
"And everywhere the Humans went, they found life..."This dazzling future history, winner of the 2000 Philip K. Dick Award, is the most ambitious and exciting since Asimov's classic Foundation saga. It tells the story of Humankind -- all the way to the end of the Universe itself.Here, in luminous and vivid narratives spanning five million years, are the first Poole wormholes spanning the solar system; the conquest of Human planets by Squeem; GUTships that outrace light; the back-time invasion of the Qax: the mystery and legacy of the Xeelee, and their artifacts as large as small galaxies; photino birds and Dark Matter; and the Ring, where Ghost, Human, and Xeelee contemplate the awesome end of Time.Stephen Baxter is the most acclaimed and accomplished of a brilliant new generation of authors who are expanding the vision of science fiction and taking itto a new golden age.
Fails for the 2020s part, but check out Greg Egan if you haven't already (his Diaspora is mentioned elsewhere in this thread). Dark Integers is a short-fiction collection. Also Vacuum Diagrams, by Stephen Baxter. These three authors are, IMHO, the absolute pinnacle of hard scifi. But be advised they are definitely kinda optimizing for being idea-dense. For more literary stuff with deeper focus on story structure & consideration of the individual characters, etc, you might want to look elsewhere.
EDIT to say, Dennis E Taylor is more recent and on reflection definitely deserves a mention. Also an ex-programmer-turned-author IIRC. The Bobiverse series is aimed at a wider audience of more casual nerds than the stuff above, and more of a recap of "big ideas" from other scifi without the head-spinning future shock of stuff like Stross and Egan. But it's solid nevertheless and easier to call it "fun". And despite the artistic license with the more dreamy far-future tech that's available to protagonists in the not-so-distant-future.. Bobiverse is kind of a "scifi procedural" flavor, so that probably makes it appealing to people who like stuff like Weir's the Martian.
1. Vacuum Diagrams by Stephen Baxter. When I think of epic hard science fiction, the Xeelee Sequence books spring to mind. With a story line that spans millions of years (and a few dozen books), this collection of short stories is a good introduction to one of the best and most underrated sci-fi series out there. Baxter's Manifold trilogy (Manifold: Space, Manifold: Time and Manifold: Origin) are also fantastic.
2. Foundation by Isaac Asimov. The whole Foundation series is wonderful, but this book is a landmark of sci-fi that should be on any fan's bookcase.
3. The Martian by Andy Weir. This book is what I've been giving the last couple years to people who don't think they like sci-fi. Everyone I've given it to has loved it.
4. Rama by Arthur C. Clarke. Another hard sci-fi staple. The rest of the Rama books he "co-wrote" with Gentry Lee are decent but become more space opera than hard sci-fi. I enjoyed them but many sci-fi fans find them polarizing.
5. Silver Tower by Dale Brown. More of a military thriller than sci-fi (Flight of the Old Dog is another favorite of mine by him) and terribly dated by modern standards (it was written when the Soviet Union was still a thing). But it's the first "adult" sci-fi book I ever read as a kid, so it'll always have a special place for me.
EDIT: Another one:
6. Coyote by Allen Steele. I love stories like this one: primitive, longshot interstellar exploration and primitive, first generation colonization. Especially for desperate reasons. The first two Coyote books were good, but I just can't get into any of the subsequent ones.