I can’t remember the exact source, but I heard a stat that the universe has doubled in size about 100 times since it was smaller than a grain of sand just immediately after the Big Bang. I was astounded, and bowed to the power of arithmetic. But what does “doubling” mean in this case? Doubled in radius? Volume? Some other measure?
*Even more fascinating to me is the fact that we’re living in a very special period in the timeline of the universe. From a “doubling” perspective, we are at the point where dark energy just surpassed matter as the dominant occupant of the universe. Since dark energy expands along with universe, and the amount of matter stays fixed, the amount of matter exceeded the amount of dark energy until just prior to this last, or (approximate) 99th, doubling.
When people speak of the Universe doubling in size, they’re referring to the scale factor, a linear (not volume) measure.
But there’s a subtle misconception lurking in Wesley Clark’s post. He says that the Universe is currently 92 billion lightyears in diameter. That’s the size of the observable universe, but that’s not what we’re talking about when we speak of the scale factor of the Universe. When we speak of the scale factor, we’re talking about the distances between objects changing. That is to say, if we take any two points in the Universe (say, two widely-separated galaxies), then if we were to wind back the tape to a point one doubling-time ago, then at that time, those galaxies would be half as far apart as they are now. Now, it may be that in the intervening time, those two galaxies entered each others’ observable universes, and it might even be that they’re still not observable to each other, but that doesn’t matter for defining the scale factor.
It should also be noted that we don’t actually know how many doublings the Universe has gone through. We can speak with some confidence of what the Universe has done since the end of inflation. And we can, assuming that our models of inflation are accurate, say something about a minimum amount of time (or doublings) that our Universe must have been undergoing inflation. But it’s quite possible, or even (according to some models) likely, that there was no start to inflation, and that the Universe had always been inflating, for an infinite amount of time prior to the time when inflation stopped.
From the context it probably does mean the linear proper dimensions of the observable Universe, but the physical meaning of the linear proper dimensions of the observable Universe doubling in size is extremely obscure. The physical meaning of the scale factor doubling in size is simple to explain physically: when the scale factor doubles in size the wavelength of a photon emitted at the initial time will be observed to be twice what it was now than when it was emitted.
