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From my layman's understanding of Hubble's law, the spacetime between 2 celestial objects 1,000,000 parsecs apart has been observed to increase at a rate of about 73 km/s. Extrapolating from this, on a much smaller scale the spacetime over a distance of 1 meter would be increasing at a rate of 8.51 × 10^-18 km/h.

My question is, has anyone ever engineered a way to manipulate spacetime in this way, either increasing that rate of expansion or even compressing space?

And if not, is such a feat even theoretically possible?

Mike
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It's simply wrong to extrapolate like this. In the early universe, matter was very uniform and Hubble's law held very accurately. Today, matter at small scales has clumped, and the clumps have been interacting gravitationally in complicated ways for billions of years. Their velocities are functions of that complicated interaction history. Only when you average over very large scales, large enough that matter hasn't had time to clump significantly at that scale, do you still see signs of the original uniform motion.

Two lumps of matter one meter apart don't "want" to be moving relative to each other at the Hubble speed or any other speed. They move according to the forces of gravity, electromagnetism, and so on, just like everything else. They are "everything else". There's no cosmological force on top of the familiar forces that is trying to make things move at the Hubble speed.

It seems to be a very common misconception, even among professional physicists, that there is such a force. I'm not sure why, but my best guess is that a lot of the time when people do calculations in general relativity, they're just calculating geodesics of particles in fixed spacetime backgrounds, and after a while they seem to forget that GR isn't just a theory of geodesics in arbitrary spacetime backgrounds, but is actually a theory of gravity. Then they assume the FLRW geometry (the approximate shape of the universe at large scales) is valid at small scales, because they think of it as just a background, and they calculate local geodesics in it and find a force. They don't notice that by assuming the FLRW geometry, they also assumed (via the GR field equations) that space is filled uniformly with matter like it was in the early universe, and the force they calculated is the gravitational influence of that matter. In reality, that matter isn't there (it's elsewhere, having clumped into stars and planets and whatnot), and so the force doesn't exist. The only real gravitational forces are the forces from the matter that's actually present, in its actual location. The "Hubble expansion force" is a made-up force arising from matter that doesn't exist via a misunderstanding of general relativity.

benrg
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  • If you re-read my question you'll notice I'm not asking about the scale of matter or forces acting on matter, I was asking about the scale of spacetime and if anyone has come up with a way of directly manipulating the scale of spacetime. I understand that local forces keep matter together. – Mike Oct 16 '20 at 22:40
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    @Mike There is no scale of spacetime; it's all part and parcel of the same mistake. The scale factor is a property of the FLRW metric and is only as meaningful as that metric. The metric is only approximate at large scales and is completely wrong at small scales. The universe is really more like a bunch of Schwarzschild patches stitched together. – benrg Oct 16 '20 at 22:43
  • Maybe it was my mistake in how I phrased the question. From reading your comment it sounds like you are saying the scale is distorted at small distances by matter, so I guess in a way the answer to my question is all matter distorts the scale of spacetime, although I'm sure that terminology is wrong. I am not a classically trained physicist so please excuse the misuse of terms. – Mike Oct 16 '20 at 22:58
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This can be done by using a convenient large mass to distort spacetime locally, and then running experiments of various sorts in the vicinity of the large mass and collecting the data.

niels nielsen
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  • Do you know if anyone has conducted such an experiment and how they went about measuring the effect on the scale of spacetime? – Mike Oct 16 '20 at 22:42
  • these tests are done under the general topic of confirming general relativity. hundreds of them have been conducted. all confirm GR. – niels nielsen Oct 17 '20 at 01:39