Some issues about hard sf, & the stories that primarily deal with these issues

Mike Brotherton's "Ten Issues for Hard Science fiction" is meant for authors, but has interesting points for fans too. Some of the stories I link are online & download link will be found at the linked page; others aren't.

1. #2 - FTL: "In the most rigorous cases, writers should realize that FTL implies time travel or at least non-causal effects and has philosophical implications about free will." For "FTL implies time travel", see James Blish's "Common Time". For "philosophical implications about free will", see Ted Chiang's "What's Expected of Us". OK - Blish's story is not about time travel, but that FTL implies unusual behavior of time that can have weird implications (& it's not quite hard sf).
2. #3 - Teleportation: "If conservation laws are violated, it could be used to create perpetual motion machines and infinite energy. If not, things get more interesting. How are energy differences made up?" Not in the context of teleporting, but in the context of time travel, Murray Leinster's "Long Ago, Far Away" looks at the issues of conservation of matter/energy (at any instant, total matter + energy in the universe is fixed; so time travel involves their exchange across times!).
3. #4 - The Fermi Paradox: "Why aren’t aliens common in the universe and already in abundant evidence here on Earth?" There are dozens of stories primarily on this subject. Am feeling a bit lazy looking for them, but interested might want to sift through this big list of first contact stories.
4. #7 - Alien communication. I can off hand think up at least 2 good stories primarily about the subject: Ted Chiang's "Story of Your Life" & H Beam Piper's "Omnilingual".
5. #8 - Alien chemistry/biology: While there are many stories that pay attention to the subject in some detail, the one that immediately comes to my mind is Murray Leinster's "Proxima Centauri". While the story wasn't among the Leinster's best & the alien bilogy was only a small part of the story, the premise is interesting: what if a plant became sentient rather than an animal? Of course, it will be as natural for it to eat humans as it is for us to eat apples!