I heard that galaxies are receding from each other because the ‘space between them is expanding’. Does this mean the ‘fabric’ of space can swell infinitely? How? Why?
Ta
In: Physics
There is no swell, its the same amount of stuff spreading out farther and farther. If you put some powder on water then drop a rock in the middle the powder keeps spreading out, eventually in the example the powder hits the shore. But there is no limit to hit in space.
As best we understand the physics, there is not a hard limit. However, the expansion is not strong enough to pull apart objects that are bound together by gravity or other forces, so as long as structures exist at all they’ll be safe from the expansion of the Universe. That said, structures aren’t totally eternal as we understand the cosmos today, and it is likely that in the very, very, very far future the Universe will consist of isolated particles (primarily photons) so distant from one another that none of them can observe any other.
As for how – well, why *wouldn’t* it be able to? It *already* stretched by a factor of untold billions just after the Big Bang, so in a sense we’re already *in* a very, very stretched-out Universe.
People talk about spacetime like it’s a physical thing you can touch, but in actuality it’s a geometric surface used in the mathematical model of general relativity. So the answer to your question is that space does not need to stretch because it isn’t a thing.
The question that follows is why do we say the universe is expanding? In GR the function for measuring distance between two objects is the [scale factor](https://en.m.wikipedia.org/wiki/Scale_factor_(cosmology)#Detail), and it includes a time component. So the distance between two very distant objects will increase with time. This lines up with real-world observations, so we say it’s expanding.
However GR does not explain how or why things work in the real world, so there’s no real answer to your question. Suffice it to say that nothing is actually stretching in the sense that you’re thinking.
Short answer is, we don’t know. We don’t even know what’s causing it.
The current theory is that small bits of space are simply randomly created. That space is empty, it’s nothing and doesn’t really mean anything.
It’s kinda like, if we’d change the definition of 1 cm or 1 in, increasing them by 25%, and all the rulers also stretch by 25% by themselves. 1 cm would still correspond to 1 cm on the ruler. (It’s just an analogy. Don’t hate…)
The thing is, over large distances the extra space somehow adds up to more than it would make sense this way. It’s like 1 cm measures 1 cm here but just 0.5 cm on the same ruler on the other side of the room.
Something weird is happening. Scientists call that weirdness dark eneegy. Whether it has any limit, we don’t know. It’s possible this process will stop, reaching its limit. Ot it will reverse like a rubber band and start shrinking again. Or maybe it will break reality apart like when the band breaks.
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