The concept of the shape of the universe is a complex topic in cosmology. When scientists refer to the "flatness" of the universe, they are not referring to the overall perimeter or boundary of the universe but rather to the curvature of space itself.
In the context of cosmology, the term "flat" refers to the spatial geometry of the universe, specifically the curvature of space on large scales. It is based on the concept of a two-dimensional flat surface extended into three dimensions. A flat universe, in this sense, means that the geometry of space on large scales is consistent with Euclidean geometry.
The idea that the universe is flat comes from observations of the cosmic microwave background radiation and measurements of large-scale structure in the universe. These observations suggest that the overall curvature of space is very close to zero, meaning that space is nearly flat on large scales. This finding is supported by the theory of cosmic inflation, which explains the uniformity and flatness observed in the universe.
However, it's important to note that the concept of the shape of the universe goes beyond our current understanding and is an active area of research. The universe may have a more intricate topology or shape that we have yet to fully comprehend. Different theories and models propose various possibilities, including a flat Euclidean geometry, a curved geometry, or even a more exotic topology.
In summary, when scientists describe the universe as "flat," they are referring to the overall curvature of space on large scales rather than the shape or perimeter of the universe. The concept of the shape of the universe is still a subject of ongoing research and exploration in cosmology.