Latest / Daniel and Kelly’s Extraordinary Universe / How can the Universe expand faster than the speed of light?

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How can the Universe expand faster than the speed of light? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, Daniel, what do you think are the goals of
0:10
Speaker 1: this podcast? You know, the same as they ever were,
0:13
Speaker 1: blowing everyone's minds by revealing the incredible universe we live in.
0:18
Speaker 1: But yeah, we've been doing this for a while. You
0:19
Speaker 1: don't think people's minds are already blown? Can you get
0:22
Speaker 1: your mind blown only once? Well, I mean, if you
0:24
Speaker 1: really blow someone's mind, it's it's kind of a once
0:26
Speaker 1: in a lifetime experience, right, or lifetime ending experience. While
0:30
Speaker 1: not literally hoping to explode people's heads. Here, Oh, I
0:33
Speaker 1: see there is another misleading physics metaphor. Yes, today, let's
0:38
Speaker 1: blow up the phrase blow your mind. Well, let's just
0:41
Speaker 1: hope we don't blow up the podcast. I actually do
0:44
Speaker 1: hope the podcast blows up. Lets you mean blow up
0:47
Speaker 1: like inflating? Then it just expands right exponentially? Did I
0:50
Speaker 1: just blow up the blowing up? Hi? Am more handed
1:08
Speaker 1: cartoonists and the co author of the book frequently asked
1:11
Speaker 1: questions about the universe. Hi, I'm Daniel. I'm a particle
1:14
Speaker 1: physicist and a professor at U C Irvine, and I'm
1:17
Speaker 1: the co author of the book We Have No Idea,
1:20
Speaker 1: A Guide to the Unknown Universe. Oh Man we're both
1:22
Speaker 1: co authors on different books. We are exactly our fourth
1:26
Speaker 1: friend by the two of us. By the way, let's
1:29
Speaker 1: not mislead people here with our authored metaphors. You just
1:33
Speaker 1: blew everybody's minds. Welcome to our podcast, Daniel and Jorge
1:36
Speaker 1: Explain the Universe, a production of My Heart Radio in
1:39
Speaker 1: which we hope to blow your mind by showing you
1:42
Speaker 1: the incredible wonders of our universe, by diving deep into
1:46
Speaker 1: the biggest questions about how the universe works, what is
1:49
Speaker 1: at the edge of it, why can we see as
1:51
Speaker 1: far as we can, and how is it possible to
1:54
Speaker 1: see things that are racing away from us. We don't
1:57
Speaker 1: shy away from asking the hardest, trickiest, deepest questions about
2:00
Speaker 1: the nature of the universe because we are desperate to understand.
2:04
Speaker 1: We think that our tiny little brains can somehow metaphorically
2:08
Speaker 1: expand to understand the entire cosmos that is visible to us,
2:12
Speaker 1: and we want to share that experience with you. That's right,
2:15
Speaker 1: because it is a pretty amazing universe. It's a pretty
2:17
Speaker 1: big universe full of amazing and incredible things to discover
2:21
Speaker 1: and to puzzle about. And the extra awesome thing is
2:24
Speaker 1: that it's actually getting bigger. The universe is expanding, and
2:27
Speaker 1: it's not only expanding, it's expanding faster every day. That's right.
2:31
Speaker 1: If you love the universe, you should be happy because
2:34
Speaker 1: you're getting more of it every year for free. You
2:36
Speaker 1: don't even have to pay extra. It just does it automatically.
2:40
Speaker 1: It's like built in inflation, right, I mean physically and monetarily,
2:45
Speaker 1: that's true. But the same amount of dollars buys more
2:47
Speaker 1: universe every year. Oh? Really? Is the universe for sale?
2:52
Speaker 1: Is it listed somewhere? You know, real estates the best investment?
2:55
Speaker 1: And who's selling it? I can sell you andometer right
2:58
Speaker 1: here today on the podcast aust If you cut me
3:00
Speaker 1: a check, all right, I'll trade you for Alpha centri
3:03
Speaker 1: done and done. Mabel Birth the co owners of both things.
3:10
Speaker 1: That would be confusing, and humanity has long been confused
3:13
Speaker 1: about how the universe works. We stare up into the
3:16
Speaker 1: night sky. We try to understand what are those brilliant
3:19
Speaker 1: little dots of light moving across our vision. We try
3:22
Speaker 1: to build a model that can explain everything that is
3:24
Speaker 1: happening out there. And along the way we learned some
3:27
Speaker 1: pretty interesting things about how the universe works. But what
3:30
Speaker 1: the rules are for things moving really really fast or
3:33
Speaker 1: things near very heavy objects like black holes. Yeah, because
3:37
Speaker 1: through most of human history we sort of like the
3:39
Speaker 1: diners and thought, hey, this is a pretty big place.
3:41
Speaker 1: But it wasn't until recently in which humans realize that
3:45
Speaker 1: the place is actually getting bigger and getting bigger by
3:48
Speaker 1: the day, faster and faster. And one thing we encourage
3:50
Speaker 1: you to do on this podcast all the time is
3:52
Speaker 1: to apply your understanding too new questions of physics. And
3:56
Speaker 1: it's one thing to hear us blah blah blah about
3:58
Speaker 1: some situation and ex in it to you, But if
4:01
Speaker 1: you want to really understand a rule of physics, you
4:04
Speaker 1: have to apply it in some new situation to see
4:06
Speaker 1: if it really clicks together for you. Are you saying
4:09
Speaker 1: people should jump into a black hole just for just
4:12
Speaker 1: for their own curiosity? Yes, please do jump in a
4:15
Speaker 1: black hole and then send me an email from within
4:17
Speaker 1: the black hole to tell me what you see. That's impossible,
4:20
Speaker 1: isn't it. Well figure out a way, right, Bring an
4:22
Speaker 1: engineer with you and figure it out, and a physicist too,
4:26
Speaker 1: so then they can be co authors on your book.
4:28
Speaker 1: That's right. You can write multiple books and I'll be
4:31
Speaker 1: co authors on all of the books, or you can
4:33
Speaker 1: just stay at home and do thought experiments. A great
4:36
Speaker 1: way to test your understanding of our ideas of physics
4:39
Speaker 1: is to bang them against each other in extreme situations
4:42
Speaker 1: to see you, do I really understand what's happening at
4:44
Speaker 1: the edge of the universe or near the edge of
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Speaker 1: a black hole? Or if I fire a laser beam
4:49
Speaker 1: into a black hole, why does it not get hot?
4:51
Speaker 1: All these kind of questions are great ways to test
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Speaker 1: the boundaries of your own understanding. Yeah, that's right. You
4:57
Speaker 1: can expand your own mind just sitting there and thinking
4:59
Speaker 1: about the things, and also by listening to this podcast
5:02
Speaker 1: and a bunch of folks right in because they are
5:04
Speaker 1: confused about two ideas they hear us talk about. One
5:07
Speaker 1: is that the universe is really strange in a particular
5:10
Speaker 1: way that nothing in the universe can travel faster than
5:13
Speaker 1: the speed of light. There's this really weird limit on
5:16
Speaker 1: how fast things can travel. And the other is that
5:19
Speaker 1: the universe is expanding faster and faster, and the further
5:22
Speaker 1: you go out, the faster that expansion seems. So things
5:25
Speaker 1: that are super far away seem to be expanding faster
5:28
Speaker 1: than the speed of light. A lot of folks want
5:30
Speaker 1: to understand these two ideas in their head at the
5:33
Speaker 1: same time. So today on the podcast, we'll be asking
5:36
Speaker 1: the question, how can the universe expand faster than the
5:44
Speaker 1: speed of light? Pretty fascinating that the universe, as you say,
5:48
Speaker 1: is expanding faster than the speed of light. I mean
5:50
Speaker 1: we when we only figured this out kind of recently, right,
5:53
Speaker 1: that's right. We've known the universe has been expanding for
5:56
Speaker 1: about a hundred years, but it's been only twenty five
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Speaker 1: years since we discovered that that expansion is accelerating. Right.
