Speaker 1: likely as a description of our universe, because it would
• 28:50
  Speaker 1: mean that one part of the universe is very different
• 28:52
  Speaker 1: from another part of the universe. The edge would have
• 28:55
  Speaker 1: to have a different nature than the rest of the universe.
• 28:57
  Speaker 1: And so far, what we've seen is that every part
• 28:59
  Speaker 1: of the of our sort of seems to be the
• 29:01
  Speaker 1: same as every other part. But you know, maybe it's
• 29:03
  Speaker 1: like we're in the very center of a huge lake
• 29:05
  Speaker 1: and we just can't see the edges, but they are there.
• 29:07
  Speaker 1: It's possible that the universe has edges, that there's another
• 29:10
  Speaker 1: kind of space, an edgy kind of space that's different.
• 29:13
  Speaker 1: That's a possibility. We can't rule it out that it
• 29:15
  Speaker 1: would be weird, right, Right, So then when we're talking
• 29:18
  Speaker 1: about the shape of base, I mean if it does
• 29:20
  Speaker 1: have walls and a boundary could be shaped by anything, right,
• 29:23
  Speaker 1: could be shaped like a cube or a banana or
• 29:25
  Speaker 1: you know, like um, like being trapped inside of a banana,
• 29:28
  Speaker 1: because that could be one way to talk about the
• 29:30
  Speaker 1: shape of the universe. But I think for our discussion today,
• 29:33
  Speaker 1: what do you actually mean by the shape of the
• 29:35
  Speaker 1: universe is like, let's assume that that's not possible. Like,
• 29:37
  Speaker 1: let's assume that the universe can't have walls or boundaries.
• 29:42
  Speaker 1: Then what are the possibilities left? Yeah, and so if
• 29:45
  Speaker 1: you assume that every point in space is the same
• 29:48
  Speaker 1: as every other point, right, you have like one kind
• 29:50
  Speaker 1: of lego piece, and you have to build a whole
• 29:52
  Speaker 1: universe with just that one lego pieast then what are
• 29:55
  Speaker 1: your options? So, for example, if I only give you
• 29:58
  Speaker 1: those flat pieces of lego, then what can you do?
• 30:00
  Speaker 1: You can just make a big flat sheet, right, just
• 30:03
  Speaker 1: like quilt together space, so that every piece is just
• 30:05
  Speaker 1: next to other pieces adjacent to it, and nothing is
• 30:08
  Speaker 1: more complicated. Space just goes on forever, totally flat. Right.
• 30:13
  Speaker 1: That's one simple idea for what space might be. Right,
• 30:15
  Speaker 1: because I think, you know, maybe a lot of people
• 30:17
  Speaker 1: might be confused with this, because you know, if you
• 30:19
  Speaker 1: tell me that the universe can have boundaries or like
• 30:22
  Speaker 1: walls or edges to it, then to me, I would
• 30:24
  Speaker 1: think that it has to go on forever. Then, you know,
• 30:27
  Speaker 1: to me, it seems like the only possibilities for the
• 30:29
  Speaker 1: universe to be infinite and have no shape. But I think,
• 30:33
  Speaker 1: you know, I physicists think about it differently, like it
• 30:35
  Speaker 1: could still be sort of infinite and continuous without boundaries
• 30:38
  Speaker 1: and still have a shape. No, I think you're right.
• 30:40
  Speaker 1: If it's flat and it's infinite, that is, it doesn't
• 30:43
  Speaker 1: repeat and there's no special edges to it, then it
• 30:47
  Speaker 1: has to be infinite. I can't think of another way
• 30:49
  Speaker 1: to arrange space. It is possible, though, for space to
• 30:52
  Speaker 1: be flat and not infinite, right. Imagine the pac Man
• 30:56
  Speaker 1: universe or the Asteroids universe, where at some point space
• 30:59
  Speaker 1: just repeat. You know, you get to some point in
• 31:01
  Speaker 1: space and it just is now connected to a spot
• 31:04
  Speaker 1: you've already been without any curvature. Right. It's just like
• 31:07
  Speaker 1: the way space is sort of the same way we
• 31:09
  Speaker 1: talked about for wormholes. Space can have non trivial connections,
• 31:13
  Speaker 1: like you can connect this bit of space to that
• 31:15
  Speaker 1: bit of space and just say, these two bits are
• 31:17
  Speaker 1: now next to each other, meaning you can step from
• 31:20
  Speaker 1: one to the other, even though in the larger fabric
• 31:23
  Speaker 1: of space they seem to be far apart. In the
• 31:25
  Speaker 1: same sense, you could have like a whole seam where
• 31:27
  Speaker 1: you connect like a whole line of space to another
• 31:30
  Speaker 1: whole line of space, effectively making like an infinitely sized wormhole.
• 31:34
  Speaker 1: I think what's interesting is that the idea that space
• 31:36
  Speaker 1: can have no it could have no boundaries, no walls,
• 31:40
  Speaker 1: but still have a shape. And I think what you're
• 31:42
  Speaker 1: telling me is that the one way it can do
• 31:44
  Speaker 1: that is if it's if space sort of wraps around itself,
• 31:47
  Speaker 1: sort of like if I keep going in one direction,
• 31:49
  Speaker 1: I eventually come back from the other direction, like somehow
• 31:53
  Speaker 1: space is curved or connected to itself in a strange
• 31:57
  Speaker 1: way so that I can't keep going forever, but I
• 32:00
  Speaker 1: sort of keep going around circles, and in that case,
• 32:03
  Speaker 1: the universe sort of has the shape, but it has
• 32:05
  Speaker 1: no edges to it. And there's two possibilities there. One
• 32:08
  Speaker 1: is space is actually curved, sort of like a sphere, right,
• 32:12
  Speaker 1: and so it's natural to put it together in that
• 32:14
  Speaker 1: way that space is curved and finite, and if you
• 32:17
  Speaker 1: keep going in one direction, you come back to where
• 32:19
  Speaker 1: you are. There's another possibility, which is that space is flat,
• 32:22
  Speaker 1: it doesn't have curvature, and yet it doesn't have edges
• 32:25
  Speaker 1: and it's finite. And that's like the asteroids or the
• 32:28
  Speaker 1: pac Man universe, where without space being curved, it's just
• 32:31
  Speaker 1: connected in that way. That's why local curvature is a
• 32:34
  Speaker 1: little bit different from this question of like global topology
• 32:37
  Speaker 1: the connectedness or shape of space, right right, But maybe
• 32:40
  Speaker 1: let's take a step back and break it down. So
• 32:42
  Speaker 1: it's possible for the universe to for it to be
• 32:45
  Speaker 1: sort of continuous all the way through, no boundaries, no edges,
• 32:48
  Speaker 1: but still be finite. And you're telling me, I think
• 32:50
  Speaker 1: that there are different ways in which that can happen.
