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My understanding of cosmological principle is this:
Space looks same in all directions and is homogeneous. But this principle was under heading of Observational Evidence for expansion of space in this article:

https://en.wikipedia.org/wiki/Expansion_of_the_universe#Observational_evidence

My question is, how does Cosmological Principle imply an expanding space?
If it's my understanding of cosmological principle which is flawed, I would highly appreciate if someone corrects me. Thanks in advance!!

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  • $\begingroup$ The cosmological principle states that at any given time the Universe is isotropic and homogeneous - which allows for constant curvature. In general relativity, there are typically 3 choices for curvature, $k=1,0,-1$. As far as an expanding universe is concerned, it's by fiat. It's consistent with Big Bang Theory - which is consistent with the Old Testament of the Bible. The Hubble data, however, looks like random sequence - it's not clear that red shifts imply velocity - and quasars which were believed to be on the "edge" of the Universe are actually in our Local Super Cluster. $\endgroup$ – Cinaed Simson Apr 4 at 7:28
  • $\begingroup$ It does if combined with other hypothesis or observations. $\endgroup$ – Alchimista Apr 4 at 8:51
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Since according to Cosmological Principle (CP) every point is space is equivalent to every other point, if the galaxies appear to be rushing away from an observer in the Milky way galaxy, the same would also appear to be to an observer from the Andromeda galaxy. So if the CP is true, the Universe must be expanding.

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