If the Universe is infinite, why is the night sky dark?

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Speaker 1: A, Kelly, Is it getting warm where you guys are? Oh my gosh. Yeah, it's Virginia and the Virginia summer is definitely here. So does that mean sunscreen for the kids every single day? Yeah? They hate it, but I'm a pretty big believer in that. And what about at night time? Do you guys put sunscreen on at night? Why would we put sunscreen on at night? What do you know, physicists? But I don't know. Well, you know, stars are actually just other suns, right, So it's still sunlight even at night. So you're trying to tell me that the stars are going to give us skin cancer. Look, I'm not trying to make astronomy scary. I'm just looking out for your kids. Uh huh. Well, I guess I gotta order a gallon to star screen. Somebody out there is going to get star cancer, oh boy. Or someone's gonna make a lot of money off of star screen. Hi. I'm Daniel. I'm a particle of physicist and a professor at UC Irvine, and it was the night sky that got me into science. I'm Kelly Weener Smith. I'm an adjunct assistant professor at Rice University. And while I prefer parasites. I do love looking at the night sky. What about parasites at night? I mean, you know, the parasites are always out there, and they're just as good at night as they are during the day. They're always fascinating. And then the super meta question, do parasites look at the stars? Can you enjoy the stars with your parasites? Oh, you know, ecto parasites can probably look at the stars. But the endo parasites, the ones like in your guts, they probably don't have as good of you. Exo parasites are those like exo planets and exo moons. Are you talking about astro parasites parasites on other planets? I said ecto E, C, T O M. But you know there could be exo parasites. Uh, it wouldn't surprise me to find out that, you know, the second organism on another planet was a parasite of the first. That would not be too surprising to be Well. Welcome to the podcast Daniel and Hoordey explain the universe where we have just invented the field of exo parasitology. Yes, job security. On this podcast we do delve deep into the mysteries of the universe, What is out there? How does it work? What squirmy little beings are living on the backs of other squirmy little beings on weird planets around other stars. We don't shy away from the biggest, deepest, squishiest questions in the universe, and sometimes we like to ask really basic, fundamental questions about the nature of the universe. We find ourselves in because we think that our tiny, little, squishy human brains are weirdly, somehow capable of understanding the deepest cosmic mysteries, of looking out into the night sky and from that information gleaning something big and deep and true about the nature of the cosmos. My friend and co host Orgy can't be with us today, but we are very happy to have our regular guest host, Kelly Smith. Kelly, thanks again for joining us today. Thanks for having me. I'm excited to be here again, and today's topic is super interesting. I can't wait to talk about it. Awesome. Well, one reason I love thinking about today's topic is because it really was the night sky that got me into science. That wasn't just a sound bite when I was a kid. I was just like all those other undergrads who write cheesy essays about staring up in the night sky and wondering about the universe. You know, it really is a very attractive and easy way to get into asking questions about the universe. Were you also a star watcher as a kid, you know, I was more looking down at the ground, and you know, as an ecologist, all of our essays start off with I always liked playing in the woods, so I mostly was looking down, but I did like looking up when it got too dark to find salamanders and stuff. And as an adult, I really like looking at the night sky. Well, the funny thing about the night sky in terms of science is that it's very attractive. It's like easy if your people to hear about, and astronomy seems like a very accessible topic. But when it gets down to it, you know, like doing science of the stars is mostly like standing around in the cold looking through a telescope. And so while I was very excited about astronomy as a young kid, when I got older and tried to get into it a little bit, I found that it wasn't really actually for me. You know, I didn't really like staying up late at night or staring through a telescope or dealing with optics, So sometimes the day to day grind of science doesn't quite match with your like fantastical aspirations. And you know, I think one thing that draws astronomy students and ecology students together is that both of those groups of students think that it's going to be one thing, like playing outside a lot, and then it ends up being a lot of math, for example, and that is where we lose a bunch of the outside enthusiasts. Is there a lot of math in ecology and then you're like putting down squares and counting how many slugs or something. Well, I mean there's counting, but then there's also trying to like, you know, write mathematical models to describe host predator dynamics or whatever. Actually I took ecology because I thought it was going to be an easy a because it's like hippies playing in the woods. But then when I found the equations, I fell in love. So for me, that's what hooked me, which I'm sure is what hooked some students on astronomy as well. Yeah. Absolutely, it is amazing to think that we can describe what's going on over there and it's fascinating to me that we can learn about the rest of the universe just from looking at our local neighborhood. You know that we have an example star right here in our solar system, and we can study it and think about and understand it, and then we can apply that knowledge to the stars to saying, oh, those are just other sons. And you know, I have to embarrass my daughter because it was only six months ago or so. I was talking to her and I said something like, you know, the stars, they're just other sons, right, And she was like, what are you serious? And then I was like, what are you serious? You didn't know that already. She was like no, I had no idea. And I was so embarrassed, Like, here I am, you know, a physicist, a science communicator, and my own daughter doesn't know this very basic thing about the universe. Yeah. I was gonna ask if you're a little embarrassed admitting that, and I, yeah, I guess you are. But you know, we all have gaps things that we just assume our kids intu it in, but they didn't, and it's you know, surprising sometimes. Yeah, she thought that they were sort of like other stars, but they were much closer and much dimmer that we were surrounded by like an ocean of not very bright suns or something. I don't think she had actually thought about it very much, and so you're right, it's just something we never actually came to the point of talking about. And now we have a fun running joke, which is, at any time something obvious comes up about the universe, I'm like, you are aware, of course that the Earth is round, right, She's like, Dad, it's important you never let kids forget if they got something wrong. You know, you