Latest / Daniel and Kelly’s Extraordinary Universe / What's the best way to measure the expansion of the Universe?

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What's the best way to measure the expansion of the Universe? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, hoory, how tall are you these days?
• 0:11
  Speaker 2: What do you mean these days?
• 0:13
  Speaker 1: Well, I don't know. It's been a while since I've
• 0:15
  Speaker 1: seen you in person and measured you up. Maybe you've
• 0:18
  Speaker 1: had that popular leg lengthening surgery.
• 0:21
  Speaker 2: Well I haven't, And even if I did see you,
• 0:23
  Speaker 2: how do you know you've been enshrin? Also because it's
• 0:27
  Speaker 2: all relative, But no, I seriously haven't measured my height
• 0:30
  Speaker 2: in like years.
• 0:31
  Speaker 1: Well, it might be that you're headed in the other direction.
• 0:34
  Speaker 1: People tend to shrink as they get older, so maybe
• 0:37
  Speaker 1: your greatest heights are behind you.
• 0:39
  Speaker 2: It could be are you saying I peaked already?
• 0:41
  Speaker 1: It's all downhills from here or down joorge?
• 0:43
  Speaker 2: Well I heard that your tallest in the morning, so
• 0:47
  Speaker 2: you know that's why I try to sleep all day.
• 0:51
  Speaker 1: Is that still true if you don't get out of
• 0:52
  Speaker 1: bed until the afternoon?
• 0:53
  Speaker 2: Yeah, just a tall tale. Hi am Hohem a cartoonist
• 1:12
  Speaker 2: and the creator of PhD comics.
• 1:14
  Speaker 1: I'm Daniel. I'm a particle of physicist and a professor
• 1:17
  Speaker 1: UC Irvine, and I'm very happy to be half an
• 1:20
  Speaker 1: inch taller than my older brother.
• 1:22
  Speaker 2: But how happy is your older brother?
• 1:25
  Speaker 1: Less happy? But who cares?
• 1:28
  Speaker 2: What about your younger brother.
• 1:29
  Speaker 1: He's at half an inch taller than me. Oh boy, yeah,
• 1:32
  Speaker 1: I remember when we were kids and we realized that
• 1:35
  Speaker 1: my older brother might not always be the taller brother.
• 1:38
  Speaker 1: He had a moment of terror.
• 1:40
  Speaker 2: Yeah, yeah, that's tough. I am taller than my older
• 1:42
  Speaker 2: brother as well.
• 1:45
  Speaker 1: And so now you get to look down on him.
• 1:48
  Speaker 2: Nice to look up.
• 1:48
  Speaker 1: To him only figuratively, though.
• 1:51
  Speaker 2: But I I just did it from a higher panthage point.
• 1:54
  Speaker 2: But anyways, welcome to our podcast Daniel and Jorge Explain
• 1:56
  Speaker 2: the Universe, a production of iHeartRadio.
• 1:59
  Speaker 1: In which we try to climb the heights of understanding
• 2:02
  Speaker 1: in the universe. We look at this great cosmic mystery
• 2:05
  Speaker 1: as a journey to some kind of understanding. We want
• 2:09
  Speaker 1: to slowly make our way up the path towards understanding
• 2:12
  Speaker 1: the universe a little bit better, wondering if there is
• 2:15
  Speaker 1: some sort of final illumination at the top, or if
• 2:19
  Speaker 1: this mountain even has a top.
• 2:21
  Speaker 2: It's right, We stand tall and try to take a
• 2:23
  Speaker 2: view of the universe from this little vantage point we
• 2:25
  Speaker 2: have in our little planet floating around in space, wondering
• 2:29
  Speaker 2: how big is this giant universe that we live.
• 2:31
  Speaker 1: In, and what's it doing? After all, the more we
• 2:34
  Speaker 1: look out in the universe, the more we are surprised
• 2:37
  Speaker 1: by what's going on, not just in our cosmic neighborhood,
• 2:40
  Speaker 1: but in the farthest reaches, the deepest parts of space
• 2:44
  Speaker 1: that we are just barely able to see. Every time
• 2:48
  Speaker 1: scientists look out with some new kind of technological marvel,
• 2:51
  Speaker 1: they come back with news that shocks us about what's
• 2:54
  Speaker 1: going on out there in the furthest reaches of space.
• 2:57
  Speaker 1: It seems like the universe is destined to.
• 2:59
  Speaker 2: Keep the Yeah, it is a very surprising universe, still
• 3:03
  Speaker 2: full of giant mysteries. There seems to be a lot
• 3:05
  Speaker 2: that we don't know about the universe at a very
• 3:07
  Speaker 2: basic level still, you know, just basic facts about the
• 3:10
  Speaker 2: universe we still don't know.
• 3:11
  Speaker 1: Yeah, on one hand, we feel fairly accomplished because of
• 3:14
  Speaker 1: our incredible technology and everything we have learned and all
• 3:17
  Speaker 1: of the science that we have mastered. On the other hand,
• 3:20
  Speaker 1: there are still very basic things about the universe that
• 3:23
  Speaker 1: we don't know. How big is it, where did it
• 3:25
  Speaker 1: come from? What's it doing right now? What is its
• 3:28
  Speaker 1: future hold? It feels like scientists or even children in
• 3:32
  Speaker 1: a few hundred years will look back in this time
• 3:34
  Speaker 1: and think, boy, those folks really didn't know what was
• 3:38
  Speaker 1: going on.
• 3:38
  Speaker 2: Yeah, Like we don't know basic things like how tall
• 3:40
  Speaker 2: is the universe, you know, and like how tall is
• 3:44
  Speaker 2: it in comparison to its brother or sister?
• 3:46
  Speaker 1: Is there another universe and the multiverse that our universe
• 3:48
  Speaker 1: has like a sibling rivalry with I'm bigger than you are?
• 3:52
  Speaker 2: Maybe yeah, But then the question would be is our
• 3:55
  Speaker 2: universe the older sibling or the younger sibling? Is it
• 3:58
  Speaker 2: trying to get attention on? Is it the mediator sibling?
• 4:02
  Speaker 1: And if you're a universe, where do you go for therapy? Anyway?
• 4:06
  Speaker 2: Boy, it'd be interesting to be like a universe psychologist.
• 4:10
  Speaker 1: There would be big problems to solve, for sure. But
• 4:15
  Speaker 1: I think that every universe should be judged on its
• 4:17
  Speaker 1: own merits and not relative to some other kind of
• 4:20
  Speaker 1: universe out there that could be bigger or smaller, or
• 4:23
  Speaker 1: faster or taller or able to get higher scores on
• 4:27
  Speaker 1: math tests.
• 4:28
  Speaker 2: Yeah, I'm sure our universes are grand Universe's favorite.
• 4:32
  Speaker 1: I like to think we're all the favorite in some way.
• 4:34
  Speaker 2: Yeah, although it's always good to have a favorite uncle
• 4:37
  Speaker 2: or aunt. Maybe there's like a cool universe out there.
• 4:40
  Speaker 2: That's our uncle or aunt.
• 4:41
  Speaker 1: Oh, I know, like a universe that doesn't care so
• 4:44
  Speaker 1: much about the rules, maybe like flaunts causality, doesn't care
• 4:47
  Speaker 1: about locality. It's just like the Rebel universe.
• 4:50
  Speaker 2: Yeah, it just showers you with gifts and stuff, takes
• 4:53
  Speaker 2: you out for ice cream.
• 4:54
  Speaker 1: Well, we are still trying to figure out which universe
• 4:57
  Speaker 1: we are in. We don't really know what the rules
• 5:00
  Speaker 1: of the universe are. All we can do is turn
• 5:02
  Speaker 1: our heads skywards and gather the information that it sends us.
• 5:06
  Speaker 1: By looking at photons and other particles that arrive on Earth,
• 5:09
  Speaker 1: we do get little bits of information, little clues that
• 5:12
  Speaker 1: tell us what's happening out there in the universe, and
• 5:15
  Speaker 1: they tell a really fascinating story, but one that we
• 5:18
  Speaker 1: are still unraveling.
• 5:20
  Speaker 2: Yeah, it is a pretty amazing universe full of interesting
• 5:23
  Speaker 2: stuff out there, like antimatter. Do you think our favorite
• 5:25
  Speaker 2: universes are anti matter universe?
• 5:29
  Speaker 1: I think that's definitely like our other twin universe that's
• 5:32
  Speaker 1: our rival.
• 5:33
  Speaker 2: But as you said, it is a giant universe and
• 5:35
  Speaker 2: it seems to be getting bigger, right, that's the idea.
• 5:38
  Speaker 2: It's a huge universe and not only is our view
• 5:40
  Speaker 2: of it expanding every day, but the universe itself space
• 5:44
  Speaker 2: itself seems to be getting bigger and bigger.
• 5:46
  Speaker 1: Yeah, we are about six billion years into a growth
• 5:50
  Speaker 1: spurt where we have been getting bigger and bigger, faster
• 5:53
  Speaker 1: and faster, and unlike your children, it doesn't seem like
• 5:56
  Speaker 1: that has an end. Scientists suspect that this might go
• 6:00
  Speaker 1: on forever until the universe is almost unfathomably large. If
• 6:05
  Speaker 1: it isn't already infinite.
• 6:07
  Speaker 2: Well, I guess that's the big question. Is how fast
• 6:11
  Speaker 2: is it growing? And does it seem like it is
• 6:12
  Speaker 2: going to keep growing forever exactly?
• 6:14
  Speaker 1: That is the question. And we look out into space,
• 6:17
  Speaker 1: which allows us to look further and further back in time.
• 6:21
  Speaker 1: It's sort of like looking at the marks you make
• 6:23
  Speaker 1: on that door jam as your kids grow up. You
• 6:26
  Speaker 1: can see not just how tall they were, but how
• 6:28
  Speaker 1: fast they grew based on the spaces between the marks.
• 6:32
  Speaker 1: We do the same thing by looking out into the
• 6:34
  Speaker 1: universe and seeing how big it is now and how
• 6:37
  Speaker 1: fast it was expanding in the past, and trying to
• 6:39
  Speaker 1: tell the whole story of the universe's expansion and extrapolate
• 6:43
  Speaker 1: that into the murky future.
• 6:45
  Speaker 2: So today on the podcast, we'll be asking the question
• 6:53
  Speaker 2: what's the best way to measure the expansion of the universe.
• 6:57
  Speaker 1: Definitely some kind of cosmic door jam, Oh.
• 7:00
  Speaker 2: My gosh, that would be a gigantic door. Where does
• 7:03
  Speaker 2: that door lead to? And how big does your pencil
• 7:06
  Speaker 2: need to be in order to make those marks exactly?
• 7:09
  Speaker 2: And is that doorjay part of the universe or part
• 7:11
  Speaker 2: of some metaverse or multiverse or multi metaverse. Well, it's
• 7:16
  Speaker 2: a door, I guess it's a doorway or doorway's part
• 7:21
  Speaker 2: of things.
• 7:23
  Speaker 1: I suppose if it's just sort of a dotted line,
• 7:24
  Speaker 1: But if you're going to make a mark on it,
• 7:26
  Speaker 1: it's got to have something in it that can hold
• 7:28
  Speaker 1: that information. So I guess then it would still be
• 7:30
  Speaker 1: part of the universe. So yeah, it's a tricky problem.
• 7:33
  Speaker 1: How do you use the universe to measure the universe?
