The recent announcement about the observation of a giant ring structure in a distant part of the universe, was widely reported as putting into question some fundamental assumptions of cosmology. But Dominik Schwarz argues such interpretations depend on a misunderstanding of the cosmological principle.

At the recent American Astronomical Society meeting, an announcement was made about the discovery of a giant ring with a diameter of 1.3 billion light years. This follows a trend of similar claims in recent years about the discovery of larger and larger structures observed in the universe. The first difficult question to ask is of course, are these structures real or just chance patterns, like for example the constellation of Ursa Major. Our brains tend to fool us and we like to see patterns in any random assembly of points. If we can somehow convince ourselves of the reality of these, the second question to ask is, are they gravitationally formed and bound, or are they formed by some other dynamical process. Especially the second question is often very hard to answer and typically requires information in many different wavelengths. For sure there will be some debate on both of these questions, but let us assume both are answered and that the announcement is correct. The third, and deeper question to ask then is, what would it mean if these structures are both real and brought together by gravity?

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In short, the cosmological principle is an assumption that  that space is maximally symmetric. In other words, whichever direction you look and from wherever you look, the universe would look roughly the same.

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Following the recent announcement about the giant ring structure, as in previous such cases, it has been argued that these structures are so large that they would challenge the cosmological principle. If that statement was true, it would be remarkable, as the cosmological principle is at the very foundation of our modern understanding of the structure of the Universe. The cosmological principle is a statement about the spatial symmetries of the Universe. However, such pronouncements stem from a misunderstanding of the cosmological principle itself.  I argue that even the largest known structure itself cannot challenge the cosmological principle, since correctly understood it is a statement about the statistical distribution of matter and light in the Universe, and not a claim about the particular size of anything.

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The cosmological principle is a convenient starting point in modelling the Universe, as it allows us to make up for our lack of knowledge of what happened soon after the Universe came into existence. In short, the cosmological principle is an assumption that space is maximally symmetric. In other words, whichever direction you look and from wherever you look, the universe would look roughly the same. This assumption was originally invoked by Einstein in his very first attempt to find a cosmological solution in the context of his new theory of general relativity. The assumption of maximal symmetry does not come out of the blue, it is a property that also holds for space in Newtonian physics, where the Euclidean geometry is used to describe a pre-existing absolute space. That absolute space does not exhibit any preferred direction and it does not have any preferred place, thus exhibiting six possible symmetry operations, consisting of all translations (move an object) and all rotations (turn an object) that you can think of. In some sense, Einstein just assumed what was a standard assumption about space at the time.