I think the best hard sci-fi can include cutting edge theories or even ideas that the author themselves are proposing, based on a reasonable understanding of the existing science.

At the time many of Greg Egan's early books from the 90s/00s were written, there were a lot more competing ideas or theories in quantum mechanics compared to now. In his books he basically explores some of those ideas, and pushes the boundaries.

The best sci-fi to me inspire future scientists or engineers to explore novel approaches.