I’m trying to wrap my head around an apparent paradox. I could not find this by searching and I apologize if this breaks the rules for hypotheticals.
The Hubble volume is a region in space containing everything we can physically interact with. Objects outside of our Hubble volume are receding away from us faster than the speed of light and are therefore impossible to interact with for us.
Now consider an object A at the very edge of our Hubble Volume. It is receding away from us near the speed of light. Near object A is object B. Object B is at a greater distance from us than A, such that B lies outside of our Hubble volume. According to my understanding of the Hubble volume, we can never interact with Object B under any circumstances.
However, imagine the following complication of this setup: Object B is a spaceship, and it maintains an equal distance to Object A at all times by firing its thrusters very rarely to make up for the distance increased between it and Object A due to the expansion of space.
Now, Object A is inside our Hubble volume. Object B is outside of it, but at a constant distance from Object A.
We now travel to Object A. By the time we reach Object A, where is Object B? Can we reach it? Similarly, could the captain of Object B launch a dropship in advance to Object A to meet us when we arrive?
In: Physics
There is a lot to this question but im going to adopt the more theoretical approach.
This is a huge complication for species at the top of the Kardashev scale that worry about galaxy hopping. Essentially yes if object B could slow itself relative to the expansion of space between A and B then we could hop to A and proceed to hop anywhere in A’s hubble volume. Interestingly, spacetime is not uniform everywhere, depending on B’s own mass as well as A’s, its possible that a large enough body could distort space to create a kind of pimple on our otherwise uniform hubble diameter.
Technically speaking, the Hubble volume does NOT mean you will never be able to interact with it, it just implies that nothing moving slower than light speed could catch up as the expansion of spacetime has reached “terminal velocity”. We do however know its theoretically possible to create bubbles of isolated space that kind of move around the expansion of spacetime but there’s a many more problems beyond being conceptually possible. (You gotta figure out how to accelerate it to beyond light speed while its completely isolated from anything it could use to gain momentum)
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