03 October 2014

The Observable Universe vs. the ENTIRE Universe

The Observable Universe vs. The Entire Universe
I was reading along in Caleb Scharf's very interesting book. «The Copernicus Complex -- Our cosmic significance in a universe of Planets and Probability,» when I came across a reference to the approximately 400 billion galaxies in the "observable universe." This is a figure and a concept I've seen before, and I've seen descriptions of the relationship between the so-called observable universe and the ENTIRE universe, but I wondered about it so I looked it up.

The "observable" universe is that (small) part of the universe where light has had time since the Origin Event (Big Bang) to reach Earth. Every place in the universe has its own "observable universe," all the same size at the present time, and they may or may not overlap with ours. It APPEARS to be roughly 27 billion light years in diameter at this epoch, but in fact is more like 45 billion, due to the expansion of space since the Big Bang. Lot of space.

So, my question (to which I'd seen various answers) was, What is the percentage of the entire universe, including the part we can't see and will never see, made up of this observable part? Alan Guth, the discoverer of cosmic inflation, estimates that it could be as much as 10 to the 23 times the size, which is truly mindboggling. That would mean the number of galaxies (each with tens to hundreds of billions of stars) in the whole universe would be a number that makes the number of grains of sand in all the beaches of the world seem as nothing. But other theoretical estimates range down to more like just 250 times. But even that... A space with a diameter 250 times 45 billion light years. Countless billions of galaxies. Untold trillions of stars. The mind simply reels at the thought.

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