• 32:53
  Speaker 1: So one of those ways has to do with curvature
• 32:55
  Speaker 1: and the other one doesn't. So let's maybe break it
• 32:58
  Speaker 1: down a little bit more about what this idea of
• 33:00
  Speaker 1: curvature is, Like, what does what does curvature mean for you?
• 33:03
  Speaker 1: Like for me, curvature means that the ground is uneven
• 33:06
  Speaker 1: or it has a slope to it. But what does
• 33:07
  Speaker 1: it mean in terms of space? In terms of space?
• 33:10
  Speaker 1: It locally, it means what is the path of a photon?
• 33:13
  Speaker 1: If you shine a laser beam through a chunk of space,
• 33:16
  Speaker 1: where does it go? And if space is curved, then
• 33:20
  Speaker 1: you can't see that curvature, but it affects the flight
• 33:23
  Speaker 1: of the photon, and so the photon will bend. It's
• 33:26
  Speaker 1: taking what's actually the shortest distance through that space, right,
• 33:30
  Speaker 1: because photons always take the shortest path. But the shortest
• 33:33
  Speaker 1: path is now something that looks bent to you because
• 33:36
  Speaker 1: you can't see the relationships between those bits of space. Remember,
• 33:39
  Speaker 1: the curvature of space is intrinsic, meaning it just changes
• 33:43
  Speaker 1: the relative distances between bits of space. So that's what
• 33:47
  Speaker 1: I mean by the curvature of space. I mean like
• 33:49
  Speaker 1: the path of a photon through that piece of space.
• 33:52
  Speaker 1: And again I think it means space time, right, or
• 33:54
  Speaker 1: I guess that the lump it all together. Yeah, Well,
• 33:57
  Speaker 1: relativity definitely mixes space and time together. There's a connected
• 34:00
  Speaker 1: effects there, but you know, space and time are also different.
• 34:03
  Speaker 1: You know, space has these properties that time doesn't have.
• 34:06
  Speaker 1: And here we're talking specifically about the spatial part of
• 34:09
  Speaker 1: space time. But yeah, you can't really think about space
• 34:11
  Speaker 1: without thinking about the four D structure that it sits
• 34:14
  Speaker 1: images space time. Well, and another thing that I think
• 34:17
  Speaker 1: I've read is that, you know, this idea of curvature
• 34:19
  Speaker 1: is not just about whether one ft like if you
• 34:21
  Speaker 1: shoot one photon and it leans to the right or
• 34:24
  Speaker 1: to the left. It's more about, like, if you shoot
• 34:26
  Speaker 1: two photons, do they curve away from each other or
• 34:29
  Speaker 1: towards each other. That's really more about the curvature of space, right, Yeah,
• 34:32
  Speaker 1: one way to measure the curvature of space is to
• 34:35
  Speaker 1: look at the path of two photons. Another way, which
• 34:37
  Speaker 1: is equivalent, is to like draw a triangle and ask, like,
• 34:41
  Speaker 1: what are the angles of that triangle. If you draw
• 34:44
  Speaker 1: a triangle on a flat surface, then it's angles add
• 34:47
  Speaker 1: up two, And if you draw triangle on the surface
• 34:49
  Speaker 1: of a sphere, it's angles are bigger than one a
• 34:51
  Speaker 1: d um and that's the same notion, right, if you
• 34:53
  Speaker 1: shoot two photons, and they have to move along the
• 34:55
  Speaker 1: surface of a sphere, then they can appear to curve
• 34:57
  Speaker 1: towards each other, for example, And if you're on a parabola,
• 35:01
  Speaker 1: then they appear to curve away from each other. And
• 35:03
  Speaker 1: it's equivalent to saying that a triangle on that surface
• 35:06
  Speaker 1: as angles less than degrees. Well, it's kind of strange
• 35:10
  Speaker 1: to think of a sears and converted between two the
• 35:13
  Speaker 1: and four D three D. But I think what you're
• 35:16
  Speaker 1: saying is like if the universe, like if the curvature
• 35:18
  Speaker 1: of the universe is flat, or at least around us,
• 35:21
  Speaker 1: things are flat, it means that if I shoot two photons,
• 35:24
  Speaker 1: they're not going to curve, or at least they're gonna
• 35:25
  Speaker 1: curve together maybe, but they're not gonna sort of move
• 35:28
  Speaker 1: away from each other or move towards each other. Right,
• 35:30
  Speaker 1: They're just gonna keep going in the same direction, side
• 35:32
  Speaker 1: by side forever. That's what it means for space to
• 35:34
  Speaker 1: be flat, Right, that's the way to test it. In
• 35:37
  Speaker 1: flat space, two photons moving in parallel will not approach
• 35:40
  Speaker 1: each other, whereas in curved space, two photons initially moving
• 35:44
  Speaker 1: what appears to be parallel to you will either approach
• 35:47
  Speaker 1: each other or divert from each other the same way
• 35:49
  Speaker 1: as if you're on the equator of the Earth, which
• 35:51
  Speaker 1: is a curved surface, and you and your friend both
• 35:54
  Speaker 1: walk north due north, which feels like you know, we're
• 35:56
  Speaker 1: moving parallel. You'll notice that you get closer and closer
• 35:59
  Speaker 1: to each other because eventually you're gonna hit the same point.
• 36:02
  Speaker 1: It's a motion constrained along a curved surface, or motion
• 36:05
  Speaker 1: through curved space can make things that initially were parallel
• 36:09
  Speaker 1: moved towards each other. So the Earth is not flat.
