# The Universe might not be flat (and cosmologists are quietly freaking out)

## Метаданные

- **Канал:** Dr. Becky
- **YouTube:** https://www.youtube.com/watch?v=Pd9P7CibbHc
- **Дата:** 30.04.2026
- **Длительность:** 15:29
- **Просмотры:** 247,761
- **Источник:** https://ekstraktznaniy.ru/video/48962

## Описание

Everything we know about the shape of the Universe could be completely wrong. 

This is one of the most fascinating unsolved problems in cosmology, and it almost never gets talked about outside of research papers. It's called the curvature tension, and it links in to the "crisis in cosmology". Astrophysicists have always thought the geometry of the Universe is flat, not its shape, its geometry: which is a very different thing. 

But back in 2019, a careful analysis of 6 years worth of data from the Planck telescope suggested the geometry of the Universe wasn't flat, and instead was closed. And since then, the debate has gone back and forth in the scientific literature, with some data pointing to a curved Universe and some to a flat one. This video will dive into the details of the curvature tension!

Papers mentioned
Di Valentino et al. (2019) - https://arxiv.org/pdf/1911.02087
Handley (2019) - https://arxiv.org/pdf/1908.09139 
Wu & Xhang (2025) - https://arxiv.org/pdf/2411.06356
Calab

## Транскрипт

### Introduction []

Everything we know about the geometry of the universe could be completely wrong. This is one of my favorite unsolved problems in all of cosmology. And yet, it doesn't get talked about outside of research papers. It's called the curvature tension, but it all links in to the crisis in cosmology that I've talked a lot about on this channel before. We've always thought that the geometry of the universe is flat, not its shape, its geometry. Those are two very different things. But back in 2019, a careful analysis of 6 years worth of data from the Planck Telescope was published suggesting that the universe wasn't flat at all and instead it was curved. So what is going on here? Well, in this video, we're going to dive into the details of the curvature tension and chat first about what is the difference between the shape and geometry of the universe. Second, why we even care about whether the universe is flat or not. Then we'll chat about how we measure if the universe is flat and then finally what the curvature tension is and what different data sets are telling us. So first what the difference is between

### I - What is the difference between the shape and geometry of the Universe? [1:05]

shape and geometry because this does trip a lot of people up myself included when I first learned about this. So I think we're all familiar with what a shape is. It's a sphere or a cylinder or a flat plane or a donut. It's the structure of something right? It's how everything is interconnected on a large scale. Geometry however is different. It's a property of space itself on a local scale and it describes how objects behave on that space. So there's three options for geometry. You can be flat, closed or open. Flat geometry means that if you draw two parallel lines on that space and extend them to infinity, they will never ever meet. Whereas closed geometry means if you draw two parallel lines and extend them across the space, eventually they will meet. And then the opposite is true for open geometry, which is when if you draw two parallel lines and extend them to infinity, they will diverge from each other and never meet. You might also see this described as if you draw a triangle, do the angle sum to more or less than 180°? A sphere is the usual example given for a closed geometry and a Pringle shape for an open geometry. And weirdly, the curved geometries are ones that people can wrap their head around. But it's the flat geometries that people really struggle with because a flat geometry doesn't mean a flat shape, a flat piece of paper. In fact, there are lots of shapes that have flat geometries. Like for example, a cylinder because if I draw two parallel lines around the edge of this cylinder, they will never meet. Now, I can make a cylinder out of a flat piece of paper because they both have flat geometries. But I couldn't make a sphere out of a flat piece of paper without cutting it, without changing the geometry of the paper. I can make like a donut shape if I roll it up like this and then curve it around to make the doughut. And that's cuz technically parts of a donut also have flat geometry as well. These are not flat shapes, but they are flat geometries. And it's a really important difference. So when we ask what's the geometry of the universe, we're not asking what shape it is. We're not saying is it a finite sphere or does it extend to infinity. Instead, we're asking what's the curvature of space. Which brings me nicely to part two. Why

