ELI5: If the universe is expanding in all directions, does that mean that the universe is shaped like a sphere?

ellinger: When people say that the universe is expanding in all directions, they don’t merely mean *at its edges*. The universe is expanding everywhere all at once. Galaxies aren’t really moving apart from one another, the space between them is expanding. If we removed that expansion from their velocities they would be practically standing still.

All of the images that you see of the Big Bang show a spherical explosion, but that’s greatly simplified for the masses. Remember that what is inside that “sphere” is *everything*. It’s the whole universe. It’s impossible to look at it from the outside because there is no concept of “outside the universe”. The idea of what it looks like from the outside is meaningless.

Why do all those animations show a sphere then? Well, in part because of the [Cosmological Principle](https://en.wikipedia.org/wiki/Cosmological_principle), which says
that the universe looks the same no matter the direction we look, and the natural way to depict that is with a sphere (and because [sphere shapes are very common in space](https://www.universetoday.com/112805/why-is-everything-spherical/)), but that’s not the way that real explosions work. Even the most perfectly packed explosions don’t generate perfectly spherical shockwaves.

Importantly, there would be no way for us to tell if the universe is shaped like, say, a giant chicken, because spacetime has no edges. We could be hip deep in chicken guts, but if we never see any feathers, we’d assume it’s guts all the way down.

TL;DR – The universe isn’t expanding at its edges because it has no edges, and the concept of what it looks like is meaningless.

stuthulhu: The universe is (theorized to be) infinite in extent, so it’s not really ‘spherical’ in shape. it may be easier to think of expansion as a ‘reduction in density over time.’

The observable universe, however, is an expanding spherical volume. But this doesn’t represent any sort of true physical boundary, so much as the volume of universe from which light has had time to reach us.

supertaquito: Not necessarily. But yes, and no. Content can expand in any direction regardless of the shape of the container, so the type of expansion should not be proportionate to the shape of it.

We tend to think in spheres when it comes to the universe because when it comes to space exploration, Earth has always been the starting point, or rather our solar system, we can only explore what is going on up to a certain distance in any direction from a 360 degree starting point, this is why the famous image of ”the visible universe” is actually in shape of a circle.

There is another theory going on, in which the universe is shaped like a doughnut, instead, the cross section is still circular, but if you keep going and going and going, you will eventually end up in the place you started. Problem with that theory is… what IS in the middle of the doughnut?

WRSaunders: The shape of the Universe could be anything, because it’s bigger than the Visible Universe – the sphere we can see. Perhaps our VU is a small bubble in a giant loaf of universe-bread. Since it’s expanding it’s not going to make a difference to us. The Universe is pretty flat around here, so it could be infinite and flat, but that’s not excellent evidence – only reasonable.

shapplesauce: Here’s a 2D example. Take a semi-inflated balloon. Glue a bunch of pennies on the surface of the balloon. The 2D surface of the balloon is an analog to our universe. If someone starts blowing up the balloon even more, each of the pennies is moving away from each other, and the surface area of the balloon is expanding. From our 3D perspective, we can see the shape of the balloon. But if your entire universe is the 2D surface of the balloon, it doesn’t really have a shape. It just is. And somehow, it’s expanding.

You can take the same principles, and add another dimension to be our universe. The planets and stars and galaxies are all of the pennies glued to the surface of the balloon.

maitre_lld: Many uninformed answers here. The truth : we don’t know the shape of the universe. The fact that it is expanding in all directions doesn’t mean it’s spherical. Just think about a balloon with an odd shape, you can still inflate it, and it can expand in all directions but still keeping an odd shape. Its surface is also unbounded, in the sense that an ant walking along it would never be stopped by any boundary. Now imagine this in one dimension higher (just imagine, but we can’t really see a 3d surface because we can’t see in 4d) : instead of the surface of a balloon, imagine our universe itself is a higher dimensional (3d) surface. We call this a 3-dimensional manifold. We know that this manifold is expanding but we don’t know it’s topology (it’s shape : what happens to ants walking on it, what kind of path they can make etc).

