What is the “cold spot” on the cosmic microwave background? Why shouldn’t it exist? What are some possible explanations for it, and why are they unsatisfying? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

I don't think the average person, and to be perfectly clear, you are far from the average person, so this will be easy for you, appreciates just how messy science is. Popular conceptions of science, and I'm looking at you in particular, science journalism, paint this simplified picture of how science works where there is a problem, a bunch of scientists get to work, they work really hard, and they produce an answer. And that theories are either right or wrong, black or white, and they're either accepted by the evidence or they're totally rejected by everybody. Either a scientific idea works or it doesn't work. But that's not even close to how the scientific process actually unfolds.

And while I'll save a full discussion of the process of science for a later episode, feel free to ask to Accelerate that timeline. I want to use something called the Cosmic Microwave Background Cold Spot as an example of how we can have a really, really solid understanding of the big picture and not be entirely sure what's going on with some of the details, not be confident what the potential answers are, and whether it's worthy of excitement or not, and not even agree about whether what we're seeing is even real, and how all that can happen, and it doesn't affect the big picture because scientific theories are not perfect. Yeah. That sounds pretty messy, But this is what science actually looks like. To be perfectly honest, in today's episode, I'm not going to focus too much on the physics of what the cold spot could be.

Spoiler alert. Our best guess is that it's a supervoid along the line of sight, but this is disputed. I actually want to talk a lot about why the cold spot might be interesting and why we think it's interesting. And so to do that, I have to dig into the physics of the CMB itself, the cosmic microwave background, and I have to discuss, like, what are the statistics of it? How do we approach our understanding of the CMB so that it tells us why the cold spot really sticks out?

And so that's what I'm gonna spend most of the episode on. And then we'll talk about some solutions and talk about how those solutions kinda, sorta don't work and how this actually isn't a problem. So for so first, let's dig into the CMB. I don't wanna say cosmic microwave background a million times in this episode, so I'm just going to say CMB. So forever, for now, you're going to hear CMB, that is cosmic microwave background.

Cosmic microwave background, CMB, it's one of the earliest predictions of the Big Bang theory. It's very straightforward. Like, once you realize that we live in an expanding universe, the CMB just pops out of it as a natural consequence. I've done entire series on the CMB. It's a wonderful aspect of our universe.

But the short version is our universe right now is big and cold. Everything spread out. If this was not the case, billions of years ago, if you go back in time, our universe is smaller and hotter and denser. At one point, eventually, far enough in the distant past, everything is so small and so hot and so dense that it's a plasma. It's like, the gas that's inside the sun or a lightning bolt.

The electrons are ripped off of the atoms, and everything's a hot mess. Now the universe back then was a plasma, and it was expanding and cooling. Eventually, it cooled off to become not a plasma anymore. That process released radiation. The radiation completely soaked the universe.

It was also really hot. At the time it was released, it was around 10,000 Kelvin. But the universe has expanded. It stretched out that light, and now it's down in the microwave, and it surrounds us. This leftover light from when the universe transitioned from being a hot plasma to a slightly less hot neutral gas.

This radiation was emitted by every point in the universe. So the room you're in right now, the space that the earth, the solar system currently occupies, was once a long time ago a plasma, and then it wasn't, and it released this radiation. And this radiation that was generated right here is now 1,000,000,000 of light years away because it went away from us. If you look at the Andromeda galaxy, a long time ago, the Andromeda galaxy was the that region of space was a hot dense plasma. It generated cosmic microwave background, CMB.

That light now went past where we are and is now on the back end, billions of light years away. What we see when we look at the CMB, when we look up in our sky and we put on our microwave goggles and we look at this, we see it as a shell that surrounds us in all directions. And what we are seeing is the light that was generated in distant parts of the universe that is just now reaching us. And and this distinction is absolutely crucial for understanding the cold spot, what it is, and why we care about it. Because the CMB was not generated in one particular place, it was generated everywhere at once.

So the metaphor I like to use, and we'll come back to this metaphor a lot in this episode, is imagine if everyone in the world started shouting. And just once. Like, we could do it right now. Like, 3, 2, 1. Like, we just shout.

Everyone shouts once. And let's say the shouts are loud enough to be heard around the world because I I need this to work for the metaphor. Everyone shouts. Everyone on the entire planet Earth shouts at the exact same time. Just once, a big burst.

