Didicosm

[2] As a child, her father shows Charlotte the night sky and wants her to realize the truth about the endless worlds and possibilities in the universe.

After her mother dies as well, Charlotte is brought to her grandmother and later wants to find the correct topology of the universe, which turns out to be a didicosm (Hantzsche-Wendt manifold).

Her own student later comes up with a theoretical explanation involving quantum gravity, concluding this shape is indeed canonical due to being the only platycosm with a finite first homology group.

[3] In 1984, Alexei Starobinsky and Yakov Zeldovich at the Landau Institute in Moscow proposed a cosmological model where the shape of the universe is a 3-torus.

The derivation is explained by Greg Egan on his website,[4] which also lists four academic papers taken for the scientific basis of the short story: „Describing the platycosms“ by John Conway and Jean-Paul Rossetti,[6] „The Hantzsche-Wendt Manifold in Cosmic Topology“ by Ralf Aurich and Sven Lustig,[7] „On the coverings of the Hantzsche-Wendt Manifold“ by Grigory Chelnokov and Alexander Mednykh[8] as well as „How Surfaces Intersect in Space“ by J. Scott Carter.

He claims that "there is a story here, but it is rather weak, and serves only as a vehicle" for the main idea, which is an "impenetrable subject for those [....] who lack a higher degree in theoretical physics or the relevant mathematics.