You might think of the Universe as infinite, and quite honestly, it might truly be infinite, but we don’t think we’ll ever know for sure. Thanks to the Big Bang – the fact that the Universe had a birthday, or that we can only go back a finite amount of time - and the fact that the speed of light is finite, we’re limited in how much of the Universe we can see. By time you get to today, the observable Universe, at 13.8 billion years old, extends for 46.1 billion light years in all directions from us. So how big was it all the way back then, some 13.8 billion years ago?
How Big Was the Universe at its Birth?