6:03
Speaker 1: We learned that it's accelerating, and that it's excelerating faster
6:05
Speaker 1: and faster every year by looking at how fast the
6:08
Speaker 1: galaxies are moving. Right, Yes, we found type one a
6:11
Speaker 1: supernova which let us measure the distance to super duper
6:14
Speaker 1: far away objects and look through the past and discovered
6:18
Speaker 1: that the expansion the universe is larger than it once was,
6:21
Speaker 1: and it's larger every year. So it's accelerating the expansion
6:25
Speaker 1: of the universe. But even without that accelerating expansion, there's
6:28
Speaker 1: something really fascinating about the expansion of the universe, which
6:31
Speaker 1: is that the velocity of far away things correlates with
6:34
Speaker 1: their distance, So it means that the further away in
6:37
Speaker 1: space you look, the faster things are moving away from us,
6:40
Speaker 1: which means if you look far enough, you look, if
6:42
Speaker 1: you look deep enough into the universe, you can find
6:45
Speaker 1: some things that are moving away from us faster than
6:47
Speaker 1: the speed of light. It seems to be an apparent
6:49
Speaker 1: contradiction with Einstein's relativity that says nothing can be moving
6:53
Speaker 1: away from most faster than the speed of light. Right,
6:55
Speaker 1: but do we know that for sure? Because it kind
6:57
Speaker 1: of depends on how far out into the universe we
6:59
Speaker 1: can look. Right, Well, we definitely can't see everything out
7:02
Speaker 1: there in the universe. Is also a horizon beyond which
7:05
Speaker 1: we just cannot see, but things within the horizon are
7:09
Speaker 1: moving away from us faster than the speed of light. Although,
7:12
Speaker 1: as I'm sure you won't appreciate, there are some asterisks
7:16
Speaker 1: and caveats about exactly what it means for things far
7:19
Speaker 1: away from us to have very higher velocities. SHO make
7:22
Speaker 1: that the title of the podcast Asterisks and Caveats. I mean,
7:26
Speaker 1: who doesn't love this? True crime and comedy and also
7:30
Speaker 1: asterixtent caveats come here. Physicists form out of contradictions by
7:34
Speaker 1: invoking cosmic loopholes. Right, But I think what you mean
7:37
Speaker 1: is that, you know, the universe is expanding, and it's
7:39
Speaker 1: a bit, but it's expanding like everywhere, like between me
7:42
Speaker 1: and you, even here in southern California. The space between
7:45
Speaker 1: us is expanding, right Like the space itself, not the
7:48
Speaker 1: stuff in it is is expanding between you and me,
7:50
Speaker 1: but just a little tiny, little bit. But if you
7:52
Speaker 1: sort of added up over long distances, that expansion gets
7:55
Speaker 1: bigger and bigger. Like between here and Jupiter, the expansion
7:58
Speaker 1: is much bigger than between here and Orange County, and
8:01
Speaker 1: between here and the next Galucia, it's even bigger and bigger.
8:04
Speaker 1: And so if you go out far enough, that expansion
8:06
Speaker 1: is huge. It's expanding, you know, thousands or millions of
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Speaker 1: kilometers per second exactly. And there's two different ways to
8:12
Speaker 1: look at that. You can look at it as the
8:14
Speaker 1: expansion of space, or you can look at it as
8:16
Speaker 1: things moving through space. There are two sort of alternative
8:20
Speaker 1: ways to view the same situation. But we'll dig into
8:23
Speaker 1: that in a minute, all right. So then the question
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Speaker 1: is here is sort of like if the universe, parts
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Speaker 1: of the universe are expanding away from us faster than
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Speaker 1: the speed of light. How can that be right? Because
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Speaker 1: one of the rules of the universe is that you
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Speaker 1: can move faster than the speed of light. Exactly. That's
8:37
Speaker 1: the puzzle, and it requires graduating our understanding of the
8:41
Speaker 1: cosmic expansion from special relativity to general relativity. Sounds special,
8:46
Speaker 1: and so, as usual, we were wondering how many people
8:48
Speaker 1: out there had thought about this question or wondered how
8:51
Speaker 1: the universe can expand faster than the speed of light.
8:54
Speaker 1: So thank you very much to everybody who is willing
8:56
Speaker 1: to answer these questions for the podcast. If this sounds
8:59
Speaker 1: like fun to you, don't be sid Please write to
9:02
Speaker 1: me two questions at Daniel and Jorge dot com and
9:05
Speaker 1: I'll set you up to answer these questions for future episodes.
9:08
Speaker 1: I think about it for a second. How do you
9:10
Speaker 1: think the universe can expand faster than the speed of light.
9:13
Speaker 1: Here's what people had to say. I have a heard
9:15
Speaker 1: it said that because its expansion of space instead of
9:19
Speaker 1: expansion in space, the universe can expand faster than the
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Speaker 1: speed of light. Nevertheless, that seems to me to imply
9:26
Speaker 1: that things are moving at the hypervelocities that should be impossible.
9:32
Speaker 1: I've heard it, but I don't understand it. You explained
9:35
Speaker 1: on a previous episode how the universe can expand faster
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Speaker 1: than the speed of light. It's basically because the speed
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Speaker 1: of light in a vacuum is the speed limit for
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Speaker 1: anything to move through space. However, with the expansion of
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Speaker 1: the universe, nothing is moving through space, new spaces being
9:55
Speaker 1: created between things. The universe expanding fashion light really makes
10:00
Speaker 1: the eyes roll back in my head. So I'm thinking
10:03
Speaker 1: it is light bread baking in the oven, and everything
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Speaker 1: expands together, and that expansion happens faster than the speed
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Speaker 1: of light, which is impossible, And now my eyes are
10:14
Speaker 1: rolled back in my head. Now, I have heard this
10:16
Speaker 1: question before and I've never really understood it. And I
10:18
Speaker 1: understand the universe is expanding fast on the light, and
10:20
Speaker 1: I think it's got something to do with inflation and
10:23
Speaker 1: the fact that the overall space is expanding similar to
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Speaker 1: how a balloon expands, and that it's a different scale
10:30
Speaker 1: or a different measuring technique to that of the speed
10:33
Speaker 1: of light. Speed of lights finite within the universe, but
10:36
Speaker 1: the universe is able to expand into other stuff. Perhaps
10:40
Speaker 1: it's not the universe that is expanding faster than the
10:43
Speaker 1: speed of light, but it's the empty spaces in between
10:47
Speaker 1: the particles that is perhaps not limited by the universal
10:52
Speaker 1: speed limit. Just a thought. I believe that the universe
10:57
Speaker 1: can expand faster than light because the speed limit of
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Speaker 1: the universe, which is the speed of light, only applies
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Speaker 1: to something that is traveling through space, and that speed
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Speaker 1: limit does not apply to space itself when it expands,
11:11
Speaker 1: because it's not expanding in space, it is space. All right,
11:15
Speaker 1: some interesting answers here. Some people are relating it to baking.
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Speaker 1: It's a great analogy raising than a cosmic bread. But
11:23
Speaker 1: the major point people seem to be making is that
11:25
Speaker 1: there's no limit in the speed of expansion, even if
11:28
Speaker 1: there's limit of motion through space. Yeah, that's kind of
11:31
Speaker 1: the big caveat in this statement, right, like the rules
11:34
Speaker 1: that you can move through space faster than the speed
11:36
Speaker 1: of light, but there's no limit to what you can
11:39
Speaker 1: do with that space. You can squish it, you can
11:41
Speaker 1: expand it technically faster than the speed of light. Yeah,
11:43
Speaker 1: And that's a nice shorthand for thinking about expansion, But
11:46
Speaker 1: there are some issues with that if you want to
11:47
Speaker 1: be precise about it, because what does it mean to
11:50
Speaker 1: be moving through space? Space itself doesn't have a frame.
11:53
Speaker 1: You can't measure your velocity with respect to space. You
11:56
Speaker 1: can only measure your velocity with respect to other things
12:00
Speaker 1: in space. And so special relativity would seem to be
12:02
Speaker 1: in contradiction with that anyway, because it says, how can
12:05
Speaker 1: somebody be measuring a distant galaxy moving away from us
12:09
Speaker 1: at two times the speed of light. We're not measuring
12:11
Speaker 1: its velocity through space, We're measuring its velocity relative to us.
12:16
Speaker 1: Sounds confusing. Going back to thing he said earlier, he said,
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Speaker 1: there's no limit to how fast this space can expand?
12:22
Speaker 1: Is that really true? Like can I expand space from
12:24
Speaker 1: zero to infinity in no time? Or even the opposite,
12:27
Speaker 1: like can I take some space and collapse it in
12:30
Speaker 1: an instant in zero time? There's no limit to the
12:33
Speaker 1: rate of which space can expand, but it does need
12:35
Speaker 1: to be continuous, right, so you can put in any number,
12:39
Speaker 1: And in fact, we've seen very dramatic expansions of space
12:42
Speaker 1: in our history of the universe. Right. Inflation is nothing
12:44
Speaker 1: more than expansion of the universe. And that was a
12:47
Speaker 1: factor of like ten to the thirty in ten to
12:49
Speaker 1: the minus thirty seconds. So as far as we understand,
12:52
Speaker 1: there are no bounds there except that it does need
12:55
Speaker 1: to be continuous. You can't have an instantaneous transformation of space,
13:00
Speaker 1: so you can have any number except for infinity essentially interesting,
13:04
Speaker 1: so literally almost instantly instantaneously, then you can collapse and
13:08
Speaker 1: expand space. Because that's kind of what happens in the
13:10
Speaker 1: Big Bang. We don't understand it, but general relativity can
13:13
Speaker 1: describe it, and we've seen it happening in the universe.
13:16
Speaker 1: We don't know what can cause it, Like we know
13:18
Speaker 1: inflation happened and it stretched the universe dramatically. We know
13:21
Speaker 1: dark energy is accelerating the expansion of the universe. We
13:24
Speaker 1: don't understand the mechanism for that, like what makes that
13:27
Speaker 1: happen and what the limits are to that mechanism. But
13:30
Speaker 1: in principle, the framework of general relativity does allow for
13:33
Speaker 1: very very dramatic expansions. Yes, all right, well let's dive
13:35
Speaker 1: into it, and let's maybe start talking first about the
13:39
Speaker 1: just the general expansion of the universe. Why do we
13:41
Speaker 1: know about that and how do we find out? So
13:43
Speaker 1: our understanding is that space is stretching everywhere. It's expanding.
13:48
Speaker 1: It's getting bigger. Between me and you, space is getting larger.