need to remind them of that all the time. No, trying not to make her feel bad. Now, it's just my opportunity to get to like insert a little bit of science into any random conversation because now I have an excuse because maybe this is another thing we just never got around to talking about. That's right, And I'm sure she doesn't find it frustrating that you do that at all. Oh, she teasings me back. She is quite capable of that. I'm glad. But this question of the stars and the night sky really has lots of fascinating layers to it. A lot of people are surprised when they learn, for example, that the stars are there during the daytime, like you could see them in principle they are shooting starlight at you and it's hitting the earth. It's just sort of like overwhelmed by the sun. I feel like that's probably something I didn't figure out until I was much older, Like that's another one of those obvious things that you should have known. But I feel like I remember, maybe like as a senior in high school, having a like, oh, they're still there moment, which I should be particularly embarrassed about. But as I said, I wasn't looking up that much. I was mostly looking down. Yeah, And there's those shame in coming to understanding of the universe. At any age. We should just be rewarding people for their curiosity and their enthusiasm. And one of the things for me that I love about the night sky and just staring at it is trying to get a mental image or conception, grappling with the idea of how ridiculously far away they are. Know, I like standing on the top of a mountain and seeing eighty miles hundred miles on a clear day even further, but looking up in the night sky is like standing on the top of a mountain. The side of the Earth and staring across billions of light years, right, those photons left those stars millions or billions of years ago, just now arriving on Earth. It's like it's the most spectacular view in the universe. My brain has so much trouble wrapping its head around distances in space. Like even just knowing that it takes like, what six months to get to Mars, that seems crazy because you know, it's like two days of driving to get across the US. So it feels to me like anything should be accessible within forty eight hours. I should be able to drive to Mars in two days, right, stopping at gas stations and getting like space snacks. I mean, I understand you're going to need a different vehicle, but it seems like it should be reachable pretty quickly. And one of the real challenges with understanding these distances in particular is that you have two things you have to grapple with. Not just are the stars super far away, but they are enormous, Like the size of a star is just flabberghastingly huge, Like the Sun for example, right, so much bigger than the Earth. Jupiter is like a thousand times the volume of the Earth, and the Sun is like a million times the volume of the Earth. Right, that's flabbergasting. It's hard to hold that many Earth's in your head. And there are stars out there there are much much bigger, And yet you look up at the night sky and they are these tiny little pin pricks, right, And so you have to have both of these things in your head. That these objects are enormous fireballs really just like bigger than you can even imagine, and yet so distant that these enormous fireballs now look super duper tiny. Right, So two things in your head you have to sort of like divide by each other, both huge numbers. How big is our son relative to some of the others you said, some of the others were much much bigger. Is our son sort of a tiny sun? Or is always like about average? Our son is not unusual. But there are many stars out there that are much bigger. For example, Beetles Juice, if you put it in our solar system, it's radius would extend out into the asteroid belt like Mars would be inside Middle Juice. We did an episode about the biggest stars in the universe. Some of them are even larger. Some of them, their radii would extend out, you know, into the outer solar system, you know. So there are monsters out there, absolute gargantuan monsters, balls of burning plasma. That's really hard for me to imagine. That's right. They're huge, they're enormous, and yet they only appear as pin pricks in our sky. And the reason, of course is math, because as those photons shoot out from that star, they go in all directions, and we only see a tiny fraction of those photons. Right. If you're standing like a meter away from a light bulb and you have a solar panel gathering some of that light, you're not gathering all of the light, right, You're gathering some fraction of the photons that come off of that light bulb. Right, it's shooting in all directions, and the fraction that you get like the area of your solar panel divided by the area of that sphere that's at your distance, right, So that's the fraction of the photons you're getting. If you take two steps away, for example, now you're twice as far. Then the area of that sphere that the photons are getting spread over is now four times as big. Because the surface of a sphere with twice the radius is four times as big. So the same solar panels now capturing one fourth of what it did before. So the luminosity of a star, the brightness of its appearance, goes like one over the distance square. And these distances we're talking about are huge, right, Other stars are not just super bright, they're really really far away, So one over their distance square becomes a really big number. But if you do that over and over and over and over again, like billions or trillions of times, shouldn't it be much brighter out there? Yeah, it's a really good question. So we're balancing lots of big numbers here. We have huge stars, so they should be admitting a lot of light, but they're really far away, so that cuts down their light a lot. But then there's a lot of them. Right, three have like three almost infinite numbers, and we're wondering, like, how how do those get fit together? You know, if the universe is in fact infinite, wouldn't all those numbers add up to be like a crazy amount of light. Wouldn't Kelly's kids get like star burns if they go out at night looking at the stars. Right, this is a question that people have asked sort of since antiquity, like why isn't the night sky like catastrophically blazing full of starlight? Thank goodness for star screen. And so today on the podcast we'll be answering why is the night sky dark? And I love this kind of question because it's such a simple, basic question, and it's the kind of thing that people can think about and have been thinking about for a long time. Right, you don't need special tools or apparatus. Is just to sit here on the surface of this rock and to ask this question, like, if I have an understanding of how the universe works, my mental model is that there are stars everywhere, then what should I expect to see in my night sky? Right? This is a perfect example of what physics really is at its core. Build a model to love the universe in your head. We make a prediction for what you should see, and then you ask, like does that agree with how the universe actually works? And if it doesn't, then you get an important clue