I'm not sure what you're referring to. Everything is moving at the speed of causality at all times through spacetime. Mass curves spacetime that's what gravity is. In order to reach FTL, the medium itself has to be altered, like an Alcubierre drive using negative mass to expand spacetime behind it.

Mathematically, there's nothing stopping spacetime itself from expanding FTL relative to another point within it. To take advantage of that for travel or communication would require new types of exotic matter that have negative mass to create a warp bubble. FTL in space is like reverse time travel.

Accessible space isn't infinite. Dark Forest hinges on the premise that FTL travel is impossible, and we're constrained to the local cluster at most if that's the case.