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> The universe is infinite Well, an open question in cosmology is whether the universe is actually infinite or not...

ajross 5y ago
Strictly that's a question about philosophy, not cosmology. Cosmology just says the universe is unbounded and leaves it at that. In fact "The Universe", meaning all the matter or energy which we will ever even theoretically be able to obvserve, is definitely finite. The distance to the point of infinite redshift (where we're looking at the big bang, essentially) is finite. It doesn't make sense to talk about anything "farther" out.
- DougN7 5y ago
  Is that true? (Honest question) Whatever the “thing” was that exploded at the big bang (which I read once was the size of a grapefruit for what it’s worth) - could there not have been other similar “things”? And what about where that “thing” came from? I’ve read theories of the universe expanding, but then eventually falling back onto itself to recreate another “thing” that will explode again. Anyway, it feels like there might indeed be more.
  - arbitrage 5y ago
    A way that I heard this described, is that what we recognize as the observable universe right now was the size of a grapefruit at the beginning of the big bang. The stuff that expanded due to whatever the big bang actually did, that stuff could have been just the size of a grapefruit and nothing more, or itself infinite. But the part we can access given speed of light restrictions was the part that was the size of a grapefruit. There might actually be a lot more stuff out there ... we can just never get to it.
    - pas 5y ago
      due to the isotropic principle we assume that the universe is infinite (because our vantage point is random, and since we don't see any orientation at the edge of the observable universe)
  - datavirtue 5y ago
    I picture the big bang as a bunch of heavy "black holes" approaching eachother discharging something that looks like lightening between them that grows more intense as they all pulled closer together. Eventually fusing into a persistent electromagnetic "explosion." Like a really bad welding accident.
- Trasmatta 5y ago
  > Cosmology just says the universe is unbounded and leaves it at that I don't think that's right. The shape and topology of the universe is an open question in cosmology, and cosmologists haven't just "left it at that".
  - ajross 5y ago
    Let me be stricter: models consistent with general relativity don't discuss the idea of a universe being "infinite", and just in general theories in science don't predict the unobservable. Open universe topologies are unbounded. They aren't "infinite".

Strictly that's a question about philosophy, not cosmology. Cosmology just says the universe is unbounded and leaves it at that. In fact "The Universe", meaning all the matter or energy which we will ever even theoretically be able to obvserve, is definitely finite. The distance to the point of infinite redshift (where we're looking at the big bang, essentially) is finite. It doesn't make sense to talk about anything "farther" out.

DougN7 5y ago
Is that true? (Honest question) Whatever the “thing” was that exploded at the big bang (which I read once was the size of a grapefruit for what it’s worth) - could there not have been other similar “things”? And what about where that “thing” came from? I’ve read theories of the universe expanding, but then eventually falling back onto itself to recreate another “thing” that will explode again. Anyway, it feels like there might indeed be more.
- arbitrage 5y ago
  A way that I heard this described, is that what we recognize as the observable universe right now was the size of a grapefruit at the beginning of the big bang. The stuff that expanded due to whatever the big bang actually did, that stuff could have been just the size of a grapefruit and nothing more, or itself infinite. But the part we can access given speed of light restrictions was the part that was the size of a grapefruit. There might actually be a lot more stuff out there ... we can just never get to it.
  - pas 5y ago
    due to the isotropic principle we assume that the universe is infinite (because our vantage point is random, and since we don't see any orientation at the edge of the observable universe)
- datavirtue 5y ago
  I picture the big bang as a bunch of heavy "black holes" approaching eachother discharging something that looks like lightening between them that grows more intense as they all pulled closer together. Eventually fusing into a persistent electromagnetic "explosion." Like a really bad welding accident.
Trasmatta 5y ago
> Cosmology just says the universe is unbounded and leaves it at that I don't think that's right. The shape and topology of the universe is an open question in cosmology, and cosmologists haven't just "left it at that".
- ajross 5y ago
  Let me be stricter: models consistent with general relativity don't discuss the idea of a universe being "infinite", and just in general theories in science don't predict the unobservable. Open universe topologies are unbounded. They aren't "infinite".