Could a huge void cause the illusion of dark energy?

00:00:07
Speaker 1: Hey, Daniel, what are we talking about today?

00:00:09
Speaker 2: On the episode, we are talking about avoid.

00:00:13
Speaker 1: As in you're gonna avoid my questions.

00:00:16
Speaker 2: No voids as in vast empty spaces.

00:00:21
Speaker 1: Is there a scientist that discovered them? And if so, is it kind of a scam because you just discovered nothing.

00:00:27
Speaker 2: I'd love to get famous for discovering nothing. That's basically what I've done my whole career.

00:00:32
Speaker 1: Same here, same here.

00:00:33
Speaker 2: For that, you got the no Prize instead of the Nobel Prize.

00:00:36
Speaker 1: Oh, you don't get to eat bells and whistles, so you do get the Nobel Prize.

00:00:41
Speaker 2: I think I want to avoid myself with these jokes.

00:00:43
Speaker 1: Avoid. I am Jorge mcgartunez, a author of Oliver's Great Big Universe.

00:01:03
Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I got into this business to make radical, universe changing discoveries. But I haven't made any.

00:01:14
Speaker 1: The whole reason we're here, it has been all for nothing.

00:01:18
Speaker 2: Well, you know, research has no guarantees. You could go out there into the vast plane of undiscovered territory and find all sorts of incredible stuff or just nothing. We have no promises from nature.

00:01:30
Speaker 1: Well, Daniel, you sound a little defeated. Did you have a lot of years ahead of you in your career?

00:01:37
Speaker 2: No, I'm not defeated. I rolled the dice and I knew there were risks, and you know, physics is a lot of fun along the way even if you don't make a big discovery.

00:01:45
Speaker 1: But don't you have like at least twenty thirty years of work still or are you just planning to coast the rest of the.

00:01:53
Speaker 2: Way on adviceive counsel. I'm going to not answer that question.

00:01:58
Speaker 1: You're like, it's called being emeritus.

00:02:02
Speaker 2: I'm rapidly approaching emeritus.

00:02:04
Speaker 1: Well, whether you're emeritus or active, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.

00:02:11
Speaker 2: In which we guide you through all of the incredible, mind shattering discoveries of the last few hundred years that have revealed a universe that's weird, that's bizarre, that's strange, and yet somehow maybe understandable, And we also try to guide you through potential discoveries in the next few years, in the next decades, may be made by me or my team, or by some of the listeners to this podcast. Or they are great great grandchildren. Whenever those discoveries come, we want you to be prepared for them by understanding what we do and don't know about the universe.

00:02:43
Speaker 1: That's right, because it is an incredibly huge and amazing universe out there, full of incredible phenomenon, astounding events, and also a whole lot of moving.

00:02:53
Speaker 2: Indeed, the universe really is something. Even if that's something is often nothing. But there's lots of mysteries remaining about the nature of the universe. As much as we've discovered in the last one hundred years about how big the universe is, about how it's expanding, about how that expansion is accelerating, about how galaxies swirl around invisible dark matter, there's still a lot of pieces that don't fit together, which tells us probably the next few decades or few hundred years will reveal even more mind blowing discoveries about the nature of our cosmos.

00:03:25
Speaker 1: Yeah, the amazing thing about this universe and about science is that all the pieces are out there for us to see, for us to discover, for us to observe, and for someone maybe you, maybe us, to put it all together and make sense of this amazing cosmos. Oh wait, wait, sorry, sorry, not Daniel, he's already given up.

00:03:45
Speaker 2: Hey you know, if it falls to my lap, I'm not saying no.

00:03:48
Speaker 1: But yeah, well it sound like like you just throw it in the trash.

00:03:53
Speaker 3: You know.

00:03:53
Speaker 2: The universe is like a giant mystery novel. Often all of the clues are out there, staring us in the face. This requires the insight, that moment of clarity to see how it all fits together into one coherent story. And currently we're struggling a little bit to understand all of those clues to tell a story that makes sense to us and that holds together mathematically. And it could be that some elements of our current story are drastically wrong. We've certainly done that lots of times in history, gone down the wrong path for decades or even centuries before having to backtrack and come up with a completely new, radical interpretation of how the universe works.

00:04:32
Speaker 1: Yeah, leaving all those other scientists to have discovered nothing. But all the clues are out there, but one of the problems is that they're really far away. That's because the universe is full of nothing, a lot of big empty void in spaces that are preventing us from reaching and touching some of the important clues that are out there. But there is maybe the idea that some of these huge voids of nothingness could maybe hold the answer to one of the biggest quest questions in the universe.

00:05:01
Speaker 2: That's right, Maybe the biggest question in modern science today is what is driving the expansion of the universe? Why is it going faster and faster every year? We have a placeholder idea we call dark energy, but as you'll hear, it's not even really a single coherent concept and we might need completely new explanations for it.

00:05:21
Speaker 1: So today on the podcast, we'll be tackling the question could a huge void cost the illusion of dark energy? So wait, what are you saying, Daniel that dark energy is not something and in fact it might be nothing.

00:05:41
Speaker 2: Wouldn't it be a huge dramatic plot twist if the answer the biggest question in science was nothing.

00:05:50
Speaker 1: I feel like we can already preview that, like what came before the universe nothing? What's going to happen after the universe nothing? We're just to blipp and the nothing is I.

00:06:01
Speaker 2: Think the answer to both of those questions is actually we don't know or we have no idea, which is a little bit different from nothing. And you know, philosophically, nothing is a complicated concept, like what does it even mean? Right? We've talked on the podcast about how space is never really empty and all of it has quantum fields and it, so what does nothing even really mean? It's actually quite a deep philosophical question.

00:06:23
Speaker 1: It is very tricky.

00:06:24
Speaker 3: Yeah.

00:06:24
Speaker 1: For example, I have a doctorate in philosophy, but I know nothing about philosophy.

00:06:30
Speaker 2: So you're saying you know nothing. Wow, you're an expert in nothing.

00:06:34
Speaker 1: Well, I guess that's something.