• 7:36
  Speaker 2: That is a pretty tricky question because I guess if
• 7:38
  Speaker 2: you think about it, we're just floating on this tiny
• 7:40
  Speaker 2: rock called Earth in a little corner of a galaxy,
• 7:44
  Speaker 2: which is in a corner of some giant supercluster of galaxies,
• 7:47
  Speaker 2: And like, how can we possibly even think that we
• 7:49
  Speaker 2: can measure the size of the universe from this little
• 7:52
  Speaker 2: vantage point.
• 7:53
  Speaker 1: It is kind of incredible and almost fantastical what we
• 7:56
  Speaker 1: can do. And astronomers are really clever. As I was
• 8:00
  Speaker 1: prepping for today's episode, I was reading some recent papers
• 8:03
  Speaker 1: about really amazing ideas astronomers have for how to measure
• 8:07
  Speaker 1: how far away things are. And boy, those are some
• 8:09
  Speaker 1: clever folks. It's almost like they are magicians.
• 8:13
  Speaker 2: Yeah, because you know, I guess we're usedeing our everyday
• 8:15
  Speaker 2: lives of measuring things directly. You know, like if I
• 8:18
  Speaker 2: want to measure how tall my kid is, you just
• 8:20
  Speaker 2: you know, line them up against the wall and make
• 8:22
  Speaker 2: a mark and then use a ruler. Or if you
• 8:24
  Speaker 2: want to measure how far away another town is, you
• 8:26
  Speaker 2: kind of have to just drive there and see how
• 8:28
  Speaker 2: long it takes you, right.
• 8:29
  Speaker 1: Mm hmm. Even when we measure the distance to the Moon,
• 8:32
  Speaker 1: we do it by bouncing a laser off of a
• 8:35
  Speaker 1: mirror that astronauts left on the Moon and measure the
• 8:39
  Speaker 1: round trip time. So even that is kind of direct.
• 8:43
  Speaker 2: Yeah, And although we also had to go to the
• 8:45
  Speaker 2: mood to put that mirror there, right.
• 8:46
  Speaker 1: Yeah, too, exactly, It's sort of like we went to
• 8:49
  Speaker 1: the Moon and unfurled a huge measuring tape on the way.
• 8:52
  Speaker 1: It's just that measuring tape is made of laser beams.
• 8:54
  Speaker 2: Ooh nice. But with the universe that's kind of harder, right,
• 8:57
  Speaker 2: I mean, you can kind of have to go to
• 8:59
  Speaker 2: the other side of the universe and then put a
• 9:01
  Speaker 2: mirror there or at the end of a measuring tape
• 9:04
  Speaker 2: in order to directly measure how big the universe is.
• 9:07
  Speaker 1: Yeah, we'd love to be able to do that, but
• 9:09
  Speaker 1: without such a cosmic measuring tape. Astronomers and cosmologists, and
• 9:15
  Speaker 1: I guess cosmetologists have figured out ways to measure the
• 9:18
  Speaker 1: expansion of the universe over time.
• 9:21
  Speaker 2: Did you just say cosmetologists? Did you loop them in
• 9:24
  Speaker 2: in the same sentence as a cosmologist.
• 9:26
  Speaker 1: I did because I was watching this hilarious video clip
• 9:28
  Speaker 1: this morning of news coverage of the James web Space telescope,
• 9:31
  Speaker 1: where they said that it was a very powerful tool
• 9:34
  Speaker 1: that helped cosmetologists understand the universe. And I've been chuckling
• 9:38
  Speaker 1: about that all morning.
• 9:39
  Speaker 2: Oh my goodness, does it like the news anchor said that.
• 9:43
  Speaker 1: Yes, exactly. Cosmetologists and astrologists all over the world are
• 9:47
  Speaker 1: excited about the James web Space telescope.
• 9:50
  Speaker 2: Well, that could be true. I guess you never know.
• 9:52
  Speaker 2: I'm sure there are many cosmetologists listeners listening to us
• 9:56
  Speaker 2: right now who are fans of the universe and excited
• 9:59
  Speaker 2: about the James Webb tel.
• 10:00
  Speaker 1: Yeah, and they're very interested in the makeup of the
• 10:03
  Speaker 1: universe haha, nice and foundational questions in science.
• 10:08
  Speaker 2: Yeah, they're ready for us to lay on some bass
• 10:11
  Speaker 2: and blush at the amazing mysteries of the universe. But anyways,
• 10:14
  Speaker 2: as usually, we were wondering how many people out there
• 10:16
  Speaker 2: had thought about this question, and we're curious about what
• 10:19
  Speaker 2: is the best way to measure the expansion of the universe.
• 10:22
  Speaker 1: So thanks very much to those of you who answer
• 10:24
  Speaker 1: these questions for this fun segment of our podcast. If
• 10:27
  Speaker 1: you'd like to hear your voice, please don't be shy.
• 10:30
  Speaker 1: Write to me two questions at Danielandjorge dot com.
• 10:33
  Speaker 2: So think about it for a second. What do you
• 10:35
  Speaker 2: think is the best way to measure the expansion of
• 10:37
  Speaker 2: the universe. Here's what people had to say.
• 10:40
  Speaker 3: What we do is we analyze the red shift of
• 10:43
  Speaker 3: a particularly distinct galaxy and we compare it from the
• 10:46
  Speaker 3: previous data. The difference is how we calculate the rate
• 10:51
  Speaker 3: of expansion.
• 10:52
  Speaker 4: It's gotta be something I do with redshift. I see
• 10:55
  Speaker 4: how much something really far away, maybe the farthest thing
• 10:58
  Speaker 4: away we know is redshift, and then check it again
• 11:02
  Speaker 4: in a couple of months, see how far it's gone.
• 11:05
  Speaker 4: You know, carry the ones.
• 11:07
  Speaker 2: I guess you could build a universe's longest measuring tape. Oh,
• 11:11
  Speaker 2: maybe let's pose us. I don't know.
• 11:12
  Speaker 5: As far as I know, the best way to measure
• 11:14
  Speaker 5: the expansion rate of the universe is using a red shift.
• 11:18
  Speaker 1: You know, you look at the.
• 11:19
  Speaker 5: Red shift of galaxies very far away and compare them
• 11:22
  Speaker 5: to the red shift of galaxies that are closer, and
• 11:25
  Speaker 5: it can probably give you the right that way.
• 11:27
  Speaker 2: All right, A lot of people think maybe using some
• 11:30
  Speaker 2: kind of light and red shift of the light seems
• 11:33
  Speaker 2: to be the best way.
• 11:34
  Speaker 1: Mm hmmm. Well, of course there was the cosmic measuring
• 11:37
  Speaker 1: tape answer, and definitely the right approach in the sense that,
• 11:40
  Speaker 1: like what is the best way? We didn't ask what's
• 11:43
  Speaker 1: the best possible way? So in terms of like the
• 11:46
  Speaker 1: most impossible ideas, cosmic measuring tape is definitely the best way.
• 11:51
  Speaker 2: Hmmm. I wonder if that's even possible. Like hmm, Like,
• 11:56
  Speaker 2: is there enough material in the universe to make a
• 11:58
  Speaker 2: universe long measuring tape?
• 12:01
  Speaker 1: You wouldn't even have to make a universe long. Even
• 12:03
  Speaker 1: if we could just measure the distance to other galaxies,
• 12:06
  Speaker 1: that would be very helpful. But you know, galaxies are
• 12:09
  Speaker 1: millions of light years away, sometimes billions, and so that
• 12:13
  Speaker 1: would be a pretty incredible construction job. By the time
• 12:15
  Speaker 1: you finished it, the galaxies would have already moved.
• 12:17
  Speaker 2: Well, I think I know what you're saying. You're saying
• 12:19
  Speaker 2: that maybe to measure the expansion of the universe, we
• 12:21
  Speaker 2: don't actually need to measure the size of the universe.
• 12:24
  Speaker 1: Oh, that's right. The size and the expansion are different.
• 12:27
  Speaker 1: The universe could be infinite when it started, in infinite
• 12:30
  Speaker 1: now and could still be expanding because the expansion is
• 12:34
  Speaker 1: an intrinsic thing. It's a relative thing. Measures the growing
• 12:38
  Speaker 1: distances between things in the universe. And many of the
• 12:42
  Speaker 1: listeners commented about redshift, which is important to all understanding
• 12:45
  Speaker 1: the relative velocity of things, but we also need to
• 12:48
  Speaker 1: know their distances. That's going to turn out to be
• 12:50
  Speaker 1: the bigger challenge. Mm.
• 12:52
  Speaker 2: But it's weird that you don't need to know the
• 12:53
  Speaker 2: size of the universe to know how fast it is expanding, Like,
• 12:56
  Speaker 2: don't you need to know how big it is before
• 12:58
  Speaker 2: you can tell how big it's getting.
• 12:59
  Speaker 1: Well, what we're interested in is sort of the relative expansion.
• 13:02
  Speaker 1: Like if you're inside a blob of raisin bread, you
• 13:05
  Speaker 1: can use the raisins to measure how fast the raisin
• 13:08
  Speaker 1: bread is expanding, even if you don't know if there's
• 13:11
  Speaker 1: a crust to it and if you're near that crust
• 13:13
  Speaker 1: or if the raisin bread goes on forever. Right, you
• 13:16
  Speaker 1: can just measure the sort of local expansion and then
• 13:18
  Speaker 1: speculate that the expansion might be the same everywhere else.
• 13:21
  Speaker 2: Well, that's kind of what I mean, Like, how do
• 13:23
  Speaker 2: you know that your universe is not just expanding around you,
• 13:26
  Speaker 2: but maybe it's shrinking everywhere else, in which case the
• 13:29
  Speaker 2: universe itself as a whole is not expanding.
• 13:31
  Speaker 1: Yeah, you're absolutely right, we don't. All we can do
• 13:34
  Speaker 1: is measure the expansion in the part of the universe
• 13:36
  Speaker 1: that we can see, and then we can wonder what's
• 13:39
  Speaker 1: going on in the parts of the universe that we
• 13:41
  Speaker 1: can't see. It would be really weird if our part
• 13:44
  Speaker 1: of the universe was expanding, and the rest of it
• 13:46
  Speaker 1: was like contracting or doing something else weird and frothing.
• 13:49
  Speaker 1: But that's actually one of the ideas people have to
• 13:52
  Speaker 1: explain the strange results we get when we do try
• 13:54
  Speaker 1: to measure the expansion of the universe.
• 13:57
  Speaker 2: All right, well, let's dig into it, Daniel. What exactly
• 14:00
  Speaker 2: do you mean then, by the expansion of the universe.
• 14:03
  Speaker 1: This is a bit of a counterintuitive idea because people
• 14:06
  Speaker 1: think about the expansion usually relative to something else, Like
• 14:10
  Speaker 1: if you were making raisin bread in your oven, you
• 14:12
  Speaker 1: might measure the expansion relative to some ruler or your
• 14:15
  Speaker 1: oven and hope that, for example, the raisin bread still
• 14:17
  Speaker 1: fits in your oven and you didn't make too big
• 14:19
  Speaker 1: a loaf. In our case, though, because we're inside the universe,
• 14:23
  Speaker 1: there is no outside the universe. There is no ruler
• 14:26
  Speaker 1: outside of it that's not also affected by the universe.
• 14:29
  Speaker 1: All we can do is measure the relative expansion of
• 14:32
  Speaker 1: the universe, meaning how far apart are things. So if
• 14:36
  Speaker 1: we're here and there's another galaxy a million light years away,
• 14:39
  Speaker 1: or we're interested in, is how far apart is that
• 14:42
  Speaker 1: galaxy in a year, or in ten years, or in
• 14:45
  Speaker 1: a thousand.
