There is the observable universe and there are good reasons to believe that there is "some universe" beyond the observable universe. The observable universe is finite, the reason for that is the geometry of spacetime and the consequences of it (cosmological horizon/particle horizon). But I would not make absolute statements about what lies beyond the horizon(s), since there is (and can be) no data that would justify a statement about the infinity of space or the falsification of it.
IIUC everything in the observable universe is sufficiently far away from anything else that any causal relationship with anything is either from before inflation or within the universe we can observe.
And given how the existence of matter seems to be rather connected to time, it self a by product of interaction, it seems to follow that there is, relatively speaking, nothing beyond the horizon.
>> IIUC everything in the observable universe is sufficiently far away from anything else that any causal relationship with anything
I don't think that is correct. Suppose you have a particle that is very very near the horizon. This particle has its own observable universe and we are at the edge of its horizon. Now, this particle can have casual contact with the other parts of the universe (the parts beyond). But by the time the interaction takes place, it has already slipped out of our horizon. So from the perspective of the particle casual contact is possible. But from our perspective: Not even a chance to detect this interaction.
To be honest, that question never made any sense to me, because this whole discussion is merely about refining a semantic detail of the common word "existing". It says nothing about the particle itself, but just about our language.
For me, a far more interesting question is: Is it relevant? (for science?)
And the answer is: As long as it doesn't interact in any way with us, it is certainly not relevant whatsoever.
> A finite universe doesn't import anything as well, and this is no contradiction whatsoever.
It's not far-fetched that a satirical sci-fi series could have multiple universes with travel between them.
The Hitchiker's Guide actually both does and doesn't if you look at the "Whole Sort of General Mish Mash" joke. Though the WSOGMM joke is pretty explicit that it's one infinite mish-mash with infinite perspectives and universes are just an illusion.
> Also, an inifinitely large "part" can still import and export, despite being infinite. For example, take an infinite universe and split it into two halfs. Those "parts" import and export, despite being infinite.
He actually covers this in the quote on WSOGMM.
> The reason they are not universes is that any given universe is not actually a thing as such, but is just a way of looking at what is generally known as the WSOGMM, or Whole Sort of General Mish Mash. The Whole Sort of General Mish Mash doesn't actually exist either, but it is just the sum total of all the different ways there would be of looking at it if it did.
> The reason they are not parallel is the same reason that the sea is not parallel. It doesn't mean anything. You can slice the Whole Sort of General Mish Mash any way you like and you will generally come up with something that someone will call home. Please feel free to blither now.
You can split the universe anyway you like, it doesn't make 2 universes, just a divide in the universe. Your 2 divides are exporting/importing but the universe as a sum is not.
Of course, we're just arguing over a joke in a fictional universe now. But if you're going to do that you should get the fictional physics right :P
This part of the Hitchhikers' Guide might not have aged well, but because it contributed to that effect.
> Area: The area of the Universe is infinite.
Right now there is more evidence for the universe to be finite rather than infinite.
But apart from that: Just because it has no borders doesn't mean it is infinite. This is a classic failure in understanding infinity. For example, the area of the surface of a torus is finite, yet when living on a torus surface you won't find any borders. Same for higher dimensions.
> Imports: None. This is a by product of infinity; it is impossible to import things into something that has infinite volume because by definition there is no outside to import things from.
This has nothing to do with infinity. This is solely due to the definition of a universe.
A finite universe doesn't import anything as well, and this is no contradiction whatsoever.
Also, an inifinitely large "part" can still import and export, despite being infinite. For example, take an infinite universe and split it into two halfs. Those "parts" import and export, despite being infinite.