00:06:37
Speaker 2: I would say. I'm no expert in nothing. Nothing is actually quite complex.

00:06:41
Speaker 1: I lost track of the negatives there, No, you didn't, No, I didn't. No, I didn't not know nothing about something?

00:06:50
Speaker 2: There you go. Well, let's check in with the listeners to see if they don't not know nothing about nothing?

00:06:54
Speaker 1: Yeah, as usual, Daniel went out there to ask people if they think maybe a huge void could be causing what we see as dark energy.

00:07:02
Speaker 2: Thanks very much to everybody who's sent in their answers. I would love to hear your voice on the podcast. That's right, I'm talking to you. You've been listening for years but never chimed in. Today's the day right to me two questions at Danielanjorge dot com.

00:07:17
Speaker 1: So think about it for a second. Do you think dark energy could be explained by a huge amount of nothingness? Here's what people had to say.

00:07:26
Speaker 4: I don't think so. Even without the accelerated expansion of the universe, it's still possible to have huge voids in space. Perhaps if we were able to detect that a huge void wasn't there before, we might be able to use that as evidence towards the cosmological constant, But I think that would take a long time.

00:07:46
Speaker 3: Perhaps the void is filled with dark energy which we cannot yet detect, So perhaps these areas could be responsible for the accelerated expansion rate, if only we could learn what dark energy truly is.

00:07:57
Speaker 5: I'm going to assume in your definition of you ascribe to quantum field theory and avoid in space would be an area where the fields from all the fundamental particles are somehow blocked from this area. One could posit that some of these fields might feel a sort of pressure to fill the void, causing an expansion. So, sure, I'll say it could.

00:08:19
Speaker 6: Aha dark energy explain. Sure it could be space expanding into a void. But then where is that void? What is that void made out of and how is it expanding into it? And what does that even mean? I don't know about this one.

00:08:36
Speaker 2: I'm confused by that comment. What's the difference between a huge amount of nothingness and a small amount of nothingness? If they're all nothing, it's like ten times zero versus one time zero.

00:08:45
Speaker 1: I think the difference is a nothing.

00:08:50
Speaker 2: Nice. Well, I like these listeners answer some really well informed thoughts here. You seem surprised, Daniel, No, I'm impressed as usual.

00:08:58
Speaker 1: Had you given up on our list as well.

00:09:01
Speaker 2: I think your reaction is based on nothing. I have nothing but respect for these listeners, their efforts to understand the universe, their willingness to contribute. I'm in awe.

00:09:10
Speaker 1: Well, they do have some interesting answers here, and it's kind of an interesting question that maybe the biggest part of what we see in the universe, ninety five percent of all the mass and energy in the universe, maybe is not really something or mass or energy. Maybe it's just something that can be easily explained with nothing.

00:09:29
Speaker 2: Yeah, that's right. In order to explain this accelerating expansion of the universe, we've had to add into our equations, something very weird, very awkward, something we really struggle with, and so people have been working for a long time to find alternative explanations, sort of more prosaic ideas that would explain the strange things we see out there in the universe.

00:09:50
Speaker 1: My question, Daniel, is why didn't we think of this before? I mean, well, when you see something mysterious, shouldn't the first thing you ask be maybe it's nothing.

00:10:00
Speaker 2: Yeah, it's a good question. This particular idea that we're going to talk about today is uncomfortable in other ways for scientists. It violates some like philosophical principles that they really hope the universe respects. And that's why it's a little bit weird and a little bit uncomfortable to consider. But hey, sometimes when the universe confronts you with data, you just got to accept it.

00:10:21
Speaker 1: I mean, after like twenty thirty years of trying to explain it in other ways.

00:10:24
Speaker 2: I guess, yeah, exactly.

00:10:27
Speaker 1: Just because it makes it uncomfortable.

00:10:29
Speaker 2: I like that the universe makes us uncomfortable. I like, when we go what it doesn't make any sense at all? How could the universe be that way? Those are the moments we really learn something when we really have to confront some fundamental assumption that is probably wrong.

00:10:43
Speaker 1: Right, right, you like it, but only if you're sitting comfortably in your couch.

00:10:46
Speaker 2: Right, yeah, exactly. Let me get comfortable before you make me uncomfortable.

00:10:52
Speaker 1: Yeah, let's draw online here. I want to be physically comfortable but intellectually uncomfortable.

00:10:58
Speaker 2: Is that what you're saying, Yes, serve me some chocolate while you tell me the results of this study. Yes, absolutely, a little bit of sugar makes the medicine go down.

00:11:07
Speaker 1: Right, right, you've given up even getting up up for your couch. But anyways, Daniel, for those of us who are sitting in our couches, wait to hear the answer. What you start us with the basics? What is this thing that we call dark energy?

00:11:22
Speaker 2: Yeah, so the thing that we're struggling to explain is something we call dark energy. But dark energy is not really like a theory as much as a description of something we observe in the universe for which we don't have a great explanation. And that's the fact that the universe is expanding, and it's not just expanding the same rate every year. It's expanding at an accelerated rate. It seems like the distances between clusters of galaxies is increasing, so that's expansion, but it's increasing faster and faster every year. This is something we discovered about twenty five years ago now, when we first learned how to figure out how far away things are in the universe, and it was really a shocker. You know, people before that were wondering how quickly the expansion of the universe was slowing down. That was the big question in science because you have the universe expanding when it's very very young, but then it's filled with matter. There's all this heavy stuff in the universe, galaxies and black holes and whatever that tends to pull stuff back together, and so people were wondering, hey, is there enough gravity to pull stuff back together to make like a reverse big bang, like a big crunch, or is there not enough and it's just going to gradually slow down for a long time to keep drifting apart. And when they went out to measure it, they discovered, shocker, neither of those were true. The universe preferred secret answer CEE, which is that things are getting further apart faster and faster. So dark energy is sort of like our placeholder name to explain how that might be happening.

00:12:49
Speaker 1: So, I think we've known that the universe is expanding for about one hundred years, right, But you're saying it's only recently we figured out it's accelerating.