• 14:46
  Speaker 2: Years, and This is a little bit different from any
• 14:48
  Speaker 2: sort of relative velocity our galaxy might have to a
• 14:51
  Speaker 2: different galaxy, Like our galaxy could be moving away from
• 14:54
  Speaker 2: or closer to the Andromeda galaxy. But that doesn't mean
• 14:56
  Speaker 2: that the universe is getting smaller. It just means that
• 14:59
  Speaker 2: we're both in out of this universe, and we both
• 15:01
  Speaker 2: happen to be moving towards each other exactly.
• 15:03
  Speaker 1: We're talking about an expansion of space itself.
• 15:06
  Speaker 2: I think you're talking about, like, what is the average
• 15:09
  Speaker 2: rate at which everything is moving closer or farther away
• 15:11
  Speaker 2: from us? Because you know, we're all sort of moving
• 15:13
  Speaker 2: inside of this universe, but on average, if things are
• 15:16
  Speaker 2: getting further apart, that kind of means that the universe
• 15:19
  Speaker 2: is expanding, right.
• 15:20
  Speaker 1: Yeah, And there's sort of two ways to think about it.
• 15:22
  Speaker 1: One is to think about space between us and other
• 15:26
  Speaker 1: galaxies expanding, like that the universe is creating more space
• 15:30
  Speaker 1: between us and other galaxies, and it's happening faster and
• 15:34
  Speaker 1: faster every year, So this expansion is accelerating. The weird
• 15:38
  Speaker 1: thing is that in those galaxies you don't feel that acceleration.
• 15:43
  Speaker 1: It's not like there's this force that's pushing on those
• 15:46
  Speaker 1: galaxies and accelerating them away from us. If you had
• 15:49
  Speaker 1: like an accelerometer in that galaxy, you wouldn't measure any acceleration.
• 15:55
  Speaker 1: And yet you see the velocities between these galaxies increasing
• 15:59
  Speaker 1: every year, So like the distances are increasing and the
• 16:02
  Speaker 1: velocities are increasing, but we don't measure any acceleration because
• 16:06
  Speaker 1: space itself is expanding. It's not like there's some explosion
• 16:10
  Speaker 1: that's pushing us further and further apart, faster and faster
• 16:13
  Speaker 1: every year.
• 16:14
  Speaker 2: But I guess that made me think about, like how
• 16:15
  Speaker 2: do you tell the difference, Like how do you know
• 16:18
  Speaker 2: if space is expanding between the two of you or
• 16:20
  Speaker 2: if you're just moving further and further away from you,
• 16:23
  Speaker 2: faster and faster. Maybe they're repulsed by us and they're
• 16:26
  Speaker 2: trying to get away from us, and they're like, it's
• 16:28
  Speaker 2: not you, it's just the space between us. We're just
• 16:30
  Speaker 2: growing apart.
• 16:31
  Speaker 1: You can think about it in two different ways. One
• 16:33
  Speaker 1: way is to think about space expanding between the different
• 16:37
  Speaker 1: galaxies and say like we have a little frame here
• 16:40
  Speaker 1: and within our galaxy everything makes sense, and they have
• 16:42
  Speaker 1: a little reference frame there and in their galaxy everything
• 16:46
  Speaker 1: makes sense, and between it. Space is expanding, And there's
• 16:49
  Speaker 1: another way to think about it, which is to put
• 16:50
  Speaker 1: the whole universe in a single reference frame and say, look,
• 16:53
  Speaker 1: I'm just going to measure the distance to stuff, and
• 16:55
  Speaker 1: I'm going to measure the distance to stuff later, and
• 16:57
  Speaker 1: I'm going to compare them, and I'm going to call
• 16:59
  Speaker 1: that velocity. And if you do that, you get weird results,
• 17:02
  Speaker 1: like things that are super duper far away seem to
• 17:04
  Speaker 1: be moving away from us faster than the speed of light.
• 17:07
  Speaker 1: And that second view of like thinking about everything in
• 17:09
  Speaker 1: terms of our frame doesn't really work because you can't
• 17:12
  Speaker 1: extend our frame to the entire universe because between us
• 17:16
  Speaker 1: and them, space is doing weird things. It's expanding, which
• 17:19
  Speaker 1: is why you get strange results like things seem to
• 17:22
  Speaker 1: be moving away from you faster than the speed of
• 17:24
  Speaker 1: light if you try to extend our frame all the
• 17:26
  Speaker 1: way to the end of the universe. So there are
• 17:28
  Speaker 1: two ways to think about it, and in some sense
• 17:30
  Speaker 1: they're equivalent. But cosmologists and cosmetologists prefer to think about
• 17:35
  Speaker 1: space expanding because then you get to have like a
• 17:37
  Speaker 1: nice little frame at each galaxy and think about it
• 17:40
  Speaker 1: expanding between frames.
• 17:42
  Speaker 2: What about astrologists, no comment, or comecologists.
• 17:47
  Speaker 1: No comment. But I didn't really answer your other question,
• 17:49
  Speaker 1: which is how can you tell the difference, And you
• 17:51
  Speaker 1: can tell the difference in terms of acceleration, Like acceleration
• 17:53
  Speaker 1: is something you can measure locally, you know, like if
• 17:56
  Speaker 1: you have a box or the ball inside of it,
• 17:58
  Speaker 1: it'll tell you whether you're accelerating because the ball will
• 18:01
  Speaker 1: get like pushed to one side. Like if you're in
• 18:03
  Speaker 1: a spaceship and you have a box or the ball
• 18:05
  Speaker 1: inside of it and the spaceship accelerates right, the ball
• 18:08
  Speaker 1: will roll to one side of the box, and if
• 18:10
  Speaker 1: the spaceship breaks, the ball will roll to the other
• 18:13
  Speaker 1: side of the box. So you can measure your own acceleration.
• 18:16
  Speaker 1: And if you're in that distant galaxy and you have
• 18:18
  Speaker 1: that accelerometer, you won't measure any acceleration, and yet your
• 18:22
  Speaker 1: velocity relative to other galaxies is increasing, So that tells
• 18:26
  Speaker 1: you that it really is the expansion of space itself
• 18:28
  Speaker 1: and not some like force that's pushing these things apart
• 18:31
  Speaker 1: and accelerating them.
• 18:32
  Speaker 2: But would you know, how do you know that that
• 18:34
  Speaker 2: other galaxy is not being accelerated? Like what if everything
• 18:37
  Speaker 2: in our in our local galaxy is being accelerated at
• 18:41
  Speaker 2: the same time.
• 18:41
  Speaker 1: Well, you're right, we haven't measured accelerometers in distant galaxies.
• 18:45
  Speaker 1: We do have accelerometers here, and we can tell that
• 18:47
  Speaker 1: there's no like Grand force pushing us all in some direction.
• 18:51
  Speaker 1: There's no overall acceleration of the Milky Way. And so
• 18:54
  Speaker 1: either we're very very unusual as a galaxy, we're the
• 18:57
  Speaker 1: only one not being accelerated and we're like at the center,
• 19:00
  Speaker 1: or none of those galaxies are being accelerated, and so
• 19:03
  Speaker 1: in general we prefer not to assume that we're at
• 19:06
  Speaker 1: the center of the universe. You can make the same
• 19:08
  Speaker 1: argument for the expansion, right. We look out in every
• 19:10
  Speaker 1: direction and we see things moving away from us. So
• 19:13
  Speaker 1: either we happen to be at the center of all
• 19:15
  Speaker 1: the expansion of the universe and everything is moving away
• 19:18
  Speaker 1: from where we are, or everything is expanding from every
• 19:22
  Speaker 1: point simultaneously, which we think is a simpler explanation and
• 19:26
  Speaker 1: less suspicious because it doesn't put us at the center
• 19:29
  Speaker 1: of the universe.
• 19:30
  Speaker 2: Right right, I guess you don't want to believe that
• 19:32
  Speaker 2: we are that repulsive that the whole universe is just
• 19:34
  Speaker 2: trying to get away from us mean, we need a
• 19:36
  Speaker 2: better cosmetologism.
• 19:38
  Speaker 1: And so we prefer to think about it in terms
• 19:40
  Speaker 1: of space expanding between us and other galaxies, because that's
• 19:44
  Speaker 1: also something that we can measure. We can look at
• 19:46
  Speaker 1: the space between us and other galaxies and we can
• 19:48
  Speaker 1: measure their velocities right now. We can look further and
• 19:51
  Speaker 1: further back in time, and we can see how that
• 19:54
  Speaker 1: velocity changes with time.
• 19:56
  Speaker 2: But it seems like it's all kind of based on
• 19:58
  Speaker 2: the idea or the discover that things are moving away
• 20:01
  Speaker 2: from us faster and faster in time, like things are
• 20:04
  Speaker 2: it seemed to be accelerating away from us, And then
• 20:06
  Speaker 2: you're saying that because there's an acceleration there, we have
• 20:09
  Speaker 2: to assume that space.
• 20:10
  Speaker 1: Is expanding mm hmm exactly.
• 20:12
  Speaker 2: But what if we had non measured an acceleration, could
• 20:14
  Speaker 2: we tell the difference? Like, what if the space was
• 20:16
  Speaker 2: happened to be expanding at a constant rate or a
• 20:19
  Speaker 2: rate that makes the velocity seem constant, would we then
• 20:23
  Speaker 2: know if things were moving away from us or if
• 20:26
  Speaker 2: space was expanding.
• 20:27
  Speaker 1: If there was no acceleration, no dark energy, then essentially
• 20:31
  Speaker 1: everything would be in one big inertial frame. And those
• 20:34
  Speaker 1: two pictures would be equivalent, but because there is acceleration,
• 20:37
  Speaker 1: you can't put everything into one big inertial frame, so
• 20:41
  Speaker 1: they really would be equivalent pictures if there was no acceleration.
• 20:44
  Speaker 1: The acceleration is what means those things really are in
• 20:47
  Speaker 1: their own separate frames.
• 20:49
  Speaker 2: All right, Well, thank you dark energy, I guess for
• 20:51
  Speaker 2: giving us a cluid that space itself is expanding. Otherwise
• 20:54
  Speaker 2: we would not know at all that it could expand.
• 20:56
  Speaker 1: Maybe yeah, Otherwise there'd be lots of different ways to
• 20:59
  Speaker 1: think about it. And you know, we would love to
• 21:01
  Speaker 1: measure the expansion in the universe by like trotting out
• 21:04
  Speaker 1: a ruler to other galaxies and measuring it and then
• 21:07
  Speaker 1: waiting a thousand years and measuring it again. But number one,
• 21:11
  Speaker 1: you could never really build that ruler. You'd have to
• 21:13
  Speaker 1: like stop the expansion of space as you stretch out
• 21:16
  Speaker 1: the ruler, which is like not practical, And of course
• 21:19
  Speaker 1: you don't want to wait a thousand years for measurement
• 21:21
  Speaker 1: number two. So that sort of measurement of the expansion
• 21:25
  Speaker 1: of space and its acceleration is not like a real
• 21:28
  Speaker 1: measurement that you could make. It's not something you could
• 21:30
  Speaker 1: actually measure, whereas thinking about it from the other point
• 21:33
  Speaker 1: of view, and just thinking about how distant galaxies are
• 21:35
  Speaker 1: moving away from us and measuring their velocity, and then
• 21:38
  Speaker 1: looking further and further back in time, the way you
• 21:40
  Speaker 1: like look down the door jam to see how far
• 21:43
  Speaker 1: away things were further back in time is the best
• 21:46
  Speaker 1: way to measure the expansion history of the universe.