Just like the CMB, it was a big burst of light. Yes. It took 10000 years for the light to actually transition and and for the process to unfold and the light to emit. But in cosmological scales, that's essentially instant. Everybody shouts.

So if you're standing, you shout, your shout now goes out into the world. You never see it again. It's gone. And from your perspective, you start hearing shouts. First, you hear the shouts from the people nearest you, and then you give a little bit of time, and then you start to hear the shouts from the people, say, in your town or your city, and then you start to hear people shouting from your state, your country.

It moves further and further. And eventually, you start hearing shouts from people over the horizon. You can't see the people that are shouting. That shout was emitted by people well over the horizon, but now their sound is finally reaching you. So as time goes on, you see this ever expanding or hear this ever expanding shell of shouts because the people nearest you started shouting or they shouted once, the shout hit you.

You heard it. Ah, you heard them. But then that sound wave moves past you. And now, there's someone standing behind them on the other side of the room, and then that sound wave hears you. So you're always hearing shouting.

You're always hearing, like, ah, this continuous shout. But it's because you're picking up people from a wider and wider circle until you reach all the way over the horizon, and then it just keeps going. You don't see the people anymore, but you can still hear their shouts because enough time has passed for their shout to wave over you. This is important because the cosmic microwave background, the CMB, is almost perfectly uniform. It has a nearly perfect temperature right around 2.7 Kelvin, which is a lot cooler than when it was emitted, and it's not perfect.

There are tiny, tiny differences in temperature. These differences in the temperature, if you start looking if you look at the CMB sky and you look in any particular direction and you try to measure its temperature, it's right around 2.7 Kelvin, but though there will be one part in a million differences from place to place. One part in a million where you look in one direction, it will be a 1 millionth cooler. Look in another direction, it'll be 1 millionth warmer. There are these tiny little imperfections.

And these imperfections are the juiciest part of the CMB. The existence of the CMB tells us that the Big Bang Theory is is probably correct. But the differences tell us so much rich information because magically, actually, scientifically, we understand why the CMB is not perfect. We actually expected tiny little differences in temperature from place to place. And there are 2 general kinds of differences that appear in the CMB.

One are tiny little density differences which translate into tiny little temperature differences seeded by inflation. This this event that we hypothesize happened in the incredibly early universe that set the stage for the growth of galaxies and large structures in the universe. They started actually, we believe, as tiny little quantum fluctuations in the in the fabric of space time itself. It's not inflation, I got to admit, is not the greatest idea, but it's the best one we got. There's there's a lot I could say about inflation, and I have in previous episodes.

This event of inflation seeded what it did, it laid the groundwork. It it took these subatomic, like, truly subatomic differences and made them, like, little tiny density differences so that tiny pockets of the universe were a little bit more dense than average, and over here is a little bit less dense than average. When the CMB was generated, we see that as tiny little temperature differences. So we get a map of what the universe looked like. By looking at these temperature differences, we get a map of what the universe actually looked like, of the density differences, the matter differences throughout the universe.

These are relatively small. They're tiny little speckles. And if you look at a map of the CMB, if you if you go into your favorite search engine and type CMB, you will not see the average 2.7 Kelvin map because, it's pretty boring. It's just a a blob of a single color. You will be seeing maps of these tiny little differences.

And there are tiny little speckles. These little speckles were seeded by inflation. These speckles grow up eventually to become galaxies, to become clusters, to become the largest structures in the universe, but they need another 1000000000 years or so from the CMB in order to grow up to that kind of scale. And so at this time, at the CMB, what we are seeing are these tiny little baby fluctuations, which is amazing. And then there's another kind you will see on the CMB map.

There are these large splotches. These large splotches are actually generated by sound waves crashing around in the early universe. I'd love to do an episode on that. So you you will see the point is you see different kinds of splotches, tiny splotches and big splotches. And we have an understanding of where these density differences, temperature differences, these splotches come from.

We we understand plasma physics. We actually understand the physics of the early universe, at least the early universe that generated the cosmic microwave background. And here's where I need to bring in my my sound wave metaphor again because we can't predict exactly what the fluctuations will look like. We can't predict, even though we understand the physics of the CMB, and we understand how the small splotches are generated, how the big splotches are generated. All the wiggles appear.

