I read a book a long time ago (Poetry of the Universe: A Mathematical Exploration of the Cosmos, by Robert Osserman) about the shape of the universe. In it he talks about quasars. If you observe 2 of them in opposite directions, each 13 billions light years from earth, then they must be 26 billion light years from each other. But the universe is only a little less than 14 billion years old so how can this be? He argues that the universe must be a hypersphere.

I read this more than 20 years ago, and I'm no cosmologist, so forgive me if I butchered that summary.