system
Given the following post, respond as if you're commenting on Reddit:
human
If I suspended two large masses in space sufficiently far apart such that they were not moving relative to one another, could I tether them and use the expansion of space between the objects to generate energy?

If two large objects were tethered to one another and a generator were attached to a string in the middle could you accelerate the two objects towards one another as space expands in between them and use this acceleration to generate energy from the work done? Where does this energy come from and is it dark energy? How much energy could you make with say, 1000kg objects 1 light-year apart? 1000 light-years? (I'm not sure at what scale the expansion of space starts beating the gravitational pull)
gpt
Yes, there would be a tension in the rope. If the rope is approximated as a spring with spring constant *k* and the current distance between the two rope-tethered galaxies is χ, then the tension in the rope is just F(t) = kχa(t). The number a(t) is the so-called scale factor which describes how proper distances increase over time. The potential energy of the rope due its displacement from equilibrium is simply due to the expansion of space, not necessarily dark energy. (This tension would be there even with zero cosmological constant.) Yes, this energy can be extracted to do work.

There is a bit of a caveat here though. This tension would not be uniform throughout the rope. Any signals within the rope travel at the speed of sound, so the central part of the rope doesn't really "know" it's being stretched until the sound wave has propagated from the ends of the rope and reached the center. In reality, the rope will break closer to the ends long before the central part of the rope ever begins to stretch.