We know how it comes about. We understand the physics of how these are created, but we can't say, oh, yeah. You look in this direction, there will be a cold spot that is, 50 micro Kelvin colder than average. And if you look in that spot, you will see, a warm spot that is 15 micro Kelvin warmer than average. You can't we can't do that.

We can't predict exactly what the CMB will look like. We can only predict the statistics, the properties of the CMB. And to go back to the sound wave metaphor, everyone shouts across the world. Once the shouts start coming from over the horizon, that once there's been enough time that you're hearing shouts from beyond the horizon, you can't say, oh, there's going to be a loud shout with this pitch coming from this direction. And if I look to the west, there'll be a slightly quieter shout coming from that direction.

You can't do that because you can't attach a sound to a person. You can't see the person that's generating the sound. And in our universe, when we look at the CMB, we can't see that part of the universe anymore because our universe has expanded. That little region, if you point in any one direction and look at the CMB, there was a part of the universe that generated the CMB light. That CMB light has now left that part of the universe and has crawled across the cosmos on its way to us.

And in the meantime, that patch of the universe has now expanded away from us because we live in an expanding universe, and we can't see it anymore. We only get the CMB light. We have no idea what's going on in that part of the universe anymore. It's beyond our observable horizon. But the light left long enough ago that it's just now reaching us, but we have no idea what's going on there now.

We can't attach a visual, like, oh, right now, there's a galaxy over there and a cluster, and so we know, the CMB light that it generated would have had this pattern. We can't do that. Just like if everyone shouted at once, at first, you could say, okay, I know I know I know that person. They're they're gonna shout really loud, and, yep, there it is. It matched it.

Okay. And but once it gets far enough away where you can't see the people anymore who are generating the shouts, you lose the ability to predict exactly what shouts are going to come from what direction, and how loud they're gonna be, and what pitch they're gonna be. And with the CMB, we can't predict exactly what the pattern will be. If you look up the CMB, we can't predict that exact pattern with with the cold spots over there, and the hot spots over there, and the sizes over here. We can't do that.

Instead, we can only predict the statistics. We can say what the average spot size is. What the average temperature difference is. What are the range of temperature differences that we can expect. What are the range of spot sizes that we can predict.

This is absolutely critical because this gets into one of the issues with the cold spot. This does introduce a fundamental limitation to the certainty of our calculations, but it's the best we're gonna get. We can only look at the CMB statistically. And just about everything with the CMB is fine and dandy. We understand where the splotches come from.

They tell us a lot about what the universe was cooking when it was very young. Over the decades, we've built ever more refined telescopes and satellites for getting a better look at the CMB. It's just about one of the biggest success stories in science and some, in fact, some argue that it's the most precise measurement ever made in science. We can debate that another time. It's a cornerstone of the big bang theory.

You're welcome to create your own model of the history of the universe, but your first job is going to be explaining the nature of the CMB, and good luck with that. You'll need it. The CMB is awesome. I've had firsthand experience working with the CMB. For a few years, I was a member of the Planck collaboration.

The Planck was a satellite launched by the European Space Agency and NASA to create the highest resolution, most detailed global sky map of the CMB ever. We did it. Good job, us. Everything's great. We actually understand the CMB.

We understand the statistics of the CMB. We understand the physical processes. We can't predict exactly which cold spot and hot spot will appear where. But we can say on average, yeah, we expect a certain number of cold spots, a certain number of hot spots. They're gonna be roughly this size or have this these there are gonna be so many small ones, so many medium ones, and so many big ones.

Like, we could do this, and then there's the cold spot. Now there are a lot of cold spots on the CMB. In fact, it's it's half the spots. The other half are hotter than average. But the cold spot stands out.

It's it even stands out visually if you look at a map of the CMB where the entire sphere of the sky is compressed into a weird vaguely oval shape. The cold spot is down in a little to the right. And if you see, like, a a bluish spot, almost always these maps are painted blue for cold. It it looks weird. You can see it on the sky.

If you look through the direction of the constellation Eridanus, you're looking in the direction of the cold spot. You might need microwave sensitive godwills to see it, but it's there. Now the cold spot has got a good name, and you know I'm a fan of solid names in astronomy and physics. The cold spot is cold. Depending on how you define the edge of the spot, it's about 70 micro Kelvin colder than average.

