Since the big bang, the universe is expanding, it doesn’t mean new space is being inhabited that was previously empty, its the existing space spreading out. How measurable is this on the human scale (meaning human dimensions and human timeline)?
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No, we aren’t. Neither is our galaxy.
Mathematical models that cosmologists use to predict universal expansion make two **huge** assumptions: That the universe is *homogeneous* (“the same everywhere”) and *isotropic* (“the same in all directions”). At first glance, these assumptions seem strange. From where I’m sitting, the universe is clearly *not* the same in all directions. Galaxies are clearly distinct from intergalactic space.
The key is that if you zoom out far enough, this assumption holds *really well*. If you zoom out far enough, the universe looks like a uniformly dense gas. The thing is, you have to zoom out *really* far, on the scale of billions of galaxies. Models that discuss universal expansion are not valid on smaller scales than that.
The biggest misconception about universal expansion appears to be the idea that there is some fabric-like backdrop to spacetime that “expands” . Balloon analogies don’t help: they give the impression that there is some mystical “fabric of spacetime” that is “stretching” and dragging matter with it.
This is not the case. The universe (ignoring dark energy) is expanding because matter has momentum leftover from the big bang, that carries it away from other matter. Galaxies and galactic clusters are parts of the universe where gravity has won out over that momentum and managed to draw everything back together.
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