If so, do we know where is the middle of the universe?
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The universe does expand at more-or-less the same speed in all directions. But because of that, there is no center to the expansion. An observer will perceive themselves to be at the center no matter where they are.
A common analogy is a loaf of raisin bread that is rising. Each raisin in the bread can be thought of as representing a galaxy. For each raisin’s point of you, all of the other raisins are moving away from it with the father ones moving faster.
There is no middle as far as we can tell.
When expansion of the universe is discussed you probably imagine a balloon that is being inflated getting bigger, right? But as far as we can tell there is no edge, like the surface of the balloon.
Instead think of an infinite mass of bread dough which is rising and therefore expanding in all dimensions.
We can talk about the size of the “observable universe” which has to do with fire much we can see sure to the store if light but don’t confuse that with the actual universe.
Yes. As far as we know there is no center. There could be a center but well look into why that isn’t required and isn’t the case.
So lets look at a universe with a center where every point races away from the center with a constant velocity. If we sit into one of the points that isn’t the center we can calculate what velocities we see for other points. Classically you just subtract velocities. I look at a point and subtract my velocity and if you draw out a few points and just look at the resulting vectors (assuming the subtraction of geometric vectors isn’t new) you’ll see that looking from either point it looks like it’s the center.
So for us to look like we are in the middle is consistent with many models. The actual expansion of the universe has no center and in that case the geometric idea we discussed also applies. Everything races away from everything. This is accelerating and is proportional to distance. Think of it like space is stretching like a rubber band. Points near to each other move apart slowly while far away points have a lot of rubber between them to stretch so they drift apart quickly.
Or you can think like space gets added, like they are cells tha multiply. The more space you have between to point the more space multiplies.
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