Universe or universe?

Solution 1:

"Universe" is not, by itself, a proper noun. So strictly speaking, "universe" is correct.

However, in literature, it's common enough to capitalize things if they are "special" enough.

For example, perhaps you have a story that features a legendary book. Consider the following passage:

"He has read the Book", said the man. You could hear the capital "B". The crowd gasped.

Clearly "book" is not a proper noun, but given the right context anything can be a proper noun.

Maybe, in your case, if you are considering a multiverse, there may be a special universe... The Universe. If you're just speaking about universes, or our universe, in general, then use the lowercase form.