• 36:14
  Speaker 1: Need slash, it's not slash, right. I mean, if the
• 36:16
  Speaker 1: Earth was flat and you and I started walking in
• 36:18
  Speaker 1: one direction, we would keep walking at the same distance
• 36:22
  Speaker 1: from each other all the time. But but that doesn't
• 36:24
  Speaker 1: happen on Earth, right. That's one way we know the
• 36:25
  Speaker 1: Earth is not flat exactly, although I don't know what
• 36:27
  Speaker 1: north would mean on an infinite flat Earth, though I'm
• 36:30
  Speaker 1: sure the flat Earth earths have figured that out somehow.
• 36:32
  Speaker 1: All right, So that's flat universe and you're seeing a
• 36:36
  Speaker 1: curved universe. Means that the two photons would eventually either
• 36:41
  Speaker 1: collide or move away from each other forever, depending on
• 36:44
  Speaker 1: the sign of the curvature. So if you're on a sphere,
• 36:46
  Speaker 1: for example, then those photons eventually will hit each other.
• 36:49
  Speaker 1: They'll come closer to each other. If you're on a
• 36:51
  Speaker 1: parabola or in three D it's called a hyperbolic paraboloid,
• 36:54
  Speaker 1: then they will eventually move away from each other. Right,
• 36:57
  Speaker 1: And that's what you mean by like positive curvature and
• 36:59
  Speaker 1: negative curvature. If the universe has positive culture, then two
• 37:02
  Speaker 1: photons would eventually hit each other, even if you shoot
• 37:04
  Speaker 1: in parallel to each other. And if it has negative curvature,
• 37:07
  Speaker 1: deligious diverge and never hit each other exactly. But that's curvature.
• 37:12
  Speaker 1: What does that tells about the shape of the universe,
• 37:14
  Speaker 1: Because the curvature of space determines what shapes are possible.
• 37:18
  Speaker 1: Like if the curvature is positive, like on the surface
• 37:21
  Speaker 1: of a sphere, then you are building up your universe.
• 37:24
  Speaker 1: You can't make a parabloh, you can't make an infinite
• 37:27
  Speaker 1: flat universe. Right, you can make a sphere. You could
• 37:30
  Speaker 1: make a donut. You can make a donut with more
• 37:32
  Speaker 1: holes in it. Right, you have to build a universe
• 37:34
  Speaker 1: where every piece of space has that curvature and sort
• 37:37
  Speaker 1: of like have it all fit together without any edges.
• 37:40
  Speaker 1: There are only a few shapes that are possible. The
• 37:42
  Speaker 1: curvature of the universe on the large scales determines the
• 37:45
  Speaker 1: shapes that are possible. So if you measure the curvature
• 37:48
  Speaker 1: doesn't tell you the shape, it tells you what shapes
• 37:51
  Speaker 1: are allowed. M I see. So you're saying, like, if
• 37:54
  Speaker 1: it's positively curved everywhere, then there's only one sort of
• 37:59
  Speaker 1: shape ish that the universe can be, which is a
• 38:02
  Speaker 1: sphere or closed right like something that wraps around itself,
• 38:06
  Speaker 1: although it could be other shapes than just a sphere
• 38:09
  Speaker 1: like a donut, but not here there's the mid of
• 38:11
  Speaker 1: a subtle point of geometry, the picture you have in
• 38:14
  Speaker 1: your mind. If a donut has lots of spots on
• 38:17
  Speaker 1: it with positive curvature, they sort of bend away from you.
• 38:21
  Speaker 1: But not everywhere on the donut has positive curvature. There
• 38:24
  Speaker 1: are spots there with negative curvature. And if you talk
• 38:27
  Speaker 1: to professional geometrs like mathematicians, they think of a donut
• 38:31
  Speaker 1: is actually like a flat plane connected on itself, like
• 38:35
  Speaker 1: the pac Man universe we were talking about. So there's
• 38:38
  Speaker 1: the donut you eat, which has curvature, and then there's
• 38:40
  Speaker 1: a mathematical donut which is actually flat on the surface,
• 38:43
  Speaker 1: and we could also consider more exotic shapes like a
• 38:46
  Speaker 1: donut with two holes in it. I don't even know
• 38:48
  Speaker 1: what you call that, like a double bagel. But yes,
• 38:51
  Speaker 1: it couldn't be totally flat if space is curved positively,
• 38:54
  Speaker 1: I see. But if we measured space to be flat,
• 38:56
  Speaker 1: then there's no shape. It can't beat has to be
• 38:59
  Speaker 1: in it kind of. That's right. A space is flat,
• 39:02
  Speaker 1: and it can't be a sphere. It could still be
• 39:04
  Speaker 1: that weird Pacman universe or as you say, it could
• 39:06
  Speaker 1: have a weird edge to it right beyond what we're measuring,
• 39:10
  Speaker 1: but it can't be a sphere, or it can't be
• 39:11
  Speaker 1: a paraboloid or a hyperboloid or anything like that. So
• 39:14
  Speaker 1: measuring the curvature of space does rule out some possibilities
• 39:18
  Speaker 1: for the shape of space. Okay, So depending on the
• 39:20
  Speaker 1: curvature that we measure around is in all space, they'll
• 39:23
  Speaker 1: determine whether the universe is shaped like an infinite sort
• 39:27
  Speaker 1: of space sheet, or whether it's shape maybe closed like
• 39:31
  Speaker 1: a ball, like a sphere, or whether it has a
• 39:33
  Speaker 1: more interesting shape like I don't. And so let's talk
• 39:36
  Speaker 1: about what we've measured out there whether the universe is
• 39:38
  Speaker 1: flat or curved. But first let's take another quick break. Okay.
