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