• 21:49
  Speaker 2: Hmmm interesting. All right, Well, let's get into how you
• 21:52
  Speaker 2: actually make that measurement, how we can confirm that the
• 21:55
  Speaker 2: universe is expanding, and what does that mean for the
• 21:58
  Speaker 2: future of our cosmos. First, let's take a quick break.
• 22:13
  Speaker 2: All right, we're talking about the expansion of the universe
• 22:16
  Speaker 2: and how we would measure that. You can't just kind
• 22:19
  Speaker 2: of like loop a belt around it. We're a measuring
• 22:21
  Speaker 2: tape around it and see how much it's lately.
• 22:24
  Speaker 1: If you have the funding for that giant measuring tape,
• 22:26
  Speaker 1: I suggest we spend it on other science projects.
• 22:30
  Speaker 2: I guess what's tricky is that, like there's no edge
• 22:33
  Speaker 2: to the universe, even from our advantage point or any
• 22:36
  Speaker 2: vantage points, So you can't just kind of like look
• 22:37
  Speaker 2: out in one direction and look at the other direction
• 22:40
  Speaker 2: and see how far apart the edges of the universe
• 22:44
  Speaker 2: are right, we have to kind of go by what's
• 22:45
  Speaker 2: inside of the universe.
• 22:47
  Speaker 1: Exactly what we have to do since there isn't like
• 22:48
  Speaker 1: a ruler laid out for us, is we have to
• 22:51
  Speaker 1: find rulers. We have to like find things in the
• 22:53
  Speaker 1: universe where we think we know how big they were
• 22:56
  Speaker 1: a long time ago and see how big they are now.
• 22:58
  Speaker 1: Or we have to do things where we figure out
• 23:00
  Speaker 1: how far away things are and how fast they're moving,
• 23:03
  Speaker 1: which lets us sort of make a picture backwards in
• 23:06
  Speaker 1: time of how fast things have been moving away from
• 23:09
  Speaker 1: us as time spools back to the very beginning. And
• 23:13
  Speaker 1: so those are the basic ideas is to try to
• 23:16
  Speaker 1: put down some measuring points where we can look back
• 23:19
  Speaker 1: in time and see how things have changed.
• 23:21
  Speaker 2: But I think the main point you were trying to
• 23:22
  Speaker 2: make before is that it doesn't make sense to measure
• 23:25
  Speaker 2: like distances or how those distances are changing between us
• 23:28
  Speaker 2: and other galaxies. It makes more sentis to look at
• 23:30
  Speaker 2: their velocititers right, because space itself it's expanding. So if
• 23:34
  Speaker 2: you sort of try to measure the space between us,
• 23:37
  Speaker 2: you're going to run into trouble because that space is changing.
• 23:40
  Speaker 1: That space is changing but we do want to know
• 23:42
  Speaker 1: the distances to things, and that actually turns out to
• 23:44
  Speaker 1: be the crucial thing we're trying to measure, because the
• 23:46
  Speaker 1: distance also tells us the time. Right, things that are
• 23:50
  Speaker 1: really far away, we're getting information from them from a
• 23:52
  Speaker 1: long time ago. A galaxy that sent us light a
• 23:55
  Speaker 1: billion years ago and that is just now arriving on
• 23:58
  Speaker 1: Earth is telling us about it's a lifevelocity a billion
• 24:01
  Speaker 1: years ago. And we're curious about how that velocity varies
• 24:04
  Speaker 1: with distance now in the universe, and also how that
• 24:06
  Speaker 1: velocity varies with distance as we go backwards in time
• 24:09
  Speaker 1: in the universe, Like are the expansion velocities changing? Are
• 24:13
  Speaker 1: they getting faster? Are they getting slower? These are the
• 24:16
  Speaker 1: kind of measurements we want to make, and so knowing
• 24:18
  Speaker 1: that distance is crucial also to understanding the time and
• 24:21
  Speaker 1: history when that measurement left that object.
• 24:24
  Speaker 2: All right, well, let's dig into it, Daniel. What are
• 24:26
  Speaker 2: some of the ways that we can measure the expansion
• 24:28
  Speaker 2: of the universe.
• 24:29
  Speaker 1: In the end, we want to look out of the universe,
• 24:31
  Speaker 1: find a bunch of objects and know their relative velocity
• 24:34
  Speaker 1: and their distance. Right, we know their relative velocity, we
• 24:37
  Speaker 1: can tell how fast they're moving away from us. That's
• 24:39
  Speaker 1: just what the velocity is. And if we know their distance,
• 24:42
  Speaker 1: we can tell when that light left them, so we
• 24:44
  Speaker 1: can put it in the right spot in history. And
• 24:47
  Speaker 1: so those are two things we want to know when
• 24:49
  Speaker 1: to look at the sky, point to the galaxy and
• 24:51
  Speaker 1: say how far away is that and how fast is
• 24:54
  Speaker 1: it moving away from us? Turns out one of those
• 24:56
  Speaker 1: things is pretty easy and the other one is very
• 24:58
  Speaker 1: very hard. So measuring the velocity is pretty easy because
• 25:03
  Speaker 1: galaxies shine at us, and that light we look at
• 25:06
  Speaker 1: has a certain spectrum, meaning the colors of that light
• 25:09
  Speaker 1: are things we understand. It's like a lot of green
• 25:12
  Speaker 1: light or less blue light or more red light. If
• 25:15
  Speaker 1: you plot like the intensity of different colors, you get
• 25:18
  Speaker 1: like a certain wiggle. We call that the spectrum. But
• 25:20
  Speaker 1: that spectrum is shifted based on the velocity. So if
• 25:24
  Speaker 1: the galaxy is moving away from us really fast, then
• 25:26
  Speaker 1: the wavelength of the light that comes towards us is
• 25:29
  Speaker 1: stretched out, it's shifted towards longer wavelength, it's red shifted.
• 25:33
  Speaker 1: And because we have a pretty good idea what the
• 25:35
  Speaker 1: spectrum looked like when it left the galaxy, because it
• 25:38
  Speaker 1: just comes from like basic physics of atomic emission spectru
• 25:41
  Speaker 1: we can tell how much it's been shifted, So the
• 25:44
  Speaker 1: velocities are pretty easy to measure just using red shifts.
• 25:47
  Speaker 2: Because I guess you're assuming that all the lighte from
• 25:51
  Speaker 2: every galaxy should basically look sort of the same when
• 25:54
  Speaker 2: it leaves the galaxy, right, Like, you're assuming that other
• 25:57
  Speaker 2: galaxies are made of the same kinds of stars that
• 25:59
  Speaker 2: we are, with the same materials, And so when light
• 26:02
  Speaker 2: in general leaves a galaxy, basically all galaxies look the same,
• 26:06
  Speaker 2: is what you're saying.
• 26:06
  Speaker 1: Almost like, not exactly that all galaxies look the same,
• 26:09
  Speaker 1: but that all galaxies are made of the same kinds
• 26:11
  Speaker 1: of stuff, and we know how that stuff shines. We
• 26:14
  Speaker 1: know how hydrogen shines, and we think it shines the
• 26:17
  Speaker 1: same way in Andromeda as it does in other galaxies,
• 26:20
  Speaker 1: And we know how oxygen shines, and nitrogen and carbon shines.
• 26:24
  Speaker 1: Different galaxies have different mixtures of those kinds of things,
• 26:27
  Speaker 1: but they all shine the same way. So when you
• 26:29
  Speaker 1: look at the spectrum of a galaxy, you can measure
• 26:31
  Speaker 1: what's in that galaxy. Oh look there's water there. Oh
• 26:34
  Speaker 1: look there's nitrogen there. And because each of these things
• 26:37
  Speaker 1: shines differently. You can break it apart and say, oh, look,
• 26:39
  Speaker 1: that galaxy is a lot of water. This one has
• 26:41
  Speaker 1: a lot of nitrogen, and you can tell how much
• 26:43
  Speaker 1: they're shifted. So there's an incredible amount of information just
• 26:46
  Speaker 1: in the spectrum of light from these galaxies. You can
• 26:49
  Speaker 1: tell the components they're made out of and how much
• 26:52
  Speaker 1: they're all shifted.
• 26:53
  Speaker 2: I think technically, like oxygen doesn't glow, does it, It
• 26:56
  Speaker 2: blocks light.
• 26:57
  Speaker 1: So there's a couple of nuances there. Some of these
• 26:59
  Speaker 1: things low and some of these things absorb light. In
• 27:02
  Speaker 1: both cases, there are characteristic lines to it. If it's glowing,
• 27:06
  Speaker 1: it's giving off light at a certain frequency. If it's
• 27:09
  Speaker 1: absorbing light, then it's subtracting that frequency from the spectrum.
• 27:13
  Speaker 1: So you're looking for like dips in the spectrum and
• 27:15
  Speaker 1: also peaks in the spectrum. All those things astronomers can
• 27:19
  Speaker 1: use to figure out what's in that galaxy, And based
• 27:22
  Speaker 1: on the location of those lines, you can tell how
• 27:24
  Speaker 1: much they're shifted because of the velocity of the galaxy.
• 27:27
  Speaker 1: So yeah, there's emission and absorption going on.
• 27:30
  Speaker 2: So we get these wiggles of the light from other galaxies,
• 27:33
  Speaker 2: and it has like certain markers stories. I think that's
• 27:36
  Speaker 2: what you're saying. Like, if you get a wiggle from
• 27:38
  Speaker 2: a galaxy, there's a certain like a little spike or
• 27:41
  Speaker 2: a little dip or oxygen usually is, for example, and
• 27:44
  Speaker 2: you can tell if that's in the same spot as
• 27:47
  Speaker 2: the oxygen wiggle from our galaxy, then it's like not
• 27:50
  Speaker 2: moving relative to us at all. But if it is shifted,
• 27:53
  Speaker 2: then it's moving at a certain velocity away or towards us.
• 27:55
  Speaker 1: Right exactly, the more light you can gather from that
• 27:58
  Speaker 1: galaxy and the broader the spectrum, like a better handle.
• 28:01
  Speaker 1: You see more examples of this. This is why, for example,
• 28:04
  Speaker 1: recent images from the James Web Space Telescope of very
• 28:07
  Speaker 1: very distant galaxies have a lot of uncertainty in their
• 28:11
  Speaker 1: recession velocity because they haven't measured a whole lot of
• 28:13
  Speaker 1: light yet, and they don't have a very long curve.
• 28:16
  Speaker 1: They only have seen a part of the spectrum. If
• 28:18
  Speaker 1: they point hubble at it and they get like a
• 28:19
  Speaker 1: longer spectrum and more light, they'll get a better measurement
• 28:22
  Speaker 1: of that recession velocity.
• 28:24
  Speaker 4: Right.
• 28:24
  Speaker 2: And so this method tells you the relative velocity of
• 28:27
  Speaker 2: those stars and those galaxies, but that doesn't tell you
• 28:30
  Speaker 2: like where it is or how far away it is
• 28:31
  Speaker 2: from you. Right, Like, if I measure something with a
• 28:34
  Speaker 2: certain redshift that's piving away from me, that could be
• 28:37
  Speaker 2: like right next door to us, or it could be
• 28:38
  Speaker 2: a bazillion light years away, right exactly.
