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I want to preface this by saying I don't disagree with the idea of the universe's expansion, this was just a random thought that popped into my head. I also apologise in advance if it's a duplicate, badly formatted, fairly simple or outright dumb (I'm very sleep deprived and haven't looked at the intricacies of the universe's expansion in a long time + I'm on mobile).

on to the question; how do we know that the redshift we observe while looking at faraway galaxies isn't a Doppler shift, caused by potential great attractors outside of the bounds of our observable universe? are there galaxies that have travelled "faster" away from us than galaxies that are in the same region of space, but further away from us? if not, could this also potentially explain the differences in the perceived rate of expansion depending on what region of space you measure?

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"how do we know that the redshift we observe while looking at faraway galaxies isn't a Doppler shift, caused by potential great attractors outside of the bounds of our observable universe?"

The attractors you propose to be outside of the boundary of the observable universe (OU) may be approximately uniform or not. It it is uniform, than it effect on what is inside the OU is zero because the gravitational field of a uniform sphere is zero with respect to any point inside the sphere. If it is not uniform then its influence on the galaxies inside the sphere would NOT create an approximate uniform motion of the distant galaxies we see from Earth: velocity away from earth is proportional to distance from Earth.

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