about how the universe might be different from your mental model. Well, you forgot about the part where you spend like a decade writing to grant so that you can get the money to try to test the model. But but yes, essentially that's how physics works. We're trying to make science sound glamourus here talking all the emails and the meetings. I'm sorry, I'm sorry, didn't mean to take away all the beauty of physics. Al Right, Well, let's tap into the apparent beauty of physics by asking our listeners if they know why the night sky is dark. So I wrote to all of them, and I asked them, if the universe is infinite, why is the night sky dark? If this sounds like a lot of fun to you, and you would be willing to participate for future episodes, please don't be shy right to us, to Daniel and Jorge dot com, and we'll send you our recent questions. So think about it for a minute. Do you know why in an infinite universe the night sky is not blazingly bright? Here's what our listeners had to say. Well, I guess that's because from the electromagnetic spectrum that's the light emitting of all stars and everything in the universe, we are only able to see just this tiny fraction that's the visible the visible spectrum. So I guess that there are photons all around the universe, but they are in different wavelength so we are not able to see them. That's why the night sky looks dark for us, because we cannot perceive it. Okay, so, first of all, the universe is not in finite. It's just insanely huge, and it expanding faster than the spade of light. So it may seem infinite, but it's not infinite. As for the question itself, I can come up with two different explanations. One is that the light coming from really distant stars and galaxies that they get that eventually they get really spread out. They're not concentrated enough for us to spot, so like in our able to see them in the night sky. The other reason that I could come up with was that although the speed of flight is fast, it's not fast enough compared to the size of the universe. So I imagine this bubble of visibility around Earth that keeps growing every second as new light reaches us from further places. But there is still a lot of the universe out there from which the light has simply not had enough time to reach us. It's because all those stars are just too spread out and they're too far away so that they can really make a strong light. And I also believe that inflation plays a role, right, because I think that in a few million or billion years there will be rarely any stars to be seen in the night sky. So maybe that's the point. Well, first of all, proved to me that we live in an infinite universe. But supposing we do, it's not just jam packed with stars all the way out, and the intensity of light fairies with the square of your distance from it, so the really far away stuff is extremely him um. And I'm sure there's some kind of math you can do that shows a curve that approaches an awsome tote or one of the axes and the zero pans out there. It's kind of like, um, if you keep going, you go one ft, and then you go half a foot, and then you go a quarter of foot, you never quite get there two feet away. And I'm glad it's not effinitely late, because um, it's nice to have some darkness every once in a while. It is not dark at all frequencies. If we shift to the right frequency, there's usually something coming from every point in the sky we look at, however, for the visual range of humans, even though at any given point in the sky. If we look deep enough in there probably is a source of light. There are dust clouds and gas clouds and lots of other things that obstruct the light before it gets to us, so within human visual range it can look very dark, but if we shift to a different frequency, there's usually some form of light coming in. So I know this one. The answer is dust. The interstellar gases are just thick enough to absorbs enough of the radiation sent by the more distant stars that we can't with our naked eye see things all the way back to the dawn of time. It's what we need things like Hubble for. So those more distant and fainter lights do get filtered out by the dust, and so the night sky appears dark to us instead of appearing light. Light from different stars takes time to travel from whatever the star is to where we are, so depending on how far the star is, the light from it might not have reached us yet, and so the space in the interim for the duration for which the light hasn't gotten here yet will be dark. Well, that is debatable if that's even true, that we live in an under infinite universe, but I guess the question is why wouldn't there be as star in every direction we look? And this would be due to um that there would be still a couple of photons arriving from any direction. I guess, um. But the brightness dissipates um over long distances UM in a sphere, so we are not getting a lot of photons um. So I guess if our eyes would be good enough, we would be able to see a totally lit up universe at night. You know, for this set of answers, I was particularly impressed with your audience. Now, of course your audience is brilliant and they always give good answers, but for this set in particular, they really had some clever answers. And so so, first of all, like when you ask this question to the listeners, do they have to answer like immediately or do they get the question, think about it for a day and then call in. That's a good question. Well, the rules are no preparation, no googling, and you're supposed to give immediate off the cuff answers. Now, I can't police them, so I don't know if they've googled or they've gone and looked up their star on the professor from college and then trying to give an intelligent answer. But I'm trust in them that these are what they immediately think that these are. They're off the cuff responses. Yeah, yeah, I think they're not just smart, but they're trustworthy and good looking and good looking, that's right, many many good features. You know, you'd have to be a great person to be listening to this podcast. I think that's called the halo effect. If anyway, getting off topic, but yeah, there were like some good skeptical answers. You know, we don't know if the universe is infinite. You've clearly taught them about what we do and don't know. And Dust came up to be honest, Dust I hadn't thought of as a potential explanation until reading the answers, and so I was I was pretty impressed with this set of answers. What did you think? Yeah, they really run the game, and there's a lot of different things to think about. And that's why this is a great question because it forces you to sort of clarify your understanding for what really is going on, what is interfering with the light that's coming to earth, how much should be arriving. There's a lot of good stuff here. There is a lot of good stuff there. And so one of the listeners said, we don't know if the universe is infinite. Is that right? What do we know about the size of the universe? That is correct, We do not know if the universe is infinite. You know. We know that our patch of the universe, what we can see, what we call the observable universe, no part of it seems to be different