### II - Why we care about whether the Universe is flat or not [3:22]

we actually care whether the universe is flat or not. Well, the geometry of the universe is like a direct fingerprint for the physics that shapes the universe in the very first few fractions of a second of its lifetime. Specifically, it's a test of something known as inflation, an unimaginable burst of expansion which took something the size of an atom and stretched it out to the size of a melon in less time than it takes light to cross a single proton. If inflation happened like our best model of the universe predicts it did, then that inflated expansion should have stretched the universe flat. It should have given the universe a flat geometry. And inflation underpins our explanation for why the universe looks the same in every direction that we look in. Why matter is so evenly distributed and why structure formed the way that it did. So, our best model of the universe is just one big house of cards with inflation being one of the things at the bottom propping it all up. Take inflation away and everything comes crumbling down. This is why cosmologists care so much about whether the universe is flat or not. Now, thankfully, there is a number that describes the geometry of the universe that we actually have a hope of measuring. And it's represented by the Greek letter omega. And that number is at the heart of the entirety of cosmology. It's technically a measure of the energy density of the entire universe. But the mass is set up so that if omega equals exactly one, it means the geometry of space is flat. But if omega is greater than one, space curves back in on itself and the universe has a closed curved geometry. If omega is less than one, it's got that Pringlelike geometry, aka an open universe. So, how do we know if omega is one or not? How do we actually measure if the universe

### III - How we measure if the Universe is flat [5:15]

is flat? Well, remember our parallel lines and how they behave differently in a flat or closed or open universe. Well, those parallel lines are essentially the paths that light will take through the universe. So, if we're going to try and measure this curvature signal, we need light that's been traveling for a really long time. So for that we need the oldest light in the universe. What's known as the cosmic microwave background given out when the universe cooled enough to become transparent to light when it was just 380,000 years old. Because light takes time to travel to us because it travels at a finite speed. We're still detecting that light today. It comes from every direction on the sky because it was emitted from every part of the universe at the same time. And that light has a different energy or temperature depending on whether it came from a hot or a cold spot in the universe when the light was released. And the size of those spots is set by the physics of what we think was going on before the light was released. The fingerprint of inflation. So our best model of the universe can give us a prediction for the sizes of those patches if the universe is flat. And therefore two light rays that were emitted parallel to each other in this cosmic microwave background travel through space for 13. 8 billion years and stay parallel to each other before they're then detected here on Earth with our telescopes. If instead the universe was closed though, then those two light rays would eventually converge and intersect, we would then extrapolate that back out and it would appear like the patches on the cosmic microwave were bigger than they were. Or if the universe had open curved geometry, then those light rays would spread apart on their journey through the universe and we would extrapolate those back so that the patches on the cosmic microwave background would look smaller than they actually were. So whether the universe's geometry is flat, closed, or open changes what we expect the cosmic microwave background to look like. The problem comes with the fact that that's not the only thing that affects the sizes of those patches on the cosmic microwave background. The density of normal matter in the universe or the amount of dark matter also does this. So to measure the geometry or the curvature of the universe, we're often measuring multiple different properties of the universe at one time. They're all intertwined. So that when we try to get a best fit model to the universe to measure omega, there's a really big uncertainty on the number that we get out. But the first experiment to actually do this was the boomerang experiment in the 1990s which flew a telescope on three high alitude flights to detect the cosmic microwave background. And they found their data showed the universe was very close to flat. At least they couldn't say with any certainty that it wasn't flat. Similarly, the first year of observations from the W map experiment in the 2000s showed that the universe was close to flat. They then slowly collected more and more data over a number of years to get a stronger signal. So that by the end of their 9-year experiment, the data still favored a flat universe. So the assumption was the universe must be flat. But then along came Easter's plank mission in the 2010s, which brings me to

