The universe as a whole is much larger than the portion we can measure. That "observable universe" (OU) has a proper radius of around 43 billion light years. The Planck mission measured the curvature density ΩK of the OU as 0.0±0.005. For the positive 1 sigma value, that suggests the whole would be a 3-sphere with a radius of ~210 billion light years but inflation suggests the curvature should be much closer to zero, hence the whole would be far larger, probably many orders of magnitude. For zero or negative curvature, the spatial extent would be infinite.

If we assume that the whole is much larger than the observable portion, we could think of many alien species scattered throughout the universe but so widely separated that there is no overlap between their respective OUs. While the universe would have a mean curvature density, there are also statistical variations, so we can think of producing a histogram of the values of ΩK for all these uncorrelated regions, each around 43 billion light years in radius.

The Harrison-Zel'dovich spectrum tells us the two-point correlation of density as a function of separation of sample points. Specifically, it would be a power law with slope 1 but quantum effects predict a slightly smaller value and it has a measured slope of 0.96 (Planck mission results, 2015). The same results also gave a null result for "running" of the spectrum, which means it appears to be linear, no quadratic term, and there is no evidence of non-Gaussianity, which means the distribution can be assumed to be the standard bell curve.

My question is: what would be the standard deviation of the samples of ΩK measured over a large number of non-overlapping regions, each the size of our observable universe?

If anyone needs more background, I can provide links to introductory articles and some undergraduate lectures. This is the relevant section in the Planck results paper:

https://arxiv.org/pdf/1502.01589.pdf#subsubsection.6.2.4

P.S. If you don't accept the standard model of cosmology or have your own alternative, don't waste your time replying, I am looking for a purely mathematical answer based on the conventional model.

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