I enjoy SF written by scientists who extrapolate their understanding to a future state. Alastair Reynolds fits that bill for me. Vernor Vinge is another. The latter comes up with some imaginative ideas and constructs. In “A Fire Upon The Deep” he has different laws of physics as you move up from the plane of a galaxy, so for example, faster-than-light travel is possible far above the galaxy disk and as you descend to the disk you start to enter “the slowness”.
Is this what one would call “hard SF”?