Companion Volumes

Three hard sf novels by Poul Anderson:

After Doomsday
World Without Stars
Tau Zero

- form not a trilogy but a "triad" (I think).

Each is a stand-alone novel about interstellar travel that develops a distinct, and different, premise:

that an FTL spaceship returns to the Solar System to find that Earth has been sterilized;

that an FTL spaceship visits a planet of a red dwarf star in intergalactic space;

that a relativistic spaceship accelerates uncontrollably.

World Without Stars and Tau Zero have a couple more points in common:

the relativistic ships passes a red dwarf with planets in intergalactic space (see Stars Between Galaxies);

in World Without Stars, Argens asks, "...must we kill through all time, until time ends when the disgusted universe collapses inward on us?" (XV, pp. 115-116)

The crew of the relativistic ship experiences that collapse.