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Name: Kevin
Status: other
Grade: other
Country: USA
Date: Spring 2012

I cannot seem to find the answer to my questions about the "observable universe" in the archives or as a novice I have confused some concepts and just do not understand the answers in the archives. Is the term observable universe synonymous with the 13.7 billion year old universe we live in and the term unobservable universe referring to possible universes other than our own? If observable universe refers only to our 13.7 billion year old universe then is there such a thing as the unobservable part of this universe? Is our universe divided into two parts, the observable part and the unobservable part?

Kevin, br>
Let us say that the universe is in fact 13.7 billion years old. Then, think of a type of measure of distance based on the distance that light can travel in space in a year (called a light-year - which is not a measure of time but a measure of the distance a beam of light travels in a year). Now suppose that the universe is larger than 13.7 billion light-years. This means that the light from a galaxy that is, say, 13.7 billion + 1 light year away from us, has not reached us yet since even if that galaxy existed at the very beginning, it's light will not have travelled the distance between us. Next year, however, -if we have the technology– we may see that galaxy.

Complications to this idea: because the universe is expanding, there are galaxies that were close enough for their light to have reached us already, but -due to the expansion– have actually gone beyond the 13.7billion light-year distance. So we can actually observe a much larger radius than the 13.7billion light-year limit. Calculations show that we can actually observe about a 93 billion light-year radius. ... I know, it boggles my mind too.

Greg (Roberto Gregorius) Canisius College

I believe what we mean by that statement is that "observable universe" usually refers to the parts of this universe that are observable to us right now. In particular this refers to things that are too far away from us for the light they emitted at some point in the past to have reached us at this point in time. So yes, there should be vast, perhaps infinite, universe beyond what we can observe. But since it is beyond what we can observe, we do not have any direct way to know!

Michael Pierce

Dear Kevin,

Good question. Yes, the observable universe is everything we can see, or observe. Since we cannot see all the way back to the Big Bang, then part of the universe remains unobservable. And we can see no other universes.

SIncerely David H. Levy

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