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