13:52
Speaker 1: Between us and other galaxies, space is getting larger. This
13:56
Speaker 1: is an expansion that's happening everywhere at the same time,
14:00
Speaker 1: not an explosion from a tiny dots some people might
14:03
Speaker 1: imagine the Big Bang started. Take a very dense universe,
14:06
Speaker 1: stretch it out to a less dense universe, a colder,
14:10
Speaker 1: more dilute universe. Right, But when you say that space
14:13
Speaker 1: is stretching everywhere or getting bigger everywhere, you don't mean
14:16
Speaker 1: like space itself is getting bigger. Like you actually mean
14:20
Speaker 1: like there's more space being created out of nothing, right,
14:23
Speaker 1: Because it's not like a kilometer is suddenly more or
14:26
Speaker 1: like what we call a kilometer is getting bigger. It's
14:29
Speaker 1: like there's just more space growing all the time out
14:31
Speaker 1: of nothingness. Right. I think both of those descriptions are accurate.
14:34
Speaker 1: I'm not sure exactly what the distinction is between them. Like,
14:38
Speaker 1: the universe is expanding at a rate we call the
14:40
Speaker 1: Hubble parameter. It's seventy kilometers per second er mega parsak.
14:45
Speaker 1: What that means is that every second a mega parsak,
14:48
Speaker 1: which is a unit of distance and astronomical unit becomes
14:51
Speaker 1: longer by about seventy kilometers. Right, So every second a
14:55
Speaker 1: mega parsak does get longer, is it getting stretched or
14:58
Speaker 1: is the universe creating new space in the middle? Doesn't
15:01
Speaker 1: really matter either way. We just describe it as an expansion.
15:04
Speaker 1: We say the universe is scaling up. Well, I guess
15:06
Speaker 1: what I mean is that, like between here and Jupiter, right,
15:10
Speaker 1: space is expanding, it's getting bigger. But between here and Jupiter,
15:14
Speaker 1: like how long it takes light to go there and back?
15:16
Speaker 1: It's not getting bigger every year, right, Like, that's pretty
15:19
Speaker 1: much stay in the same. So if you take two
15:21
Speaker 1: mirrors and you put them a megaparsec away from each other,
15:23
Speaker 1: what's a megaparsic? So a mega parsk is about three
15:27
Speaker 1: million light years. It's a really really big distance. So
15:30
Speaker 1: it's how far light travels in about three million years.
15:33
Speaker 1: So set up two mirrors three million light years apart,
15:37
Speaker 1: one megaparsec apart from each other, it would take light
15:39
Speaker 1: a certain time to go there and back. Right now,
15:42
Speaker 1: wait for the universe to expand, those mirrors will be
15:44
Speaker 1: further apart from each other, and so we will take
15:47
Speaker 1: light longer to get there and back. The space really
15:51
Speaker 1: is expanding, either by creating new space or by stretching
15:54
Speaker 1: that space relative to this standard of the speed of
15:57
Speaker 1: light between here and Jupiter. Is something else going on.
16:01
Speaker 1: That's because the Sun is holding Jupiter in place. In
16:03
Speaker 1: the case the two mirrors, there isn't any gravity to
16:06
Speaker 1: keep the two mirrors at a certain distance. But the
16:08
Speaker 1: distance between us and Jupiter isn't expanding because the Sun
16:11
Speaker 1: is holding Jupiter in place, the same way that the
16:13
Speaker 1: Earth is holding you in place, even though the space
16:16
Speaker 1: between you and the Earth is expanding. Right. I guess
16:18
Speaker 1: maybe a more concrete example would be like, let's say
16:21
Speaker 1: I stretched the cable, a steel cable that doesn't stretch
16:24
Speaker 1: between here and Jupiter. Right. I measure that the cable
16:27
Speaker 1: to be a certain length, and as the space expands
16:29
Speaker 1: between here and Jupiter, the cable is not stretching, right,
16:32
Speaker 1: It's staying the same the length, right, Or the spa
16:36
Speaker 1: the distance between its molecules is staying in the same.
16:38
Speaker 1: So does that mean that the space that it's in
16:41
Speaker 1: got thinner somehow or stretched, or they're just like more
16:45
Speaker 1: space grew in it. That's right. The length of the
16:47
Speaker 1: ruler is determined by the bonds of the stuff inside
16:50
Speaker 1: the ruler, the atoms holding themselves together with those electrons,
16:55
Speaker 1: so that doesn't change. You put that ruler out into
16:58
Speaker 1: space and it's just floating there. The space around it
17:00
Speaker 1: will expand you can still use that to define a distance.
17:03
Speaker 1: You can say my ruler is one megaparsec long. But
17:06
Speaker 1: if you started out with mirrors at either end of
17:08
Speaker 1: your ruler, and then you waited a second, and the
17:10
Speaker 1: mirrors would no longer be at either end of the
17:13
Speaker 1: rulers because there's nothing holding those mirrors together. So space
17:16
Speaker 1: is expanding between the mirrors, pushing them apart. So you
17:20
Speaker 1: have this ruler which no longer covers the distance between
17:23
Speaker 1: your two mirrors. Al right. Maybe an easier way to
17:25
Speaker 1: ask my question is like, if space doubles in size
17:28
Speaker 1: as we say it's doing, does that mean there's twice
17:31
Speaker 1: as much space as there was before or is there
17:34
Speaker 1: the same amount of space? It's just like stretched out dinner.
17:37
Speaker 1: I say space doesn't get thinner, right, there's no like
17:39
Speaker 1: density to space I would require measuring like with respect
17:43
Speaker 1: to something else. This is an intrinsic expansion, which means
17:46
Speaker 1: it just changes the relative distance. There's no meaning for
17:49
Speaker 1: the density of space. So the first answer is the
17:52
Speaker 1: more accurate description. There's more space between those two points.
17:55
Speaker 1: Oh I see, So there's just more space popping up
17:57
Speaker 1: everywhere in the universe, even between like the space of
18:00
Speaker 1: my fingers. There's more space always popping up all the time.
18:04
Speaker 1: That's right, And the expansion is actually really quite weak.
18:07
Speaker 1: We're talking about seventy kilometers per second over three million
18:11
Speaker 1: light years, so between your fingers that's really really tiny,
18:14
Speaker 1: and almost anything is powerful enough to overcome it. Gravity,
18:17
Speaker 1: the bonds in your fingers. It's also basically nothing for
18:21
Speaker 1: our solar system, even for our galaxy, it's almost negligible.
18:25
Speaker 1: But as distances get really really large, then it becomes overwhelming,
18:29
Speaker 1: and between superclusters of galaxies it's the dominant thing, all right,
18:33
Speaker 1: So maybe let's bring it down for people. Are you
18:35
Speaker 1: saying over three hundred million kilometers over three million light years,
18:40
Speaker 1: over three million light years Every second that distance grows
18:44
Speaker 1: by seventy kilometers, So right now it's three million light years,
18:49
Speaker 1: and then and now it's three million and seventy kilometers long,
18:53
Speaker 1: and now it's three million and a hundred and forty kilometers.
18:56
Speaker 1: Is that what kind of what you mean, yes, exactly,
18:58
Speaker 1: So every second, each mega parts that grows fractionally by
19:01
Speaker 1: a tiny, tiny, tiny amount. It's one point zero zero
19:05
Speaker 1: zero zero zero zero zero zero zero zero zero zero
19:08
Speaker 1: two megaparsex after a second. That's really tiny, because the
19:13
Speaker 1: Milky Ways is not even a megaparsic, right, it's like
19:15
Speaker 1: a hundred thousand light year exactly. It's a tiny fraction
19:19
Speaker 1: of a megaparsec. Okay, but you're saying that over large
19:22
Speaker 1: distances like businesses between galaxies and the size of these superclusters,
19:27
Speaker 1: it starts to get pretty significant, right, It starts to
19:29
Speaker 1: get pretty significant. And since the universe is really really big,
19:33
Speaker 1: many many millions and billions of light years wide, and
19:36
Speaker 1: if you go far enough away, this is really significant.
19:38
Speaker 1: And the rate at which things are moving away from
19:41
Speaker 1: us because of this expansion starts to get very very large.
19:44
Speaker 1: So you can measure this velocity. We call it the
19:46
Speaker 1: recession velocity. How fast is something moving away from us
19:49
Speaker 1: because of this expansion and you go to the other
19:52
Speaker 1: side of the Milky Way, the recession velocity is tiny, right,
19:54
Speaker 1: if you go to the next galaxy, it's not tiny.
19:57
Speaker 1: If you go really, really far away then starts to
20:00
Speaker 1: be large, and if you go far enough away, it
20:03
Speaker 1: actually gets to be larger than the speed of light.
20:06
Speaker 1: So things are technically moving faster away from us than
20:10
Speaker 1: the speed of light, which is the big question in
20:13
Speaker 1: this episode. So let's get into that actually really means
20:17
Speaker 1: and how we can make sense of it. But first
20:19
Speaker 1: let's take a quick break. All right, we're talking megaparsex
20:35
Speaker 1: and inflation and recession, which just a reminder, this is
20:39
Speaker 1: not a financial podcast, not yet, at least not until
20:44
Speaker 1: it blows up, until it crashes. We are talking about
20:48
Speaker 1: bubbles though, right, Yeah, exactly. We are talking about spheres
20:52
Speaker 1: in the universe. And there's lots of ways that we
20:54
Speaker 1: like to think about spheres and put ourselves at the
20:56
Speaker 1: center of them, and we tend to cosmologically to think
21:00
Speaker 1: that's a mistake there. That's not the center of anything,
21:02
Speaker 1: but it is at the center of what we can observe,
21:05
Speaker 1: and so in some senses, it does make sense to
21:08
Speaker 1: build a universe with us right in the middle of it. Yeah,
21:11
Speaker 1: and there's also a lot of speculation, uh, in these theories.
