When scientists refer to the "flatness" of the universe, they are not referring to its spatial dimensionality. The term "flat" in this context relates to the overall geometry of the universe on a large scale, specifically in terms of its curvature.
In cosmology, the concept of flatness refers to the spatial curvature of the universe, which is related to the distribution of matter and energy within it. It is based on the assumption that the universe follows the principles of general relativity, as proposed by Albert Einstein.
According to the theory of general relativity, the curvature of space is influenced by the distribution of mass and energy. If the density of matter and energy in the universe is precisely balanced, the spatial curvature can be flat. In this scenario, parallel lines would remain parallel and the angles of triangles would add up to 180 degrees, just like in Euclidean geometry.
However, if the density of matter and energy in the universe deviates from this precise balance, the spatial curvature can be either positively curved (closed), meaning it curves back onto itself like a sphere, or negatively curved (open), meaning it curves away from itself like a saddle. These are the alternative possibilities to a flat universe.
Current observations suggest that the universe is very close to being flat. Studies of the cosmic microwave background radiation, large-scale structure formation, and other cosmological measurements indicate that the spatial curvature of the universe is nearly zero within the margin of error. This is consistent with the idea of a flat universe.
It is important to note that when scientists talk about the universe being three-dimensional, they are referring to the number of spatial dimensions we are familiar with (length, width, and height). The concept of flatness, on the other hand, relates to the overall geometry of the universe on a much larger scale, encompassing the curvature of space itself.