Where does the extra space come from as the universe expands? and if it’s just stretching, does that mean it somehow is getting thinned out?
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It is completely unknown.
One theory that I like says that the reason for the universe’s expansion is that dark energy is a fundamental property of spacetime. It’s the nature of space to grow, and as it does, there exists more space which also has its own dark energy.
In this way, space expands faster and faster as more space comes into existence. It increases in volume forever while all the matter and energy inside is gradually pulled apart.
We don’t actually know for certain. It’s speculated that new “Planck Volumes” of space (the speculated smallest possible chunks of space) randomly pop into existence everywhere.
The space is added uniformly in the existing space. The mass remains the same so the density is reduced overall.
So here’s what happens with you add a little space inside the Earth. Gravity is still a thing, so gravity returns the Earth to it’s proper size, leaving a tiny bit of extra space just outside the region of the Earth’s influence.
The space between things expands, right, but not the things themselves. So what of the space within things? Do solid objects have some force opposing the expansion of space within them? Do they even need a force? Is the expansion rate big enough to even notice at the scale of physical objects like stars and planets?
Yes.
What’s getting stretched?
*Time*.
If I may just ride on to this question. Is there an explanation or rather theory of what happens if our universes’ expansion collides into another universes’ expansion?
Physicists have a tendency to talk about space as if it were a physical fabric you can touch. But space is not a thing, it’s a mathematical construct. So space doesn’t have to come from anywhere because it isn’t an object.
However there is an energy associated with spacetime geometry, and it seems reasonable to ask where this energy is coming from in an expanding universe. The answer is that energy is not conserved in an expanding universe so the energy doesn’t have to come from anywhere, it can just appear.
General relativity doesn’t actually talk about ‘extra space’ as if it’s some substance that is coming into being. That’s an analogy, since a lot of things in GR can be analogized by some kind of fluid flow or stretchy material. There are some mathematical parallels, but fundamentally GR only says that lengths and time intervals can change depending on the structure of the universe.
When we say the universe expands, we mean that if you have two non-interacting objects floating in space, with no forces on them from any direction, then over time the distance between them increases. Everything else, like rubber sheets, is a physical analogy – something we can create in the real world that has similar behavior to the space of GR. The only relevant feature of this analogy is that distances can change.
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