• 39:54
  Speaker 1: I know we are talking about the size and shape
• 39:55
  Speaker 1: of the universe, and now we're doing something even router,
• 39:58
  Speaker 1: which is actually measuring the size in shape of the universe. Like,
• 40:01
  Speaker 1: wouldn't that be rude? Or something came up to and
• 40:02
  Speaker 1: be like, hey, I wonder how big you are? Let
• 40:04
  Speaker 1: me measure? Well? You know, I think if you're a tailor,
• 40:06
  Speaker 1: for example, you ask somebody to waste size and then
• 40:09
  Speaker 1: you measure it because you want to deliver a suit
• 40:11
  Speaker 1: that fits, not an aspirational suit that they hoped to
• 40:14
  Speaker 1: one day fit into. Oh I see our physicists, the
• 40:17
  Speaker 1: tailors of the universe. What kind of suit are we
• 40:20
  Speaker 1: making for the universe? I don't know, But if we
• 40:22
  Speaker 1: asked the universe it's size and it gives us an answer,
• 40:24
  Speaker 1: I'd still want to go out and measure just to
• 40:26
  Speaker 1: make sure it wasn't being modest. Oh man, you think
• 40:29
  Speaker 1: the universe is lying to you? Maybe you always got
• 40:32
  Speaker 1: a double check and then double check your double checks.
• 40:34
  Speaker 1: I guess the universe could have been eating more dark
• 40:36
  Speaker 1: chocolate than it thought, and you know its measurements are outdated.
• 40:40
  Speaker 1: That's right. Maybe in between the time it's sent in
• 40:42
  Speaker 1: its measurements, it had a bunch more snacks, and so hey,
• 40:44
  Speaker 1: we just want to make sure to make a suit
• 40:46
  Speaker 1: that fits. We were talking about how knowing the curvature
• 40:49
  Speaker 1: space tells you at least not maybe the total or
• 40:52
  Speaker 1: exact shape of the universe tell you kind of what
• 40:54
  Speaker 1: kinds of shape the universe can be, whether it's sort
• 40:57
  Speaker 1: of closed like a sphere, or open like a infinite sheet,
• 41:01
  Speaker 1: or maybe has some weird shaped like a doughnut or
• 41:03
  Speaker 1: a hyperbola. And so I guess the big question now
• 41:06
  Speaker 1: is how do we measure the curvature of space? Like,
• 41:08
  Speaker 1: how do we measure if its curving one way or another? Yeah,
• 41:11
  Speaker 1: the best way to do it would be to have
• 41:13
  Speaker 1: a huge triangle, right, to like draw out a huge
• 41:16
  Speaker 1: triangle in space and to measure its angles. You know,
• 41:20
  Speaker 1: shoot photons in straight lines between three points and then
• 41:24
  Speaker 1: measure the opening angles between those photons, and that would
• 41:27
  Speaker 1: tell you just like if you did that on the
• 41:28
  Speaker 1: surface of the Earth, you could literally measure the curvature
• 41:31
  Speaker 1: of the Earth because you would measure that a triangle
• 41:34
  Speaker 1: actually has more than eight degrees in it if you
• 41:36
  Speaker 1: laid it out on the surface of the Earth. So
• 41:39
  Speaker 1: that would be the best way to do it. You know,
• 41:40
  Speaker 1: huge lasers really really far apart deep in space. Oh
• 41:44
  Speaker 1: I see. I mean, like you send out three satellites,
• 41:47
  Speaker 1: put arrangement a triangle, and you shoot lasers at each other. Yeah,
• 41:52
  Speaker 1: you make sure they're in a triangle by shooting lasers
• 41:54
  Speaker 1: at each other. This is totally fantastical, right, you never
• 41:56
  Speaker 1: actually do it. Imagine one satellite in this galaxy and
• 42:00
  Speaker 1: and Andromeda one and another galaxy, and they shoot lasers
• 42:03
  Speaker 1: at each other so you know they're lined up, and
• 42:05
  Speaker 1: then you just measure the angles between those lasers. So
• 42:07
  Speaker 1: like if I'm shooting an Andromeda from here and I'm
• 42:10
  Speaker 1: shooting at Alphas, another gassy from here, I would literally
• 42:13
  Speaker 1: just us take like a protractor and measure the angle
• 42:16
  Speaker 1: between the lasers. That would tell me whether the universe
• 42:18
  Speaker 1: is curved. Yeah, in the same way you could on
• 42:21
  Speaker 1: the surface of the Earth. Right, You've got two friends,
• 42:23
  Speaker 1: you guys stand in the triangle and you spool out
• 42:26
  Speaker 1: string between yourselves. You could just measure the angle between
• 42:29
  Speaker 1: those strings, and that would tell you the curvature of
• 42:31
  Speaker 1: the Earth same way you could do it with photons
• 42:34
  Speaker 1: deep in space. What you want is a really really
• 42:36
  Speaker 1: big triangle because the curvature, if it exists, it's going
• 42:39
  Speaker 1: to be pretty small. Just like on the surface of
• 42:41
  Speaker 1: the Earth. You'd want a really big triangle because it's
• 42:43
  Speaker 1: pretty hard to measure the curvature of the Earth in
• 42:46
  Speaker 1: just your bedroom. I see, Well, technically you could do
• 42:48
  Speaker 1: it in your bedroom. You just need like a super
• 42:50
  Speaker 1: precise protractor, right, Yeah, but then you're also measuring the
• 42:54
  Speaker 1: curvature of your bedroom instead of the Earth. But if
• 42:57
  Speaker 1: your bedroom follows the perfect curvature of the Earth Earth,
• 43:00
  Speaker 1: then yes, and so that's what you'd like to do ideally.
• 43:02
  Speaker 1: Obviously we can't do that. So what we have to
• 43:05
  Speaker 1: do is sort of look for existing triangles in space,
• 43:09
  Speaker 1: Like you know, is there a place where two photons
• 43:12
  Speaker 1: were sent to us from really distant locations and we
• 43:15
  Speaker 1: can sort of reconstruct like an existing cosmic triangle. Mmmmm.
• 43:20
  Speaker 1: It's so meaning like looking at galaxies, are looking at
• 43:22
  Speaker 1: sort of like the background of the universe. Yeah, so
• 43:24
  Speaker 1: the oldest light in the universe is a really helpful
• 43:27
  Speaker 1: resource for this. So we're talking about light from the
• 43:30
  Speaker 1: cosmic microwave background back when the universe was really hot
• 43:34
  Speaker 1: and really dense and only a few hundred thousand years old,
• 43:37
  Speaker 1: there was a moment when those particles came together and
• 43:40
  Speaker 1: became transparent, and we still see light from that plasma glowing.