00:12:58
Speaker 2: Yeah, if you want to go further back into history, around the time that Einstein was developing the theory of relativity, we thought the universe was static. We thought that there was just like our galaxy hanging out there in empty space and that was it. Then Edwin Hubble and Henrietta Levitt developed this way to measure the distance to things in the sky to tell how far away they were, And what they discovered is that some smudgets we saw in the sky that we thought were just like clouds of gas in our galaxy were actually much further away. There were further than any of the stars in the sky. There were actually other galaxies. And on top of that, those galaxies were moving away from us. So we discovered that the universe was not only just our galaxy, it was filled with galaxies, and that the universe was expanding. Everything was running away from us. That was about one hundred years ago.

00:13:44
Speaker 1: And how do we know they were moving away from us from the red shifting the Doppler.

00:13:48
Speaker 2: Effect, Yeah, exactly. You can look at the light from those galaxies and you can see how its frequency shifted things that are moving away from us. As you said, the Doppler effect, the light from those things is shift in frequency, and you can tell because we have an idea for what frequency of light should be emitted by galaxies to have these particular fingerprints, and so when they're shifted over twenty nanometers or one hundred nanimeters or whatever in wavelength, then we can tell. And that we use that to measure the velocity.

00:14:17
Speaker 1: And so back then, did we think or measure that everything was moving away from us at the same speed or we just didn't know enough to know at what rate things were moving away from us.

00:14:29
Speaker 2: Back then, Hubble's discovery allowed us to measure the distance to other galaxies, but sort of only in us nearby sphere. Hubble couldn't tell that the universe was expanding and accelerating because he couldn't measure the distance to far away enough stuff. He and Henriette Levitt developed this technique to measure the distance to stuff looking at these particular variable stars called cephids. But you had to be able to see them, and if galaxies were far enough away, then you couldn't see those stars in the galaxies, so he could sort of only see a little bubble. But about twenty five years ago we developed a new technique that allowed us to see much further and to measure the distance much further, and so then we could see the change over time as we look back into the history of the universe, and we noticed this effect. So Hubble couldn't see it because they didn't have enough data. It didn't have a long enough lever arm in history to see things changing.

00:15:17
Speaker 1: It was being able to tell how far away things were and then measuring on top of that their velocity that let us figure out that things we're accelerating.

00:15:25
Speaker 2: Yeah, exactly. It was the development of this technique using type one a supernova supernova of course, the implosion of stars leading to an extraordinarily bright explosion, so bright that they outshine the galaxies therein. And so you can measure these things for very very distant galaxies, and understanding the physics of it, and understanding how bright they are at their source without knowing how far away they are, lets you figure out how far away they are by looking at how bright they are here on Earth, because things here are dimmer when they arrive, so they'll lets you know how far away things are, and that's absolutely crucial because knowing how far away some thing is tells you when the light left it right. And so as we look further into space, we're looking further back in time. So that's how we can see the history of the expansion of the universe. We can see how fast is stuff moving away from us that's close by i e. Recent history, and then we can ask how fast it was stuff moving away from us earlier in the universe by looking further away. So this is absolutely crucial to understanding how dark energy works and also how it might be wrong. What we see is that the expansion is faster for things that are closer to us, and we interpret that to mean the expansion is speeding up, so deeper into space the expansion looks slower. Closer to us, the expansion looks faster. That's what leads us to conclude that the expansion is accelerating.

00:16:45
Speaker 1: So things we can tell are really far away don't have as much of a red shift as the things that we can see that are closer to us.

00:16:53
Speaker 2: They're further away, so their actual distance, the red shift is larger, but hasn't accumulated over time, So it's the acceleration is like the slope of that velocity curve. So those things are very far away, they are very very red shifted, but the change in that slope we can see over time as we look at these things from further away to closer in.

00:17:14
Speaker 1: All right, So then that tells us that the universe is expanding more and more rapidly. And we have a name for that, which is dark energy, or at least the thing that might be causing this acceleration. That's what we call dark energy.

00:17:27
Speaker 2: Yeah, we call it dark energy. And I feel like there's a lot of confusion out there about what we do and don't know, and I think this is really interesting insight into sort of the way physics works. Like we often begin thinking about an idea before we have it all worked out. We just sort of like start fleshing it out the way you might like design your house, starting from what you wanted to look on the outside before you figured it out, Like, hey, what are all the structural supports and can I really have pipes over here? And is it possible to have a shower on the roof or whatever. You know, you haven't always figured out the details, some of which might be absolutely essential. And so if you look at the beginning days of the.

00:18:01
Speaker 1: Development, I know every house you would have a shower on the roof.

00:18:05
Speaker 2: My wife is a big fan of an outside shower. She's always wanted to put one in. So I'm always suggesting, let's put it on the roof. And I don't know why that's not a popular idea. But anyway, if you look at the history of the development of big ideas and physics, this is how it works. And so we've done that with dark energy. We're like, wow, this is weird. How do we explain this? Okay, first, let's give it a cool name. Now, let's start to think about how it might be possible. And we definitely don't have all the details worked out. We have some conception of how in general relativity you might generate this effect, we definitely don't have all the pieces together.

00:18:39
Speaker 1: And we say that it's ninety five percent of the energy, mass, and energy of the whole universe, because that's how much energy you need to explain something that's accelerating the entire observable universe.

00:18:52
Speaker 2: Yeah, it's something like seventy percent. Dark matter and dark energy together make ninety five percent, but the dark energy portion is set tventy percent. Yeah, and you're exactly right. And the way we get that seventy percent is a few different ways. But one way is to say, all right, how in general relativity could you cause the universe to expand? Because usually gravity causes things to come together. Right, you have two masses in space, they curve space, they tend to move together. But general relativity is much more complex than Newtonian gravity includes all sorts of complicated terms and effects in there. And one of the possible effects in general relativity is if you have a field in space, like the electromagnetic field or the weak field or the electron field. We know space is filled with these fields, but if these fields have a lot of potential energy, meaning they have like stored energy inside of them because of their configuration the way the Higgs field does. For example, having a lot of potential energy in space will generate this expansion. Just like part of the mathematics of general relativity. You have a negative sign in front of this term in the equations, and so it generates expansion of space. So if you had a field with a lot of potential energy, it would generate the expansion. So we calculate how much potential energy do you need, and that comes out to be about seventy percent of the energy in the universe.