21:14
Speaker 1: So let's get into this idea then, of things moving
21:17
Speaker 1: away from us faster than the speed of light. So
21:20
Speaker 1: you're saying that the universe is expanding a little bit
21:23
Speaker 1: at a time. You each spot everywhere in it, and
21:26
Speaker 1: so over long distances things are the distance between here
21:31
Speaker 1: and that thing that's out there is growing faster than
21:35
Speaker 1: the speed of light. Yeah, this is Hubble's basic discovery.
21:37
Speaker 1: He was looking at stuff that was further and further
21:40
Speaker 1: away and measuring its velocity, and he found this linear relationship.
21:45
Speaker 1: Things that are further away move away from us faster.
21:47
Speaker 1: If you plot velocity versus distance, you get a straight
21:50
Speaker 1: line and you can just keep going further and further
21:53
Speaker 1: distances faster and faster velocities away from us. So this
21:57
Speaker 1: was Hubble's big discovery, like more than years ago, now
22:00
Speaker 1: that things are moving away from us at in this
22:02
Speaker 1: linear relationship. And that's the Hubble parameter. We used to
22:06
Speaker 1: call it the Hubble constant, except it's not really constant,
22:08
Speaker 1: so now we call it the Hubble parameter. It's a
22:10
Speaker 1: relationship between the distance and the speed the slope of
22:14
Speaker 1: that line. If you go far enough away, those velocities
22:17
Speaker 1: start to get greater than the speed of light. And
22:20
Speaker 1: so that's the basic conundament is how can something be
22:23
Speaker 1: moving away from us faster than the speed of light,
22:26
Speaker 1: And I guess maybe the isn't the answer there in
22:28
Speaker 1: that world, like, it's not technically moving away from us, right,
22:31
Speaker 1: it's just sitting there, but there's more space growing between
22:34
Speaker 1: us and them? Is it technically moving when you can
22:36
Speaker 1: measure It all comes down to how you measure velocities. Right,
22:40
Speaker 1: If you take a really big ruler and you measure
22:42
Speaker 1: the distance from here to there, and then you do
22:44
Speaker 1: it again in ten seconds, you can measure velocity, and
22:48
Speaker 1: that velocity does appear to exceed the speed of light,
22:51
Speaker 1: and so from that perspective, that does break that rule. Well,
22:54
Speaker 1: I guess it's where it all gets relative, because I
22:56
Speaker 1: guess the question is is that thing out there in
22:59
Speaker 1: space actually moving within this space it's around? Like, gets
23:02
Speaker 1: it technically moving? Or like, let's say we have something
23:05
Speaker 1: that is maybe right next to me, and over trillions
23:08
Speaker 1: of years, the space between me and it expands, and
23:11
Speaker 1: so it gets further and further out, maybe to the
23:13
Speaker 1: point where distance is growing fast in the speed of light.
23:15
Speaker 1: But did that thing ever experience any acceleration? Right? So
23:18
Speaker 1: let's talk about what special relativity actually allows and what
23:22
Speaker 1: it doesn't allow. Right. Special relativity says that nobody can
23:26
Speaker 1: measure a relative velocity to be greater than the speed
23:30
Speaker 1: of light in their local inertial frame. Okay, and those
23:35
Speaker 1: are the loopholes. We're going to drive this huge expanding
23:38
Speaker 1: universe through local inertial frame. Right. What that means is
23:43
Speaker 1: that you define a frame of reference, you define or
23:45
Speaker 1: and you say, I'm here at the core of my X,
23:48
Speaker 1: y Z, and I'm the one making measurements, and I'm
23:50
Speaker 1: not accelerating with respect to this frame. I'm just sitting
23:53
Speaker 1: here at the origin and I'm making a bunch of measurements.
23:56
Speaker 1: The other key thing is the word local. Right. In
23:58
Speaker 1: special relativity, we can we make measurements of things that
24:01
Speaker 1: are nearby. We say that the speed of light never
24:04
Speaker 1: exceeds see when it goes near you. Things that are
24:07
Speaker 1: much further away are not in your local inertial frame,
24:10
Speaker 1: and so the rules of special relativity don't apply. That's
24:13
Speaker 1: when you need to apply general relativity for things that
24:15
Speaker 1: are not in your inertial frame, like when the universe
24:18
Speaker 1: is curved or expanding, or when things are really really
24:21
Speaker 1: far away. What are you saying that the rules are
24:23
Speaker 1: different depending on where you are in the universe, or
24:26
Speaker 1: that our theories don't always work the same way in
24:30
Speaker 1: different parts of the universe. The rules are the same
24:32
Speaker 1: everywhere in the universe. A special relativity only applies to
24:36
Speaker 1: the things you can see near you. It can't describe
24:39
Speaker 1: the whole universe because there is no global inertial frame
24:43
Speaker 1: for the universe. Like the universe has wiggles and curves
24:47
Speaker 1: and all sorts of stuff, and that breaks special relativity.
24:49
Speaker 1: You can't have a global inertial frame for the whole universe.
24:53
Speaker 1: You have to use general relativity, which gives you another
24:56
Speaker 1: way to look at the universe and make these measurements
24:58
Speaker 1: and fitted it all together. You're saying, that's the pull.
25:00
Speaker 1: What do you mean by that? So, one reason we
25:01
Speaker 1: can measure these galaxies to be moving away from us
25:04
Speaker 1: faster than the speed of light is that they are
25:06
Speaker 1: not in our inertial frame of reference. Right, It doesn't
25:09
Speaker 1: really make physical sense to measure their velocity relative to
25:14
Speaker 1: us because we're not in the same frame. It's like
25:16
Speaker 1: a frame over here near us, and there's a frame
25:18
Speaker 1: over there where they are. But we can't really compare
25:21
Speaker 1: velocities in hours to theirs general relativity tells us there's
25:25
Speaker 1: no global frame. We don't know how to make those comparisons.
25:28
Speaker 1: In fact, the definition of velocity and general relativity is
25:32
Speaker 1: a little bit fuzzy for things that are far away
25:34
Speaker 1: from you. Well, I feel like I'm getting a little
25:36
Speaker 1: bit confused here because of this difference between things moving
25:39
Speaker 1: and space expanding. Whether it's something in expanding space is moving,
25:43
Speaker 1: and I feel like maybe you're saying it's the same thing,
25:46
Speaker 1: but I guess it's not quite clicking in my mind.
25:48
Speaker 1: So maybe let's go back to that example of saying, like,
25:50
Speaker 1: if I start with something next to me, and over
25:53
Speaker 1: trillions of years, the space between me and it expands
25:56
Speaker 1: so that to the point where it's now that thing
25:58
Speaker 1: is super far away from me, and maybe even that
26:00
Speaker 1: the distance between me and it growing faster than the
26:02
Speaker 1: speed of light. Is that thing technically moving away from me?
26:06
Speaker 1: Or is it just sitting in space and the space
26:09
Speaker 1: is growing around it? You know what I mean? Like,
26:11
Speaker 1: did anything ever push that thing? Did that thing or
26:14
Speaker 1: me experience acceleration? And if not, then maybe may it's
26:17
Speaker 1: not moving. It's maybe it's just sitting there and space
26:19
Speaker 1: is growing around it. Here's the technically accurate and totally unsatisfying.
26:23
Speaker 1: Answer is that both pictures are accurate. Right in one picture,
26:28
Speaker 1: the thing is moving away from you, it has velocity
26:30
Speaker 1: relative to you, and the other picture, there is no
26:32
Speaker 1: relative velocities. There's just the expansion of space. Everything is
26:36
Speaker 1: just floating, motionless, and space between them is expanding. Those
26:39
Speaker 1: two pictures are actually equivalent. And what I meant a
26:42
Speaker 1: moment ago when I said that in general relativity we
26:44
Speaker 1: don't have a well defined meaning of velocity is exactly
26:47
Speaker 1: that that both of those pictures can describe what we see,
26:50
Speaker 1: even though physically they sound very different. So let's step
26:54
Speaker 1: through them one at a time. Right, In one picture,
26:56
Speaker 1: we talk about the distance between us and some other objects,
27:00
Speaker 1: and that means it like if we could freeze time
27:02
Speaker 1: and somehow take a huge ruler and measure the distance
27:06
Speaker 1: to this distant object, and we could write it down
27:09
Speaker 1: on our piece of paper, and then we could wait
27:11
Speaker 1: a minute or a year and do it again. We
27:13
Speaker 1: could use that to measure the velocities. Now, no, that's
27:16
Speaker 1: not really physical, right, because you have to freeze the
27:18
Speaker 1: universe and like trust out your ruler and take this measurement.
27:22
Speaker 1: That's not possible because things are expanding as you're doing it.