from any other part of it. That it's the universe seems to be sort of homogeneous, and so that suggests that no part of the universe should be special, and it implies that perhaps the universe is infinite. We don't know that. We've tried to measure things like we can measure the curvature of space, how much space itself is bent. Though we talked about in a recent episode about whether the universe is shaped like a doughnut or a sphere or an infinitely flat plane, we can't actually tell. We've measured the universe is curvature to be consistent with zero curvature, like as if it was flat. But the universe could be flat and also not be infinite. It could like tie together at the edges sort of like the screen and pact Man or asteroids. So there's lots of things we don't know, but so far it's consistent with us seeing a finite patch of an infinite universe. But absolutely we don't know. What do you mean by homogeneous like that? Because you know, when I think about galaxies, they sort of seem like clumps. But I guess, just like in general, it's everything somewhat evenly distributed. Is that what that means by homogeneous? We don't mean literally exactly homogeneous, because there are parts of the universe where there are galaxies in parts where there are not. We mean that if you zoom out far enough, like sort of on the biggest scales, everything seems basically smooth. There's no clusters and superclusters, but there's no real structure beyond that, and the rules of the universe seem to be the same everywhere. There don't seem to be any special locations. There's no center to the universe. As far as we can tell, everyone in the universe seems to be similar to everywhere else in the universe. That's what we mean. And so I thought that everything was moving away from a central point, But is that not true? There's no like point that we know that everything is moving away from, like expanding out. Everything is moving away from everything else. So the expansion is also homogeneous, like every point in the universe is expanding just like every other point. No, no matter where you are in the universe, it will look like everything is moving away from you. So either you are at the center of the universe and there's just expansion away from that one point, or everywhere in the universe is expanding, which is much more likely. I am probably the center of the universe, but maybe I'm not. I can accept that. Okay, you're the center of your kids universe, I'm sure. But this question about why the night sky is dark is actually a really old question and really predates any modern understanding of sort of like the size and shape of the universe, and it helped spark some of these questions like is the universe infinite? Could we possibly tell? And a long time ago people thought it was obvious that the universe was infinite. One of my favorite things about the history of astronomy is seeing how like conceptions of what is obvious or natural change with time. Right, like a long time ago, people thought the universe was obviously infinite, it should just go on forever. And it was just sort of sprinkled with stars. There were no galaxies at all. They just thought, here's a star, there's a star. The whole universe is just like a vast sea of stars. So when did we start learning enough to start having a good answer to these questions. Well, it was like a hundred years ago with Hubble and lots of others who discovered that there are galaxies and that those galaxies are moving away from us and the universe is expanding. And then it became later much more natural to imagine that the universe might be finite. In age, it seemed like obvious that the universe should have a beginning, whereas before that people that I was obvious that the universe had existed forever and had no beginning and went on forever. Right, So what seems to be like obvious and natural changes with time, which I think it's really fascinating, an important lesson, you know, for like making assumptions about the universe. Anyway, thank yourself. Back to like h fifty years ago, when people thought, Okay, the universe is infinite, there are stars everywhere. There's sort of like dotted in this vast ocean. So pretend you're an astronomer two fifty years ago, and that's the sort of understanding. People were asking themselves a question like how much starlight should expect to fall on the Earth in that situation, in an infinite universe filled with equally spaced stars, And so, I mean, of course you look out at the night sky and you sort of know what you get. What did they think that the answer was right? So obviously they knew that the night sky was dark, right, That wasn't the mystery. The question was like, why is the night sky dark? Because when they sat down to do the calculation, they were quite surprised. They couldn't explain it right. Their calculation suggested that the night sky should be infinitely bright if there are an infinite number of stars out there. Okay, well, I am dying to know what explanation they came up with to explain the discrepancy between what they thought they should see and what they were seeing. But first, I think we need to take a break, all right, and we're back. So before we left, you were telling us that in the past there was this big question about like, well, how much light should we be seeing, and why aren't we seeing a lot more of it? And and what sort of answers did they come up with? Hopefully, hopefully the answers we have now are better. Well. The interesting thing is they were trying to figure out whether the nice guy should be dark or not. They knew something about how light was transmitted that they didn't really understand it on a microscopic scale, but they knew, for example, that as you got further away from something, it's light dimmed by a power of one over the distance squared. And so you know, you apply that math to for example, like Proxima Centauri. That's the nearest star to Earth, but it's really still very very far away. It's only a few light years. But in terms of like distances in our Solar system and au, for example, is the distance between the Earth and the Sun, proximate Centauri is two hundred and seventy thousand a U. Right, so it's two d seventy thousand times further from the Earth than the Sun is. So that means that proximates Centauri is light gets reduced by one. Were tend to the eleven relative to the Sun, right, So our distance to the Sun means the light from the Sun is reduced by a certain amount, but proximate centauri is one over ten to the eleven times dimmer than the light from the Sun because the distance between here and there is so large. Okay, so the equation where they were trying to figure out that like light dims at one over the distant square. That must have been done on Earth when they were looking out at space, where they assuming that like space sort of had the same atmosphere and the same sort of dynamics, or like, how did they deal with that sort of uncertainty? Oh yeah, great question. It wasn't always obvious to people that the rules here on Earth also applied to space, that you could say, like things were fundamental and physics was universal. Newton was one of the first people to do that, to