00:20:05
Speaker 1: So this whole idea that dark energy is seventy percent of the universe comes from the assumption, if I'm understanding right, the assumption that what's causing the acceleration of the universe is something kind of baked into space itself.

00:20:17
Speaker 2: Yeah, exactly, And there's some supporting evidence for it, and there's also a bunch of stuff that doesn't work about it. Like one supporting piece of evidence is that we can go out and measure all of the energy in the universe, because it affects the overall curvature of the universe, Like is the universe overall flat or curve negatively or curved positively. That depends very sensitively on how much energy is there in the universe. And we go out and we measure that, and then we measure the energy out there in the universe to be exactly what we need to make it flat and to add up very nicely with this seventy percent number. So the dark matter, the normal matter, and the dark energy in the universe all adds up perfectly to make the universe flat, which is what we observe. That's very cool.

00:20:56
Speaker 1: Oh interesting, All right, Well, let's get into the things that don't work about dark energy and why maybe nothingness could be the key to understanding how it all works. So let's dig into that. But first let's take a quick break. All right, we're talking about dark energy and giant voids in the universe, and could those two things be related. Could dark energy just be a whole bunch of nothing. Could it be explained by perhaps the absence of things in space? And so we talked a little bit about dark energy what it is, but we know about it. But Daniel, there's something wrong with that picture of dark energy. What's wrong with it.

00:21:47
Speaker 2: What's wrong with it is that we have no idea where all this potential energy would be coming from. In order to have the universe expand and accelerate at the rate that it has been, we need space to be filled with some kind oftential energy.

00:22:01
Speaker 1: Well, here's a question, like, does dark energy need to be something that's like a field or baked into space, or could it be some maybe hidden property of gravity perhaps or electromagnetism, or is that the same thing.

00:22:13
Speaker 2: It could definitely be something totally brand new, some new kind of physics we've never seen before, or change in general relativity. That's definitely possible, and there are people out there working on other crazy ideas. But so the mainstream effort is to say, like, well, can we incorporate this into general relativity, because it's kind of interesting that general relativity has this capacity already. It's something that Einstein's relativity can do expand space and accelerate that expansion. The problem is that to make that happen, you need to fill space with all of this energy. Either you say, look, that's just the way it is. You just put a number in there, and that's called the cosmological constant and you say, like, it's not explained, that's just the nature of space. It just has this energy, kind of unsatisfying, or you find the source of that energy you say, oh, look it's this particular quantum field that can do it. And we look through our list of quantum fields we ask, well, how much potential energy do they have? And one of them, has a lot of potential energy. The Higgs field is filled with potential energy. Remember that most of the fields in the universe like to relax down to zero. They like to be in their lowest energy state. But the Higgs field is really weird, has this wrinkle in it, so it sort of gets stuck at high potential energy when the universe is cooling. The Higgs field doesn't collapse down to zero the way the other ones do. It has all this potential energy. All right, So this is really exciting, right. Dark energy needs some field filled with potential energy to explain the accelerating expansion in the universe, and over here we have a field filled with potential energy. Awesome. So people sat down to do the calculations and say, does the Higgs field have enough energy to explain the accelerating expansion the universe. The answer is no, and it's not even close. The answer is off by ten to the one hundred and twenty. So the fields we have can definitely not explain the expansion of the universe as we see it.

00:24:02
Speaker 1: But I guess if it's not a field, if it's something else, would it still account for seventy percent of the universe, or like, what are some of these other things that could.

00:24:11
Speaker 2: Be ooh not a field, man, That blows my mind. Currently, our theories of the universe are that everything's a field, right, Like all the particles are ripples in fields, and all the energy out there is stored in some kind of field. So mostly people are developing like new kinds of fields. Maybe there's some other kind of field, not the Higgs field, something else out there in the universe that contains all of this energy, and then you have to explain, like why we wouldn't have seen it and has no other effects that we could detect. For example, that's the way a lot of people are going. Non field explanations for the accelerating expansion in the universe. Those are pretty far out of the mainstream, but you know, it could be right. Physicists tend to work like within the framework of the ideas we have. Doesn't mean that that's where we're going to find the answer.

00:24:55
Speaker 1: Or what if it's like a part of gravity itself or general relativity like maybe Grouty gets it turns the negative if you get really far away from something, would that still be a field? Technically?

00:25:07
Speaker 2: I don't think that's compatible with general relativity. So in order to have that be the explanation, you have to change general relativity. And the thing people like about this dark energy framework is that you don't have to change general relativity because it's been tested up and down the wazoo and it really is accurate for huge distances or huge distances yeah, I mean, it explains an enormous amount about the structure of the universe as we see it, and so changing general relativity is pretty uncomfortable, except in places where it hasn't been tested, like inside black holes or the very very early universe, things we haven't been able to observe yet, basically where quantum mechanics arises. So it's difficult to make changes to general relativity without messing up a lot of other.

00:25:48
Speaker 1: Stuff that would just be uncomfortable and also require a lot of work.

00:25:54
Speaker 2: And it could be the path to huge discoveries. But you know, the way physics works is, hey, let's try the easiest thing first. And so there is this door open in general relativity, like, let's see if we can explain it using the framework we have, which requires discovering this field that explains where all the energy is and that we don't have an answer for.

00:26:12
Speaker 1: So then that's the problem with our current theories about dark energy is that you think it requires a field, but we don't know what field it could be, because it needs to have more energy than anything we know about.

