**Definition: **The "flatness problem" is a problem in

cosmology about how scientists can explain that, when viewed on very large distances, the universe appears to have very little curvature (i.e. it is "flat").
According to

general relativity, three types of curvature are allowed for the structure of

spacetime:

- a positive curvature
- negative curvature
- no curvature at all

At present, all evidence suggests that spacetime in our universe has virtually no curvature. This is very unlikely. If the universe had begun with a slight positive or negative curvature, then models predict that this curvature should increase over time, not go away.
Many cosmologists feel that

inflation theory offers a likely solution to the flatness problem. According to the theory, the process of inflation pushes the universe toward a flat universe the longer it goes on.