Flatness of Space

General relativity allows the universe to curve: positively like a sphere, negatively like a saddle, or not at all. Measurements of the cosmic microwave background show space is flat to within 0.4%.

Inflation provides that mechanism. Rapid expansion stretches any starting curvature flat, just as inflating a balloon makes its surface look increasingly flat up close. Blow up a patch of space by a huge enough factor and any bend in it becomes too gentle to detect — exactly what inflation predicts.