Would anyone like to explain the Poincare conjecture about the 3-sphere and its relevance to the shape of the universe?
My only stipulation is that any such explanation be intelligible to a wider audience than just professional mathematicians. The “wiki” explanation unfortunately fails this test.
I will try my best…
I personally dislike the name 3-sphere because it can lead people to the wrong conclusion. A 3-sphere is a
four-dimensional object and should be called a 4-sphere, but since mathematicians are mathematicians, they consider only the
unit radius and thus reduce the dimensionality by one.
That is, a circle is usually given by coordinates r & theta, but if r=1 then we only need theta; this is the 1-sphere. A ball has coordinates r, theta, & phi, but r=1 so we only need theta & phi; this is the 2-sphere. And so on down the line to the 3-sphere which is actually a 4D object. The error in this naming convention is when you consider Cartesian coordinates of these object, then you still require the
correct number of dimensions: a circle is defined by x & y, a ball is defined by x, y, & z, and a “3-sphere” defined by x, y, z, & w. With regard to cosmology, we use time (t) in place of w and we put it first, so a position in space-time is given by t, x, y, & z.
When we want to measure the dimensions of some object, we pull out a ruler. This works for the most part because we are not
inside this object. Since we are inside the universe, we cannot make a definitive measure of its shape in this manner. What we do instead is measure the
curvature (literally the amount of deviation from flatness, generally defined as the inverse of the radius: k = 1/R) and from the curvature, we can determine the shape of space–this requires the use of a
metric (a mathematical function that gives the distance between two vectors).
I do not know that we have pinned down a value for k, but I do believe that we are fairly certain that it is very close to 0, indicating a fairly flat universe. By flatness, we mean that if we took two (extremely long) strings, laid them out parallel to another another, and managed to have them go all the way around the universe, the lines would never intersect. In a non-flat (i.e., curved) universe, the parallel strings would/could intersect with one another.