• 28:41
  Speaker 1: And we're interested in this relationship between distance and velocity
• 28:45
  Speaker 1: and how that relationship is changing over time. So we
• 28:48
  Speaker 1: really need to know the distance to these objects. And
• 28:51
  Speaker 1: that's hard because in general, if you don't know how
• 28:54
  Speaker 1: bright something actually is, you can't tell the difference between
• 28:58
  Speaker 1: it being like kind of dim and close by or
• 29:01
  Speaker 1: really really far away and super dup or bright. Those
• 29:03
  Speaker 1: two things look the same if you don't know how
• 29:06
  Speaker 1: bright it is originally, like what the true brightness is
• 29:09
  Speaker 1: of these objects, And so measuring the distances is much
• 29:13
  Speaker 1: more challenging, and that's what people have been doing a
• 29:15
  Speaker 1: lot of creative work, coming up with really clever.
• 29:17
  Speaker 2: Techniques, right because like if you just get a photon
• 29:20
  Speaker 2: from a distant galaxy, like you don't know where that
• 29:22
  Speaker 2: photon has been basically, right, that photon could have come
• 29:26
  Speaker 2: from a star really really really far away or close by,
• 29:30
  Speaker 2: Like the intensity of the time doesn't tell you much, right,
• 29:33
  Speaker 2: It could be from a dim star that's close by
• 29:35
  Speaker 2: or a super bright star that's really far away. Like
• 29:38
  Speaker 2: you wouldn't know just from the photon.
• 29:40
  Speaker 1: Yeah, well, intensity is the key. An intensity of light
• 29:43
  Speaker 1: comes from the number of photons.
• 29:45
  Speaker 3: Right.
• 29:45
  Speaker 1: A single photon isn't intense or non intense. It just
• 29:48
  Speaker 1: is a photon. It's really about a blob of photons,
• 29:51
  Speaker 1: a bunch of photons. You got ten photons from this star.
• 29:55
  Speaker 1: Is that because it's pretty close by and it's sent
• 29:57
  Speaker 1: one hundred and you got ten of them? Or is
• 29:59
  Speaker 1: it beca because it's super far away and it made
• 30:01
  Speaker 1: a zillion of them and you only got ten of them.
• 30:04
  Speaker 1: You can't tell how many went other directions. How diluted
• 30:07
  Speaker 1: is this packet of photons? As you get further and
• 30:10
  Speaker 1: further away from a star, you get a smaller and
• 30:12
  Speaker 1: smaller fraction of its number of photon outputs. The intensity
• 30:16
  Speaker 1: of your viewing dims as you get further away. So
• 30:19
  Speaker 1: that's the whole ambiguity. You can't tell if you're nearby
• 30:21
  Speaker 1: to something pretty dim or really far from something really bright.
• 30:25
  Speaker 2: All right, Well, what are some of the ways that
• 30:26
  Speaker 2: we can use to measure distance? Out there in a
• 30:29
  Speaker 2: big old space.
• 30:30
  Speaker 1: The classic way is a distance ladder. We use a
• 30:33
  Speaker 1: bunch of different methods to try to like extrapolate from
• 30:35
  Speaker 1: here to other galaxies. For very very close by stuff,
• 30:39
  Speaker 1: we can actually measure pretty directly how far away it
• 30:42
  Speaker 1: is just by seeing how it wiggles in the sky
• 30:45
  Speaker 1: as the Earth goes around the Sun, Because as the
• 30:48
  Speaker 1: Earth goes around the Sun, we get sort of like
• 30:50
  Speaker 1: a different view of a star. If it's pretty close by,
• 30:53
  Speaker 1: then we'll sort of see a different side of it.
• 30:55
  Speaker 1: It looks like it's in a different part of our sky.
• 30:58
  Speaker 1: If it's really really far away, then it won't change,
• 31:01
  Speaker 1: just the same way that you can measure the distance
• 31:03
  Speaker 1: like a basketball somebody has thrown you, because your two
• 31:06
  Speaker 1: eyeballs get different views of it. They see like different
• 31:10
  Speaker 1: parts of it, and your brain automatically reconstructs that and
• 31:12
  Speaker 1: tells you, oh, that basketball is really far away, or
• 31:15
  Speaker 1: the basketball is pretty close by. Or if you hold
• 31:18
  Speaker 1: up your finger and look at it with one eye
• 31:20
  Speaker 1: and then the other eye, you see that it changes,
• 31:23
  Speaker 1: and that change is greater as the finger gets closer
• 31:26
  Speaker 1: to your face, and the change is smaller as the
• 31:29
  Speaker 1: finger gets further from your face. So that's called parallax.
• 31:32
  Speaker 1: We can do that for pretty nearby objects.
• 31:34
  Speaker 2: It's also called triangulation in a way, right, because you're
• 31:37
  Speaker 2: making a triangle between, for example, and the basketball. You're
• 31:40
  Speaker 2: making a triangle between your left eye, your right eye
• 31:43
  Speaker 2: and the basketball. And because you form a triangle there
• 31:46
  Speaker 2: and you can measure those angles, you can tell how
• 31:48
  Speaker 2: far away the basketball is. You can sort of do
• 31:51
  Speaker 2: that with like the Earth and one side of the
• 31:53
  Speaker 2: Solar system, and the Earth and the other side of
• 31:56
  Speaker 2: the Solar system. You kind of form two points of
• 31:58
  Speaker 2: a triangle. And then depending on where the star looks
• 32:01
  Speaker 2: like it is, you can make the triangle and measure
• 32:04
  Speaker 2: its distance exactly.
• 32:05
  Speaker 1: And if the star is super duper far away, you
• 32:07
  Speaker 1: won't notice any difference, but if the star is pretty
• 32:10
  Speaker 1: close by, it has a pretty big effect. This is
• 32:12
  Speaker 1: actually a really fun story about how the Greeks got
• 32:15
  Speaker 1: it wrong. You know, the Greeks saw that the Earth
• 32:17
  Speaker 1: was at the center of the Solar system because they
• 32:19
  Speaker 1: figured if the Earth was moving around the Sun, they
• 32:22
  Speaker 1: would see this parallax effect. Like there were masters of geometry,
• 32:25
  Speaker 1: triangles were not going to escape them. And they figured, look,
• 32:28
  Speaker 1: we look up in the night sky, and we don't
• 32:29
  Speaker 1: see any stars wiggling. Therefore the Earth is not moving.
• 32:32
  Speaker 1: And their mistake was that they thought the stars were
• 32:34
  Speaker 1: all pretty close by, so they figured they should all
• 32:36
  Speaker 1: be wiggling if we're moving. They didn't realize the stars
• 32:39
  Speaker 1: are much much further away than they actually were. And
• 32:42
  Speaker 1: that's the thing about parallax. It only works for pretty
• 32:44
  Speaker 1: close by stars. Even still, the wiggle is pretty subtle.
• 32:48
  Speaker 1: We didn't detect it until like the nineteenth century.
• 32:50
  Speaker 2: Yeah, it's also trigger because what if there's a giant
• 32:53
  Speaker 2: three D glasses out there space, Then you get fooled
• 32:56
  Speaker 2: into thinking at a certain distance always concern.
• 33:00
  Speaker 1: So that's the sort of like most direct way we
• 33:02
  Speaker 1: can measure the distance to pretty nearby stuff. And then
• 33:05
  Speaker 1: about one hundred years ago, Henrietta Levitt figured out a
• 33:08
  Speaker 1: way to measure the distances to other kinds of things.
• 33:11
  Speaker 1: That there's special kind of stars called cephids. Cephids are
• 33:14
  Speaker 1: stars that do something really cool. They vary in their brightness,
• 33:18
  Speaker 1: like they get brighter and dimmer, brighter and.
• 33:21
  Speaker 2: Dimmer because of something that's going on in the stars right,
• 33:24
  Speaker 2: Like there's some process that seems to happen not just
• 33:26
  Speaker 2: in one star, but in a certain kind of star.
• 33:29
  Speaker 1: Exactly. It has to do with the internal like dynamics
• 33:32
  Speaker 1: of the star. They get opaque and then they absorb
• 33:36
  Speaker 1: their own radiation which puffs them out and they get
• 33:38
  Speaker 1: dim and then they collapse and they get brighter again,
• 33:40
  Speaker 1: and then they absorb that radiation. So there's this cycle
• 33:43
  Speaker 1: that goes on. And the really interesting thing is that
• 33:46
  Speaker 1: there's a close connection between how long that cycle takes
• 33:50
  Speaker 1: to happen between like the bright and the dim moments,
• 33:52
  Speaker 1: and how bright it is at its origin. So if
• 33:55
  Speaker 1: you measure the period, if you measure how long it
• 33:57
  Speaker 1: is between like peaks of brightness, then you know how
• 34:00
  Speaker 1: bright it actually is, which means you can tell how
• 34:04
  Speaker 1: far away it is because you measure how bright we
• 34:06
  Speaker 1: see it, and you know how bright it is if
• 34:08
  Speaker 1: you were really close by, and you can extrapolate.
• 34:11
  Speaker 2: But I guess we had to know how far away
• 34:13
  Speaker 2: they were before to make that connection right exactly.
• 34:17
  Speaker 1: So to calibrate this, to make sure this really works,
• 34:20
  Speaker 1: you need some sephids whose distance you can measure using parallax.
• 34:25
  Speaker 1: So there's a few stars where it overlaps. There are a
• 34:27
  Speaker 1: few sephids that are close enough where we can measure
• 34:30
  Speaker 1: their distance using parallax, and we can measure their distance
• 34:33
  Speaker 1: using their period, and we see that the two things agree.
• 34:36
  Speaker 1: So that's why it's called the distance ladder because we
• 34:38
  Speaker 1: have like a little bit of overlap. And then we
• 34:40
  Speaker 1: assume the sephids and like other galaxies, operate the same way,
• 34:44
  Speaker 1: and that way we can measure the distance to other
• 34:46
  Speaker 1: galaxies where parallax doesn't work.
• 34:49
  Speaker 2: So it's thanks to these sephods that we have a
• 34:51
  Speaker 2: better view of how far things are, right, because it
• 34:54
  Speaker 2: just so happens that because of the mechanics as the star,
• 34:56
  Speaker 2: those two things are related the period of their blinking
• 34:59
  Speaker 2: and kind of like their size or how bright they
• 35:02
  Speaker 2: are exactly.
• 35:03
  Speaker 1: But you know, it's a big extrapolation, right. We are
• 35:05
  Speaker 1: talking about like things we measure in our galaxy and
• 35:08
  Speaker 1: we're extrapolating to distant galaxies and we're assuming that we
• 35:11
  Speaker 1: understand how this works. But we're relying on those stars
• 35:14
  Speaker 1: where we can check it, those sephids where we can
• 35:16
  Speaker 1: have parallax measurements, and there's not a lot of them.
• 35:19
  Speaker 1: There's like ten or twelve, right, So this whole distance
• 35:23
  Speaker 1: ladder is calibrated on like a handful of stars in
• 35:26
  Speaker 1: the overlap region, and we.
• 35:27
  Speaker 2: Have pretty good measurements of that parallax where we're confident
• 35:31
  Speaker 2: we know where they are. But I guess maybe you're
• 35:32
  Speaker 2: not confident that ten sephids really represents all sephords in
• 35:36
  Speaker 2: the universe.