00:26:21
Speaker 2: Yeah, that's the major problem, sort of the conceptual problem. There are also a few smaller problems, experimental problems, like our picture of the universe as being mostly dark energy and a big chunk dark matter and a little bit of baryonic matter and normal matter. It explains an enormous amount. It explains how the universe started out filled with radiation, and then that radiation got diluted and it was matter dominated, and it was expanding, but that expansion was slowing down. But the expansion led to more dark energy, which then accelerated the expansion, and the universe became dark energy dominated like six billion years ago. It's an amazing, beautiful story that explains an enormous amount, but it doesn't quite explain everything. Like there's this discrepancy between our observation of how fast the universe is expanding now and how fast it was expanding the very early days. This is called the Hubble tension because the Hubble constant is kind of a measure of the expansion rate, and that's not something we can really explain. We don't really understand how the expansion the early universe connects for the expansion in late times. At first, it was very fuzzy measurements. We thought, I will figure it out. But as we make more and more precise measurements, they don't really agree. So that might require a tweak to this, or it might be a crack that undermines the whole theory. So it's not like it's a perfect description of everything we see, even if you knew where this energy was coming from.

00:27:38
Speaker 1: So then I guess what's the alternative if it's not something like a field.

00:27:43
Speaker 2: So one of the alternatives is to question one of the basic assumptions of this whole picture of the universe that we've been talking about. We usually talk about the universe in terms of density, Like when we say seventy percent or twenty five percent, we're saying, take a random chunk of the universe, how much of that is dark energy that is normal energy. When you do that, you're kind of making an implicit assumption that all chunks of the universe are about the same the stuff is spread out through the universe basically evenly, like we know, it's not completely evenly. There's definitely clumpiness, like we're a big clump in the universe. The Earth, the Sun, the galaxy is a clump. But that if you zoom out far enough, the universe is evenly spread out on the giant scales. The universe is basically the same everywhere. That's kind of a philosophical preference. They call it the cosmological principle or the Copernican principle. But one of the ideas to explain the accelerating expansion of the universe is to say, maybe it's not expanding in an accelerating way. Maybe it just looks like it is because we're stuck at the center of a huge void where there's less stuff in this bubble than outside of it, and that gives us the illusion that the universe's expansion is accelerating, even if it isn't.

00:28:50
Speaker 1: Whoa, whoa, whoa Wait a minute, how does this even work? Is the idea then that maybe outside of the absorbable universe there's a whole bunch of nothingness for a while. Is that kind of what the picture you're thinking about.

00:29:02
Speaker 2: The idea is that the bubble we're living in, even though it has galaxies and stars and whatever, is less dense than the stuff outside some bubble.

00:29:10
Speaker 1: But then wouldn't we be able to see those things outside the bubble and tell if it's more or less dense.

00:29:17
Speaker 2: Yeah, and we can get into the observations and whether this idea actually lines up with what we see in the universe. But if this bubble exists, it'd be a little tricky to see because we'd be looking really, really far out, and our sort of three D map of the universe is best for close by stuff, and it sort of fades out a little bit as you get further out. It's hard to make these big three D maps of the universe at this void, if it exists, would basically describe everything we see so far. So it's sort of suggesting that out there in the deep dark reaches of space where we haven't really mapped very well yet, galaxies might be a lot denser than they are here. Then that might be confusing us about the expansion of the universe.

00:29:58
Speaker 1: Wait, I think you're saying that maybe you know the galaxies we've seen to come to the conclusion that the universe is expanding in an accelerated way. You know, we use galaxies we've seen, but maybe those galaxies we seem don't go all the way to the edge of the observable universe. Is that what you're saying.

00:30:13
Speaker 2: We've used those galaxies and they do go all the way to the edge of the observable universe, but it might be the things change as you go from here to the edge of the observable universe, and that could be confusing us about this accelerated expansion. The key concept is, remember that we decided that the expansion of the universe was accelerating because we saw that changing as we look further and further into space, right, And we decided that's because things are changing over time. But what if things aren't changing over time, they're just changing over space. Right. What if the universe is different as you go further away than it is here. What if the universe is denser further away and then it is closer by, and that's what's creating this effect. We see an effect in space, and we're assuming that it means an effect over time, But what if it's just an effect over space. What if the universe is different further away.

00:31:04
Speaker 1: I see what you're saying. I think you're saying that some of those galaxies we're using to measure the expansion of the universe maybe where they're not where we think they are.

00:31:12
Speaker 2: I'm saying that we see faster expansion close by and slower expansion further away, right, And we interpret that to mean that there was slower expansion earlier in the universe, and there's faster expansion later in the universe. So the expansion is accelerating. But what if the universe is just denser further away, and so it's not expanding as fast because there's more gravity, it's holding stuff together. And so what if we're confusing nearby faster expansion for expansion increasing more recently, right, what if it's not increasing more recently. It's just we're in a bubble that's expanding faster. If you go further away from us in the universe, things are expanding more slowly because the universe is denser there there's more gravity to hold it together. So instead of saying, oh, the universe was expanding slower further away, back further in time, and it's expanding faster closer to us, more recently, what if it's not a function of time, it's just a function of space because we're in this bubble that happens to be under dense and so has faster expansion because it's less stuff to hold it together.

00:32:13
Speaker 1: Who I feel like my mind is a void right now, Daniel.

00:32:19
Speaker 2: Yeah, if you have more stuff in the universe, if these more galaxies, for example, then there's more gravity to battle against this expansion. Stuff stays closer together.

00:32:27
Speaker 1: So there is still an expansion of the universe.

00:32:29
Speaker 2: Yes, there would still be an expansion, but it wouldn't be accelerating. It's just the expansion depends on how much stuff you have around you, which makes perfect sense.

00:32:37
Speaker 1: So you're saying that there is an expansion of the universe that's still happening. Yeah, but you're saying, maybe, what if it's maybe even and not accelerating. Maybe it's just a constant, steady expansion of the universe, which would still be a mystery. Though.

00:32:50
Speaker 2: Right now, you can have the expansion of the universe without any sort of like weird dark energy in the universe.

00:32:57
Speaker 1: Okay, so then we are assuming still a steady expand But you're saying that maybe the acceilerit that expansion looks faster around us, even though it's not. It just looks faster.

