The story opens with Torris, a young man of a Neolithic-level tribe living in a giant tree on a comet in the Oort Cloud, about to undergo his tribe's rite of passage. I didn't initially find this encouraging; how many previous books have I read featuring humans in the far future reverted to primitivism in unlikely settings? It was well done, however, and I kept reading.
And that was a good thing, as I discovered the starship Time's Beginning, in orbit around a planet named Rebirth, in a distant galaxy. Joorn Gant, captain of Time's Beginning, and his friend Delbert Karn, the ship's leading astrophysicist, have dramatically different views of what Time's Beginning should do, once this latest human colony is firmly established. Two of the oldest members of the crew, among a dwindling number born on Earth before departure, Gant wants to lead the Homegoing faction back to Earth, where Sol, now a red giant, will be shrinking again, with Earth emerging from its photosphere, while Karn wants to lead his even smaller faction of hardcore astrophysicists on an expedition to the edge of the universe, where the earliest galaxies formed--the original intent of the ship's name, before this became an expedition to seed human colonies as widely as possible, to ensure the survival of the species.
The colonists would prefer neither of these expeditions to happen; they want to keep the ship and dismantle it for its resources, including its remaining habitats, which increase the survival odds of the new colony by providing additional secure living spaces while they terraform.
The colonists don't prevail.
Gant and Karn reach a bargain, to take the ship back to Earth, and leave the Homegoers with some of the habitats, while the scientists take the ship and the remaining habitats off on their voyage of scientific discovery.
It's not that simple, of course, and the conflicts of competing interests reaches to the Sol system, and encompasses even the comet dwellers and their trees. There's also the conflict between Torris's tribe and the tribe on their nearest cometary neighbor, currently approaching close enough that bride raids are imminent.
Both the ship culture and the cometary culture are interesting, and the characters are interesting and worth getting to know. There's also a solid plot that keeps moving. A completely enjoyable light read.
I received a free electronic galley of this book from the publisher via NetGalley.