Brace yourselves for gravitational waves

[Alex K]

Meaduring lengths in relativity is a subtle thing because there is so much freedom to choose coordinates. The simplest way to understand it is that you have two freely floating test masses at a distance which get hit by a gravity wave. The wave changes the distance measure between the masses just as cosmic expansion increased the distance between galaxies.

... and because the Hubble Constant is a known factor, they can discriminate the additional motion and adduce it to gravitational waves?

Oh, I just meant that it seems to be a similar physical effect, but that's really a great question.
So I'd say even if the hubble constant were large enough to be seen in detectors (and I doubt that but with those astonishing sensitivities I have to check again) it would maybe result in a constant outward force on the mirrors which would shift the equilibrium point of the mirror suspension by a tiny amount... but I really have to check again. The effect is probably totally eclipsed by tidal forces etc, since the earth is a big mass itself and we are not at the center of mass, and it is rotating and moving around the sun etc. A good homework question for a cosmology lecture