And that's compared to the, you know, average not important cold spot that's only roughly 18 micro Kelvin colder than average. So right there, it's about 4 to 5 times colder than the average cold spot on the CMB. And in its deepest parts, in the very center of the cold spot, it's about a 140 micro Kelvin colder than average, which is a lot for CMB. I know it's like 1 part in a 100000, 1 part in a 1000000, you know, getting around in that ballpark. But for the CMB, that's a lot.

That's almost 10 times colder than average. And the cold spot is also big. It's about 5 degrees across, which doesn't sound like a lot, but that's about 10 full moons lined up side to side. The average spot in the CMB is less than 1 degree across. So not only is the cold spot weirdly cold, it's also weirdly big.

And this is where things get tricky. We first spotted the cold spot with the WMAP probe in the late 19 nineties, early 2000, and then the Planck satellite. And then we wondered, like, okay, maybe maybe there was some measurement error. You know, maybe aliens were messing with us. Maybe there was just something weird happening with our instrument when it happened to look in that direction of the sky.

But then the Planck satellite confirmed the existence of the cold spot. So it wasn't just a fluke of the instrument or measurement error or weird alien interference. It was a real thing on the real sky. So that eliminates my personal favorite explanation for anything weird that happens in science, which is measurement error. And it means we actually have to get some work done.

And the next question, why I spent so much time talking about the statistics and the shouting metaphor and all that, I swear there is a madness to my method. The next question is, do we care? Folks, I need to take a quick break and mention that this show is sponsored by BetterHelp. Can you believe the year is already halfway over? I swear sometimes you blink and it's just, what is time?

I should do a 10 part series on the nature of time and especially the human perception of time because it is. It's a little bit challenging to understand. And with that passage of time comes memory, regret, pride, anticipation, you know, all the human emotions surrounding time. And so now that we're halfway through the year, maybe we should pause to see where we are and if we're we're reaching the goals and achieving the vision that we wanted to achieve this year. And one way to do that is with therapy.

I've benefited from years of therapy, and I think you can too. Give it a shot. If you're thinking of starting it, give BetterHelp a try. It's all online. You just fill out a brief questionnaire, you get matched to a licensed therapist, and you're off to the races.

You can switch therapists at any time for no additional charge. Take a moment. Visit betterhelp.com/ spaceman today to get 10% off your 1st month. That's betterhelphelp.com/spaceman. Okay.

So there's a big cold spot on the CMB in one particular direction. Do we care? Remember, we can't say for certain what splotches on the CMB will appear where. We can only predict the statistical information. There is no theory or observation that could tell us that when we look through the direction of that particular constellation, that there will be a cold spot of that size.

We can't make that kind of scientific prediction. We don't have information. We don't have access to that. It's beyond our horizon. Just like you can't say, I am going to listen in the northeast direction, and I expect in the next 5 minutes to hear a a very high pitched loud scream.

You can't say that anymore because you don't know who's over the horizon. You can only predict the statistics. So we have to play a little dance here. We see a large cold spot. We have to decide.

Is the cold spot too big and too cold based on what we reasonably expect the CMB to look like? Let's say you're listening to these shouts as they roll across you from over the horizon, and, randomly, out of nowhere, you hear a shout that's 10 times louder and 10 times higher pitched than anything else you've ever heard. You have to ask yourself, is this just a weird statistical fluke, like, there's one crazy loud human way over the horizon that you didn't expect? Or is this crazy loud shout telling you something funky is happening that maybe you don't understand human voices as well as you thought you did? Because you built a statistical model, you know how humans work, you know how loud they can be.

I know for this metaphor to work, humans have to be way louder than they actually are. Roll with me. We got this. Is the cold spot too cold? Is this human voice that you hear that's 10 times louder, is that too loud?

Is that impossible based on what you know about human voices? Is the cold spot too cold based on the statistics of what we know of how the CMB works? There's been a lot of back and forth on this, as is usual, with how messy science actually is in practice, but the general consensus is that, yes, we should not reasonably expect the cold spot to be so cold and so big just out of random chance. And that based on our understanding of the physics of the early universe is just way too out of line. Yes.

Randomly big cold spots should occasionally appear, but our chances of just seeing one out of pure random chance is less than 1% and might be even lower depending on who you ask. And so so while we could say we just got super unlucky and got a cold spot, it's rare enough that it demands some attention. So it's not measurement error, and it's probably not random chance. What is it? Well, it might be something mundane, something strange ish and interesting ish, but still fitting within the framework of known physics.

