Why would it only expand in one plane not every direction like you’d expect?
Also how is a flat universe even possible? Surely since we live in 3 dimensions the universe needs to be a 3 dimensional shape.
Im probably misunderstanding what physicists are trying to say but that’s why im here.
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Just because it’s a single point doesn’t mean the force was equal in all directions. Imagine a cucumber in space with increasing pressure inside. When it finally breaks, it’s not going to break equally all over, it’s going to break at the weakest point first which is in the middle. There’s no rupture to the top and bottom, so the explosive forces don’t go in those directions. The explosive force would travel in one plane perpendicular to the length of the cucumber.
I’m not saying the big bang happened from a cucumber shaped object, but clearly, the poles were stronger (or not compromised), and we had a similar rupture along the equator.
A pool table is a 3 dimensional object, but balls stay in a plane because there’s no force that moves them out of that plane. Occasionally, you’ll have a ball that gets some odd physics and a ball jumps. Same thing with the universe. I’m sure there are some celestial objects that have gone up or down if the forces are just right.
Throw it all out, they just disproved the big bang… we’re back to square one, kinda exciting
Good answers here, but most are above an eli5. Let me give it a try:
When scientists use the word “flat” they are not saying the universe is flat like a piece of paper. Rather they are implying that (to the best observations we have) it “does not curve”.
To understand this wording we can take a look at our own planet. From our perspective, standing on the ground, the earth seems pretty flat. However, if you starting traveling in a straight line, you would eventually end up back at the same place. Zooming way, way out – you can see why. They earth is actually a sphere and you have walked all the way around the curve.
The larger universe, on the other hand, appears to be “flat”. It you took off in a rocket, you would continue to move away from your original point. You would not “curve” around and end up back where you started.
The universe isn’t flat. Galaxies are flat. If the universe was flat, you’d only see stars on the horizon. Just look outside at night and it’s obviously not flat.
A galaxy is flat because it’s spinning. Same with solar systems. They spin like a disk. That disk can be any angle though and there are other galaxies at completely different angles to our own. You can’t spin something in 3D. It needs a single axis of rotation. So you get a disk as things spread out. That’s just the nature of spin.
Where did you get the idea that the universe expanded from a single point? There is nothing in modern theoretical cosmology that says this.
The universe was simply denser as we go far back in time. It could always have been infinite in size. If you extrapolate back to t=0 (you never can in any known theoretical framework), then the physical distance between all points is zero, but at time t=0+epsilon, this isn’t true.
What you think of spherical is how the observable universe is. “Flat” just means the universe lies on a relatively constant plane, rather than the universe having fundamental curvature, which would influence how far we see something from something else. You could think of a “donut” or taurus where seeing one “end” of the universe could be seen from the other “end. We don’t see things like that, or reasons to believe it works like that.
When we talks about the universe being flat we don’t mean flat like a sheet of paper rather than having volume like living inside a sphere. Think of the difference between a plane and the surface of a sphere. The curvature were talking about is an extension of that difference to three dimensions. It describes the geometry of that space.
In school you mostly learn flat geometry so the fact that there are others can be confusing at first.
Start with a square. Extend two opposite sides as lines towards infinity. If those lines get closer together as you look further away from your square your universe has positive curvature. If they get farther apart, it had negative curvature. If they stay the same distance forever, your universe has zero curvature (i.e., is flat).
>If the universe expanded from a single point
That was one hypothesis that gained a lot of steam before being proven false. I’m not up-to-date on the latest theoretical physics, but last I heard the best hypothesis is that it started expanding from everywhere all at once.
“Flat” in this context only means that space itself isn’t curved. You can have a sphere in a flat metric space. In fact, the spherical universe you are imagining would be considered “flat.” In this context, the term doesn’t say anything about the number of dimensions.
Imagine a laser that shines two parallels lines away from the earth. You can make a statement about the geometry of space based on the behavior of these two lines.
One of the three will be true: The lines will either intersect, diverge or remain parallel indefinitely. The latter is what is meant by “flat.” Flat just means that space doesn’t fold in on itself or fold outwards.
The universe appears to be *flat* because observations are consistent with two parallel lines remaining parallel for as far out as currently observable.
Ok, so no analogy is going to be perfect, but this gets the point across very well. Imagine our universe was 2D. Everything we know is living on the surface of a sheet of paper. You drip a drop of ink into the middle and BANG! A circle starts expanding outward in all directions equally. This creates a curved shape: a circle. A circle is curved. But the piece of paper is still flat. You can pick up the piece of paper, giving it curvature, rolling it, crumpling it up, whatever, but the circular splot on the 2D surface of the paper hasn’t really changed.
Spherical vs flat space refers to the curvature of spacetime itself. But any object (including our universe) which lives in the 3D space can be any shape (including spherical) without changing the overall curvature of space.
The universe ≠ space. I think that might be the misunderstanding. The universe is a thing that exists in space. Just like Earth is a thing that exists in space. The Earth is spherical (ish) but it doesn’t mean that space itself is spherical.
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