27:25
Speaker 1: So in some sense, even the definition of distance there
27:28
Speaker 1: doesn't really make sense. But that's sort of like our
27:30
Speaker 1: intuitive idea of distance. So from that perspective, this thing
27:34
Speaker 1: really is moving away from you faster and faster every year.
27:38
Speaker 1: That velocity you measure is larger every year, And I
27:41
Speaker 1: think what you're saying is that, like, maybe initially, if
27:43
Speaker 1: it's right next to me, I'm going to measure that
27:45
Speaker 1: it has zero velocity because it's just sitting next to me.
27:48
Speaker 1: But maybe next year, when it's further away from me,
27:50
Speaker 1: I'm going to start measuring, like, hey, now it's further
27:52
Speaker 1: away from me, the distance grew, and I can divide
27:55
Speaker 1: that by the time that passed and say, oh, now
27:57
Speaker 1: it has a velocity. So you're saying that that's true,
27:59
Speaker 1: even though nothing pushed it, even though it didn't feel
28:02
Speaker 1: any acceleration, it's velocity grew between last year and today,
28:06
Speaker 1: exactly because the universe is expanding, right, and so it's
28:10
Speaker 1: moving these things away from each other. It's increasing the
28:12
Speaker 1: relative velocity of these things, and you can measure that
28:15
Speaker 1: physically also in the red shift, right, things that have
28:18
Speaker 1: a relative velocity give you a red shift. The photons
28:20
Speaker 1: from these things are red shifted. It starts out a
28:23
Speaker 1: certain frequency. By the time it gets to you, it's redder,
28:26
Speaker 1: and we use that to infer the velocity of these
28:29
Speaker 1: things all the time. Right. So that's one picture, right,
28:31
Speaker 1: But the thing itself didn't never felt any exceleration, right, Like,
28:34
Speaker 1: if I was sitting in that rock that's moving away
28:36
Speaker 1: from me, I would never feel like anything happened, right, Like,
28:39
Speaker 1: I wouldn't feel pressed against the wall, or I wouldn't
28:42
Speaker 1: feel like somebody pushed me. I would just be sitting there,
28:44
Speaker 1: and my velocity the relative to me, it's just magically, magically,
28:48
Speaker 1: it's just growing. It's like free velocity. Yes, it's free velocity.
28:51
Speaker 1: The universe is just creating this. Right. And the other picture,
28:54
Speaker 1: which is a bit more physical, though a little harder
28:56
Speaker 1: to think about, is think about each of these objects
28:58
Speaker 1: in their own frame, whether each at rest, right, and
29:01
Speaker 1: now these frames have space expanding in between them. So
29:04
Speaker 1: from that point of view, right, everything was at rest.
29:07
Speaker 1: Everything is still at rest, but space is just sort
29:09
Speaker 1: of like expanded between them. There's no velocity, it's just
29:13
Speaker 1: that space is expanding. So this is another frame. It's
29:16
Speaker 1: like it requires you to have a coordinate system that's
29:18
Speaker 1: growing with the universe, like stretch the ruler as the
29:23
Speaker 1: universe grows. These are called co moving distances, if you
29:27
Speaker 1: want to google it and read more about it. And
29:29
Speaker 1: so in this picture there is no velocity. It's just
29:31
Speaker 1: that space itself is expanding. I think that's what I
29:34
Speaker 1: was asking earlier. It's like in this other picture, and
29:37
Speaker 1: now a kilometer is actually kind of longer, right, technically
29:41
Speaker 1: a kilometer is now longer. And you can wonder, like
29:44
Speaker 1: what about the red shift? The red shift is something
29:46
Speaker 1: I see. It shouldn't depend on like what a physicist
29:48
Speaker 1: is thinking about, whether they're thinking about two different frames
29:51
Speaker 1: with the one big frame. It shouldn't depend on that.
29:54
Speaker 1: And the red shift that these two pictures predict is
29:56
Speaker 1: the same. And the first one, the red shift comes
29:58
Speaker 1: from velocity in the set, second one, it comes from
30:01
Speaker 1: the expansion of the universe, which also lengthens the wavelength
30:04
Speaker 1: of the light. So in the second picture, where there
30:06
Speaker 1: are two frames and spaces expanding between them, then the
30:10
Speaker 1: photon that leaves one galaxy is stretched out by the
30:13
Speaker 1: expansion of the universe, not by the relative velocity because
30:16
Speaker 1: there is none in that second picture, and I think,
30:19
Speaker 1: how could both of these things be true? Well, these
30:20
Speaker 1: are two different pictures of the universe, and what general
30:23
Speaker 1: relativity tells us is that there's no unambiguous way to
30:26
Speaker 1: decide between these two pictures. That's why relative velocity for
30:30
Speaker 1: very distant objects is not well defined in general relativity. Well,
30:34
Speaker 1: I guess maybe the maybe the difference. I wonder if
30:36
Speaker 1: the difference between the two pictures is that in one
30:38
Speaker 1: of them, the speed of light is technically staying the same,
30:42
Speaker 1: but in the other one, I feel like the speed
30:44
Speaker 1: of light is kind of getting diluted, right a little bit,
30:46
Speaker 1: because if a kilometer grows, but the time it takes
30:50
Speaker 1: light to grow to that kilometer is no longer than
30:54
Speaker 1: or shorter than that, somehow the speed of light went down. Well,
30:56
Speaker 1: the speed of light is the standard, right, there's no
30:59
Speaker 1: other metric, So the time it takes light to go
31:02
Speaker 1: between the galaxies definitely increases. And they used to be
31:05
Speaker 1: a kilometer apart. Now we just say that there are
31:07
Speaker 1: five kilometers apart, and that's because it takes light longer.
31:10
Speaker 1: Light is the way that we measure these distances, after all,
31:13
Speaker 1: there's no other external metric, right, that we can use
31:16
Speaker 1: to measure the distance between things other than the time
31:19
Speaker 1: it takes light to go between them. I thought you
31:21
Speaker 1: said that in the second scenario, a kilometer is now longer.
31:24
Speaker 1: The kilometer space is now two kilometers of space. It's
31:27
Speaker 1: not like the kilometer itself is getting diluted in some way. Right,
31:30
Speaker 1: you have more space. The space and that's out there
31:33
Speaker 1: is now longer, so it takes longer for light to
31:35
Speaker 1: go through it. Right, So it would still be a kilometer,
31:38
Speaker 1: It just takes light longer to go through it, which
31:40
Speaker 1: means the speed of light technically went down. No, no,
31:43
Speaker 1: it's not still a kilometer, right, it's more space now.
31:46
Speaker 1: But I thought we said we stretched the kilometer like
31:48
Speaker 1: the kilometer stretched. Oh, we don't stretch the definition of
31:51
Speaker 1: a kilometer, right, that's comes from the speed of light.
31:54
Speaker 1: We just stretched the space which used to be one
31:56
Speaker 1: kilometer of space, and now it's a bigger serving of space.
32:00
Speaker 1: We don't change the definition of the kilometer. Space itself expands,
32:03
Speaker 1: so the kilometer stace is same now there's just more kilometers.
32:07
Speaker 1: We define the kilometer in the same way, right, it's
32:09
Speaker 1: how far light goes in a certain amount of time, right,
32:12
Speaker 1: so we don't change the definition of the kilometer. I
32:14
Speaker 1: guess we can go around the circles. But I think
32:16
Speaker 1: the main point is that the universe is expanding and
32:20
Speaker 1: the stuff that's really far aways moving faster than the
32:22
Speaker 1: speed of light. The universe really is expanding, and it
32:24
Speaker 1: really is creating space between us. And if you try
32:27
Speaker 1: to think about relative distances in terms of like one
32:30
Speaker 1: mega ruler where you're gonna measure these distances, then you're
32:33
Speaker 1: sort of falling into one of the loopholes of special relativity,
32:36
Speaker 1: which says that that's not a meaningful physical thing. You
32:38
Speaker 1: could never actually measure that distance. Like if you're looking
32:41
Speaker 1: at a galaxy that's billions and billions of light years away,
32:43
Speaker 1: you can never actually make the measurement we're talking about
32:45
Speaker 1: where you try out a ruler and measure the distance
32:48
Speaker 1: and then wait and measure the other one. Because space
32:50
Speaker 1: is expanding at the same time, it requires you to
32:52
Speaker 1: like artificially freeze the universe. So those things are not
32:55
Speaker 1: in our local inertial reference frame, so special relativity doesn't
32:59
Speaker 1: really apply. So it's possible for these recession velocities to
33:02
Speaker 1: be faster than the speed of light without breaking special
33:05
Speaker 1: relativity because it doesn't really apply to things that are
33:07
Speaker 1: not in our inertial frame, right. I think you've always
33:10
Speaker 1: phrased it as it's not possible to move through space
33:13
Speaker 1: faster than the speed of light, but it is possible
33:16
Speaker 1: to make space faster than the speed of light, which
33:19
Speaker 1: I think usually explains it. But here I feel like
33:21
Speaker 1: we're trying to be more specific about and say there,
33:24
Speaker 1: it's basically the same thing. Kind of, It's like it's
33:26
Speaker 1: the same thing, and so you can go faster than
33:28
Speaker 1: the speed of light. There are two different ways to
33:30
Speaker 1: look at it in general relativity, yeah, and I think
33:33
Speaker 1: the more intuitive way is the second with co moving
33:36
Speaker 1: way to say, the universe is expanding. Everything is at
33:38
Speaker 1: rest in its local frame, and the universe is expanding
33:41
Speaker 1: between those two things, and there's no limit to how
33:44
Speaker 1: fast the universe can expand. And interestingly, that means that
33:47
Speaker 1: there are things out there that are moving away from us,
33:50
Speaker 1: apparently faster than the speed of light. Means that a
33:53
Speaker 1: photon leaving that galaxy is not getting closer to us, right,
33:58
Speaker 1: It's moving through its local space, but that local space
34:01
Speaker 1: is expanding away from us faster than the speed of light.