say gravity here on Earth also applies in space and can describe the motions of the planets. So you're right, that is kind of a big leap. Plus, they're assuming that light can travel through those distances, that there isn't like dust or gas blocking it. We'll get into the details of whether or not gas and dust can explain it in just a minute, but first let's start with sort of the simpler vision, like let's just imagine that light is not blocked by anything. It's just a question of distance and the number of stars. And that's sort of like simpler model. How much light do we expect to fall on Earth? Okay, so they had an understanding then about how light should fall off with distance. Did they have like an estimate for how many stars were out there? So they didn't know how many stars were out there, but they were assuming that the stars were uniform right there, like every chunk of space had on average the same number of stars. And so you can take the fact that stars get dimmer as they get further by one over distance squared and the fact that there are an infinite number of stars, and you can do the math and you can ask, like which one wins out? Does the infinity of the number of stars win out? Or does the growing distance to those stars suppress the light faster than like more stars get added, And so then because the night sky is dark, that means the distance must win. Is that the conclusion they came to, not the conclusion they came to. Actually, they did the math and they discovered that the number of stars should win. It's actually not that hard to do the math yourself. Brightness of a star goes like one over distance squared, and at a given distance, the area of a sphere around the Earth also goes like distance squared. So that means that, like any shell of universe at an arbitrary distance from the Earth, the stars from that shell get suppressed by one over distance squared, but the number of stars in that shell goes like distance squared, So those two numbers both canceled. So that means that, like any shell of universe an arbitrary distance away from you should have a constant brightness, like every shell should have the same amount of brightness. Because you have, like a shell of universe, further and further away, each star in it gets dimmer, but the number of stars grows perfectly to match that. So that means that every shell of universe around us should be equally bright. Further ones have many many more stars, each of which are dimmed, and the closer ones have fewer stars, but they're not dim as much. Okay, but his wifind physics so frustrating. But but but the night sky is dark? So what what what were they missing? What? What was not in the equation that should have been there. You're absolutely right, the night sky is dark, and so something is wrong with this calculation. But just to underline it for the listeners, you know, that means that every shell of the universe is a fixed brightness. But there's an infinite number of shells, right, so all those shells should add up to be an infinite amount of light. And so this is like Heinrich Olbers. Here's a guy in the seventeen hundreds. He's doing this calculation, and you know he gets this number. He's like, hold on a second, something must be wrong, as you say, because if there's an infinite number of stars, even if distance dims them like one over distance squared, the night sky should be infinitely bright. You know. His calculations suggests that if you go out at night you should be brighter than the daytime. You should get like infinitely crisped up, you know, by all of these stars. Right. So did he start like the Olbers Prize to be like I try to solve this famous problem or like how many people cared about about this problem? At the time. A lot of people were thinking about this, and Olbers thought that this was proof that the universe wasn't infinite, right, thought, oh, well, something must be wrong. One of the assumptions that go into this calculation, right, that stars go like one of a distant squared and that the universe is infinite and uniformly filled with stars. One of those assumptions must be wrong. And he thought it was clear that the universe couldn't be infinite because I would solve the problem. You can't get infinite light on Earth if there aren't infinite stars out there. So he was convinced that that means that the universe is not infinite. Okay, that makes sense. But apparently, well you've told us that that is a question that we're still trying to figure out the answer to, so it must be more complicated. Yet it is in fact more complicated. It's possible to live in an infinite universe without an infinitely bright night sky. Right. There are other solutions out there to this problem, but first let's tackle one of the ones that the listeners brought up that you mentioned earlier, which is gas and dust. Another suggestion people had, not just our listeners, but people over the centuries, was imagining that not all the light is getting to the Earth, that maybe some of it is getting absorbed by gas and dust. Basically, it's being blocked before it gets here. And so I mean, I guess when I think about space, I don't think about there being a lot of gas and dust. But of course the International Space Station needs to have what is it called a whiffle barrier because it gets hit by all of this space dust that you want to make sure it doesn't get into the International Space Station. So there must be dust up there. But is there enough dust up there when you get out past like low Earth orbit to be blocking the light? Absolutely, most of the mass of the galaxy is in terms of gas and dust. Right, Stars do not make up most of the stuff in the galaxy. Even if we're just talking about barrionic matter, we're not even talking about dark matter. They're vast clouds of gas out there which might in the future become stars, and they're huge expanses of dust from old solar systems that blew up up And so there's a lot of gas and dust out there, and people who study the universe have to map that gas and dust. If they're looking at a star and they're trying to figure out how bright it should be, they have to know how much stuff is between us and that star. There are people whose entire theses are like maps of dust in the galaxy because it's so important and it's one reason, for example, that we can't see through the galaxy. One of the big mysteries in astronomy these days is the Great Attractor, this weird source of gravitational attraction that seems to be on the other side of the Milky Way, and we can't see what's there because the Milky Way is so gassy and dusty that is basically blocking our view. So it's definitely something that people have to take into account when they're figuring out how bright and individual star should be when seen from Earth. Are the people who study that like not invited to the parties of people who are excited about space stuff because to me, when I think about space dust, I think you can't travel to another galaxy because that space dust is going to be like bullets going through your capsule or whatever. On the wage with other galaxies where these like not popular people because they kill dreams. No, they're super