### IV - The curvature tension and what different data is telling us [8:33]

part four. this curvature attention and the confusion with all these different data sets because another thing that also affects the size of the patches on the cosmic microwave background is gravitational lensing. If the light encounters massive galaxies of billions of stars as it travels through the universe, those massive galaxies can act like lenses, bending the light around them, again changing the path that the light takes, whether the universe is flat or curved. Now the scales that this happens on in terms of the sizes of the patches on the cosmic microwave background are very small and earlier experiments like the WMAP mission couldn't pick out those kinds of details but issa's plank mission could. This was a telescope that launched in 2009 which had three times higher resolution than WMAP. And as it slowly collected its data over the next few years, it became clear that our predictions for the sizes of those smaller patches on the cosmic microwave background after you've taken into account all the fact that gravitational lensing should be happening didn't actually match the data from plank. What's more is that they didn't even match within the uncertainties on the data either. This is what's known as the plank lensing anomaly and it is what set excited alarm bells ringing in the cosmological community. It was de Valentina and collaborators in 2019 who pointed out that this could be explained if the universe wasn't flat and instead had a closed geometry. Here's the figure that shows how the curvature and the amount of lensing are intrinsically linked. And that while there is still a chance it could be that the universe is flat just with more lensing happening, the more likely model is a closed universe at greater than 99% confidence. Similarly, Handley in 2021 calculated the odds against a flat universe in the plank data at more than 50 to1. But that was 5 years ago. And now plank isn't the only high precision cosmology experiment we have. The Atakama Cosmology Telescope or ACT is a groundbased telescope sitting at about 5,000 meters up in the Chilean desert which has also slowly been taking data over the past 6 years building up a stronger and stronger cosmic microwave background signal. In particular, we got much better data on the polarization of the light in the cosmic microwave background from ACT. Its noise level is like three times better than plank. Polarization is the orientation of the light wave essentially. Is it waving this way or some which direction in between? And the reason we care about polarization is because the lensing that the galaxies do to the light twists the polarization pattern in a very predictable way. So measuring that twist with the polarization gives you another way of getting at how much lensing happens so that you can separate it from the curvature of the universe. So the act findings were published last year with this paper by Louisian collaborators and I want you to look at this plot. It shows the plank data with the orange points and then the act data in blue. The gray line there shows the prediction for a universe with a flat geometry. The top represents the energy of the cosmic microwave background and the bottom the polarization of it. And you can see they mostly agree in the energy or temperature data. But that weird anomaly found by Plank that's really obvious in the polarization data in the orange points has completely disappeared in the ACT data shown by the blue points. You can see that really clearly here too with these distributions that show the most likely value that you get for the amount of lensing going on and then the curvature that you get when you fit it to each data set. Plank says it's definitely closed and Axe says it's very likely flat. Okay. So, is there even a curvature tension? Right? We've got better technology now. It's 5 years later. Is act not just better data leading to better model and we can just disregard the plank data. Sadly, no. We still need the plank data and we need to understand what's going on here. ACT is a groundbased telescope, so it only covers about 40% of the sky. The parts that are visible from Chile. Plank was a space satellite that covered the entire sky. That's a big deal because it means the larger scales in the cosmic microwave background can only be measured from space and away from the interference of the atmosphere. These large scales of the cosmic microwave background feed into almost every single parameter in your cosmological model of the universe. So we need them and ACT can't give us that. And so often people combine plank with their groundbased data that they've got like ACT. And so if you do that, you still really need to understand what's going on with the plank data. So really what's happened now is that the curvature attention has shifted from ooh new exciting physics, maybe the universe is curved and not flat to we really need to understand what's going on with the plank data and if there's some weird thing wrong with the data or the processing of the data or there's a systematic there that's throwing us all off. And this is where it ties into the crisis in cosmology, which if you've been following me for a while, you'll know is the fact that we have two main ways to measure the age of the universe. And they're giving us two very different answers. And one of those ways has to use the cosmic microwave background data from Plank. But we can't go around pointing fingers just yet. We actually need to confirm whether there really is an issue with the Plank data or if what we're seeing here is real. It's a very complex problem and there's been a lot of people looking into it for years now, but nothing's been found just yet. And I think that's what I love most about these cosmological tensions because with hindsight, it might end up just looking completely and stupidly blindingly obvious what was wrong. Or it might just quietly disappear with better data. But that's science for you. It doesn't always come trumpets ablazing with brand new discoveries, but instead with like a long, sometimes uncomfortable process of not knowing all the details yet. Because remember, measuring the curvature of the universe isn't really about the geometry itself and whether the universe is flat or not. It's about whether inflation happened or not. It's whether we're right about the physics of what happened in the first few fractions of a second of the universe's life.

### Bloopers [15:01]

This is one of my favorite unsolved pop. Just noticed this is open over here. I'm looking at me. Then a long claim a long claim. A long claim plank necessarily mean I can't pick up this piece of paper was curved. It's a very Oh, also in my eye, but there's birds tweeted outside right at the end of that. Hopefully that's not in it. Let's do it again.