34:03
Speaker 1: So as time goes on, the photon is not getting
34:06
Speaker 1: closer to us, even though it's moving through its local
34:09
Speaker 1: space at the speed of light. Right, the distance between
34:12
Speaker 1: us and that photon is not decreasing. What do you
34:14
Speaker 1: mean the distance is not changing because it is changing,
34:17
Speaker 1: isn't it. So the photon is emitted by that very
34:19
Speaker 1: distant galaxy which is moving away from us faster than
34:21
Speaker 1: the speed of light. Right or equivalently, the space between
34:25
Speaker 1: us and it's local spaces expanding faster than the speed
34:27
Speaker 1: of light. Right now, those photons are moving towards us
34:31
Speaker 1: in their local space, but the distance between us and
34:34
Speaker 1: those photons is not decreasing because space is expanding between
34:38
Speaker 1: us and that photon. So if a photon is moving
34:40
Speaker 1: through space which is super luminately receding, then it's not
34:44
Speaker 1: making progress towards us. So some things that are super
34:47
Speaker 1: far away that are moving away from us faster than
34:49
Speaker 1: the speed of light, their photons will never reach us
34:52
Speaker 1: no matter how long you give them, because space between
34:55
Speaker 1: us and them is expanding faster than the speed of light. Right, Yeah,
34:58
Speaker 1: I remember we talked about the before. It's sort of
35:00
Speaker 1: like if someone is zooming away from you on a
35:03
Speaker 1: spaceship and they, you know, shoot a nerf gun at you,
35:06
Speaker 1: that nerve bullet is never going to get to you
35:08
Speaker 1: because it's not being shot at you faster than then
35:11
Speaker 1: the rocket is moving away from you. That's right, And
35:13
Speaker 1: that's for like a Galilean transformation. Right, we're just linearly
35:16
Speaker 1: adding the velocities. Light is weird and different though, and
35:18
Speaker 1: light always moves at the speed of light. So somebody
35:21
Speaker 1: in our local frame who's moving away from you, if
35:24
Speaker 1: they shine a flashlight at you, that light still moves
35:26
Speaker 1: towards you at the speed of light. Doesn't matter how
35:29
Speaker 1: fast they are going, so it's not their relative velocity. There.
35:32
Speaker 1: Here's the expansion of space, right, And interestingly, there's some
35:36
Speaker 1: things that are so far away from us that their
35:38
Speaker 1: light will never reach us. There are other things that
35:41
Speaker 1: have always been moving away from us faster than the
35:43
Speaker 1: speed of light, but their photons do eventually reach us.
35:47
Speaker 1: All right, Well, let's get into that mystery and how
35:50
Speaker 1: that's possible. But first let's take another quick break. All right,
36:06
Speaker 1: we are expanding our minds here, trying to learn things
36:09
Speaker 1: faster than the speed of light. And trying to understand
36:12
Speaker 1: these concepts. I think I sort of understand what you're
36:14
Speaker 1: saying about this contradiction between the two things. I mean,
36:17
Speaker 1: we've always explained it as you know, nothing can move
36:20
Speaker 1: through space faster than the speed of light, but space
36:23
Speaker 1: can grow. You can make new space at a rate
36:25
Speaker 1: that's faster than the speed of light. That's how things
36:28
Speaker 1: that are really far away from us in an expanding universe,
36:31
Speaker 1: the distance between us is growing faster than the speed
36:33
Speaker 1: of light. But nothing in that spacing between us can
36:36
Speaker 1: move faster than the speed of light in their sort
36:38
Speaker 1: of local space. Right. But I think what you're saying
36:40
Speaker 1: is that for a physicist, there's really no distinction between
36:44
Speaker 1: space being created between us and using the word that
36:46
Speaker 1: it's moving away from us at a certain rate. Right,
36:49
Speaker 1: like it. It sort of makes into the sense if
36:51
Speaker 1: you make that distinction. But I think what you're telling
36:53
Speaker 1: us here today is that to a physicist, the fact
36:55
Speaker 1: that there's more space growing between us is the same
36:58
Speaker 1: thing as if it was moving away of us. Yeah,
37:00
Speaker 1: that's exactly right, And in your local inertial frame, there
37:03
Speaker 1: really is no difference. You try to extend that to
37:05
Speaker 1: the whole universe, and to say I'm going to talk
37:08
Speaker 1: about the velocity of things that are really really far away,
37:12
Speaker 1: you end up with this weird calculation that gives you,
37:14
Speaker 1: like the velocity of these things is two times a
37:16
Speaker 1: speed of light or three times the speed of light.
37:18
Speaker 1: And the reason that's strange is that you're using this
37:20
Speaker 1: concept of a distance and velocity and how you measure velocity,
37:23
Speaker 1: which doesn't really work over super duper long distances because
37:27
Speaker 1: you could never actually measure those things. You can't really
37:30
Speaker 1: measure those velocities of things. There's an apparent velocity if
37:33
Speaker 1: you make simple assumptions and just like translate redshift into velocity,
37:37
Speaker 1: but that velocity doesn't have a physical meaning. It makes
37:40
Speaker 1: just much more sense to talk physically about the expansion
37:42
Speaker 1: of space between us and them. I wonder if another
37:45
Speaker 1: way to look at it is to say that this
37:47
Speaker 1: expansion of the universe, this like free velocity, basically breaks
37:51
Speaker 1: all the rules that we thought were true, right because technically,
37:55
Speaker 1: because of this expansion, as you're saying, things are gaining
37:59
Speaker 1: velocity are free, you know, nothing's pushing it, No energy
38:02
Speaker 1: is actually causing this change in velocity. If it's not
38:05
Speaker 1: equals m a in this case, right, it doesn't break
38:08
Speaker 1: all the rules. It just breaks the rules that we're
38:10
Speaker 1: used to applying even the things in our cosmic neighborhood.
38:13
Speaker 1: It definitely doesn't break general relativity. General relativity can definitely
38:16
Speaker 1: describe it, but it requires a very different picture of
38:19
Speaker 1: the universities and when that we're not used to, and
38:21
Speaker 1: very specifically, you can't have a whole frame of reference
38:24
Speaker 1: for the universe and talk about like velocity of things
38:26
Speaker 1: across the universe. General relativity says you can only really
38:30
Speaker 1: make local observations, and the things that are really far away,
38:33
Speaker 1: you can't even really talk about what their velocity means.
38:36
Speaker 1: They're in their own frame over there, and we're in
38:38
Speaker 1: our frame over here. So you a tempted to like
38:41
Speaker 1: put your mind at the center of the universe and
38:43
Speaker 1: expand that picture out to include everything out there. But there,
38:46
Speaker 1: it really is no unified picture like that that makes sense,
38:50
Speaker 1: because you can never actually make those measurements. Well, I
38:52
Speaker 1: guess maybe I'm confused about what you mean that you
38:54
Speaker 1: can't talk about it. I mean, we're talking about it.
38:56
Speaker 1: We've just spent fifty minutes talking about I guess it's
38:59
Speaker 1: really we can't talk about it. Are you saying that
39:01
Speaker 1: the laws don't apply to the laws note work? Are
39:04
Speaker 1: you saying that it doesn't make sense in our current
39:07
Speaker 1: kind of way of thinking about things. It definitely works
39:10
Speaker 1: and the laws makes sense, right, But there's no unique
39:12
Speaker 1: way to picture it. Like in special relativity, you can
39:15
Speaker 1: pick a unique frame and you can say, I'm gonna
39:17
Speaker 1: put myself in the origin, I'm at rest, I'm gonna
39:19
Speaker 1: measure everything relative to me. But you can't do that
39:22
Speaker 1: for the universe, right. You can't say, here's how I'm
39:24
Speaker 1: gonna define everything, and this is the right way to
39:27
Speaker 1: do it. You can't have a single frame with everybody
39:29
Speaker 1: in it. In order to talk about the velocities of
39:31
Speaker 1: things that are really really far away, when space is
39:34
Speaker 1: curved between us and them, or when the space is
39:37
Speaker 1: expanding between us and them, you have to make some
39:39
Speaker 1: arbitrary choices. You have to choose your coordinates, and different
39:43
Speaker 1: people could choose different coordinates and they get different velocities.
39:46
Speaker 1: Like we said earlier, one person could say, I'm gonna
39:49
Speaker 1: put the Earth at the center, and I'm gonna measure
39:51
Speaker 1: velocities using an unphysically long stick. I'm gonna get crazy
39:55
Speaker 1: velocities two times of speed of light. Another person you
39:58
Speaker 1: could say, now I'm gonna choose coordinate that grow with
40:00
Speaker 1: the universe, so everything is at rest, so nothing has
40:03
Speaker 1: any velocity. Actually, and both of those things can be true, right,
40:08
Speaker 1: and there's no way to pick between them in general relativity.