popular because they enable the rest of astronomy. You know, there's so many people who need to know how much gas and dust is there between this object I'm studying for my thesis. So I think people are very grateful that somebody has gone out there and like devoted five years their life to mapping dust in the galaxy. And you know, they also get zillions of citations. I see these These sometimes have like five thousand citations because it's such vital work just to understand like what's out there between us and other stuff. Well, I'm happy for them, But unfortunately this idea that gas and dust is dimming the otherwise infinite light from stars that can't explain why the night sky is dark. The problem is that gas and dust also absorbs energy. Right, so if the universe was filled with effectively infinite light from infinite stars had infinite time to get here, then that gas and dust would be super duper hot. Right. If it's blocking infinite light, then it's absorbed being infinite light and basically gets to be the same temperature as the stars, and it should also be glowing. So you can't explain the non infinitely bright night sky with gas and dust, and that would make those theses much easier if mapping it just meant you had to look for the glowing penches exactly, and yet we know that the night sky is not dark, and so we have to think more deeply about what those assumptions are and which one might be wrong. And one of my favorite bits of history is that an early thinker on this question is somebody you might not imagine, somebody you all know very well but probably don't think of in terms of astronomy, and that's Edgar Allan Poe. What's no way he was at UVA for a little while. You can go to the dorm room that he was in briefly before he left. But I don't know if I had highlight the fact that he was only here briefly so much during the tour. But anyway, so did he generally think about astronomy questions or is this like the one area of astronomy that really drew him in. No, he was really prolific, and he wrote about all sorts of stuff. But you know, a guy who writes horror movies spends a lot of time up it at night, and so I think he spent a lot of time thinking about the stars and going for long crisp walks, you know, in the evening. So let me redo this quote from Edgar Allan Poe. He says, we're the succession of stars endless. Then the background of the sky would present us a uniform luminosity like that displayed by the galaxy, since there could be absolutely no point in all that background at which would not exist a star. So he's saying, basically, in any direction you look, you should be able to see a star. If the universe is infinite, then every line you make you should hit a star, and so every point in the sky should basically look like a star. Then he goes on to say, the only mode, therefore, in which under such a state affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all. Oh, so he's saying, everything is so far away that the light hasn't gotten here yet, gotten here yet exactly. And that's a really vital point. He's thinking about the time it takes light to get here from those stars, and he's assuming that it hasn't had an infinite amount of time, because in Olber's calculation, he not only assumes that the universe is infinite in size but also infinite in age, And that's necessary because if a star is super duper far away from us, it's light will take a long time to get here. If the universe is infinite in age, then it will get here, and it will already be here because that stars existed infinitely in the past and it's had plenty of time to get here. But if the universe is not infinitely old, if it started a certain number of years ago, that means that there are some stars who are so far away that the light from them has never reached us, that not a single photon from that star has arrived here on Earth. It's always surprising to me how quick physics goes from math to like existential questions. So was Poe sort of on track with what other physicists at the time we're thinking, or was he the first one to sort of think about it this way? Or should we wait until after a break and then find out if Poe was right. Let's leave our listeners in deep Dark Edgar Allan Poe suspense and come back after the break. All right, I've been dying to know. Was Edgar Allan Poe right. Edgar Allan Poe was totally correct, and he was also sort of a vanguard of thinking at the time. Remember that until Hubble saw that the universe was expanding, this is the beginning of the nineteen hundreds, people thought it was very likely that the universe was infinitely old, that it didn't have a beginning. The natural concept was that everything is just hanging in space. Nothing is changing. I mean, you don't see stars blinking out, our new ones appearing. They thought the universe was a turn and static, and so it wasn't until we saw that the universe was expanding and that we could sort of wind that history backwards to some time when the universe was infinitely dense, before which it didn't even make any sense to talk about time. So this whole idea of the age of the universe being finite, being about fourteen billion years old, that's only about a hundred years old. Before that, even mostly thought the universe was static and infinitely old. So I was sort of ahead of his time here imagining this as an explanation for why the night sky was dark. He's so great. He has a story the title of which I forget, where essentially one guy locks or says like, oh, let's go find this wine which is in this basement, thank you, yes, and he locks the other guy in the basement and then he leaves. And a collaborator and I were talking about how that's sort of the way, sort of like how parasitoids lock their hosts inside the goals that they make. Anyway, you know, we we love connecting literature who are wasps whenever we can, and that was one of our connections. But anyway, I digress. That means that Edgar Allan Poe is sort of like our two degrees of separation, right. He wrote about parasitology and he wrote about cosmology, so he connects us well, except he didn't think he was writing about parasitology, whereas he did think he was writing about cosmology. But sure, yes, let's go with it. He was just so far ahead of his time he didn't realize he was breaking new ground in every field. Okay, so we are back to an area where my brain sort of has trouble keeping up with the distances and the speeds and the sizes. So okay, so the universe is expanding, and there we're looking at like the speed of expansion relative to the speed of light. So does this mean that, like, at some point in the future, the light is gonna catch up to us and the night sky is going to be bright? Or am I missing something fundamental? I think I'm missing something fundamental. No, you're absolutely not. That's a great question. So let's take it one step at a time. So first we say the universe is not infinitely old, which means that light from some places hasn't reached us yet. So you can imagine maybe the universe is infinite, maybe it goes on forever, but we can only see a part of it. The part of it that we can see is determined by the speed of light and the age of the universe. So