Ring by Stephen Baxter

A sprawling far-future epic that follows multiple generations of humans as they confront cosmic-scale threats and pursue a gargantuan alien megastructure — a ring of unimaginable size and purpose — created by a far older, godlike species; the novel weaves hard science, political maneuvering and personal stories across vast stretches of time as humans and their allies and enemies navigate relativistic travel, strange physics and the desperate quest to understand and, if possible, use the Ring to secure their species’ future.