• 35:37
  Speaker 1: Yeah, there's a lot of uncertainties there, like the uncertainties
• 35:40
  Speaker 1: on the parallax for those sephids, and are those typical
• 35:43
  Speaker 1: and is there some uncertainty due to like how much
• 35:46
  Speaker 1: metallicity there are in these sephids. There's a lot of
• 35:48
  Speaker 1: work going on to try to like nail that down
• 35:51
  Speaker 1: more precisely. And then there's another step in the distance
• 35:53
  Speaker 1: ladder because cephids are great and they're in distant galaxies,
• 35:57
  Speaker 1: but they're not that bright, so for like really far
• 35:59
  Speaker 1: away galaxy you can't see them. And then about twenty
• 36:02
  Speaker 1: years ago people found another element to add to the
• 36:05
  Speaker 1: distance ladder, which were type one, a supernova sort of
• 36:09
  Speaker 1: like cephe's. You can tell how bright they are in reality,
• 36:12
  Speaker 1: like if you were close by by looking at how
• 36:15
  Speaker 1: their brightness fades. So these are stars that are very
• 36:18
  Speaker 1: very bright because it's a supernova it's like as bright
• 36:21
  Speaker 1: as the galaxy that contains it, very very briefly, and
• 36:24
  Speaker 1: then it fades away, and by looking at the rate
• 36:26
  Speaker 1: of which it declines, you can calculate how bright it
• 36:29
  Speaker 1: is in reality.
• 36:31
  Speaker 2: I guess you're assuming that you know the laws of
• 36:33
  Speaker 2: physics are the same here as they are in other
• 36:36
  Speaker 2: parts of the universe and other galaxies, and so you're
• 36:38
  Speaker 2: saying that when a star goes super nova with this
• 36:41
  Speaker 2: type one A, it usually happens the same way, and
• 36:44
  Speaker 2: it happens in a way that tells you like, oh,
• 36:47
  Speaker 2: if it's decaying, if it's the brightness of that flashes
• 36:50
  Speaker 2: slower or faster, it tells you like how explosive that
• 36:53
  Speaker 2: supernova was.
• 36:54
  Speaker 1: Exactly All these techniques have the same basic strategy, which
• 36:57
  Speaker 1: is find some other way to predict how bright it
• 37:01
  Speaker 1: is at the source, assuming that the physics is happening
• 37:04
  Speaker 1: the same way there and here, And if you can
• 37:07
  Speaker 1: do that, you can predict how bright it actually is,
• 37:09
  Speaker 1: and you can compare it to how bright you see
• 37:11
  Speaker 1: it to be, then you can tell how far away
• 37:13
  Speaker 1: it is. And people knew this for a long time.
• 37:15
  Speaker 1: People understood type one A supernova might be a good
• 37:18
  Speaker 1: technique for this, but again we didn't have enough overlap.
• 37:21
  Speaker 1: It wasn't until Hubble launched and we got a bunch
• 37:23
  Speaker 1: of like far away sephids that we could calibrate these
• 37:26
  Speaker 1: Type one A supernova. So now we have again just
• 37:29
  Speaker 1: a handful of galaxies that have both sephids and Type
• 37:32
  Speaker 1: one A supernova in them where we can cross calibrate
• 37:35
  Speaker 1: and add like another plank to our distance ladder.
• 37:38
  Speaker 2: I feel like it's not so much a ladder that
• 37:40
  Speaker 2: you're building, but like it's a stack of stools, you
• 37:44
  Speaker 2: know what I mean. Like you start with a short
• 37:46
  Speaker 2: stool or stepping stool, and then you're not attaching another step.
• 37:50
  Speaker 2: It's like you're just putting another stepping stool on top
• 37:53
  Speaker 2: of your first stepping stool, and the whole thing is
• 37:56
  Speaker 2: kind of shaky.
• 37:57
  Speaker 1: The whole thing is pretty shaky, yeah, exactly. And this
• 38:00
  Speaker 1: lot of uncertainty and how these things overlap, because there's
• 38:02
  Speaker 1: not a lot of data where we have things on
• 38:05
  Speaker 1: both kinds of stools, right, And another big uncertainty is dust,
• 38:09
  Speaker 1: Like there are other ways things can get dim. It's
• 38:11
  Speaker 1: not just being far away. There's a big dust cloud
• 38:14
  Speaker 1: between us and one of these galaxies that'll make it
• 38:17
  Speaker 1: look dimmer, which would make it look further away. So
• 38:21
  Speaker 1: unless you know exactly where dust is in the universe,
• 38:23
  Speaker 1: that really complicates these measurements.
• 38:25
  Speaker 2: M interesting. All right, Well, let's get into other ways
• 38:28
  Speaker 2: that we can measure distances out there, and let's see
• 38:31
  Speaker 2: how shaky this ladder of stepping stools can get and
• 38:34
  Speaker 2: what that tells us about the expansion of the universe.
• 38:37
  Speaker 2: But first, let's take another quick work. All right, we're
• 38:52
  Speaker 2: talking about the expansion of the universe and how you
• 38:55
  Speaker 2: would actually measure how fast the universe is expanding, Because
• 38:59
  Speaker 2: I guess we're people, Daniel, we want to know if
• 39:02
  Speaker 2: the universe is getting bigger or smaller.
• 39:06
  Speaker 1: We definitely want to understand because it affects the fate
• 39:09
  Speaker 1: of everything. You know, is the universe going to collapse
• 39:11
  Speaker 1: into a big crunch and squish us all? Is it
• 39:13
  Speaker 1: going to tear us all apart? Is it going to
• 39:15
  Speaker 1: leave us as isolated islands to collapse into our own
• 39:18
  Speaker 1: individual black holes? Like it matters?
• 39:20
  Speaker 2: Plus we're curious, Well, it doesn't matter to us because
• 39:23
  Speaker 2: you're talking about things that would happen billions of years
• 39:25
  Speaker 2: from now. But you know, our great great great great
• 39:27
  Speaker 2: great great great great grandkids might need to think about
• 39:31
  Speaker 2: their retirement plans.
• 39:34
  Speaker 1: And I care about my super super great grandkids. You know,
• 39:37
  Speaker 1: in fact, there's almost certainly going to be some kid
• 39:40
  Speaker 1: deep in the future who's going to have both me
• 39:42
  Speaker 1: and you as an ancestor.
• 39:43
  Speaker 2: Oh boy, hopefully I'll be the its favorite ancestors, or
• 39:47
  Speaker 2: at least a taller one.
• 39:49
  Speaker 1: Well, if it listens to the podcast to learn that
• 39:51
  Speaker 1: you don't really care about their future, but I do.
• 39:53
  Speaker 2: I didn't say that, I say it wasn't my problem,
• 39:57
  Speaker 2: but that I mean I don't care about their problem.
• 40:01
  Speaker 1: Well, great great great grandkid, your problems are my problems,
• 40:04
  Speaker 1: all right.
• 40:04
  Speaker 2: Well, we're talking about different ways to measure distances out
• 40:07
  Speaker 2: there in a big space, with all this uncertainty and
• 40:10
  Speaker 2: all this dust in the universe and these unfathomable distances.
• 40:15
  Speaker 2: So far, we've been using three D glasses, certain kinds
• 40:20
  Speaker 2: of stars called sephids, and type A supernova's. How else
• 40:23
  Speaker 2: can we measure distances in space?
• 40:25
  Speaker 1: So this is a big cottage industry recently, so people
• 40:27
  Speaker 1: have been figuring out lots of different ways to measure it,
• 40:30
  Speaker 1: to try to understand whether these measurements are correct or
• 40:33
  Speaker 1: not because it tells a different story. So another way
• 40:36
  Speaker 1: people have been measuring distances it is not looking at
• 40:38
  Speaker 1: supernova but looking at moments when red giant stars get
• 40:43
  Speaker 1: really really bright. Red giants are stars near the end
• 40:46
  Speaker 1: of their life when they have been burning hydrogen for
• 40:48
  Speaker 1: a long time and collecting helium ash at their core,
• 40:52
  Speaker 1: but they aren't hot enough to burn that helium yet.
• 40:55
  Speaker 1: Then near the end of their life, suddenly they get
• 40:57
  Speaker 1: hot and dense enough to burn that helium and all
• 41:00
  Speaker 1: happens very very quickly. It's a huge flash of light
• 41:03
  Speaker 1: from this helium burning. So these are peaking red giants,
• 41:07
  Speaker 1: and when they do that, they're almost always the same
• 41:10
  Speaker 1: similar news type one a supernova or similar to sepids.
• 41:13
  Speaker 1: You can tell basically how bright they are from other
• 41:16
  Speaker 1: characteristics you measure about them, like their spectrum. So these
• 41:19
  Speaker 1: are called tip of the red giant branch because astronomers
• 41:21
  Speaker 1: think about all these stars on like a big branch
• 41:24
  Speaker 1: of luminosity versus size, and so they use these stars
• 41:29
  Speaker 1: to measure the distance to those galaxies that contain them.
• 41:32
  Speaker 2: Because I guess all red giant stars are basically the same,
• 41:35
  Speaker 2: like if you have a red giant, it means you
• 41:37
  Speaker 2: they're a certain size, Like there aren't an infinite number
• 41:40
  Speaker 2: of kinds of red giants, right.
• 41:42
  Speaker 1: Yeah, exactly. They tend to do it in basically the
• 41:44
  Speaker 1: same way. These things are a little harder to find,
• 41:47
  Speaker 1: so there aren't as many examples, but recently been working
• 41:50
  Speaker 1: really hard on this, using it to measure independently the
• 41:53
  Speaker 1: expansion rate of the universe. It's also sensitive to dust,
• 41:57
  Speaker 1: like the first measurement we talked about, but it's differently
• 42:00
  Speaker 1: sensitive to dust because the best red giant candidates are
• 42:04
  Speaker 1: old stars that are on like the outskirts of galaxies,
• 42:07
  Speaker 1: which tend to be less dusty, and so it's like
• 42:10
  Speaker 1: less sensitive to dust. People think in that distant galaxy.
• 42:14
  Speaker 2: And you can actually see these in distant galaxies because
• 42:17
  Speaker 2: you know, usually when you look at a distant galaxy
• 42:18
  Speaker 2: it just looks like a fuzz. You can actually make
• 42:20
  Speaker 2: out little pinpoints in them.
• 42:22
  Speaker 1: Yeah, you can actually make out these pinpoints because they're
• 42:24
  Speaker 1: very bright when it happens, and it's sudden, doesn't last
• 42:27
  Speaker 1: for very long. So if you're watching that galaxy, you
• 42:29
  Speaker 1: can see a change in the galaxy that sudden peak
• 42:33
  Speaker 1: of brightness the same way you can see a type
• 42:35
  Speaker 1: one a supernova in that galaxy, or you can see
• 42:37
  Speaker 1: cephids in distant galaxies because they have a period.
• 42:41
  Speaker 2: I see, but you're still looking at the overall light
• 42:44
  Speaker 2: from the galaxy. You're not looking at like, oh, that
• 42:46
  Speaker 2: little corner of this galaxy flashed up, that must be
• 42:49
  Speaker 2: a red giant. You're looking at the whole thing, right
• 42:51
  Speaker 2: or not.
• 42:51
  Speaker 1: You're looking at the whole thing. But you can resolve
• 42:53
  Speaker 1: these individual red giants. Yeah, not all galaxies are so
• 42:57
  Speaker 1: far away that you can't resolve them.
• 43:00
  Speaker 2: All right, Well, are some other ways that we can
• 43:01
  Speaker 2: measure distance.
• 43:02
  Speaker 1: So people are trying to develop waves that are independent
• 43:05
  Speaker 1: of this, that are like less sensitive to dust. For example.
