In article ,
(Eric Flesch) writes:
The current Standard model is underpinned by the "flat universe",
a spatial manifold of zero curvature with local perturbations.

Why do you think that? Ned Wright's cosmology calculator
http://www.astro.ucla.edu/~wright/CosmoCalc.html
works fine for curved space.

Existing observations show that space is flat to extremely small
tolerance. What I found quickly was Fig 26 at
https://www.aanda.org/articles/aa/fu...a25830-15.html
but I'm sure I've seen better plot and more extensive discussion. The
topic has been studied in great detail, and a wide variety of
observations are relevant.

(1) The value of lightspeed (c) varies with spatial curvature.
Hyperbolic space would look to us as the same as "flat", but if you
travel in it you will find that your destination is closer

Which distance did you have in mind? All of them change with spatial
curvature, but changing speed of light would be new physics.

The possibility that lightspeed is decreasing with universal time...

Isn't this ruled out by observations? Changing speed of light
changes the ratio of frequency to wavelength. That would mean
grating spectrographs, which measure wavelength, would get different
results than radio observations, which measure frequency. In other
words, redshifts would differ between radio and optical measurements.

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