Purely theoretically, could it be possible to extract energy from spacetime's expansion? Like, say, imagine two celestial bodies far away enough so that they are not bound enough by gravity, they get farther away from each other due to spacetime expansion. We tie them to each other with a rope. As they get further apart the stretching of the rope gets converted to energy. Or would the rope itself get stretched in the same scale? Would a rope 1 km long become 100,000 km long due to space expansion without any gain in energy?