• 43:08
  Speaker 1: One really cool way is to use gravitational waves. Because
• 43:12
  Speaker 1: this doesn't use light at all, right, it just uses
• 43:15
  Speaker 1: gravitational waves. And if you watch two neutron stars, for example,
• 43:19
  Speaker 1: and you see them spiraling in towards each other so
• 43:22
  Speaker 1: that they're going to collide, you get a gravitational wave signature. Remember,
• 43:26
  Speaker 1: everything in the universe that accelerates makes waves in its
• 43:29
  Speaker 1: gravitational field, and we can measure those on Earth with
• 43:32
  Speaker 1: very powerful interferometers and as they spiral in, they go
• 43:36
  Speaker 1: faster and faster and faster, so the gravitational wave gets
• 43:39
  Speaker 1: faster and faster, and by watching that frequency change, you
• 43:42
  Speaker 1: can calculate the mass of those objects. You can tell like, oh,
• 43:46
  Speaker 1: this is a neutron star of that mass, or that
• 43:48
  Speaker 1: was a black hole of the other mass, And from
• 43:51
  Speaker 1: knowing the mass of those things, you can tell how
• 43:53
  Speaker 1: big the waves should be. So you watch sort of
• 43:56
  Speaker 1: the speed of the wiggles, which tells you how big
• 43:58
  Speaker 1: the objects are, which tells you how high the wave
• 44:02
  Speaker 1: should go. And then you measure how high the wave
• 44:04
  Speaker 1: is that you got, and that tells you how far
• 44:07
  Speaker 1: away it is, because, just like with light, as the
• 44:09
  Speaker 1: wave gets further and further away, it gets dimmer and dimmer.
• 44:12
  Speaker 1: So by measuring the gravitational wave frequency, you can sort
• 44:14
  Speaker 1: of predict the intensity of the gravitational wave as it
• 44:17
  Speaker 1: was emitted and compared to the intensity you measure here
• 44:20
  Speaker 1: on Earth.
• 44:21
  Speaker 2: So if we get a gravitational wiggle wave from two
• 44:24
  Speaker 2: neutron stars crashing, you're saying that we can tell how
• 44:27
  Speaker 2: far away it is because they all always happen kind
• 44:30
  Speaker 2: of the same way. But then how do you know
• 44:32
  Speaker 2: where it happen, right, because we're just listening to these
• 44:35
  Speaker 2: gravitational waves. How do you know where in the universe
• 44:38
  Speaker 2: that crash happens.
• 44:39
  Speaker 1: We can tell the direction these things come from because
• 44:41
  Speaker 1: we have multiple ears. Essentially, we have one in Louisiana,
• 44:44
  Speaker 1: one in Washington, and one in Italy. And as the
• 44:46
  Speaker 1: wave passes over the Earth, it doesn't arrive at all
• 44:49
  Speaker 1: these things at the same moment. So you can use
• 44:51
  Speaker 1: that to tell the directionality. But the distance measurement is different.
• 44:55
  Speaker 1: The distance measurement comes from the intensity of it, like
• 44:57
  Speaker 1: how loud was it? By looking at the free you
• 45:00
  Speaker 1: can see the wiggles. You can tell how loud it
• 45:02
  Speaker 1: was when it was created, and we can measure the
• 45:04
  Speaker 1: loudness as it arrived on Earth, and so we can
• 45:06
  Speaker 1: tell how much it's been quieted by its flight through
• 45:10
  Speaker 1: the universe. And that tells us the distance.
• 45:12
  Speaker 2: It tells us the distance between us and where those
• 45:14
  Speaker 2: two neutrons stars crashed. But what does that tell us
• 45:17
  Speaker 2: about if anything else. It just tells us that the
• 45:20
  Speaker 2: two neutrons stars crashed at a certain distance from us
• 45:22
  Speaker 2: in a certain direction. But does that tell you, like
• 45:25
  Speaker 2: the velocity or how galaxies around her are moving.
• 45:29
  Speaker 1: Well, if you know where it was in space, you
• 45:31
  Speaker 1: know which galaxy those neutron stars were in, So you
• 45:34
  Speaker 1: can point to that galaxy and say, oh, it was
• 45:36
  Speaker 1: in this galaxy, and now we know how far away
• 45:39
  Speaker 1: that galaxy is. In the same way that if you
• 45:41
  Speaker 1: spot a galaxy and you see a supernova blow up
• 45:43
  Speaker 1: in that galaxy, you know how far away that galaxy is. Now,
• 45:47
  Speaker 1: if you spot a galaxy and you see two neutron
• 45:49
  Speaker 1: stars collide inside that galaxy, you can use that to
• 45:52
  Speaker 1: measure the distance to that galaxy.
• 45:54
  Speaker 2: Do we know the directionality that with that much accuracy?
• 45:57
  Speaker 2: Like is our stereo or hearing of rotational waves that
• 46:01
  Speaker 2: accurate to tell like, oh, that wiggle came from that galaxy?
• 46:04
  Speaker 2: Because there are so many there's billions of galaxies out
• 46:06
  Speaker 2: there in space.
• 46:06
  Speaker 1: Right, there are lots of galaxies out there in space,
• 46:08
  Speaker 1: and the directionality of this is not great. You're right,
• 46:12
  Speaker 1: because we only have three ears and sometimes they're consistent
• 46:14
  Speaker 1: with like a few different directions, so there's a lot
• 46:17
  Speaker 1: of uncertainty in this measurement. It's one people are excited
• 46:20
  Speaker 1: about because it's very independent from the other measurements, like
• 46:23
  Speaker 1: not affected by dust at all, but it's not one
• 46:26
  Speaker 1: that yet provides a measurement that's competitive at all. It's
• 46:29
  Speaker 1: like has big error bars for all the reasons you
• 46:31
  Speaker 1: lay it out, and also because we just don't hear
• 46:33
  Speaker 1: many gravitational waves compared to other things. So it's something
• 46:37
  Speaker 1: that we think in the future is going to help.
• 46:39
  Speaker 1: It's a cool new technique, but it hasn't yet provided
• 46:41
  Speaker 1: a measurement that compares with the uncertainty of the other measurements.
• 46:45
  Speaker 2: Right, where are some other ways that we can measure distance.
• 46:47
  Speaker 1: One of the ways that's most amazing and impressive is
• 46:49
  Speaker 1: using something called a mazer. So a mazer is like
• 46:52
  Speaker 1: a laser, but it emits in the microwaves. So what
• 46:55
  Speaker 1: they do is they see these blobs of water orbiting
• 46:58
  Speaker 1: a black hole in a distant galaxy, and so for
• 47:01
  Speaker 1: these blobs of water, what they can do is they
• 47:04
  Speaker 1: can measure the distance between the blob of water and
• 47:06
  Speaker 1: the central black hole. And they do it in two
• 47:08
  Speaker 1: different ways, and one is that they look at how
• 47:11
  Speaker 1: these microwave light from this water blog changes as it
• 47:14
  Speaker 1: goes around the black hole, Like as it's going around
• 47:17
  Speaker 1: the backside of the black hole, it's accelerating away from you.
• 47:19
  Speaker 1: As it's coming around the other side of the black hole,
• 47:21
  Speaker 1: it's accelerating towards you, so it's either like red shifted
• 47:24
  Speaker 1: or blue shifted as it goes around this black hole.
• 47:27
  Speaker 1: So by measuring that acceleration and doing like a little
• 47:29
  Speaker 1: bit of like Kepler's laws, you can figure out what
• 47:32
  Speaker 1: is the radius of its orbit around this black hole.
• 47:35
  Speaker 1: And then they actually point telescopes at these things and
• 47:38
  Speaker 1: measure the radius. They can like see these spots orbiting
• 47:42
  Speaker 1: black holes in distant galaxies, so they know the true
• 47:45
  Speaker 1: radius from like the wiggles, and then they can actually
• 47:48
  Speaker 1: measure the radius in a telescope, and they can compare
• 47:51
  Speaker 1: those two things and tell how far away that galaxy
• 47:54
  Speaker 1: actually is.
• 47:55
  Speaker 2: WHOA wait, how do you measure the radius of something
• 47:59
  Speaker 2: orbiting a black hole in another galaxy?
• 48:01
  Speaker 1: It's hard. They have these very long baseline interferometers that
• 48:04
  Speaker 1: can actually resolve these things. They can like measure the
• 48:07
  Speaker 1: locations of these water blobs. When I was first reading
• 48:10
  Speaker 1: about this, I didn't believe it. I had to go
• 48:12
  Speaker 1: back to the papers and see. But in those papers
• 48:14
  Speaker 1: you can see they actually do measure like the distance
• 48:17
  Speaker 1: of each water blob from the black hole itself. It's
• 48:20
  Speaker 1: incredible what we can do with very long baseline interferometers MM.
• 48:25
  Speaker 2: And by water blob, you don't mean like an actual
• 48:27
  Speaker 2: like blob of water. Probably you mean like a cloud
• 48:30
  Speaker 2: of H two O molecules, right.
• 48:32
  Speaker 1: Yeah, exactly. You have some big cloud that's hot and
• 48:34
  Speaker 1: it has a lot of water in it, and so
• 48:36
  Speaker 1: it's emitting light at a characteristic frequency more like a
• 48:39
  Speaker 1: cloud maybe, yeah, like a cloud of water, like water vapor.
• 48:43
  Speaker 1: And by seeing how that frequency is shifted, we can
• 48:46
  Speaker 1: tell whether it's like going around the backside or accelerating
• 48:49
  Speaker 1: towards us or on the front side. So these megamsers
• 48:53
  Speaker 1: they're called, are a totally separate way to measure the
• 48:56
  Speaker 1: distance to these galaxies.
• 48:57
  Speaker 2: Cool, but how many of these can we see or
• 49:00
  Speaker 2: have we seen enough of them to like calibrate this method.
• 49:03
  Speaker 1: There are enough of them to calibrate these methods, but
• 49:05
  Speaker 1: it's not as accurate yet as the other one because
• 49:08
  Speaker 1: we haven't seen that many. In order for this to work,
• 49:10
  Speaker 1: the masers can't be that far away or you just
• 49:13
  Speaker 1: can't see these water blobs going around the black holes.
• 49:16
  Speaker 1: But it's useful because it's a very independent measurement. And
• 49:19
  Speaker 1: the problem is we have lots of different ways to
• 49:21
  Speaker 1: measure the expansion of the universe, but some of them
• 49:23
  Speaker 1: don't agree. A whole other way to measure the expansion
• 49:25
  Speaker 1: of the universe is to look for evidence very early
• 49:28
  Speaker 1: on in the universe from like the cosmic microwave background radiation,
• 49:32
  Speaker 1: and compare that to what we measure from these kind
• 49:35
  Speaker 1: of measurements like in the universe today. We make those
• 49:37
  Speaker 1: measurements and they don't agree, and we don't understand the difference.
• 49:40
  Speaker 1: So having as many independent measurements as possible is really important.
• 49:45
  Speaker 2: Right, you want to stack as many stepping stools on
• 49:47
  Speaker 2: top of your stepping stools as you can. Right, the
• 49:52
  Speaker 2: more stepping stools, the bigger the structure of stepping stools
• 49:55
  Speaker 2: you can convert.
• 49:56
  Speaker 1: Well, if it is one universe, then it all makes
• 49:58
  Speaker 1: sense to us. Then it should be telling us one story.
