Originally Posted by dcl

I'll offer my point of view on your original question. I believe it will simplify the picture considerably. It was, "Is the expansion of space constant?"

The simple answer is, "No", but that answer won't mean much to you without some essential details.

For just a moment, picture the Universe as two- instead of the three-dimensional entity that we believe it to be. Picture it as occupying the surface of a balloon that is expanding. Galaxies are tiny dots distributed uniformly over its surface. The distance between any two dots is increasing as time goes on. Way back in the past, 13.7 billion years ago, the balloon was a mere dot, but over the course of time, it expanded to its present size. We have no idea how large it is, but we suspect that we can see only a tiny part of it, only the dots within a short distance of the dot representing our own galaxy. Light hasn't yet had time to reach us from the remaining dots.

The actual Universe isn't two-dimensional like the surface of the sphere, but three-dimensional. But it is still somewhat like the surface of a sphere, only the sphere is four-dimensional instead of three-dimensional, and our Universe is the "surface" of the four-dimensional HYPERSPHERE. Just as all points on the surface of the sphere are equidistant from the center of the sphere, all points in the three-dimensional space comprising the "surface" of the four-dimensional hypersphere are equidistant from the center of the hypersphere. Just as inhabitants of the two-dimensional surface of the sphere can visualize only of other points on that surface and cannot visualize the direction to the center of their sphere, we cannot visualize the direction in the fourth dimension to the center of the hypersphere on whose three-dimensional "surface" we live. But every point in our three-dimensional Universe is moving away from that point at the center of the hypersphere.

It used to be assumed that the hypersphere was expanding at a uniform rate. It was discovered within the last ten years that the rate of expansion has been increasing for at least the last seven billion years or so because of "dark energy" that space itself apparently possesses as an integral property of itself. By "the expansion of space", we mean the expansion of the three-dimensional "surface" of the hypersphere as the radius of hte hypersphere increases. Dark energy is believed to be a mysterious intrinsic property of space itself.