Raft
Stephen Baxter
 The few thousand humans survive in a nebula of relatively breathable air, existing in divided communities. The society is highly stratified, with the elite living on the "Raft" (the remains of the starship that contains almost all the high technology), workers/miners living on various "Belt" worlds (where they mine burned-out star kernels), and the "Boneys", a nomadic band of "unmentionables" who live on worlds created out of corpses. It is unknown exactly how the humans came to the universe, but hints within the story indicate that the Raft ship came through a rift in our universe into this alternate reality (though whether intentionally or unintentionally is never specified). A glimpse of the high-gravity universe is seen in the book Ring, implying that the humans in Raft came from the main universe of the Xeelee Sequence, although during which time period they escaped is not clear. The alternate universe the humans live in follows same laws as our universe, except that it has a gravitational constant which is orders of magnitude larger than our own universe. The physics of the alternate universe has slowly turned the nebula into an increasingly hostile environment and the humans, along with the bizarre native species, are suffering the effects of environmental collapse.