I see these terms as being subsets of one another.
The 'universe' is everything that came out of 'our' Big Bang, and includes regions of space that are in principle knowable, but will require billions of years for us to ever receive signals from galaxies in these regions, and the visible universe which includes all of the galaxies that we can now see.
I do not distinguish between 'universe' and 'knowable universe'.
The knowable universe includes the portion that we are now able to see, and also the portion which is still beyond our current information horizon. Big Bang cosmology says that there does exist a larger portion of the universe 'out there' beyond the limits to our visible universe, but that the universe is not yet old enough for the light signals to reach us from there yet. As the universe gets older, more of this currently unobservable, but knowable, universe will come into view. There are, however, NO observations we can now make that prove that this other part of our universe exists at all.
As for the term 'universe', some feel it should include other 'universes' and other spacetimes. I disagree. A new term is required to describe these other places because they have, at least mathematically and physically, no connection with our universe. This is why 'multiverse' is coming into increasing usage in some sectors of astronomy. Although predicted theoretically, they are not a part of what came out of our Big Bang, and they are forever unknowable. Curiously, mathematics and 'logic' allow us to speculate what these other universes might look like so they are unknowable from the observational sense, but not beyond logical speculation.
Copyright 1997 Dr. Sten Odenwald
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