ANDnowmewatchbeguns: Homer Simpson once said it was shaped like a donut and I think Stephen Hawking backed him up

polska_kielbasa: How can it be shaped like anything if it’s infinite?

neugo: Your mother is expanding in all directions, does that mean she’s spherical?

And there you have your answer.

thunts7: The best explanation I’ve seen that makes sense would be that the universe is the 3d equivalent to the surface of a torus so that you can travel on the surface forever but it is not infinite. Although the size of the universe is bigger in light-years than the distance light has been able to travel since the beginning of the universe so you can yet see the repeat and probably never will be able to due to expansion

thepacifist20130: Take a deflated balloon. Now take a sharpie and mark dots on the balloon.

When you start inflating the baloon, the dots start moving away from each other.

That’s a 2D representation. If you can somehow now imagine this in 3 dimensions, that’s the universe expanding

Ch5se: Looks like a donut 🍩 but if you were on the inside of a donut. If you were traveling fast enough you could travel in a complete loop around it (like traveling around the world) though it’s expanding so quickly that we’d probably never be able to reach the speeds necessary to complete a revolution.

myz94: tl;dr nobody fucking knows.

we just have varying skill levels when it comes to citing publications that support a difficult topic we barely understand

swiftcrane: Not sure how accurate this is but I always imagine it as a comparison of dimensions. Since we can only see and properly comprehend 3d and below it is beneficial to choose a dimension as a demonstration 1 lower than what we use.

Imagine something on a plane, up and down doesn’t exist for it like it does for us.

Now imagine that plane wrapped around or in the shape of a sphere. The sphere is blowing up like balloon and all the things on the balloon are getting pulled away from each other. However, to the 2d “people” that live on the surface, the actual expansion/shape of their universe isn’t spherical at all. It’s just infinite infinite in all directions (although I guess technically its looped).

Now upscale the dimensions. Make the sphere 4 dimensional and the surface 3 dimensional (thats us!)

So it’s not really a sphere for us because we don’t live in it or on it but rather IN the plane – which I guess upscaled now isnt a plane but a 3 dimensional space.

So it’s not really expanding in the general sense to us. The problem is we think of expansion as overall the borders getting further apart, but for the distance between the galaxies to increase (which is pretty much what the expansion is) there doesn’t have to be borders. In fact, there aren’t borders, just the distance between things is increasing.

I’m not sure how accurate this is – because I think they actually use more complex multidimensional shapes to describe the expansion, but this analogy is a good way to understand the basics I think.

Kurgon999: I like to think of it as a sphere, but only if the radius is Time. The entire universe “at this moment in time” would be the surface of the sphere.

LogicalComa: This doesn’t completely answer your question but I hope it helps. https://youtu.be/DsXsJtOQnTY

ingruberti: None can ever answer that.

We can suppose that if t=0, it is the Big Bang… then t= 1 (infinitely small unit) the explosion might have been directed in every direction, thus creating a sphere of momentum.

For absurd,
if the explosion generated more momentum in some directions and less in others, this distortion could be today noticed by observing far stars.
Their light would be bent and our triangulation wrong.
Ergo the absurd is false as we have light path trajectory math proven.
Ergo the explosion was uniform.

At t=2, while the initial push STILL uniform in all directions, interactions might have formed among mass/energy expanding, thus “relatively” moving faster in some directions.
Ergo losing the “sphere” shape for an observer positioned at the Big Bang explosion in t=0

Far_King_Penguin: Yes and no. It is theorized that if you keep going in one direction fast enough and long enough in the universe that you will wind up where you started. This is just one of many theories on sych things though.

It would make sense for the universe the be spherical in shape like everything else in the universe, however the universe is also infinite. The magnitude of infinity is a hard concept for humans to understand, it is never ending. An example to demonstrate the size of infinity is that if you halved infinity you would still have infinity left over, if you took 1/10^9 of infinity you would still have infinity.

Since the universe is infinite it means it can’t really develop a shape since shapes require a definite size and boundaries and as such inifity does not have boundaries as it just goes on forever. Although if you needed to give the universe a shape I would say spherical would be the way to go but remember this grossly misrepresents the magnitude of the universe.