00:33:08
Speaker 2: It is faster around us. It's just that it's faster around us because there isn't as much stuff, not because the expansion is changing over time. There's less stuff near us, and so there's less stuff to hold stuff together, and so that's why it's expanding faster closer than further away.

00:33:23
Speaker 1: Oh, because you're saying that gravity somehow affects the expansion of space.

00:33:27
Speaker 2: Absolutely, Yes, gravity holds stuff together. If there was more stuff in the universe, then things expand more solely because gravity is pulling on them.

00:33:34
Speaker 4: Right.

00:33:35
Speaker 1: Well, but space itself, it doesn't hold space together, or does it. Is that what you're saying, that it somehow holds space itself together?

00:33:41
Speaker 2: Absolutely, the more stuff you have in the universe, the slower things move apart from each other. Remember, we talk about creating a new space, but really we're talking about increasing the distance between galaxies, and that's all we can measure. Right, Space itself is not something where you can measure distances relative to space. It's always distance is measured between objects.

00:33:58
Speaker 1: The things are being held together because in the same way that I'm being held together with the Earth. Or are we talking about some you know, relativistic or something something.

00:34:11
Speaker 2: No, the first way. Yeah, the same way that you're being held near the Earth.

00:34:14
Speaker 1: Oh okay, but I always thought that, like I'm being held to Earth, and so I'm not expanding away from the Earth, but the space that we're both sitting in is expanding.

00:34:22
Speaker 2: Yeah, there's two effects happening there, right, Gravity is holding you together, and dark energy is expanding that space. But gravity winds because gravity is much more powerful over great distances, Gravity weakens, right, and you can't hold things together, so dark energy takes over.

00:34:37
Speaker 1: Right, But like right here on Earth, I'm sitting here on Earth and being held together on Earth. I'm not expanding away from the Earth, but sort of like somebody's moving the chair under me or somebody's stretching the rug under me, the space we're sitting in is expanding, isn't it.

00:34:49
Speaker 2: Yeah that's a helpful way to think about it, but it can also be misleading, right, Like, really, what we're saying there is if you deleted the Earth and replaced it with like a point particle proton, for example, dark energy be increasing the space between you and that point particle that proton. Yes, absolutely, because there is dark energy everywhere. That's the point of that comparison. But these are like two things happening in the equations at the same time. The net effect, because gravity dominates, is that you and the Earth stay close to each other. You can't really separate them out and say they're both simultaneously happening. It's like two force vectors that you add together. If you have a huge amount of force on an object from two directions, it adds up to zero force, right this net zero force. You can talk about what would happen if you were removed one, or you could talk about what happened if you remove the other, but you can't really say that there's any force on that object because they balance each other out in the same way. Dark energy tends to push things apart and curvature tends to pull things together. So you could talk about what would happen if you only had one or the other, but really what's happening to you is a combination of the two.

00:35:52
Speaker 1: All right, So then the idea is that maybe we're wrong that the universe is expanding faster and faster. It's just sort of looks like it because things are denser out there, and so they're moving apart from each other less.

00:36:05
Speaker 2: Yeah, exactly. So as we look further into the universe, which is further back in time, we see less expansion. But maybe that expansion isn't changing with time, maybe it's fixed with time, but it's happening less further away than closer by because there's more stuff further away. Maybe we happen to be in some huge bubble where the stuff near us is under dance. This less stuff in our bubble, and so things can expand further and further away because there's less mass to sort of counteract that expansion.

00:36:36
Speaker 1: I guess what's confusing me is that if it's denser out there in the far reaches of space, things are being held together there. But wouldn't I still see an acceleration or not an acceleration from relative to me.

00:36:50
Speaker 2: An acceleration of the expansion.

00:36:52
Speaker 1: Yeah, Like I always thought that the acceleration of the expansion is because like, the thing I see really far away is moving away from me slower than the things that are closer to me.

00:37:04
Speaker 2: Yeah, that's exactly right. But maybe the reason things are moving away from you. Closer is because of an under density, not because of a change in the expansion over time. Right, Maybe everything has the same expansion over time. The things that are farther away have a smaller expansion because of the conditions over there, not because of some change in the universe over time.

00:37:25
Speaker 5: Right.

00:37:25
Speaker 2: We can only see this one slice of the universe in space and time. We can look further back in time. We can see the universe really far away as it was a long time ago, and we don't know what is that part of the universe like now, And so we see this one particular slice where we can see close by stuff recently and far away stuff a long time ago. So it's hard to tell the difference between things that change over space and things that change over time. And we see a change in that story, and we interpret that as a change over time, but it could be that it's the same over time. It's just changing over.

00:37:55
Speaker 1: Space, meaning that we are measurements of the distance of those things. How far away they are from us or from each other is wrong.

00:38:03
Speaker 2: How far away they are from us? Yeah, imagine you looked out into the universe and you saw that aliens on nearby planets were all eating white chocolate, and as you look deeper and deeper in the universe, you discovered, oh, they're all eating dark chocolate. And you say, well, oh, well, that's older information. The dark chocolate information has taken a long time to get here, and so maybe what's happened over time is that everybody switched from dark chocolate to white chocolate. And that explains why my older observations from distant galaxies the aliens are eating dark chocolate, and my recent observations from nearby galaxies the aliens are eating white chocolate. And then another scientist goes, no, no, no, maybe they just prefer dark chocolate out there and the deep reaches of space, and we live in a white chocolate bubble where everybody seems to eat white chocolate. Those two explanations both work. One is a change over space, the other is a change over time, and we can't tell the difference because we have this one set of observations linked in space and time.

00:38:57
Speaker 1: I'm not sure adding flavor to the this is helping. So then you're saying, like, maybe what's explaining dark energy is that there's more stuff in the periphery of the universe there's more stuff in the perimeter of our vision or of what we can see. Yeah, exactly, but still within the observable universe.

00:39:15
Speaker 2: Still within the observable universe. Yes, of course this is a big idea, and if it's true, you have to make sense of it and why we would be living in the middle of this big void and what else it would mean. And there's lots of ways we can check this idea.

