How do scientists measure the shape of the universe?

To measure cosmic geometry, scientists look at the size of patterns in the cosmic microwave background and compare them with predictions from general relativity. They also study baryon acoustic oscillations and supernova distances across the universe. Current data indicate that space on the largest scales is very close to flat, meaning parallel lines would remain parallel in a simple geometric sense. Small deviations are still possible, but if they exist, they are tiny.