On a planet-sized ball of dirt (the earth without oceans, mountains, etc.), carve a bunch of straight (great-circle) foot paths from one pole to the other, all of a fixed width (say 1 meter). They are the same phenomenon described with respect to different coordinates. There is no difference in general relativity between "expansion of space" and simple relative motion. Otherwise you are correct, our atoms would also be expanding and we would see no shift in the atomic thumbprints. ![]() It is the fact that matter is bound by forces that are not affected by the expansion that allows us to measure the expansion. The very idea of the actual "metric of space changing!" would seem to be that, those of us internal to that metric of space would have no clue that any such expansion is happening: the scale is just changing for everything. The expansion of the universe changes their momentum and thus the atomic spectra arrive shifted towards the infrared. Photons are elementary particles that have to obey locally energy and momentum conservation. Gravity is strong enough to keep even clusters of galaxies unaffected and given some assumptions on the energy density and solution of the general relativity equations gravity can fight the expansion and lead to the big crunch. This assures that atoms, matter in general up to the size of galaxies keep their structure, the raisin bread analogy. In the hierarchy of forces, the gravitational force is the weakest. The model that fits these facts is General Relativity, which predicted the behavior What are the observational/experimental facts:ġ)Atoms have definite spectra, with a fixed pattern, a fingerprint of the atomĢ) The further away ( measured by luminocity) galaxies all around ours the more shifted the fingerpring pattern towards the red part of the spectrum. $that would just mean the universe is changing from one set of units to another, which would (as you point out) be physically and philosophically undetectable, and therefore meaningless. There is no exterior, it is the universe. Imagine simply a meter cube in a video game with a few things in it. Indeed, it would seem to me that you would only see redshift (or if you prefer, time dilation of far-away things) strictly in the case of "everyday" motion within the metric of space the very idea of the actual "metric of space changing!" would seem to be that, those of us internal to that metric of space would have no clue that any such expansion is happening: the scale is just changing for everything. The usual better explanation is that "space itself is expanding." (Of course, on scales below clusters, gravity pulls "smaller" structures together.)Īn even more up-to-date explanation is that the conceptual "metric of space" is "expanding" ( here's a typical pedagogic example) which can perhaps be summarized as the "scale is changing".īut why? Everything's just expanding - the very metric of spacetime is expanding. ![]() The "kid's" way of understanding the expanding universe is that: "space" is totally "ordinary", and all the galaxies are expanding through it (like an explosion).
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