Location from deaths-end by
A stable 1km-radius bubble universe outside our own — Yun Tianming's gift to Cheng Xin and Guan Yifan, where they wait out the heat death of the world.
Pocket Universe 647 is one of millions of micro-universes built by various civilizations as lifeboats from the dark forest war and the eventual heat death. Yun Tianming, with the cooperation of Sophon and the resources of the Trisolaran fleet, prepares 647 as a refuge for Cheng Xin and Guan Yifan. They live there in domestic quiet for decades while the home universe burns down to its last cinders. At the end, the universe broadcasts a request to all pocket-universe owners to return their stored mass so the next big bang can occur. Cheng Xin and Yifan choose to return theirs and re-enter the dying universe, leaving only a memory and a fish for whoever comes next.
Inside: a small green planet with one small white sun, surrounded by a bright blue sky that ends at a finite distance — the universe is one kilometer across. A modest house, a garden, a stream. Time runs differently inside than outside; what is decades within is eons without. From the boundary, our home universe appears as a single dim point of light.