Originally posted by KellyJay
Nope do not understand that, and I'm not trying to be difficult or to jerk
yours or anyone else' chain on this. I'm quite serious I don't see how you
or anyone else can say something is expanding if it is isn't moving into
new areas prior to expanding, hence the word, "expanding".
Kelly
What we call "distance" is an internal property of the universe, which is to say it can be wholly defined without reference to anything "outside" the universe. Both mathematically and physically a universe is expanded by increasing the distances between its points, and still no reference to an external realm is needed. From a simplistic standpoint one could understand this by saying we are increasing the number of spatial coordinates that lie between any two existing spatial coordinates. Say the distance between two neighboring points is defined to be 5 (maybe because it takes us 5 time units to hop from one to the other). In the universe U={0,8} (the universe consisting of two "space points" denoted by 0 and 8), the distance between 0 and 8 is 5 since 0 and 8 are neighboring points. But in the universe V={0,2,4,8} the distance between 0 and 8 is 15, since it is 5 units from 0 to 2, another 5 from 2 to 4, and yet another 5 from 4 to 8. However, we wouldn't say the "expanded" universe V has actually pushed itself out to cover a greater span of the real number line, right? It's still "confined" to the interval of real numbers from 0 to 8, denoted by [0,8]. The universe W={1,2,3,4,5,6,7,8} is even more "expanded," and yet still is confined to [0,8]. This analogy isn't perfect, because here I'm drawing in more space points from an external source (the real number line), but it need not be so in general. I could pull in more space points by giving them arbitrary symbols and defining an order for them.
You can think of an expanding universe as a universe that is adding more space points (in the manner described above) on the line segment between any two points P and Q in the universe, with the time it takes light to travel from one space point to the next remaining a constant.