there are things out there that have sent us photons that are on their way screaming at the speed of light across billions of light years, but have not yet arrived, but will in the future arrive. Right, We'll get here at some point. While that explains why the night sky is not infinitely bright right now, it makes you wonder, like, does that mean it will be much brighter in the future. Right in that sort of scenario, you can imagine the nights guy getting brighter and brighter every year is sort of like more stars appear in the night sky, so that eventually it's sort of like fills in all the gaps, right, and then you really need Star Screen. I feel like we've got to start this company before this episode goes out, because otherwise somebody else is going to pick up on this idea and we're gonna be really grumpy that they make a billion dollars off of star Screen. I agree. I think this is going to be the thing that makes us forget our books and everything else. Star Screen start exactly endorsed by astronomers. We need like seven out of a astronomers agree or something. Yeah, that's right, that's right. Let's convince people that are have to worry about medical doctors, they should listen to uh any old specialist in anything, as you might worry about that the future of the infinitely bright night sky. And while I might want you to believe in that, so you buy a big tub of star screen and smear on your kids, it's not actually something we think is going to happen. We actually think the opposite is happening. That as time goes on, the nights, guy gets darker and darker and that's for the reason that you mentioned earlier, that the universe is expanding, Right, So even in the universe that wasn't expanding, where the stars are just sort of like hanging there, but the universe is not infinitely old. You wouldn't expect the nights guy to be infinitely bright, but it would be getting brighter and brighter as time goes on, and the size of the observable universe, the fraction of it that we can see got larger and larger. But what's happening is that the universe is expand ending, and so things are rushing away from us, and they're rushing away from us faster and faster every year. So it's this sort of weird effect where we can see a larger portion of space, but things are sort of moving out of that space faster than light itself. Right, So imagine like a sphere that's growing. That's our observable universe. We can see anything in that sphere, but things are rushing out of that sphere faster than the sphere itself is growing. So the expansion of the universe, the creation of new space between us and other galaxies, is decreasing the number of things in the observable universe, increasing the darkness of the night sky. So is it that as it moves away super fast, that our square distance is increasing so much that it dissipates before it gets to us, or the light never gets to us period because it's moving away so quickly. Both effects actually are happening. There's some things that we can see now. So their photons have reached Earth, and some of the photons they have sent in the meantime will reach Earth. But there are some photons that are traveling through space, and that space is expanding faster than the speed of lights, so those photons will never reach us. It's like if Hussain Bolt is running at you, but somebody's laying new track between you and him faster than he's running, He's never gonna get it to you, no matter how fast he is. Right, you have to be laying pretty fast. And so there are some things out there that we will eventually see that we have not seen yet, things that are about sixty two billion light years away. There are some photons they admitted that will get here and we will be able to see them, But anything past sixty two billion light years we will never see it because the space between us and it is expanding so fast that all of its photons are basically moving away from us. So they're moving locally through space to the speed of light, but globally, because that space is expanding between us and them, they're actually moving away from us, which is really weird because like, photons pointing towards us are actually getting further away from us. For a year, nothing about physics is intuitive, or plenty of it is, I guess, but plenty of it's not okay, And so then that is the final answer, Like is that an answer that we definitely know to be true or is this sort of on the Like we revisit this in a couple of years and people will be talking about how cute it was that Daniel and Kelly thought this was the answer, Like, how confident are we that this is the truth? Oh, you're definitely trying to trap me here. You want to sound by so in five years you can go like, man, Daniel, you were so overconfident. We should definitely always be taking these things with a grain of salt, because our understanding of them is fairly recent. Our measurement of the expansion of the universe and its acceleration is only twenty years old, and that's a blink of an eye in terms of cosmology and science, right, And so while we're fairly certain that the universe is expanding and that that expansion is accelerating, we have no idea what's causing that. We call this dark energy, but that's just like a name we give it because we have no concept of what could be causing it. Is nothing in our equations that can't explain it. You can put a number into Einstein's theory of general relativity to describe this happening, but that doesn't explain why it's happening, Like what part of the universe is doing this? Why does it have to happen? Could it stop and turn around and do something else in the future. We just don't know. So there's a heavy dose of like, oh that should be added to this, But it's sort of like our current understanding of what might be going on with the universe. And so does this answer help us answer the question about the universe being infinite or no? Because you it can be infinite well expanding, because infinity is also complicated exactly. Infinity is though complicated. Unfortunately, what this tells us is that the question of whether the night sky is dark can't tell us whether the universe is infinite, because an infinite universe and a non infinite universe both can give you dark night skies, so you can't tell the difference. Sorry, olders, but that's not even the full answer. There's another reason why the night sky is dark in an expanding universe, and I think you i'd have mentioned earlier, which is while space is expanding, it doesn't just affect the space between us and other galaxies. It also stretches that light. So if space is expanding, if it's getting larger, that means like new spaces being created, and so photons that are flying through that space, they get stretched, their wavelengths get longer, they get red shifted. So your photon, for example, that was emitted from the cosmic microwave background, this super hot plasma that existed just after the Big Bang. That plasma was really really hot, and the light that it generated was very very high frequency, short wavelength. But it's been flying through the universe ever since and it's been getting stretched out to very long wavelengths. So does that