• 43:44
  Speaker 1: That light is really really helpful because it's so old,
• 43:46
  Speaker 1: so it's traversed a lot of the universe and we
• 43:49
  Speaker 1: can kind of put it together in a sort of
• 43:51
  Speaker 1: cosmic triangle to get a sense for whether the universe
• 43:54
  Speaker 1: is curved in one way or another or whether it's
• 43:57
  Speaker 1: flat right. And this light is kind of interesting because
• 44:00
  Speaker 1: it's like the earliest light in the universe, so it's
• 44:02
  Speaker 1: very kind of like primal right like it it's sort
• 44:05
  Speaker 1: of like the o G light, like the original light
• 44:08
  Speaker 1: of the universe, like when it was born, they didn't
• 44:10
  Speaker 1: have a curvature to it. And you can tell from
• 44:12
  Speaker 1: the you said, the wiggles of the this light. What
• 44:15
  Speaker 1: we do is we look in this light for lumps.
• 44:17
  Speaker 1: We don't really just measure the angles between things. What
• 44:19
  Speaker 1: we look for is the size of lumps that are
• 44:22
  Speaker 1: evidenced in the cosmic microwave background light. So if you
• 44:24
  Speaker 1: look in one direction. If you look in another direction,
• 44:27
  Speaker 1: you see this light, But it turns out this light
• 44:28
  Speaker 1: has slightly different temperatures if you look in different directions.
• 44:32
  Speaker 1: It's really really minute, like one part in tens of thousands.
• 44:35
  Speaker 1: Mostly it's just the same, But there are these little wiggles,
• 44:37
  Speaker 1: and if you look for the size of those wiggles,
• 44:40
  Speaker 1: like how big is a hot spot or how big
• 44:42
  Speaker 1: is a cold spot, you can use the apparent size
• 44:44
  Speaker 1: of those wiggles to tell you whether the universe is
• 44:48
  Speaker 1: curved and how it's curved. Yeah, this is pretty cool
• 44:50
  Speaker 1: because I was thinking about this. It's almost like the
• 44:52
  Speaker 1: universe is acting like a lens, right, Like if the
• 44:55
  Speaker 1: universe is curved in one way positively, say, for example,
• 45:00
  Speaker 1: and maybe the universe is sort of like a sphere,
• 45:02
  Speaker 1: then the universe could acts like a magnifying glass. Right,
• 45:04
  Speaker 1: So you should see these um spots in the background
• 45:07
  Speaker 1: radiation kind of bigger than they actually are exactly, And
• 45:10
  Speaker 1: we have an idea for how big we expect those
• 45:13
  Speaker 1: spots to be, right, That's key. We have some idea
• 45:16
  Speaker 1: for like how matter and dark matter and the photons
• 45:19
  Speaker 1: were all slashing back and forth together against each other,
• 45:22
  Speaker 1: how big should those lumps be, we can predict how
• 45:25
  Speaker 1: big they should be, and then we can go and
• 45:27
  Speaker 1: measure how big they are. And as you say, if
• 45:30
  Speaker 1: the universe is curved, then photons, for example, from really
• 45:33
  Speaker 1: far apart, will curve towards each other, and so those
• 45:36
  Speaker 1: lumps will appear to be slightly bigger than they actually
• 45:39
  Speaker 1: are because you look in one direction, you look in
• 45:42
  Speaker 1: another direction, you'll be seeing like two sides of the
• 45:45
  Speaker 1: same lump. But really those edges were closer than what
• 45:48
  Speaker 1: you're seeing spaces flat than the photons are just flying straight.
• 45:51
  Speaker 1: And so that's sort of like a cosmic triangle because
• 45:54
  Speaker 1: you have these like two different sources of light all
• 45:56
  Speaker 1: coming at you, and you can look at the angle.
• 45:59
  Speaker 1: Basically we know those photons and tell whether they've been
• 46:01
  Speaker 1: bent together or bent apart or flown true right, And
• 46:05
  Speaker 1: like if the universe has negative curvature, then it sort
• 46:09
  Speaker 1: of acts like what's the opposite of a magnifying glass
• 46:12
  Speaker 1: glass if you're near sighted or something magnifying glass, you know,
• 46:16
  Speaker 1: like a piece of glass, it has the opposite kind
• 46:18
  Speaker 1: of curve, like the curse inwards in the middle, then
• 46:21
  Speaker 1: things sord have looked smaller through it. Right, so the
• 46:24
  Speaker 1: spots to look smaller, and we can do these simulations.
• 46:27
  Speaker 1: We say, here's where the hot spots and cold spots
• 46:29
  Speaker 1: should look like if the universe was flat, And then
• 46:31
  Speaker 1: we can dial up the curvature of the universe. And
• 46:33
  Speaker 1: in our simulations we can say, oh, look the spots
• 46:35
  Speaker 1: get bigger. And then we can dial down the curvature
• 46:38
  Speaker 1: to negative and say, oh, look the spots get smaller.
• 46:40
  Speaker 1: And then we go out and we look in the
• 46:41
  Speaker 1: actual universe and we say which of these best describes
• 46:45
  Speaker 1: what we see? You know, is the universe look like
• 46:48
  Speaker 1: it's flat or does it look like it's curved one
• 46:51
  Speaker 1: way or the other. So this is a very powerful
• 46:53
  Speaker 1: way to measure the curved to the universe over a
• 46:56
  Speaker 1: very very large scale, because even though those photons started
• 47:00
  Speaker 1: out not that far apart from each other, they traversed
• 47:03
  Speaker 1: a huge amount of the universe because of that expansion. Right, yeah,
• 47:07
  Speaker 1: so is the universe amplifying or shrinking the cosmic microwave background?
• 47:12
  Speaker 1: And so what we found when we measure this is
• 47:14
  Speaker 1: that the universe seems to be very very close to flat,
• 47:18
  Speaker 1: within zero point four percent. It's flat, Like this is
• 47:21
  Speaker 1: a number we measure, so it's not an exact thing.
