When scientists refer to the data indicating a "flat universe with an error of 0.4%," it means that the measurements and observations are consistent with the concept of a flat universe, and the uncertainty or margin of error in this determination is around 0.4%.
In cosmology, the term "flat" refers to the geometry of space. A flat universe means that the overall geometry of space is Euclidean, similar to a flat plane, as opposed to being curved like a sphere or a saddle shape. The concept of a flat universe is closely related to the overall density and energy content of the universe.
The error of 0.4% indicates the level of uncertainty in the measurement. It represents the range within which the true value of the parameter being measured (in this case, the curvature of the universe) is likely to lie. A 0.4% error suggests that there is a small level of uncertainty, but the data and observations strongly support the idea of a flat universe.
It's important to note that the error or uncertainty does not imply a specific probability that the universe is not actually flat. Instead, it quantifies the level of confidence scientists have in their measurement and the range of possible values consistent with the data. In this case, the data suggests a flat universe with a high degree of confidence, but there is always room for further refinement and refinement of our understanding as new observations and data become available.