Re: Does the universe have a diameter of 30 billion light- years?

Well, there is more than one thing meant by the "radius of the Universe".

One is the radius of curvature. This quantitity has the dimensions of a distance (interval to be precise), but does not correspond to any actual distance in space (4-space). In the analogous problem of 2-D geometry on the surface of a curved object, say a sphere, the radius of curvature can be thought of as an actual distance in a higher-dimensional, flat space, for example the radius of that sphere, which of course does not lie on the surface of the sphere at all.

Another meaning of "radius" is the "horizon radius" which measures how far we can see out in space (and back in time). In standard GR models, we never can see the whole universe, just the parts that are inside our backward light cone. If the expansion of the universe is slowing down, then we are able to see more and more of it as time goes by.

Both "radius" quantitities vary with time, and are calculable from the basic cosmological parameters (often stated in the form Omega_0, H_0, Lambda_0) plus some basic assumptions about the cosmological model to use (General Relativity for example). The order of magnitude of both radii is typically cT, where T is the time since the "Big Bang" (for models that have a BB). You can find some more details and references at http://super.colorado.edu/~michaele/evol.html