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ChatGPT and wikipedia have informed me that the primary evidence for the theory that the universe is expanding is the fact that photons that arrive to our planet from all directions are being redshifted, and this redshift is directly proportional to the distance from our earth to the origin of the source of the light.

Although universal expansion does explain this observation

Could it also be explained by having a ring of unimaginably dense concentrations of matter all around us, far away from what we can observe, which cause the redshift of the photons we detect by pulling the sources of those photons, (stars, galaxies whatever they may be) by gravitation?

this would imply two main problems of course:

(1) The issue that the amount of redshift is approximately proportional to the distance of the source. Perhaps the current model of gravitation breaks down at such scales and gravity behaves differently, if so this would also explain why the redshift we can determine of incoming photons is greater the further the origin is, because the net gravitational effects and thus the acceleration on that photon source would be greater as it moves closer and closer to a particular section of the perimeter, thus intensifying the doppler effect

(2) Why don't we or other observable astronomical bodies accelerate away too? Perhaps we are in a sweet spot where the gravitational effects by this ring of masses roughly cancel out.

Or perhaps we are accelerating towards a particular section on the perimeter of this ring, along with our entire galaxy we just don't realise it.

I look forward to a retort to this idea which has sat in my brain for a while now, googling didn't really help because I could not think of a "googlable" way of formulating it.

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    $\begingroup$ Aside from the highly contrived nature of this idea and the fact that it wouldn't work... it's not even an alternative to cosmic expansion. If it worked, it would just be a source for cosmic expansion. $\endgroup$
    – Sten
    Apr 21, 2023 at 18:52
  • $\begingroup$ How would it cause expansion? $\endgroup$
    – Hisham
    Apr 22, 2023 at 19:39
  • $\begingroup$ If the idea is to pull everything away from us, that's just accelerating cosmic expansion (albeit in an inhomogeneous way). That's all cosmic expansion means: everything is separating. $\endgroup$
    – Sten
    Apr 22, 2023 at 20:15
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    $\begingroup$ (sitting here, whetting Occam's razor ...) ;) $\endgroup$
    – Karl
    Apr 22, 2023 at 20:58
  • $\begingroup$ @sten I thought the defining feature of universal expansion is that... the universe expands, not just things getting further away. me and you and sit in a room of fixed volume and decide to walk away from each other that doesn't mean the room got bigger $\endgroup$
    – Hisham
    Apr 22, 2023 at 21:27

3 Answers 3

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You have identified the issues.

The model does not explain the redshift-distance relationship, which is one of the primary pieces of evidence.

Simply to say "our ideas about gravity are wrong", so we invent some magic which makes things work isn't how science is done. We already have good evidence that our ideas about gravity do work very well, even in extremely strong gravity situations near black holes.

Perhaps more fundamentally, the model doesn't produce any redshift at all! The gravitational redshift due to concentrations of mass is proportional to their relative gravitational potential. Newton's shell theorem tells us that the gravitational field everywhere inside a spherically symmetric shell is identically zero. Therefore there is no gradient in gravitational potential and no gravitational redshift caused by this arrangement anywhere inside the shell.

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    $\begingroup$ The latter paragraph's point is IMHO the better one: the suggested solution is no solution at all. $\endgroup$ Apr 21, 2023 at 11:10
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    $\begingroup$ Doesn't it also violate the mediocrity principle, that we're not located in a special place? For the effect to be the same in all directions, we'd presumably have to be in the center of that ring, right? $\endgroup$
    – Barmar
    Apr 21, 2023 at 14:41
  • $\begingroup$ @Barmar; I suspect that your objection might be a central aim of the proposition, i.e. to promote the idea that Earth -is- in a privileged position, privileged by the sanction of some "higher power". I don't know that, of corse, but I've seen such arguments before. $\endgroup$
    – JohnHunt
    Apr 22, 2023 at 5:29
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    $\begingroup$ @JohnHunt Good point -- we really are the center of the universe, that's why everything is expanding away from us. $\endgroup$
    – Barmar
    Apr 22, 2023 at 5:30
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    $\begingroup$ “we invent some magic which makes things work isn't how science is done” I would argue that is exactly how science is done, often with success and sometimes without direct evidence: antimatter, neutrinos, inflation, dark matter, dark energy, etc. $\endgroup$
    – Matt
    Apr 22, 2023 at 7:09
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The massive ring would also need to be increasing its mass to explain dark energy redshift which increases with time.Where would that extra mass come from? Also the mass distribution in the visible universe is not perfectly symmetrical and it's gravitational pull on the ring would make the ring less than perfectly symmetrical and this in turn would be altering the symmetry in the visible universe

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  • $\begingroup$ Do you mean redshift measurements of photons from the same source show more redshift the later they are measured? I.e if we measured the redshift of photons from Galaxy A today, and then measured them again a year later with the same instruments, we would measure more redshift? $\endgroup$
    – Hisham
    Apr 22, 2023 at 8:33
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    $\begingroup$ @Maalik Yes the same Galaxy at different times. Space is exp[anding at an increasing rated with time so redshifts are increasing by a larger amount for each unit of time. $\endgroup$
    – user50623
    Apr 22, 2023 at 13:26
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If so, there would be an area in the middle of the sphere/ring where the galaxies are not moving away from each other, and it would be easy to detect the gradient of movement from the center of the ring to the periphery. In reality everything is moving away from everything else at an equal ratio.

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  • $\begingroup$ is the sun also redshifted? $\endgroup$
    – Hisham
    Apr 22, 2023 at 21:35
  • $\begingroup$ The sun is redshifted relative to other stars, because they are redshifted to the sun's region. Actually the galaxy's 100bn stars are not caught in the expansion because they have local gravitation, and planets are not expanding either. Only gravitationally unbound structures are expanding. If you want, start a new question to ask about the different ways that measure that the expansion is the same locally as it is far away. $\endgroup$ Apr 24, 2023 at 10:52

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