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There are some theories on variable cosmological constants: https://en.wikipedia.org/wiki/Variable_speed_of_light

Universal constants changing slowly over the lifetime of the universe is one of the things I remember quite well from undergrad physics. Not as anything that has been proven, but as a meachanism to try and fix some of the astrophysics measurements.

Of course, the radius of the universe has clearly changed with time- this could mean that a0 did or that c did.

Complete guess: it might not be a constant, but a function of the average curvature of the universe.