• 47:23
  Speaker 1: You can never get like zero point zero zero zero
• 47:26
  Speaker 1: zero zero with infinite zeros. We measure is zero point
• 47:29
  Speaker 1: zero zero four curvature, meaning that as far as we think,
• 47:33
  Speaker 1: the universe is not curved, it's flat. That's right. It's
• 47:36
  Speaker 1: consistent with zero that we measure something a little bit
• 47:39
  Speaker 1: above zero. It's like if you've flipped a coin a
• 47:41
  Speaker 1: thousand times and it's a fair coin, you'd expect on
• 47:44
  Speaker 1: average to get of those to be heads, but you know,
• 47:47
  Speaker 1: sometimes you get a fluctuation up or down. So that's
• 47:50
  Speaker 1: what we see here. Space is either perfectly flat or
• 47:53
  Speaker 1: it's slightly positively curved, but it's consistent with flat. But
• 47:57
  Speaker 1: I guess you know, one question I would have as
• 47:58
  Speaker 1: a skeptic is that you just told me that this
• 48:01
  Speaker 1: is based on our measurement of the universe compared to
• 48:03
  Speaker 1: what we expect, But isn't what we expect also sort
• 48:06
  Speaker 1: of warped by our view of the universe, Like what
• 48:08
  Speaker 1: if what we expect is somehow distorted to absolutely this
• 48:12
  Speaker 1: could be wrong. Right, It's one measurement, and there are
• 48:15
  Speaker 1: always assumptions built into any measurement. We have other ways
• 48:18
  Speaker 1: to measure the curvature of the universe, and those other
• 48:20
  Speaker 1: ways agree with this measurement. Another way to measure the
• 48:23
  Speaker 1: curvature of the universe is to go back to something
• 48:25
  Speaker 1: we were talking about earlier, like what stuff is there
• 48:28
  Speaker 1: in the universe? You sort of like way the whole universe,
• 48:31
  Speaker 1: Like add up all the mass of all the stuff
• 48:33
  Speaker 1: that's in the universe, the matter, the dark matter, the
• 48:36
  Speaker 1: dark energy, and that will tell you like, is there
• 48:38
  Speaker 1: enough stuff in the universe to be consistent with flat
• 48:41
  Speaker 1: or is there too much stuff in the universe that
• 48:42
  Speaker 1: the universe has to be closed, has to have positive curvature.
• 48:46
  Speaker 1: And so that's another completely separate, totally independent way to
• 48:49
  Speaker 1: measure the curvature of the universe, and that also comes
• 48:52
  Speaker 1: up consistent with flat. Well, wait, you teld me earlier
• 48:55
  Speaker 1: that the curvature of the universe doesn't depend on the
• 48:57
  Speaker 1: stuff in it. Well, the overall shape, the way it's connected,
• 49:00
  Speaker 1: whether it's infinite or looped or finite, that cannot be
• 49:04
  Speaker 1: changed by the stuff in the universe. But the stuff
• 49:07
  Speaker 1: in the universe can change the amount of curvature. Collapsing
• 49:11
  Speaker 1: a large sphere into a smaller one, for example, if
• 49:14
  Speaker 1: it has more than the critical density. And knowing the
• 49:17
  Speaker 1: amount of stuff in the universe helps us measure the curvature,
• 49:21
  Speaker 1: which determines what shapes it can be. Mm hmmm, I see.
• 49:25
  Speaker 1: So okay, so we we've measured out the cosmic microwave
• 49:29
  Speaker 1: background and we it fits our predictions for a flat universe,
• 49:32
  Speaker 1: which means that probably the universe is flat. What does
• 49:35
  Speaker 1: that mean for the shape of the universe. Probably the
• 49:37
  Speaker 1: universe is flat, which means probably is infinite and boring
• 49:42
  Speaker 1: in that sense and goes on forever and that sort
• 49:45
  Speaker 1: of naive sense. But you know, people look at these
• 49:47
  Speaker 1: measurements and they see some weird stuff in there, Like,
• 49:50
  Speaker 1: first of all, it's flat, but you know, there's a
• 49:53
  Speaker 1: little bit of positive curvature there, so maybe it's something different.
• 49:56
  Speaker 1: And when they look in more detail at these wiggles,
• 49:58
  Speaker 1: they see something a little bit unexpected, something a little surprising,
• 50:02
  Speaker 1: which makes them think about donuts, Are you sure less
• 50:06
  Speaker 1: much just because it's lunchtime maybe, or you need to
• 50:09
  Speaker 1: sugar fix But wait, it's like I said, bag, you said,
• 50:12
  Speaker 1: so we measured the universe speed probably flat, mostly flat,
• 50:15
  Speaker 1: and what does that mean for the shape of the universe, Like,
• 50:17
  Speaker 1: if it is flat, what does that mean? It means
• 50:18
  Speaker 1: it doesn't have a shape, right, It means that it
• 50:20
  Speaker 1: can only be infinite and go on forever, meaning doesn't
• 50:23
  Speaker 1: have a shape. Well, I mean that is a shape, right,
• 50:26
  Speaker 1: you know, mathematically speaking, that the shape If the universe
• 50:28
  Speaker 1: is flat and has no boundaries and goes on forever,
• 50:32
  Speaker 1: that's a shape. Yeah, I see. And if it was closed,
• 50:35
  Speaker 1: if it had positive curvature, it would be a sphere.
• 50:37
  Speaker 1: But we are not measuring that. Yeah, although remember, positive
• 50:40
  Speaker 1: curvature can also be consistent with a donut. A donut
• 50:43
  Speaker 1: is positively curved on its surface. That's for a physical donut.
• 50:46
  Speaker 1: A mathematically donut is technically flat. So remember that little subtlety. Okay,
• 50:51
  Speaker 1: so we're measuring the university a little positive curvature. Is
• 50:54
  Speaker 1: that what you're saying? It has a little bit of
• 50:55
  Speaker 1: a positive curvature, a little bit of a positive curvature.
