This week I decided to review Ultima by Stephen Baxter.
When the story opens, Yuri Eden has been transported to a universe where the Roman Empire never fell but competes with the Xin Empire, based in China, while loosely allied with the Brikanti .nation centered on the British isles While he and those with him struggle to integrate into the crew of the ship that found them, a second ship fleeing an apocalyptic war in the Sol system finds itself transferred to this new universe. Its crew become part of this Earth's society with the AI Earthshine aiding in efforts to terraform Mars by diverting an asteroid Ceres.
But Earthshine has the asteroid on a course to destroy Mars, so the refugees from the UN-ruled timeline and a group of Romans are sent to intercept. They make contact with Earthshine and discover his goal is to force contact with the lifeforms inhabiting Mars by threatening their survival. In the end, the Roman ship and the refugees are shifted to a third universe where the Incas rule humanity after destroying Rome, with most of humanity living in orbital habitats.
Again, all the refugees struggle to integrate into a new society with some marrying and having children. In time they realize that in each new timeline they find themselves in humanity has gone further in replicating and spreading the Hatches, alien light speed deportation devices found on Per Aruda, a planet orbiting Proxima Centuari. Earthshine believes that the creators of the Hatches have been manipulating humanity across all of the universes. Eventually, the group finds itself on Per Aruda in the distant future when the collision between the Milky Way galaxy and the Andromeda galaxy is growing near. There they discover a horrifying truth. Now with the end of the universe they are in imminent, the group must struggle to find a way to escape the timeline they are in and decide whose survival should be a priority if they find a way.
I give the book 7 out 10. I like the various societies introduced in the new universes and some of the technologies shown are interesting, but I didn't find the characters very gripping and I wish the series had been longer. I think the story would have been much better if there had been a whole book focused on the Roman universe and another focused on the Inca universe, thus allowing each timeline and the characters introduced by that timeline to be developed in more detail.