40:11
Speaker 1: So that's what we mean when we say it's not
40:13
Speaker 1: meaningful to talk about those velocities because they're a little
40:15
Speaker 1: bit arbitrary. They depend on the coordinates that you pick,
40:18
Speaker 1: and you're free to pick any coordinates you like. In
40:21
Speaker 1: general relativity, maybe what you're saying, it's a general relativity
40:23
Speaker 1: allows you to use non inertial frames kind of or
40:26
Speaker 1: frames that are growing. Absolutely, general relativity can describe non
40:29
Speaker 1: inertial motion, things that are accelerating, things that are expanding,
40:33
Speaker 1: even if that expansion is accelerating, and you can use
40:35
Speaker 1: non inertial frames, and in general relativity, absolutely, yes, general
40:39
Speaker 1: relativity you can describe all sorts of things like that.
40:41
Speaker 1: All right, Well, let's get into that mystery. You said
40:43
Speaker 1: earlier that there are things that are growing away from
40:47
Speaker 1: us faster than the speed of light, but that maybe
40:49
Speaker 1: and you would think you can never see them because
40:51
Speaker 1: if they're moving away from as fast in the speed
40:53
Speaker 1: of light, we will never see their light. But you're
40:54
Speaker 1: saying that it is possible for us to see their
40:56
Speaker 1: light on some things. Yes, it's possible to see things
41:00
Speaker 1: that are moving away from us faster than the speed
41:03
Speaker 1: of light, which seems a little strange, but it's because
41:05
Speaker 1: this recession velocity we're talking about is not constant. The
41:09
Speaker 1: expansion of the universe is not happening at a constant rate.
41:12
Speaker 1: It's changing with time, So that changes over time which
41:16
Speaker 1: parts of the universe are moving away from us fast
41:18
Speaker 1: in the speed of light and slower than the speed
41:20
Speaker 1: of light. So some things were moving away from us
41:22
Speaker 1: faster than the speed of light. Always other things were
41:25
Speaker 1: moving away from us faster than the speed of light,
41:27
Speaker 1: but then no longer were, and then might be again
41:29
Speaker 1: in the future because of the acceleration. There's a whole
41:32
Speaker 1: sort of different outcomes based on how far away from
41:34
Speaker 1: us you are. Wait, I thought that the universe has
41:39
Speaker 1: I mean, the universe started expanding at the Big Bang,
41:41
Speaker 1: and it's always been expanding. That hasn't been contracting in
41:44
Speaker 1: any point, right, It hasn't been contracting, but it was
41:47
Speaker 1: decelerating right. Expansion the universe has a few different phases.
41:50
Speaker 1: There's inflation, which is a very very rapid expansion in
41:52
Speaker 1: the very beginning, and then this sort of a quiet
41:54
Speaker 1: period where things are expanding slower and it's actually decelerating.
41:59
Speaker 1: The expansion are is just driven by like the density
42:01
Speaker 1: of the universe. Solve the Einstein equations, you get a
42:03
Speaker 1: certain expansion based on how much stuff there is in
42:06
Speaker 1: the universe. But because the density of the universe decreases
42:10
Speaker 1: as it expands, then that expansion changes. So the expansion
42:13
Speaker 1: was actually slow in the universe, was decelerating until recently
42:17
Speaker 1: when dark energy took over and it kicked it back
42:19
Speaker 1: up in the gear again, and the expansion has been
42:21
Speaker 1: accelerating since. So you have these sort of three different
42:24
Speaker 1: phases in the history of the universe, and that change
42:27
Speaker 1: is like sort of what's been moving away from us
42:29
Speaker 1: fast in the speed of light and the fate of
42:31
Speaker 1: all of those photons. Okay, you're saying that before dark
42:34
Speaker 1: energy kind of pressed the accelerator and made the universe
42:37
Speaker 1: really start expanding faster and faster as we see it today.
42:41
Speaker 1: Before that happened, maybe there are things that we're moving
42:45
Speaker 1: the universe is sort of decelerating at some point, right,
42:48
Speaker 1: because it wasn't growing as fast, which means that there's
42:50
Speaker 1: kind of a window there for us to see things
42:52
Speaker 1: that we're once moving faster than the speed of light,
42:54
Speaker 1: but then at some point we're not. So maybe we
42:56
Speaker 1: can see those photos for a little bit, but then
42:57
Speaker 1: eventually we won't be able to see them again exactly.
43:00
Speaker 1: And the key thing to understand is this concept of
43:02
Speaker 1: the hubble sphere. Inside the hubble sphere, things are moving
43:05
Speaker 1: away from us less than the speed of light. Outside
43:08
Speaker 1: the hubble sphere, the apparent recession velocity is greater than
43:11
Speaker 1: the speed of light. So it's just a definition. It says,
43:13
Speaker 1: let's draw boundary where things are moving away from us
43:16
Speaker 1: at the speed of light, and everything further away from
43:19
Speaker 1: that is outside the hubble sphere. That's what the hubble
43:21
Speaker 1: sphere is. That's how you define it. Yeah, that's the
43:23
Speaker 1: definition of the hubble sphere, the bubble of space where
43:26
Speaker 1: things are expanding away from us the less than the
43:29
Speaker 1: speed of light. And that's different than the observable universe, right,
43:32
Speaker 1: It's very different from the observable universe. And this hubble
43:35
Speaker 1: sphere depends on the Hubble constant. Right, The Hubble constant
43:37
Speaker 1: tells you how fast things are moving away based on
43:40
Speaker 1: how far away they are. So for a given Hubble constant,
43:43
Speaker 1: that defines a Hubble sphere. But the Hubble constant is changing,
43:46
Speaker 1: it's not really a constant. As we said earlier, the
43:48
Speaker 1: universe was decelerating for a long time because it was expanding.
43:52
Speaker 1: That changes the energy density, which then changes the expansion.
43:55
Speaker 1: So the Hubble constant was actually decreasing, right, which grows
43:59
Speaker 1: the hubble sphere. Means you have to go further away
44:02
Speaker 1: to find objects that are now moving away from you
44:04
Speaker 1: at the speed of light. So this hubble sphere, the
44:07
Speaker 1: portion of the universe inside of it, which is moving
44:09
Speaker 1: away from us at less than the speed of light,
44:11
Speaker 1: is expanding as time goes on. And is it is
44:15
Speaker 1: that growing with the expansion of the universe or is
44:18
Speaker 1: it that it does it have its own kind of
44:19
Speaker 1: like growth rate as its own growth rate, that it
44:22
Speaker 1: is linked to the expansion of the universe because it's
44:24
Speaker 1: the expansion of the universe that changes a density, which
44:26
Speaker 1: then changes the Hubble parameter. So it's a bit of
44:29
Speaker 1: a complicated relationships like nasty differential equations in there, but
44:33
Speaker 1: they don't exactly track each other, which is why the
44:35
Speaker 1: hubble sphere can grow to encompass things that we're once
44:38
Speaker 1: moving away from us faster than the speed of light,
44:40
Speaker 1: including and here's the key. You can find photons which
44:44
Speaker 1: were once in superluminately receding regions and put them inside
44:48
Speaker 1: the hubble sphere so that we can now see them. Right.
44:50
Speaker 1: I think what you're saying is that if the universe
44:52
Speaker 1: is expanding faster and faster, the hubble sphere shrinks. And
44:57
Speaker 1: if the universe slows down and and starts to not
45:01
Speaker 1: grows fast, and the hubble sphere grows. So it depends
45:04
Speaker 1: a little bit on the coordinates. Right in physical units
45:07
Speaker 1: where you're like trotting out a ruler to measure things,
45:10
Speaker 1: the hubble sphere always just grows. It just grows faster
45:13
Speaker 1: or slower. It's growing because the universe is expanding. It's
45:16
Speaker 1: growing because the Hubble parameter is decreasing right as the
45:19
Speaker 1: universe expands and the density drops because the universe is expanding. Yes,
45:23
Speaker 1: the universe expansion is driving it. But what I say
45:26
Speaker 1: was still true right Well, in physical units, the hubble
45:28
Speaker 1: sphere never shrinks. It only grows. It does shrink in
45:31
Speaker 1: the co moving units, so it's always growing. The whole sphere.
