Lorne Booth
Sep 1, 2018 · 1 min read

Another more abstract way of looking at this conundrum is to remember that spacetime (time mainly) is curved. When we measure vast distances over time, we are measuring around a curve, a distance “as the photon flies” over a curved 4D surface. If we could measure a straight distance (a comoving distance), then we would determine that the distance hasn’t changed at all since the earlier time when the photon was emitted. We can’t do that. This has some similarity to measuring the distance between two faraway cities through the center of the Earth rather than over its surface in 3D. Isn’t cosmology fun?

    Lorne Booth

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