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Further Thoughts On Homogeneity/Isotropy
Phil Helbig has posted the following highly interesting comment
regarding homogeneity/isotropy on Sean Carroll's blogsite. "Third, the assumption that we can extrapolate what we observe in the observable universe to all of the universe, namely homogeneity and isotropy on large scales, to the entire universe, might not be right. In that case, what we observe might tell us nothing about whether the universe is finite or not." I appreciate this new candor and hope to see it often. RLO http://www3.amherst.edu/~rloldershaw PS: All-Caps is definitely shouting. Italics are indicated by astericks before and after, as in *qed*. Hope this helps! [[Mod. note -- I have seen different conventions for what *asterisks*, /slash marks/, and _underscores_ denote -- all usually mean some sort of emphasis, but which might be bold vs italics vs some other typographical distinction may vary. Indeed, I think that over the years I myself have used different conventions. In any case, ASCII typography is at a minimum quite peripheral to our newsgroup's focus (research in astronomy/astrophysics), and it could reasonably be said to fall outside our newsgroup charter altogether. So, let's focus on discussing astronomy/astrophysics instead. -- jt]] |
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Further Thoughts On Homogeneity/Isotropy
In article ,
"Robert L. Oldershaw" writes: Phil[lip] Helbig has posted the following highly interesting comment regarding homogeneity/isotropy on Sean Carroll's blogsite. "Third, the assumption that we can extrapolate what we observe in the observable universe to all of the universe, namely homogeneity and isotropy on large scales, to the entire universe, might not be right. In that case, what we observe might tell us nothing about whether the universe is finite or not." I appreciate this new candor and hope to see it often. It's not new. It is also obvious: by definition, we can observe only the observable universe. Anything we say about what is outside is an extrapolation. Of course, if the extrapolation is sensible, then we can have some confidence in it. For example, it would be strange if an assumption happened to break down just at the edge of the observable universe, especially since the size of the observable universe changes with time, so it would be even stranger if it broke down at the edge of the observable universe now. The remark was an answer to the question whether the observed flatness of the universe implies that it is infinite. The question is thus about the largest possible scale in the universe, which might be infinite. Had we observed significant positive or negative curvature, then we could make a statement about the finiteness or lack thereof of the universe with some confidence, since a wrong extrapolation would imply that the universe beyond the particle horizon (the edge of the observable universe) is substantially different to what is inside. However, what we observe is a universe which is at least very close to flat (i.e. the sum of the density parameter and cosmological constant, Omega+lambda, is equal to 1 to within a per cent or better), so just a small difference between the observable universe and the rest of it could in principle lead to the opposite conclusion. (If the universe is exactly flat then, of course, we could never prove this exactly by observation. If it is not exactly flat then, in principle, we could hope to observe non-zero curvature, i.e. rule out the flat case with high confidence, though the caveat discussed above still applies.) |
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