I understand that k describes positive, negative, or no curvature. However, why can't there be, for example, +0.5 (semi-positive) curvature, etc?
Answer
The continuum of curvatures does exist, but we find it more convenient to put it elsewhere. The crucial part of the metric that encodes the curvature is a factor 1−Kr2, where r is the radial coordinate (using any point as the origin), and K is any real number, positive or negative. By dimensional analysis, there is some length L such that either K=1/L2 or K=−1/L2, so we can rewrite our formula as 1−k(r/L)2, where k is just the sign of K, or 0 if K=0. The case k=0 corresponds to L equal to infinity.
This new variable k can only have the values ±1 or 0, but that's okay because L still can be any length, so we have the whole range of curvatures. L is known as the radius of curvature of the universe, and a larger L implies a smaller curvature. k determines whether this curvature is positive or negative.
Now, and this is a bit of a technical point, we can make that L go away if we measure our coordinate r in units of L. In our formula, we can set x=r/L to get just 1−kx2. The continuum of curvatures is still there but it is hidden inside of x, because the physical interpretation of x depends on L: x is how many times L fits in your distance. So if for example L=1 light-year, x=2 is a distance of 2 light-years, but if L=3 light-years then x=2 is actually a distance of 6 light-years. Mathematically, the price we pay is that L now shows up elsewhere in the formulas, in the part we use to calculate lengths.
To sum up, the curvature can indeed take any value: the closer it is to zero, the closer space is to being flat. The fact that k can only be ±1 or zero is just a matter of convenience: we use k to label the three qualitatively different scenarios of positive/negative/zero curvature. L just sets the size scale for the universe.
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