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In ancient times, the arguments for geocentrism were primarily from observation. It looks like the heavens revolve around the Earth, so "heavens revolve around Earth" is a reasonable first model. Subsequent theory finds that a better model that more closely matches observations of heavenly bodies is that the Earth is simply rotating. The argument about which celestial body was at the 'center' of the universe then raged for some time. Modern physics has revealed that motion is relative to a reference frame, and that imposing a fixed coordinate system with respect to the relative motions of bodies is actually a poor model.

However, our knowledge of the large-scale static-ish distribution of matter in the universe is largely driven by observation. The observations we have been able to make in modern times of the deep universe suggest that the universe is, on average, evenly distributed around our particular location. Since the universe appears to be, on average, uniform, a reasonable model is that it appears that way from every point in the universe.

Nevertheless, since we are confined to our tiny bubble, it is unlikely that this hypothesis will be tested anytime soon. Relating this back to the original question, it seems that, currently, it looks like the Earth is at the center of the universe. We are unable to verify the hypothesis that the universe would look more-or-less this way from every point experimentally. So, what are some modern arguments against a geocentric (or heliocentric, or, more generally, an "our-local-neighborhood-centric") viewpoint? Specifically, are there any counterpoints, backed up by experiment, to the argument that "it looks like we're at the center, therefore we are at the center"? Alternatively, what specific, tested predictions of prevailing models is this incompatible with?

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    $\begingroup$ Saying it looks like we are in the center simply because we are unable to look from anywhere else seems like a stretch $\endgroup$ – Hagen von Eitzen Apr 22 at 16:17
  • $\begingroup$ @HagenvonEitzen Said basically what I was about to. From what I understand, things appear as we would expect if we weren't the centre, and just one of many with a similar perspective. I don't see what shows that we are the centre. $\endgroup$ – JMac Apr 22 at 16:19
  • $\begingroup$ @HagenvonEitzen, possibly it is a stretch if you are, e.g., trying to get published in a respectable physics journal. However, when I'm standing in the center of an empty room, it looks the same in all directions. When I am not in the center, it does not. The fact that things look the same in all directions seems, naively, to suggest that we are in the center. $\endgroup$ – Scott Apr 22 at 16:40
  • $\begingroup$ The alternative and accepted notion that "center" is meaningless with regards to our universe is incredibly abstract and, as I pointed out, difficult to verify. Possibly it is easier to simply refute the claim that we are located in the center. Thus, I am interested in simple experimental results that demonstrate that we are not in the center of the universe. $\endgroup$ – Scott Apr 22 at 16:41
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Specifically, are there any counterpoints, backed up by experiment, to the argument that "it looks like we're at the center, therefore we are at the center"? Alternatively, what specific, tested predictions of prevailing models is this incompatible with?

There are a vast number of statements that one could make that one cannot reasonably rule out observationally. We can only see so far, and anything could be behind our cosmological horizon. It's not impossible that the Earth sits exactly at the center of a structure that is far larger than our Hubble sphere, and we would be none the wiser. It's just another Russell's teapot.

As a practical, working assumption, we take the entire universe to be described by the FLRW metric, which has no "center" for the Earth to be at. This is a mathematically simple option that fits the data. However, the crucial thing is that, even though the FLRW metric extrapolates beyond our Hubble patch, it is only used to make statements about our Hubble patch. In other words, the validity of conventional cosmology only require our universe to be locally like the FLRW metric, not globally, and we can test the former assumption perfectly well. (It is true that people sometimes elide the two in pop science, but just about everybody in the field knows the distinction, at least tacitly.)

The point is, cosmology doesn't make definite statements about your argument. But within the current formalism, having the Earth be at the center of a giant unobservable structure would be mathematically clunky, artificial, and unnecessary to explain any current experimental results.


Incidentally, we could conclude that the Earth was (1) at the center of the universe, (2) not at the center, or that (3) the question was meaningless, if the universe was finite in extent, and small enough to see from one end to the other. We could detect this by seeing multiple images of the same object in different directions in the sky. Depending on the particular shape of the universe and the data, this could support any of the three options. (The actual technique is a bit more complicated, since we can't make out specific objects in the CMB, just cold and warm spots; instead we look for "matched circles" with similar temperature profiles.)

There have been several searches for this kind of structure, with one popular candidate for the shape of the universe being the Poincare sphere. However, to date all such searches have come up negative (for a recent review, see here), which means that either the universe is infinite in extent or finite, but too large to see the shape of. In either case, for now we can't give a definite answer to your question.

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  • $\begingroup$ "There are a vast number of statements that one could make that one cannot reasonably rule out observationally.... It's just another Russell's teapot." This is fair, and I am personally inclined to agree. $\endgroup$ – Scott Apr 22 at 16:57
  • $\begingroup$ "Incidentally, we could conclude that Earth was, or was not at the "center of the universe" if it turned out that the universe was finite in extent, and we could see from one end to the other. " I don't see how being able to determine a center follows from the universe being finite. Taking the Earth's surface as analogy, it is finite, but there's no "center". $\endgroup$ – Acccumulation Apr 22 at 17:25
  • $\begingroup$ @Acccumulation Yes, observations could in principle resolve the question in either direction (that's what I meant by "was, or was not"). If you had something analogous to a sphere, the question would be simply meaningless, and hence fall into the "was not" category. $\endgroup$ – knzhou Apr 22 at 17:28
  • $\begingroup$ The phrasing "was not" implies there is a center that the Earth is not at. $\endgroup$ – Acccumulation Apr 22 at 17:30
  • $\begingroup$ @Acccumulation That makes sense, I edited. $\endgroup$ – knzhou Apr 22 at 17:32
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it seems that, currently, it looks like the Earth is at the center of the universe.

This doesn't really make sense. You haven't defined what it would look like for an observer not to find themselves at the center. E.g., would they observe that light from their own galaxy was Doppler shifted? That would be silly.

We are unable to verify the hypothesis that the universe would look more-or-less this way from every point experimentally.

Not true. We do surveys of galaxies, and we find that on large scales, the universe is homogeneous. This is a falsifiable hypothesis. If we did a more careful survey and detected some inhomogeneity, it would be falsified.

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  • $\begingroup$ "on large scales, the universe is homogeneous" -- this is not the same as the universe looking the same from every point. The surface of the table in front of me is largely homogenous. However, an observer on the table would see something different from the center and from the edge. I'm not arguing that the universe is analagous to a tabletop, only that the homogeneity we observe from Earth does not imply viewpoint relativity. $\endgroup$ – Scott Apr 22 at 16:23
  • $\begingroup$ "You haven't defined what it would look like for an observer not to find themselves at the center." This is an equally good argument that the Cosmological principle is not falsifiable. Interestingly, that same article has a quote from a connoisseur of falsifiability (Karl Popper) that is to this effect. $\endgroup$ – Scott Apr 22 at 16:33
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    $\begingroup$ @Scott: The surface of a table is not homogeneous in the sense meant in cosmology. In particular it is not invariant under translations, because it has edges. $\endgroup$ – tfb Apr 22 at 16:37
  • $\begingroup$ @tfb True. However, the only homogeneity that we actually observe is the homogeneity from our particular location. My only argument here is that homogeneity observed from one point of view does not imply homogeneity from every point of view. $\endgroup$ – Scott Apr 22 at 16:44
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    $\begingroup$ @Scott: that's not the case. As Ben says, we do surveys of galaxies and we find that things are homogeneous on large scales. $\endgroup$ – tfb Apr 22 at 17:31

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