00:39:28
Speaker 1: All right, Well, let's dig into how we're testing this idea. Is it a stretch or is it a whole bunch of nothing as well? So let's dig into that. But first let's take another quick break. All right, we're talking about nothing, Dan, Is this like the Seinfeld episode of the podcast? Just about nothing?

00:40:00
Speaker 2: Long as that can be Sifeld instead of George.

00:40:01
Speaker 1: Than sure, YadA YadA, YadA the universe.

00:40:07
Speaker 2: Or maybe I should be Kramer. I don't know, as long as I'm not Newman, oh Man.

00:40:12
Speaker 1: I want to be Lean. She's the coolest one.

00:40:15
Speaker 2: She definitely is. It is a really fun idea, and it's always cool to think about how your conception of the universe could be totally different from what everybody's been imagining. Right the biggest ideas are the sexiest because they also give you those moments you're like, Wow, the universe is this way and not some other way. And this would be a pretty big idea. I mean, this would be putting our galaxy near the center of an enormous cosmic feature in order to explain the redshifts that we are seeing. To fit this to the data, you have to have a big void, something that's three billion light years in radius a giga parsec.

00:40:57
Speaker 1: And that's the only way this works. Or what does that mean?

00:40:59
Speaker 2: That's the only way this works.

00:41:00
Speaker 1: See only nothing explanation that fits what we currently see.

00:41:04
Speaker 2: Yeah, because when we look out into the universe, we see the sort of same history in every direction. Right, So if you look one direction, you see things closer by expanding more quickly than things further away. If you look in another direction, another direction, another direction, it's very isotropic. And so to explain that by like a coincidence of the density, you need the universe to be more dense basically in every direction, which requires a huge void, and it requires us to be at the center of it, and it requires that void to be basically spherical, So you have the same weird density as a function of distance effect in every direction, and so yeah, that's kind of a big cosmic coincidence if it's.

00:41:47
Speaker 1: True, meaning like we're right smack in the middle of this giant void.

00:41:52
Speaker 2: Yeah, and like it doesn't have to be exactly in the middle to within like a centimeter, but on cosmic distance scales, we'd have to be very close to the center because we've measured this expansion effect in lots of different directions and it seems the same in every direction. So yeah, we'd have to be at the center of it. And this really violates this Copernican principle, this idea that like the universe is basically the same everywhere, and like, yeah, there's a little bit of clumpiness, but there's no huge features that make this neighborhood totally different from that neighborhood. And you know that's not something we know, it's just something we've assumed. It makes sense, it fits with the way we like to think about the universe. Until nature shows us that it's not true. We're kind of going to go with it because we like the idea doesn't mean that it is true, right, It doesn't mean that the universe has to be the same everywhere.

00:42:39
Speaker 1: I guess you know. What's confusing me is like, couldn't we tell if we were in a void, Like you know, we can look with our telescopes and we can see galaxies all the way out to the observable universe. Wouldn't we have noticed by now that there are more of them further away than what we can see close to us.

00:42:55
Speaker 2: Yeah, absolutely, we could tell if we had seen all the galaxies out there and map the density. But you know, things that are really far away are hard to see. In order to see things super duper far away, you have to point a very powerful telescope at them, and you have to look for a while because these galaxies are very very dim and they're very very red shifted. So we have a telescope that can do that, Like James Web has been breaking records and seeing galaxies really really far away into the early universe. But James Web is very expensive, and we have one of it, and it can only point in some directions at a time, and space is extraordinarily vast. The number of galaxies we're talking about are huge, so we basically just haven't really looked far enough like we've seen nearby stuff. And we'd like to think that we've seen to the edge of the observable universe. We only have really done that in a few tiny directions, and so like our detailed three D map of the universe definitely does not extend all the way out to the observable universe. There are lots of places out there where it's very fuzzy.

00:43:53
Speaker 1: But I guess you wouldn't need to see all of the universe. You just need to see whether things are dense or out there near the edge of the abstorbable universe than they are here. Isn't that sort of a easy check?

00:44:07
Speaker 2: Well, we don't see any evidence of that, right, But they could still make the data fit. Because our observations are still limited. They can make this data fit. And so you can come up with a theory of the universe that respects general relativity and describes all of the redshifts that we see from type one A supernova and our observations of galaxy density so far, and doesn't require any dark energy. But it does require this sort of like bubble where the density of the universe is smaller in her neighborhood. But you can still do that and be consistent with all of these observations. I know that sounds crazy.

00:44:41
Speaker 1: Oh, I see what you're saying. You're saying like maybe it could be that all of are checks of the density of the universe out there where somehow we just looked in the wrong places kind of, So what you're saying.

00:44:52
Speaker 2: Kind of and I think probably people overestimate how much we've looked into the deep universe. We really don't have that much data. So it's not that hard to squeak it a little bit and still be consistent with the deep images of the distant universe, because those are pretty rare. But there are other ways that we could tell whether we were in a bubble. There are other impacts this theory has on things that we can measure much more precisely than just like actually looking and measuring the density. This idea has consequences for other things that we have very sensitive probes for.

00:45:23
Speaker 1: All Right, what are some of those ways.

00:45:25
Speaker 2: Well, maybe the most powerful is the cosmic microwave background radiation. Like we look at the structure of the universe. Now, as you're saying, how much density is there? Where are the galaxies? But all that structure is seeded in little density fluctuations from the early universe. Right. The whole reason we have structure, the reason we have a galaxy here and not a galaxy there, is because in the early universe there was a little spot that was the denser, and gravity gathered stuff together to make galaxies, for example. So we can predict how stuff should be distributed in the universe today based on those fluctuations in the cosmic microwave background radiation. That light from the very early universe, and that life from the very early universe is very, very smooth.

00:46:03
Speaker 3: Right.

