It’s not as naive a question as it initially seems.
13.8 billion years ago, the Big Bang occurred. The Universe was filled with matter, antimatter, radiation, and existed in an ultra-hot, ultra-dense, but expanding-and-cooling state. By today, the volume containing our observable Universe has expanded to be 46 billion light years in radius, with the light that’s first arriving at our eyes today corresponding to the limit of what we can measure. But what lies beyond? What about the unobservable Universe? That’s what Gray Bryan wants to know, as he asks:
We know the size of the Observable Universe since we know the age of the Universe (at least since the phase change) and we know that light radiates. […] My question is, I guess, why doesn’t the math involved in making the CMB and other predictions, in effect, tell us the size of the Universe? We know how hot it was and how cool it is now. Does scale not affect these calculations?