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So what is expanding? Just space but my meter-stick stays the same? So there is some kind of selection of what gets longer what does not?
actually the distance between any two heavenly bodies are increasing at any time. but not the particles on the planet.
So the vacuum is expanding?
objects in the vacuum are expanding............
In general, the universe is expanding refers to the fact that the galaxies are moving away from each other continuously..
So I could but my meter-stick in vacuum and therefore it would expand!?!?!?
There are two ways to think about this. First, in the easy way, where you consider matter separate from the space it occupies, you can say that, yes, your meter stick expands -- or rather, starts to, but every time the space between the atoms and molecules increases by a hair, the strong forces between the atoms and molecules immediately pull them back to the correct equilibium distance, and nothing changes overall. You might say the meter stick is continually sliding back into equilibrium as the underlying space expands. The other way, more true to Einstein, looks upon matter as just a manifestation of the local curvature of space. From this point of view, the expansion of the universe is just the universe becoming, on average, less dense. But as it turns out, matter resists becoming less dense -- because it attracts itself. So the way the universe on average can become less dense is if the reduction in density is confined to those regions that are already nearly empty. Regions in which matter is already dense stay dense. The inhomgeneity of the universe steadily increases. Indeed, from this point of view the universe ends up as an infinitely dilute gas of black holes, with every bit of the universe either perfectly empty (between the black holes) or infinitely dense (within the black holes).
Ah now I get the idea. The reason for this expansion is dark energy/unknown?