• 50:58
  Speaker 1: And also some details the cosmic microwave background radiation are
• 51:02
  Speaker 1: really interesting and tantalizing, and I've led some people to
• 51:06
  Speaker 1: suspect that maybe a doughnut is the best description of
• 51:09
  Speaker 1: our universe. As the universe expands, we get these lumps
• 51:12
  Speaker 1: in the cosmic microwave background. Right, These lumps come from
• 51:15
  Speaker 1: quantum fluctuations in the very early universe during inflation. But
• 51:19
  Speaker 1: as the universe is inflating, you're getting fluctuations all the time,
• 51:22
  Speaker 1: So you should see lumps at all different sizes. It's
• 51:25
  Speaker 1: like the biggest lump. And that's when we're using to
• 51:27
  Speaker 1: measure the curvature. You should see big lumps, you should
• 51:30
  Speaker 1: see small lumps, you should see very very small lumps.
• 51:32
  Speaker 1: And so when we look at these fluctuations the amounts
• 51:35
  Speaker 1: of lumpiness in the universe, we should see big lumps
• 51:38
  Speaker 1: and small lumps, and smaller lumps and even smaller lumps.
• 51:40
  Speaker 1: And so when people go out and measure this stuff,
• 51:42
  Speaker 1: they see that mostly, but they see that sort of
• 51:44
  Speaker 1: like the biggest scales, some of those lumps seem to
• 51:47
  Speaker 1: be missing, Like you don't see necessarily correlations between things
• 51:51
  Speaker 1: that are as far apart as you expect. Wait, let's
• 51:54
  Speaker 1: maybe take a step back. We measure the unms be
• 51:56
  Speaker 1: a little bit positive curvature, which means that it can
• 51:59
  Speaker 1: still be a spear. Couldn't it connects to still be
• 52:01
  Speaker 1: a sphere? Could be a sphere? I see, but there's
• 52:03
  Speaker 1: something about the doughnut that makes it makes you think
• 52:05
  Speaker 1: that it could be a donut. What's special about a
• 52:07
  Speaker 1: donut that would fit what you're saying, Well, what we're
• 52:10
  Speaker 1: seeing is that at very very large angles, there aren't
• 52:14
  Speaker 1: as many correlations as we expect. You know, we're talking
• 52:17
  Speaker 1: about a very small effect here, and above sixty degrees
• 52:21
  Speaker 1: in the sky we should see fluctuations about a hundred microclevins,
• 52:25
  Speaker 1: but what we see is fluctuations like twenty microclevin's. So
• 52:28
  Speaker 1: it's a small effect. But a donut topology turns out
• 52:32
  Speaker 1: to suppress these large scale fluctuations because it makes the
• 52:35
  Speaker 1: universe finite. And donuts have a sort of a smaller
• 52:39
  Speaker 1: radius than a sphere for the same curvature. A donut
• 52:42
  Speaker 1: has sort of like shorter length scales, right, then a
• 52:44
  Speaker 1: sphere does. Well. I guess maybe we need to talk
• 52:47
  Speaker 1: about the differences between donuts and meatball standing because a
• 52:51
  Speaker 1: donut is interesting, right because it's it's a close shape, um,
• 52:55
  Speaker 1: but it has a hole in it, and it's kind
• 52:57
  Speaker 1: of interesting because like if you go in one direction,
• 52:59
  Speaker 1: like a around the wide rim of the doughnut, you
• 53:02
  Speaker 1: go in a circle. And also if you go towards
• 53:04
  Speaker 1: the center of the doughnut, you also go in a circle,
• 53:06
  Speaker 1: so it sort of loops around in all directions, sort
• 53:08
  Speaker 1: of like a sphere, but it's not a sphere. It's
• 53:10
  Speaker 1: more like a like a closed cylinder. Yeah, and it's
• 53:13
  Speaker 1: got two different length scales, as you say, It's got
• 53:15
  Speaker 1: like the one way around and the other way around,
• 53:17
  Speaker 1: whereas a sphere only has one length scale, it's just
• 53:20
  Speaker 1: the radius. And you know, even if the universe is
• 53:24
  Speaker 1: curved and it's a sphere or a donut, we're talking
• 53:25
  Speaker 1: about something very slightly curved, so really really huge, right,
• 53:30
  Speaker 1: really un enormous scales, just just so nobody is confused.
• 53:33
  Speaker 1: We're not like the little prints here standing on a
• 53:35
  Speaker 1: tiny donuts, the ginormous donut. Yeah. But they did a
• 53:37
  Speaker 1: bunch of simulations and they discovered that a donut is
• 53:40
  Speaker 1: better at causing the sort of lack of ripples that
• 53:42
  Speaker 1: they see in CMB. You know, in the cosmic microwave
• 53:45
  Speaker 1: background radiation, we don't see some of the bigger ripples
• 53:48
  Speaker 1: that we expect, and a donut is better suppressing those
• 53:51
  Speaker 1: because it has these two length scales, the long way
• 53:53
  Speaker 1: around and the short way around and so you just
• 53:55
  Speaker 1: sort of like don't get as big lumps on a
• 53:57
  Speaker 1: donut as you do on a sphere, and so that
• 54:00
  Speaker 1: makes people wonder maybe the universe is shaped like a donut, right,
• 54:03
  Speaker 1: So it's the difference between these lengths the scales that
• 54:06
  Speaker 1: it is giving you the clues like you're seeing some
• 54:09
  Speaker 1: weird kind of like oblong shaped and the cosmic microwave background. Yes,
• 54:12
  Speaker 1: the fact that you don't see as big lumps as
• 54:15
  Speaker 1: you expect from a flat universe or even from a
• 54:17
  Speaker 1: spherical universe. It's not impossible to suppress these long distance
• 54:21
  Speaker 1: effects on a sphere. It's just that you would need
• 54:24
  Speaker 1: a radius of curvature that's not consistent with what we
• 54:27
  Speaker 1: have measured. A doughnut gives you another length scale to
• 54:31
  Speaker 1: play with, so it's easier to suppress the long distance correlations. Now,
• 54:35
  Speaker 1: this is not a smoking gun. This is like a
• 54:37
  Speaker 1: weird little hint that could just be random noise, right.