The observable universe is something else entirely and does have a more or less spherical shape but that is just what we can see, not the universe it’s self.

paul_maybe: Late to the party, but this might help you visualize it.

Think of an unbaked loaf of raisin bread, with raisins spread out inside the dough. As the bread bakes, all the raisins start speeding away from each other. The farther away the raisin in from you, the faster it seems to be moving away. The universe is like that, but the raisins are galaxies (more or less).

What’s the shape of the universe? Well, we really can’t tell because we don’t see an edge. If there is an edge, it’s too far away. On the other hand, there probably isn’t an edge at all, which sounds weird, but think about the Earth. If you think the Earth is flat, then you would think there would be an edge. The Earth is actually a different shape, round, so it has no edge. What is the shape of the universe? Well, it probably curves back on itself somehow in some multidimensional way that we have trouble visualizing, so it has no edge.

young_vet1395: also, when we say the world is expanding in all directions, we do not specify the rate at which it is expanding, which can differ in directions or “locations in space”. a more accurate representation of the shape of the universe is an ellipsoid.

Idenwen: The expansion can be pictured like an image of the universe on the surface of a balloon that you inflate. Nothing moves but space itself (balloon rubber wall) expands in itself. The balloon wall is only 2D – but same happens in the universe in at least 3D

Nothaus1967: Do you think if it was you’d be able to see the same stars they have been seeing for thousands of years? The constellations? How is it if everything is moving in opposite directions at all directions and expanding away from each other in incredible speeds, why are the stars are not changing? I don’t know, just asking?

I_assed_you_a_Q: I always wondered about this idea. What is the viewpoint from which we could hypothetically observe this expansion? How does that work? Any observation from “outside” of the expanding, “edge” (I suppose?) Doesn’t exist, right?

What does the edge of a universe even mean exactly? Is it the edge of the presence of gravity? I am unclear on the mechanics and ways of describing of all of this.

jadenPete: When people say the universe is internally expanding, are they saying that the density of atoms is getting bigger? Kind of confused how this is possible.

tomthehippie: The best way to think about it is like a balloon. The surface is our universe. As the balloon expands you get more surface area.

This is essentially what is happening to our universe. More “space” is being created.

seven2112: I had heard that we were close to where the big bang started, is that true? Would that make our species one of the oldest and maybe why we’ve not found any further advanced life? I understand our scope is pretty limited. I was just curious, I thought this would be a good place to ask. Thanks for your time.

AndyM_LVB: I’ll always remember a great analogy that made me understand this… The universe is 3-dimensional, and is all “space” and time. This means if you travel in a straight line you will eventually end up where you started. Our brains can’t visualise this, but you can step down a dimension and imagine a balloon. Now imagine that this 2d universe is the *surface* of the balloon. As you put more air into the balloon it expands and the surface expands in all directions at the same time. There is no centre of expansion as far as the 2d surface is concerned. If you travel along the 2d surface you will never reach an edge, you will just end up where you started. The universe is like that – but you need to add a dimension.

defnotalizardperson: Currently there is no way to determine the exact size or shape of the universe. We can only estimate, most likely it is not a perfect sphere, but a similar shape, a roundish expanding shape that emminates from the center (big bang) It is very unlikely that every galaxy is equally spaced apart and moving at the same rate of speed in order to form a sphere.

This is speculation, but I would think that it might resemble a golf ball, roundish, and “dimpled” because of the differences in speed and location. Or maybe it’s shaped like a potato. We may never know

GaryBettmansRightNut: Is the universe expanding faster than the speed of light? In Neil Degrasse Tyson’s latest book, Astrophysics for People in a Hurry, Tyson explains how the universe went from very small to larger than our solar system in a fraction of a second. I thought the speed of light was the universe’s speed limit, so I’m a bit confused.

grasshoppa80: If you had one monkey, and an infinite amount of time, it would write every single works of Shakespeare.

Contrary, if you had an infinite amount of monkeys, one would at some point, write every single work of Shakespeare.

Basically, infinity solves it for you. Space is infinite.