45:35
Speaker 1: It's always growing, but sometimes it grows faster and sometimes
45:37
Speaker 1: it grows slower. Okay, And meantime the universe is expanding,
45:40
Speaker 1: but sometimes it grows faster or slower. And so you're saying,
45:43
Speaker 1: there's this kind of like this relationship between the two spheres,
45:46
Speaker 1: the observable universe fear and the hubble sphere, and there
45:49
Speaker 1: they have this kind of dance like sometimes one is
45:52
Speaker 1: bigger than the other and sometimes when it's smaller than
45:54
Speaker 1: the other. So if you think about what happens for
45:56
Speaker 1: objects at various distances, you can get an idea for
45:58
Speaker 1: how these interplays determine the fate of a photon which
46:01
Speaker 1: is shot at us from those distances. Right, So I
46:05
Speaker 1: think the end result then is that there are photons
46:08
Speaker 1: coming towards us, which at some point we're like, oh,
46:10
Speaker 1: I'm never gonna get to horrid. He's never going to
46:12
Speaker 1: see me. But now then then the universe kind of
46:14
Speaker 1: shifted gears and it's like, oh, oh, now I'm gonna
46:16
Speaker 1: make it too hori after all. Kind of exactly because
46:18
Speaker 1: the radius of the hubble sphere is increasing. Some photons
46:22
Speaker 1: that were initially in a super illuminally receding region and
46:25
Speaker 1: a portion of space that was expanding away from us
46:27
Speaker 1: faster than the speed of light, can now find themselves
46:30
Speaker 1: in a sub speed of light receding region, right, And
46:34
Speaker 1: not because they make it there, but because the hubble
46:36
Speaker 1: sphere expands to sort of engulf them, and the objects
46:39
Speaker 1: that admitted those photons have now moved to larger distances,
46:43
Speaker 1: and so they're still receding superluminally. So something that has
46:47
Speaker 1: always been moving away from us faster than the speed
46:49
Speaker 1: of light, we do have a window to see it,
46:51
Speaker 1: because it's photons cross into the hubble sphere and make
46:55
Speaker 1: it into this portion of the universe that they can
46:57
Speaker 1: then zip through and actually make progress towards us. Right.
47:00
Speaker 1: I guess maybe the part that it's hard to grasp
47:03
Speaker 1: because we don't have the math in front of us
47:05
Speaker 1: is that, you know, the hubble sphere is not a thing, right,
47:08
Speaker 1: It's just something you were using to describe it. I
47:10
Speaker 1: think the point is that there are regions of space
47:12
Speaker 1: that are stretching faster than other regions, and so sometimes
47:15
Speaker 1: the photon might be in a region that is stretching
47:18
Speaker 1: faster than the speed of light. But if it gets
47:19
Speaker 1: into a region where it's threatened not stretching as much,
47:22
Speaker 1: then maybe it has a chance to catch up to
47:24
Speaker 1: that stretching and get to us. Yeah. Well, the stretching
47:26
Speaker 1: is having at the same rate everywhere in space. It's
47:29
Speaker 1: just not the same rate everywhere in time. And then
47:32
Speaker 1: remember the recession velocity is a function of distance from us.
47:35
Speaker 1: So as the phoneton is moving to the universe and
47:37
Speaker 1: time is passing, this hubble sphere can sort of sweep
47:40
Speaker 1: across it. You say, the hubble sphere is not a thing.
47:42
Speaker 1: It's just like where things are traveling away from us
47:45
Speaker 1: faster or slower than the speed of light. And so
47:47
Speaker 1: then you can find itself in a portion of space
47:49
Speaker 1: that's within our hubble sphere, then you can make it
47:52
Speaker 1: to us because space between us and it is no
47:54
Speaker 1: longer expanding faster than the speed of light. Well, you
47:56
Speaker 1: just said something that raised something with me here. He
47:59
Speaker 1: said that the expansion of the universe is the same
48:01
Speaker 1: everywhere in space. Is that really true? I mean, aren't
48:03
Speaker 1: there spots in space that are denser than others wouldn't
48:06
Speaker 1: the maybe the expansion be a little uneven. Well, that's
48:08
Speaker 1: a great point. We think that the contribution to the
48:11
Speaker 1: expansion is the same everywhere, and that's accurate only in
48:14
Speaker 1: the picture of the universe is like totally homogeneous right
48:16
Speaker 1: where there are no like lumpy bits. Of course, once
48:19
Speaker 1: you add in lumpy bits, things are different, and we
48:22
Speaker 1: mostly ignore those when we talk about like big cosmological things.
48:25
Speaker 1: We don't think about like the gravitational impact of Jupiter
48:28
Speaker 1: or even a galaxy or a black hole, and most
48:31
Speaker 1: of those things we can only solve in flat space,
48:34
Speaker 1: assuming that the universe is not expanding, So we mostly
48:37
Speaker 1: just ignore those little bits when we talk about like
48:39
Speaker 1: really big cosmological questions. Right, but that there are clusters
48:43
Speaker 1: and walls and bubbles of superclusters. Right, are you saying
48:46
Speaker 1: the expansion of the universe is the same in the
48:48
Speaker 1: big empty spaces between clusters of galaxies and it is
48:51
Speaker 1: in the within the clusters. That's our theory, and we
48:54
Speaker 1: think that it's the same everywhere. We think it's totally homogeneous.
48:57
Speaker 1: It doesn't vary with space, does vary with time, because
49:00
Speaker 1: it depends on the overall density of matter and radiation
49:03
Speaker 1: in the universe, and that does evolve, but we don't
49:06
Speaker 1: think it varies with space. So the Hubble constant, we
49:09
Speaker 1: think is constant across space, but not across time, or
49:12
Speaker 1: at least you assume it's clossant across space. Yes, we're
49:15
Speaker 1: assuming that, and all of our observations are consistent with that,
49:18
Speaker 1: but you know, we don't really know, and again we
49:19
Speaker 1: don't really understand the expansion of space or its acceleration.
49:23
Speaker 1: So certainly possible that dark energy could be doing different
49:26
Speaker 1: things in different parts of the universe. We don't see
49:28
Speaker 1: any evidence of that, but it's certainly possible. Well, it's
49:30
Speaker 1: kind of hard to measure what's going on in empty space,
49:32
Speaker 1: isn't it. Well, we can measure what happens in some
49:35
Speaker 1: of those voids by looking at photons that passed through them.
49:38
Speaker 1: We see photons from the very early universe, and we
49:40
Speaker 1: can sort of measure the density of space between us
49:42
Speaker 1: and where they were generated and the expansion of that
49:46
Speaker 1: space by seeing how they're red shifted and blue shifted
49:48
Speaker 1: as they go through like denser and less dense regions.
49:51
Speaker 1: So we have one way to probe that space because
49:53
Speaker 1: photons do pass through them, and we talked once about
49:56
Speaker 1: really cosmic voids. If you remember, we talked once about
49:59
Speaker 1: hotspots and could spots in the universe as those photons
50:01
Speaker 1: passed through those big voids, so we do have one
50:04
Speaker 1: handle on them. Cool. Well, my mind is definitely expanded
50:07
Speaker 1: and um, but I'm not quite sure my understanding. You're
50:10
Speaker 1: caught up to the speed of light here. But I
50:13
Speaker 1: think the main interesting picture here is that the universe
50:15
Speaker 1: is almost like a living thing, like it's growing. It
50:18
Speaker 1: has phases of fast growth and phases of slow growth,
50:20
Speaker 1: and you know, and we don't know how what's going
50:23
Speaker 1: to happen in the future, which means that it's sort
50:25
Speaker 1: of unpredictable how much of the universe will be able
50:27
Speaker 1: to see right, It is a little bit unpredictable. In
50:30
Speaker 1: our current models. We have some ideas for what those
50:32
Speaker 1: numbers are, and they're sort of mind boggling. We think
50:35
Speaker 1: the things that are now forty six billion light years away,
50:38
Speaker 1: if they emitted light at the very very beginning of
50:41
Speaker 1: the universe, that light is just reaching us now. And
50:44
Speaker 1: we think that things sixty two billion light years away
50:47
Speaker 1: if it admitted light at the very beginning of the
50:49
Speaker 1: universe it will eventually reach us at the very very
50:51
Speaker 1: end of the universe, but anything further than that will
50:55
Speaker 1: never enter the hubble sphere, will never reach us. Will
50:59
Speaker 1: always be moving through space that's expanding faster than the
51:03
Speaker 1: speed of light. So even though locally those photons are
51:06
Speaker 1: pumping away at a crazy speed, they're not actually making
51:09
Speaker 1: any progress towards us. Right, although never say never, right?
51:13
Speaker 1: Didn't you say earlier in another episode, And haven't you
51:16
Speaker 1: always said that dark energy is kind of unpredictable. It
51:19
Speaker 1: might you know, reverse course or shift gears or something
51:21
Speaker 1: like that might make the universe crunched down again. Yeah,
51:24
Speaker 1: great point. This is all assuming our current understanding of
51:28
Speaker 1: the universe and the blend of dark energy and dark
51:30
Speaker 1: matter is correct. We could learn one day that dark
51:32
Speaker 1: energy is something totally different, and you could flip around
51:35
Speaker 1: and compress the universe, and then we could see much
51:37
Speaker 1: much more of it one day before we eventually get
51:40
Speaker 1: squished too much of it, seem Yeah, the universe crunches
51:43
Speaker 1: down into a tiny dot again. Will all be very
51:45
Speaker 1: familiar with every light that's ever been admitted, right exactly
51:49
Speaker 1: just before it fries us to a crisp All right, Well, um,
51:52
Speaker 1: stay tuned. I guess as we learned more about the
51:54
Speaker 1: universe and what dark energy is, and we'll know more
51:58
Speaker 1: about how the universe is changing. Well, thanks for joining us.
52:01
Speaker 1: Hope you enjoyed that. See you next time. Thanks for listening,
52:12
Speaker 1: and remember that Daniel and Jorge explained the universe is
52:15
Speaker 1: a production of I Heart Radio. For more podcast for
52:18
Speaker 1: my heart Radio, visit the i heart Radio app, Apple Podcasts,
52:22
Speaker 1: or wherever you listen to your favorite shows.