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Hi,

I have a very simple question about GRT. I use a picture which I personally like. Let us have a shpere (2D, embedded in 3D). Physics do only work on this sphere in this theoretical universe, so the forces work only along geodatics (there is no "above" or "below"). The distance of objects depends on the radius of this sphere R(t) (which is in GRT determined by the energy-momentum tensor, but this not important here). Now we measure the distance to far objects by the redshift which means that we do not measure the radius but just the distance. For this theoretic universe we would need to follow that ds is always < c dt. The moving bodies are not allowed to be faster than c. If they would move with c we would never see them. This would hold for every part of this space since we have a sphere. Conclusion: The expansion allows to see objects, meaning it is always below c - but this is still not a radius we measure. Statistically it would be sufficient to measure some redshifts to have a measure for R(t). This seems to be different from the 4D space, embedded in intself. Nevertheless: The only think I can do on the sphere is to use a high resolution telescope to look as far as possible and potentially see myself if I look around.

Why do we call it in the reality a radius since our measurement again is only along geodectics in space-time? Isn't it just the maximum on a geodetic we can look so far?

Thanks!

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# Radius of the Universe

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