And there are a lot of options because there's a lot of stuff between us and the CMB. The CMB was generated over 13000000000 years ago. That light came from a distant patch of the universe, so distant that we can't even see that part of the universe anymore. It had to travel. It had to travel billions of light years to get to us.

And so that light has been filtered. It has been altered. And, yes, we're aware of this in our studies of the CMB. All of the work that goes into analyzing the CMB is in separating the the raw cosmological stuff from 1,000,000,000 of years ago from all the interference effects in between. That light has been affected in tiny, tiny ways.

And one of the ways that that light has been affected is that some of the light passes through cosmic voids. The cosmic voids are big patches of nothing. And it turns out these big patches of nothing leave an imprint on the CMB that has nothing to do with the CMB itself, but everything to do with the fact that we have to look through the voids to get a view of the CMB. And now I'm going to give you a gift. If it's near your birthday or other notable holiday, consider this my personal gift to you.

The gift is the integrated Sachs Wolf effect named after Rayner Sachs and Arthur Wolf. Cosmologist, that's a mouthful. We just say I s w, integrated Sax Wolf effect. If you wanna bust that out to impress friends and family, loved ones, by all means, give it a shot. Does it hasn't worked on any of my friends and family and loved ones, but you can you can try it.

Integrated Sacks Wolf effect. This is how this is a mechanism where voids can leave an imprint on the CMB and actually lead to a colder spot. Here's the idea. The the idea rests on two facts. 1, we live in an evolving universe.

The universe is constantly changing as the light is making its way from the CMB to us, it's not just encountering a static universe that's fixed. Stuff is actually happening over the 1000000000 of years that this light has made its way to us. Structures have grown. Galaxies have grown. Clusters have grown.

The voids have emptied out. Things have happened. And it relies on the fact that dark energy exists. Now, we don't know what dark energy is, but it is an observational reality. The universe is accelerating in its expansion.

What this means is that the voids themselves, which are super empty, change with time. They actually get bigger with time. And they get bigger faster than expected because of the existence of dark energy. So what happens? The voids are big.

The minimum void size is like 20,000,000 light years, and they're most of them are bigger than that. They're gigantic. So imagine you're a photon. You're a bit of light released from the CMB, and you start traveling through the universe, and you're headed in the direction of earth. You're going to reach earth in, like, 13000000000 years.

So you've you packed a sandwich, and you're trucking along. You're going. As you're traveling, structure is evolving. The galaxies are emerging. Clusters are emerging.

The voids are emptying out. So let's say you encounter a void. When you first encounter a void, you reach, like, the entrance to that desert. The void is kinda small. It's just those it's it's a young void.

Okay. So you enter the void, and you actually gain a little bit of energy as you enter the void. It's like rolling downhill. You you gain a little bit of energy as you enter the void because there's less stuff around you. The density is lower.

And so your photon, you respond to the curvature of space time like that. You actually gain a little bit of energy as you fall into the void. And then you travel across the void, and it's super empty, and nothing much happens to you. Then you reach the other end of the void. Now if the universe was static, you would then exit the void.

You would come back up that hill. You would you would reach like a wall of galaxies, and then you would feel that slight little density difference, and you would lose your energy again. In in a static universe, everything would be perfectly the same. You would gain as much you would gain energy entering the void because you're rolling downhill into the void. You're entering a low density region.

And then you would come back up into a high density region, and you'd give that energy back and everything would be great. But we don't live in a static universe. We live in a dynamic universe, and especially we live in a universe with dark energy that is really dynamically affecting the voids in a measurable way. It means that in the time it takes for you as a photon to cross this desert, this void, the void has gotten bigger. And the walls on the other edge of the void have piled up.

All the matter has left the voids. So in your journey, the time it takes you to make this crossing of a void, the void has gotten bigger. There is more matter piled up on the far side. So you gain a little bit of energy entering the void. You enter a low density region.

You go downhill a little bit. Then you cross the void, and then you hit the other wall, but that wall is much bigger. The void is much bigger. The walls on the edge have been piled up from all the material of all those 1,000,000, maybe even 100 of 1000000 of years it took for you to cross the void, the void has gotten bigger. And so you lose energy on the other side, and you lose more energy than you gained.

