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Comment by cygx | Hacker News Reader

What is the expansion of the universe?

The distance between galaxies (that aren't gravitationally bound to each other) increases over time.

Or should I first ask, is there a center?

Probably not. If you inflate a balloon, any point on its surface will move away from any other point on the surface. And yet, no surface point can claim to be central.

Note that in the balloon example, there is of course a central point within the volume of the balloon. This is where the analogy breaks down: In case of the universe, we do not assume the existence of an ambient higher-dimensional space.

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A perfect geometric surface of a sphere that is expanding has no center, but a balloon has a center. That's obviously the nozzle. The analogy breaks down in both places.

But who knows? Maybe the analogy is correct. I don't think we can say. In fact I would even wager that a "center" is more likely... because can you even imagine a real world analogy where something expanding has no center?

If you can't then there probably is some sort of center.

There's nothing special about the nozzle; it's just another point. The nozzle isn't the center. But if you can't get past that, imagine a balloon without a nozzle, with a canister of compressed gas inside, and the compressed gas is leaking out of the canister.

I mentioned a perfect geometric sphere that is expanding has no center on that surface.

I also stated that no real world analog of a perfect expanding sphere exists. Because we can't imagine an analog it's much more likely that the universe DOES have a center similar to the nozzle.

Try to imagine a real world analog to the expanding geometric sphere without a point of discontinuity. You can't. And because you can't maybe it doesn't exist. Maybe it does. But not being able to imagine a real world analog is slight evidence that there there is a center.

As soon as you can imagine and reason about infinity, you might be able to reason about our universe. “Center” makes sense for most bounded things but might be completely meaningless in an unbounded universe.

Example: the number line. You might be tempted to say 0 is at the center but… there are an infinite number of things on either side of any number you choose.

Does that mean nothing is the center of the number line?

Or everywhere is the center of the number line. Some people argue 0 is the center based on symmetry. My original point is that it is non-trivial to reason about infinite spaces. Since we do not know if we live in a bounded space or not, we can’t assume there is one center. Furthermore, even if we do live in a bounded space, there are different ways of measuring center. Eg: center of mass vs geometric center of a volume. Knowing both and comparing them would be pretty interesting!

0 is the center of an infinite number line.

Additionally nobody knows if the universe is infinite.

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