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Cake day: March 3rd, 2024

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  • Crude, yes. Fair, no.

    Consider a balloon. Uninflated, make a mark on opposite sides, and then make a third mark right next to one of those. When you inflate that balloon, the two points on opposite sides of it become farther apart because of the stretching of the whole balloon, but the two marks right next to each other don’t become nearly as far apart, because they are only experiencing “local” expansion.



  • So all space is expanding. Locally, that’s “just a teeny tiny bit,” and the force of gravity is plenty strong enough to keep things up to about the size of galaxies (maybe galaxy clusters) gravitationally bound. Andromeda, for example, is the only galaxy that is heading towards us.

    But all of space is quite big. Over the vast distances of space, all of the “teeny tiny” local expansions add up. This means that the galaxies which are furthest away from us are also receding from us most quickly. This is not because those galaxies are moving through space; it’s because of all the expanding space in between them and us.

    The speed of light (in a vacuum) is the fastest anything can move through space.





  • If we are considering “the universe” to mean the spacetime that we exist in, there could be an “outside,” but we just don’t know, and there’s no indication of such an outside, or anything about what it would be like.

    By way of infinite spacetime, yes, there is only a part of spacetime that we can observe, because the farthest part is moving away from us faster than the speed of light. I seem to recall there having been some estimations of how large all of spacetime is, observable and unobservable, and that it has a finite size.

    That said, there does not appear to be a limit to the size of spacetime. Based on what is currently known, spacetime is expanding, the expansion is accelerating, and there is no limit to the expansion.