• 54:40
  Speaker 1: It could be theorists getting too excited and eating too
• 54:43
  Speaker 1: much chocolate and thinking about donuts. But it's sort of
• 54:46
  Speaker 1: like a fun hint, and we'll get more data and
• 54:48
  Speaker 1: we'll see if this holds up. But it's fascinating to
• 54:51
  Speaker 1: me that this question is the universe flat or is
• 54:54
  Speaker 1: it it's deer or is it even a donut is
• 54:56
  Speaker 1: still unanswered. It's still something we don't know the actual
• 54:59
  Speaker 1: answer to. Oh, but it seems like we have a
• 55:02
  Speaker 1: sort of a big clue, which is that the university
• 55:05
  Speaker 1: is flat around us. Could it be that we're just
• 55:07
  Speaker 1: measuring local curvature, like are we just a dimple or
• 55:11
  Speaker 1: like a flat spot in a you know, teacup shape universe? Absolutely,
• 55:16
  Speaker 1: you know, what we're doing is we're assuming that the
• 55:18
  Speaker 1: curvature we measure here is the same as the curvature
• 55:20
  Speaker 1: we're measuring somewhere else. And we're hoping that that's true
• 55:23
  Speaker 1: because we're measuring in lots of different ways, and we're
• 55:25
  Speaker 1: trying to measure in different directions. But in the end,
• 55:28
  Speaker 1: our vantage point is limited, right, And so it could
• 55:32
  Speaker 1: be that we're a flat spot on a very large doughnut,
• 55:35
  Speaker 1: or that we could be a slightly curved spot on
• 55:37
  Speaker 1: otherwise flat universe. Um, you know, we can't really confidently
• 55:41
  Speaker 1: extrapolate path what we can see. The Remember that the
• 55:45
  Speaker 1: CMB measurements do cover a lot of space. It's not
• 55:48
  Speaker 1: that local. Interesting or so you're saying kind of stay tuned,
• 55:52
  Speaker 1: like we actually don't know what the shape of the
• 55:55
  Speaker 1: universe is. That's right. If you're betting, I think flat
• 55:57
  Speaker 1: is the best bet. It's the simplest, it's harmonious. It's
• 56:01
  Speaker 1: what most physicists believe. If you ask them if the
• 56:04
  Speaker 1: is the universe slat, they would say yes. But you know,
• 56:06
  Speaker 1: the data say it's either flat or slightly positively curved.
• 56:10
  Speaker 1: And there are these interesting wiggles that are more consistent
• 56:13
  Speaker 1: with a donut shape than a sphere or a flat universe.
• 56:16
  Speaker 1: So yeah, stay tuned. We still got big realizations ahead
• 56:19
  Speaker 1: of us. So if you're the gamping type, maybe invest
• 56:22
  Speaker 1: in donuts, that's what you're saying. And if you win,
• 56:26
  Speaker 1: eat a donut, and if you lose, eat a doughnut.
• 56:28
  Speaker 1: Either way you win said. If you don't win, you don't.
• 56:32
  Speaker 1: You win, you don't, but you don't don't donut. If
• 56:34
  Speaker 1: you win. If the is still dark chocolate, how would
• 56:39
  Speaker 1: that change the curvature, Daniel M. I think would make
• 56:41
  Speaker 1: it more closed exactly. Dark chocolate is pretty dense stuff
• 56:45
  Speaker 1: and so we tend to compact on itself. And how
• 56:47
  Speaker 1: would mean the universe is not infinitely filled with chocolate,
• 56:50
  Speaker 1: and eventually my snacking days must end, right, But if
• 56:53
  Speaker 1: it's closed, then there's a finite amount of dark chocolate,
• 56:56
  Speaker 1: So eventually you might eat physicists might eat at all.
• 56:59
  Speaker 1: That's right, I better go ahead and get started on
• 57:01
  Speaker 1: my snack. You might might want to buy some new
• 57:03
  Speaker 1: belts or pants, assuming you wear pants. I mean, we
• 57:06
  Speaker 1: talked about social conventions and physicists. Who knows, right, he
• 57:09
  Speaker 1: talked about not making assumptions, right, you know, we're exploring
• 57:12
  Speaker 1: the university in mind. What shape is Daniel or Daniel's
• 57:15
  Speaker 1: pants he doesn't have pants. Maybe it's a good question.
• 57:19
  Speaker 1: There actually is a pants diagram for black hole mergers.
• 57:22
  Speaker 1: We're going to talk about a few weeks, all right.
• 57:24
  Speaker 1: Is that rude to ask what black Hole's pants are?
• 57:29
  Speaker 1: We'll find out? Are they bill bottoms or tapered low
• 57:32
  Speaker 1: rising and they wear shorts over their socks and sand
• 57:36
  Speaker 1: neither mom jeans or that cargo pants. All right. Well,
• 57:42
  Speaker 1: it's interesting to think that, you know, from our little
• 57:44
  Speaker 1: spot in the universe, we can, you know, have these
• 57:46
  Speaker 1: conversations about what the shape of the universe is when
• 57:49
  Speaker 1: it's so far away, you know, nineties, sixty billion light
• 57:53
  Speaker 1: years away. We're still we can still have conversations about it,
• 57:55
  Speaker 1: and we we might be right, which is the crazy thing. Yeah,
• 57:58
  Speaker 1: and thank you very much duper users, Leo, Stein and
• 58:01
  Speaker 1: Evans gonna be echo for consulting with us on this
• 58:03
  Speaker 1: tricky topic. Any remaining inaccuracies and ambiguities are our responsibility,
• 58:08
  Speaker 1: not there. And no matter how big and how crazy
• 58:11
  Speaker 1: the question is, we can eventually always think about some
• 58:15
  Speaker 1: way to try to answer it, some clever wrinkle in
• 58:18
  Speaker 1: the nature of the universe that forces it to answer
• 58:21
  Speaker 1: our questions. But even the biggest, hardest, craziest questions were
• 58:26
  Speaker 1: the craziest answers. That's right, even the tasty and crazy
• 58:29
  Speaker 1: answers about the universe. Well, we hope you enjoyed that.
• 58:32
  Speaker 1: Thanks for joining us, See you next time. Thanks for listening,
• 58:43
  Speaker 1: and remember that Daniel and Jorge explained. The universe is
• 58:45
  Speaker 1: a production of I Heart Radio. For more podcast from
• 58:49
  Speaker 1: my Heart Radio, visit the I Heart Radio app, Apple Podcasts,
• 58:53
  Speaker 1: or wherever you listen to your favorite shows. No