When a photon crosses a void and the void is growing, especially if the void is growing because of the influence of dark energy, you lose energy on the other side. This thought process argues that the cold spot is due to a supervoid between us and the CMB. That turns a regular spot that we wouldn't otherwise care about into a giant freezing monster. That gets us all a little bit worried. So that when we see the cold spot in that direction, what we're not looking at is something primordial, something generated in the CMB itself.

That's why it looks so weird because it's not generated by the statistical processes, by the physics that we understand of the early universe. It's created by something else, by the fact that there is a giant void between us in that direction that is forcing all the light from the CMB coming from that direction to appear much colder through this effect known as the integrated effect. Where voids grow, they grow with time, And in the time it takes for a photon to cross a void, it's gotten bigger. It entered it, and it thought the edge of the desert was 50 miles away. It turns out it's a 100 miles away, and you have to, oh, like, here's a metaphor.

I'm making up on the spot. You enter a desert and say the desert is a valley. And there's a mountain range at the beginning of the desert and a mountain range at the end of the desert. So you come into the desert and you come down the mountain range. You gain energy by coming down the mountain range, then you skate across the desert, and then you go up the mountain range on the other side.

And in a perfectly symmetric universe, the mountain ranges would be exactly the same size on the other side. So you gain energy going into the desert, and then you lose energy coming out, but then everything's equal. But in a dynamic universe, you know, imagine it took you 1000000 of years to cross this desert that the landscape is going to change. The mountains on the other side might be getting bigger. The desert might be growing larger.

And so you fall into the desert. You come down off of one range. You enter the desert. You have a little bit of extra energy, but by the time you finish crossing it, the mountain range on the other side is now even bigger. And you have to climb up that, and you end up overall losing energy.

That is the integrated SaxWolf effect. And that's a possible explanation for the cold spot. The thing is, this has to be a super void. This thing would have to be gigantic, like a 1000000000 light years on a side kinda thing, making it one of, if not the largest single object in the universe. And now you can rightly ask, like, okay.

This cold spot on the CMB is so statistically improbable that it can't be generated in the early universe. What about super voids that are a 1000000000 light years across? Can they be generated in our universe, or does that stretch our understanding of physics? Is it all turtles all the way down? And the answer there seems to be this is harder to get at because the physics of void formation is is more complicated than the physics of the CMB.

So the statistics here are looser and goosier. The general consensus at the current state at the time of recording, after having read, like, all the papers on this, is that the existence of such a supergiant void is maybe kind of sort of a problem, but definitely less of a problem than the existence of the cold spot itself. So that's that's kind of an advancement, and we'll take it. It's not much, but but we'll take it. It appears that the existence of such a giant void, supervoid, in this direction is not threatening enough to be worried about.

It's big and it's kinda weird, but not big enough and weird enough to say there's no way our universe could generate a void of that size. But is that it? Okay. Is that it? Did a is a supervoid responsible for the cold spot?

Well, if you snuck a glance at the remaining runtime of this episode, then you know that there's more than enough time for a Patreon ad. That's patreon.com/pmsutter. Thank you so much for all of your contributions. I really do. I sincerely appreciate every single contribution to keep this show going.

That's patreon.com/pmsutter. Anyway, you know there's more to the story. Here's the thing. We can't seem to agree on whether there's actually a void in the direction of the cold spot. I know I literally just told you about the supervoid sitting between us and the CMB in that direction, leading to the wonderful integrated Sacks Wolfe effect in the appearance of a cold spot that doesn't really belong there.

But that's a hypothesis, and we'd like to test it like any good hypothesis by rummaging around the galaxies in that direction to see if we can actually find a supervoid there. And this is actually somewhat challenging because the super void is huge, so we need to scan a massive volume in order to capture it. But we're only focused in one particular direction, so we're limited in scope. This is a challenging survey thing where you have to look in one particular direction, but get enough volume so you can map out this giant void. And the giant void isn't just big left to right.

It's also big in-depth. And we have lots of surveys of lots of galaxies out there in the universe, and some of them do cover the sky in this direction, but they all have limitations. They can only map so far back. They can only capture galaxies of a minimum brightness. They only map certain types of galaxies.

So all the surveys in this direction are incomplete. We don't have a full accounting of all the galaxies in the direction of the cold spot. And we have great tools for reliably mapping out voids. You can thank of certain p Sutter for that contribution to science. But supervoids are much trickier.