mean that the night sky could be bright but just at a wavelength that we don't see. So, yeah, what does that mean. It means the night sky actually is kind of bright, but just in wavelengths we cannot see. So some of the starlight that's been emitted has been red shifted outside of the visible spectrum. Like, there are stars out there that are shining that are bright, but your eyeballs cannot see them in optical telescopes cannot see them because their wavelengths have been shifted into the infrared. So the night sky is actually brighter than you see, right, because there are all these invisible stars out there that are shining at us in the infrared that our eyeballs cannot see. Don't some species see in the infrared? So like, are there some species out there for which the night sky is bright? That's a great question. I should ask my biology friend about that, because I don't know not this one. But we did an episode recently about eyeballs and how they work, and we do know that different species are sensitive to different wavelengths of light. But you know, we are very ingenious as a species, and we're capable of building other kinds of eyeballs, and so we can build telescopes that can see in the infrared and can look up at the night sky in the very very long wavelengths. And something that's really fun to realize as we think about this question is that if you look deep enough into long enough wavelengths, the sky actually is very very bright cosmic micro background radiation that's everywhere, that's in every direction. No matter what direction you look at in the sky, you will see some of it. So if your eyeballs could see the cosmic microwave background radiation is light from the very very early universe plasma plasma that filled the whole universe, then the night sky would be bright, all right, So we gotta find a star screen angle to like, we gotta make sure everybody knows this and then explain to them that the star screen protects against these other parts of the spectrum, and that's where the millions are going to come from. Right, and not to confuse the listeners, the nice guy is not infinitely bright in the cosmic microwave background radiation. Right. Ober's paradox, this question of why we weren't seeing light in every directions really suggested that the nice guy should be infinitely bright. The cosmic microwave background is almost uniformly bright because it comes not from like little lots of stars, the way optical visible light does, but because it came from this plasma which filled the whole universe back before the universe was itself transparent. And so we can still see those photons, and we see them in every direction because that plasma used to be filling all of space in every direction. So if you were a little creature like a parasite that looked up by the night sky, you could only see the microwave, then at night the sky would be uniformly bright. Wouldn't be infinitely bright, right, but it would be uniform It would look very different from our night sky. It's important to know how the parasites see things. So I've had like three four, maybe five existential crises during the span of our talk here. So what what does this all mean then? So what it means is that while we can think about the size of the universe and what it might be made out of, and we can play these games trying to figure out how the universe actually looks, we can't actually tell what's out there past our observable universe. You know, this felt like maybe a trick, like maybe a way to figure out what was in the deep dark ancient depths of the universe, because it should be contributing to our night sky. Really, what this tells us is that we're forever in a bubble. We can't ever see what's past that bubble, so we'll never really know what's out past the observable universe. And even worse than that, our bubble is sort of shrinking, while like physically in terms of length, it's getting larger and larger spaces expanding faster than that bubble is expanding. So it's sort of like our bubble of the universe is shrinking and shrinking, and as time goes on it gets smaller and smaller. It's like a fraction of the universe. So our children in the far far future will see an even smaller fraction of the universe than we are seeing today. So if we want to answer those questions, we better get on it. Like sat, well, I'm feeling slightly depressed now, but I guess I'm going to go ahead and hope that this is one of those like a hundred years from now, they'll be like Daniel, didn't think we could ever study everything, but now we know how to study everything. So I suppose in this particular area, I'm hoping that you're wrong, but you're probably not, and I agree we should be. We should be studying as much of this stuff we can. Now. Well, our great grandchildren will be sitting in their mansions paid for by the Star Screen Empire, and they'll be laughing all the way to the bank. That's they'll be thanking us, and it'll be great. So we still don't know if the full universe is infinite or if it's finite and wraps around on itself forever. It's a question people are still thinking about. People will still be thinking about. Maybe someday in the future people will find a way to probe this question, to sort of like extend our minds past the edge of the observable universe. Currently it feels like that's impossible according to the current laws of physics. But hey, let's hope future Daniel proves today Daniel wrong, and that's how you'll get your Nobel Prize. That's my plan, a good plan. I wish you luck. In case the Star Screen doesn't work out, you can, you know, make all the money that the Nobel Prize winners make. But I think The larger lesson aside from this particular question of why isn't the night sky dark, is that the larger strategy physics is employing a building a model in your head, thinking about the consequences of it, and then asking yourself like, well, why doesn't that work? What does that mean that the ideas I have don't describe the universe I'm seeing? That's generally the best way to win a Nobel prize is to try to find some discrepancy there, to discover some thread to pull on to unravel some deep mystery of the universe. It definitely is beautiful the way these models that you create in physics can have these amazing existential implications the more you think about them, and that in our minds we can imagine what would happen in an infinitely old universe with an infinite number of stars beaming an infinite number of photons to us. Somehow the math all hangs together. It's incredible. Well, thanks everyone for going on that journey with us to answer the question why is the night sky dark? After all? Why don't you need to buy a big tub of star scream to protect your children from star cancer? Don't tell them that in the end we believe in honesty and advertising. So thanks everyone for join us, and thanks again Kelly for coming along, asking great questions and contributing a lot of laughs. Thank you for having me a head fun as always. Thanks everybody until next time. Alright, tune in next time. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio. For more podcast For my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.