00:46:04
Speaker 2: It tells us that there should be no huge features. It tells us exactly how big those density fluctuations should be, and it lines up with what we see. Right. The stuff we see in the universe, both close and far away, has just about the right density fluctuations, meaning like galaxies and clumps of galaxies and actually big voids between those galaxies. We have seen huge voids between clusters of galaxies, not as big as the one this requires, but there are big voids out there in space, and all that is perfectly described by the cosmic microwave background radiation. And a huge megavoid that we're at the center of is not consistent with what we see in the CMB. There's no fluctuations in it that would give such an enormous.

00:46:46
Speaker 1: Feature, Like you don't see other voids in the cosmic microwrate background. But would you see this void, this potential giant void we're in in the cosmic micro rate background or does it tell you that there isn't avoid the.

00:46:58
Speaker 2: Cosmic microwave background tells you that there should be no huge void, and it does predict other voids. It does predict that we should see big gaps between galaxies, and we see those and we measure those, but it suggests that you should not get voids this big. Right. Basically, the size of the wiggles in the early universe limits how big the features can be in our current universe. Right, you'd need huge wiggles in the CMB to make huge wiggles now. We only see small wiggles in the early universe, So we should only have small voids in small clumps now, And that's basically what we see.

00:47:31
Speaker 1: Wait, it tells you that it's impossible or does it tell you that it's rare for us to be in a huge void like that?

00:47:37
Speaker 2: I think technically it tells you that it's very very unlikely because the light we're seeing from the CMB is not the light from the plasma that was here that formed our structures. It's the light from the plasma that was very far away, and it's been traveling to us the whole history of the universe. So we're not actually seeing the patterns that led to the formation of our structure. We're seeing the patterns that led to the formations structure that's very far away now, and so we can't actually see like the blueprints that led to our structure. We can just see the blueprints that led to other structure, and we see nothing like that anywhere else in the universe, and so it'd be very unlikely for that to happen here if it's never happened anywhere else in the universe. So that's sort of the argument.

00:48:19
Speaker 1: M all right, So the CMB says probably not.

00:48:24
Speaker 2: Yeah, And there's another argument, which has to do with how elements are made. The early universe was very very dense, dense enough briefly to cause nuclear fusion to make protons and for some of those protons to make helium, and the density of that really determines what elements were made. We have a very good understanding of how that works, and if it lines up very very well with what we see out there in the universe, how much helium and hydrogen and lithium there was made during the Big Bang. This whole field is called Big Bang nucleosynthesis. And so that's a very sensitive probe of the density of the early universe in our neighborhood. And so if we got that wrong somehow, if the universe weirdly was under dense in our region, you would see that in the amount of helium made in the Big Bang, and we don't, and so that's pretty hard to reconcile with what we see. Also, we've more recently measured a lot more Type one A supernova. This idea of the big void was very popular, like maybe ten years ago, when we had many fewer measurements of the supernova and there were some gaps, and so it was easier to sort of like fit this to the data. But more recent measurements by the Slowing Digital Sky Survey, for example, make it much harder to explain the red shifts using this sort of like weird void bubble thing.

00:49:38
Speaker 1: Meaning we have better data about where things are out there.

00:49:41
Speaker 2: Yeah, exactly. We sort of filled in some gaps by taking more and more measurements of type one A supernova, and that makes it harder and harder to even explain the red shifts using this.

00:49:51
Speaker 1: Void meaning we're more confident that things are not dens or out there, yeah, or we're just more confident by where things are.

00:49:57
Speaker 2: We're more confident about where things are, and that makes it harder to come up with a consistent picture of us being at the center of a void as an alternative explanation for the redshifts that we're seeing. But you know, there's lots of remaining to be understood, Like some of the voids that are out there in space. We don't understand how they formed and how big they got. And there's still the hubble tension like the expansion of the universe. So it's not like the explanation we have is perfect. There's lots of holes in it, lots of things that still don't work, lots of opportunities for other ideas. But I think this void theory, as cool as it sounds, works less well than the current mainstream dark energy idea.

00:50:31
Speaker 1: This idea that there's this mysterious invisible energy that we can't explain either exactly.

00:50:38
Speaker 2: There's still a lot of work left to do to make that even like a coherent theory. But it's sort of like our best current idea.

00:50:44
Speaker 3: Hm.

00:50:45
Speaker 1: Well, I guess if it has so many counts against it, why are people even considering this? Why do we you just spend an hour talking about it.

00:50:52
Speaker 2: I think people are still considering other ideas because the whole concept of our dark energy does have flaws, and it is a big idea, And it's healthy to maintain different directions of research because we could run into a big problem. You know, in sketching out the details of dark energy, we could discover a fundamental flaw in the whole plan, so that doesn't hang together.

00:51:12
Speaker 5: You know.

00:51:13
Speaker 2: It's like when you build a house and you discover while the plumbing is just not going to fit with the electrical like, we got to go back to the drawing board. So it's important to keep your mind open and consider other ideas.

00:51:23
Speaker 1: Plus, it's fun, right, right, other ideas for other people to try to figure out their tree right exactly, just not you?

00:51:30
Speaker 2: Yeah, maybe I should You're done. Maybe I should just sit on the couch, eat chocolate and let somebody else figure it.

00:51:36
Speaker 1: Out, right, But dark chocolate or white chocolate? Any which part of the universe are you in?

00:51:42
Speaker 2: If our part of the universe is turning into white chocolate, then I'm thinking about moving.

00:51:46
Speaker 1: What sounds like you already gave up on the whole universe, so now you're just giving up on the local universe exactly.

00:51:54
Speaker 3: All right?

00:51:55
Speaker 1: Well, an interesting discussion about maybe the biggest mystery in the entire universe. What is dark energy? What's costing the universe to expand faster and faster. Is it just all a big illusion or is there really some sort of mysterious energy out there?

00:52:10
Speaker 2: Well, I think the theory of dark energy is on the right track. It's really important to think about other ideas, and it also helps us understand the strengths and the weaknesses of dark energy, what we do and what we don't really know.

00:52:22
Speaker 1: We hope you enjoyed that. Thanks for joining us. See you next time.

00:52:30
Speaker 2: For more science and curiosity. Come find us on social media where we answer questions and post videos. We're on Twitter, This, Org, Instant and now TikTok. 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.