Not only are they big, they're not very deep. They're very shallow and broad, and that makes it difficult to identify if you just have a map of galaxies. It's actually challenging for supervoids to identify the presence of a void that big, which means that over the years, you have competing claims and counterclaims. Some papers saying there's definitely 100% a giant void there. Other papers saying, nuh-uh.

There's no void there, you big dummy. It just goes back and forth. And if that wasn't enough just to throw yet another wrinkle into this, even the biggest, deepest, most superlative, super void that is anyone has ever claimed to find in the maps of galaxies over in that direction, even that isn't big enough, deep enough, or super enough to give that big of an integrated SaxWolf effect on its own, Which means we need a lucky coincidence. You need a super giant void just happening to sit along a line of sight to an already colder than average spot, combining their powers to form the cold spot. But the entire point of this exercise was to remove the need for weird coincidences.

So maybe maybe it's something crazy. Maybe it's not random chance. Maybe it's not a supervoid. Maybe it's telling us something about the universe. There's this crazy idea out there that it's what we see as the cold spot is the intersection point with another universe.

That this is evidence for the multiverse that a long time ago, our little bubble universe was bumped up against another little bubble universe. Now we're widely separated, but for a while, we were like little conjoined twins, and that left an imprint in the CMB in the form of the cold spot. I'm not going to spend a lot of time at on that idea. In fact, like, zero time on that idea because, it doesn't really work. It doesn't really explain the properties of the cold spot.

It's an interesting idea. It's an awesome idea. But if it's interesting, say it with me kids. If it's interesting, it's probably wrong. And it doesn't really explain the properties of the cold spot, but we can't toss out in general crazy ideas because all the non crazy ideas aren't exactly working either.

In summary, we don't know what's causing the cold spot. We don't know if it's a big deal, and we don't even know if we should care about it. That is why we are screaming in my metaphor because we don't understand the cold spot. The moral of the story goes back to the way I introduced the episode, which is that science is messy, and we often don't have all the answers, and that we can nail the big stuff while still worrying about the small details. Does the cold spot invalidate the Big Bang Theory?

No. We understand the CMB. The existence of the CMB itself is a prediction of the Big Bang Theory. We understand the statistical fluctuations, the hot and cold splotches to a obscene level of precision. You can't use the cold spot to throw out the Big Bang Theory.

Is the cold spot worth looking into? Almost certainly. There's a chance that there's nothing interesting there. There's a chance there's something really fun going on there that we don't fully understand. Will we ever conclusively figure out what it is?

Possibly no. Generations from now, we may not have a satisfactory answer to the cold spot. This is the way science is. It's never perfect. There's always some little thorn in some theory's side.

Sometimes those thorns blossom to reveal new kinds of physics, in this case, a multiverse. Sometimes those thorns just wither away as scientists slowly chip away at it. Maybe we get better at mapping the voids in that direction. Maybe we discover some more nuances to the integrated Sacks Wolfe effect. Maybe after enough decades of work, the the cold spot problem just fades away and maybe it just sits there.

Never fully resolved, never fully answered, but never rising to the level of needing more attention. No matter what, it's okay by me. Why? Because nothing is perfect in this universe. Not even our understanding of it.

Thank you to Eric S, Martin N, SAHM, and Michael C for the questions that led to today's episode. And thank you, of course, to all of my top Patreon contributors. I really cannot thank you enough. All of your contributors' contributions are amazing. That's patreon.com/pm Sutter.

But let me give a shout out to my top contributors this month. We've got Justin g, Chris l, Alberto m, Corey d, Stargazer, Robert b, Tom g, Nyla, Bike Santa, Sam r, John s, Joshua, Scott m, Rob h, Scott m, Lewis m, John w, Alexis Gilbert m Rob w, Demetrius j, Jules r, Mike g, Jim l, Scott j, David s, Scott r, bbjj108, Heather, Mike s, Michelle r, Pete h, Steve s, what what bird, Lisa r, and koozie. That's patreon.com/pmsutter. Keep those questions coming. That's askaspaceman@gmail.com.

Go to the website. There's a form you can fill out there. Askaspaceman.com. Check out the archives. Please give this show a review on iTunes.

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And I will see you next time for more complete knowledge of time and space.

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