Becus we are Specials!!!
And that would be radius of the universe, not the full diameter. It would 13.7 * 2, or 27.4 billion light years. But really, those are all theoretical models - some scientists would say it is a perfect sphere, some would say it is relatively flat, some would say it is hyperbolic. Some would say the matter expanded in just 1 direction, not in every direction. You can't say that it is larger than the speed of light allows it to be, though.