• 50:00
  Speaker 1: But right now it's telling us several stories. Like measurements
• 50:04
  Speaker 1: from the cosmic microwave background radiation say the universe is
• 50:07
  Speaker 1: expanding at one rate, and measurements from quoisars and cephids
• 50:11
  Speaker 1: and type on a supernova and masers and all these
• 50:13
  Speaker 1: other things tell us a different story, and the tip
• 50:16
  Speaker 1: of the red giant branch that tells us a story
• 50:19
  Speaker 1: that's right in between them. So we have like three
• 50:21
  Speaker 1: different groups of measurements that kind of overlap but kind
• 50:24
  Speaker 1: of disagree with each other. It's a big problem right
• 50:27
  Speaker 1: now in cosmology. People don't really understand what story this
• 50:31
  Speaker 1: is telling us. Are we measuring these things incorrectly or
• 50:34
  Speaker 1: is the story more complicated than we imagined?
• 50:36
  Speaker 2: Right, But the except the cosmetologies are all unified and
• 50:39
  Speaker 2: they're telling us the same story, so they don't have
• 50:42
  Speaker 2: a problem.
• 50:42
  Speaker 1: They just want to cover up these blemishes with more makeup.
• 50:45
  Speaker 2: That's all that's right, just said lay on that foundation. Well,
• 50:48
  Speaker 2: you just kind of confused me because it seems like
• 50:51
  Speaker 2: there's different things giving us different stories. So you're saying
• 50:55
  Speaker 2: that one story is being told by this idea of
• 50:57
  Speaker 2: measuring objects out there how far they are, and then
• 51:01
  Speaker 2: measuring their velocity using the red shifting of their light.
• 51:04
  Speaker 2: It's one way, but you're saying there's sort of a
• 51:06
  Speaker 2: second general class of methods to measure the expansion of
• 51:09
  Speaker 2: the universe that uses the cosmic microwave background.
• 51:12
  Speaker 1: Yeah, the cosmic microwave background is light left over from
• 51:16
  Speaker 1: very early on in the universe, right when the universe
• 51:19
  Speaker 1: cooled down, so that photons that had been emitted by
• 51:21
  Speaker 1: the hot plasma all of a sudden saw the universe
• 51:24
  Speaker 1: as transparent. So that light is still flying around today
• 51:28
  Speaker 1: and we can measure it, and it tells us something
• 51:30
  Speaker 1: about what was going on early on in the universe,
• 51:32
  Speaker 1: including the expansion. It has encluded in the wiggles of
• 51:35
  Speaker 1: those photons in the hot spots and in the cold spots,
• 51:38
  Speaker 1: how the universe was expanding back then. It's very useful
• 51:41
  Speaker 1: because it captures like a really wide swath of the universe,
• 51:45
  Speaker 1: which since then has expanded very broadly. So it sort
• 51:48
  Speaker 1: of like looking at a baby picture of the universe,
• 51:50
  Speaker 1: and we can measure from the wrinkles on it how
• 51:52
  Speaker 1: much it was expanding back then, and we get a
• 51:54
  Speaker 1: different number, and so we don't understand why the early
• 51:58
  Speaker 1: universe measurements like the cosmic mic or background radiation tells
• 52:01
  Speaker 1: a different story about the expansion than the late measurements
• 52:05
  Speaker 1: like the ones we've been talking about with all these
• 52:07
  Speaker 1: different distance ladders.
• 52:08
  Speaker 2: Wait, the cosmic microwave background radiation tell us is how
• 52:11
  Speaker 2: the universe is expanding when it was little, when it
• 52:13
  Speaker 2: was a baby, not how it's expanding today.
• 52:15
  Speaker 1: That's right exactly. But we can extrapolate and we say,
• 52:18
  Speaker 1: if it was expanding at that rate back then, what
• 52:21
  Speaker 1: should we be measuring today with tapewone supernova and cephids
• 52:24
  Speaker 1: and red giants and all that stuff. And those two
• 52:27
  Speaker 1: things don't agree.
• 52:28
  Speaker 2: Well, how do you extravolate?
• 52:29
  Speaker 1: Yeah, you extrapolate using your model of how the universe expands.
• 52:33
  Speaker 1: And maybe that model is wrong. That's what I mean
• 52:36
  Speaker 1: by we need to tell a different story. We have
• 52:38
  Speaker 1: a model for how the universe should expand using various
• 52:41
  Speaker 1: components matter, radiation, dark matter, dark energy, et cetera.
• 52:46
  Speaker 2: You mean they're just guessing.
• 52:48
  Speaker 1: Well, it's a pretty simple model, but it's been working
• 52:50
  Speaker 1: really really well so far, and this is the first
• 52:53
  Speaker 1: sign of strains. It's really been showing. So maybe that
• 52:56
  Speaker 1: model that compares what happened early on and what we
• 52:58
  Speaker 1: should see today is wrong, or maybe one of these
• 53:01
  Speaker 1: measurements is wrong. We're just not sure.
• 53:03
  Speaker 2: All right, Well, so then what are the two stories
• 53:05
  Speaker 2: that we're getting. You're saying that there are conflicting stories
• 53:07
  Speaker 2: between all these measurements.
• 53:09
  Speaker 1: Yes, so the late measurements the ones from type one
• 53:11
  Speaker 1: A supernova. They measure a hubble constant of like seventy
• 53:14
  Speaker 1: three kilometers per megaparsek per second, whereas the early measurements
• 53:18
  Speaker 1: from like the cosmic microwave background radiation and other measurements
• 53:21
  Speaker 1: from the early universe that agree measure like sixty seven
• 53:25
  Speaker 1: kilometers per second per megaparsek.
• 53:27
  Speaker 2: What are these numbers and units mean? That means that
• 53:29
  Speaker 2: for every megaparsec, that's like a measurement of the size
• 53:35
  Speaker 2: of space, the universe is expanding seventy three kilometers each second.
• 53:39
  Speaker 2: Is that? What that means? That's velocity. That's not acceleration,
• 53:42
  Speaker 2: is it.
• 53:43
  Speaker 1: It's not exactly a velocity. It's velocity per size. Right,
• 53:46
  Speaker 1: kilometers per second is velocity. This is kilometers per second
• 53:49
  Speaker 1: per megaparsec. And so it's a measurement of the expansion
• 53:54
  Speaker 1: rate of the universe every second, every megaparsec grows by
• 53:58
  Speaker 1: seventy kilometers. But a megaparsec is really really long.
• 54:02
  Speaker 2: Oh, I see. You're assuming that locally space is expanding
• 54:05
  Speaker 2: at a constant rate like seventy three kilometers per second
• 54:09
  Speaker 2: for megaparsek. But overall, because the whole universe does happening everywhere.
• 54:13
  Speaker 2: Are you saying that this expansion is accelerating because you're
• 54:17
  Speaker 2: kind of like aggregating all of these local measurements, but
• 54:20
  Speaker 2: locally it's a constant, or you think it's a constant.
• 54:22
  Speaker 1: We think it's a constant in space. We think everywhere
• 54:25
  Speaker 1: in the universe has the same expansion rate. We don't
• 54:27
  Speaker 1: think it's a constant in time. We think it varies
• 54:30
  Speaker 1: in time because it depends on the density of stuff
• 54:33
  Speaker 1: in the universe, Like how much stuff is in the
• 54:36
  Speaker 1: universe affects how the universe is expanding, So as the
• 54:39
  Speaker 1: universe gets less dense, this number decreases, but it is
• 54:43
  Speaker 1: a number that we can measure, And seventy kilometers per
• 54:46
  Speaker 1: second sounds a lot, but a megaparsek is three million
• 54:50
  Speaker 1: light years. So every second, a chunk of space that's
• 54:53
  Speaker 1: three million light years long gets bigger by seventy kilometers,
• 54:58
  Speaker 1: which is like a tiny, tiny, tiny fraction of a megaparsec.
• 55:02
  Speaker 1: But over very very long distances, it does add up.
• 55:05
  Speaker 2: Because there are a lot of megaparsecs in the universe.
• 55:08
  Speaker 1: Oh yeah, we got lots of megaparsecs.
• 55:11
  Speaker 2: Okay, So what does that tell us that all these
• 55:13
  Speaker 2: measurements are disagreeing. Does it mean that things have been
• 55:17
  Speaker 2: changing with time or it just means that there's too
• 55:19
  Speaker 2: much uncertainty in our measurements.
• 55:21
  Speaker 1: It means that maybe our measurements are wrong. But people
• 55:24
  Speaker 1: have been refining these measurements over time and they've been
• 55:26
  Speaker 1: getting better and better. And now we have like alternate
• 55:28
  Speaker 1: ways to make some of these measurements which agree with
• 55:31
  Speaker 1: each other. And so the story's getting more and more precise,
• 55:34
  Speaker 1: but the disagreement is not going away. Sometimes you get
• 55:37
  Speaker 1: a bunch of measurements and they're all kind of sloppy
• 55:39
  Speaker 1: and they don't really agree with each other, and then
• 55:41
  Speaker 1: people make the measurements more precise and they sort of
• 55:43
  Speaker 1: like come into line. That's not what's happening here. As
• 55:46
  Speaker 1: we resolve these things more finely, the disagreement seems to
• 55:49
  Speaker 1: be growing, which means something basically we've misunderstood, Like maybe
• 55:54
  Speaker 1: there's some reason we're making a mistake in these measurements.
• 55:57
  Speaker 1: That seems unlikely as we get more more like very
• 56:00
  Speaker 1: different ways to make the measurements, or there's something wrong
• 56:04
  Speaker 1: about this story about the universe expanding, and maybe it
• 56:07
  Speaker 1: expanded faster early on than it is now because something
• 56:10
  Speaker 1: else happened. We had an episode about early dark energy,
• 56:13
  Speaker 1: which might explain it. Or as you said very early
• 56:15
  Speaker 1: on in the podcast, maybe we're extrapolating from our bubble.
• 56:19
  Speaker 1: Maybe our part of the universe is expanding more slowly
• 56:21
  Speaker 1: than everything else because it's less dense than the rest
• 56:24
  Speaker 1: of the universe. So something has to change in our
• 56:26
  Speaker 1: story of the universe to make sense of these measurements.
• 56:29
  Speaker 2: Interesting, Well, I guess the answer then is it kind
• 56:32
  Speaker 2: of stay tuned right. We're we're fining our measurements of
• 56:35
  Speaker 2: the universe, and with that we are getting a better
• 56:38
  Speaker 2: picture of how the universe is expanding, which might tell
• 56:41
  Speaker 2: us how the universe might end eventually exactly.
• 56:44
  Speaker 1: As we keep building better and better facilities, we develop
• 56:47
  Speaker 1: more techniques for measuring this expansion. We come up with
• 56:50
  Speaker 1: clever ways to see things happening in other galaxies that
• 56:52
  Speaker 1: we can calibrate and so we can measure the distance
• 56:55
  Speaker 1: to them, and so our picture of what's happening out
• 56:57
  Speaker 1: there in the universe gets more precise. As things get clear,
• 57:01
  Speaker 1: more mysteries always emerge.
• 57:03
  Speaker 2: Yeah, because it's very important. We really want to know,
• 57:05
  Speaker 2: is our universe growing faster or slower than our brother
• 57:10
  Speaker 2: or sister universe. The sibling race is on.
• 57:13
  Speaker 1: That's right. We want our common great great great great
• 57:15
  Speaker 1: great great great grandkids to have a leg up over
• 57:18
  Speaker 1: the ones in our sibling universe.
• 57:20
  Speaker 2: All right, well, we hope you enjoyed that. Thanks for
• 57:22
  Speaker 2: joining us, See you next time.
• 57:32
  Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain
• 57:35
  Speaker 1: the Universe is a production of iHeartRadio. For more podcasts
• 57:39
  Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
• 57:43
  Speaker 1: you listen to your favorite shows.