x1 is the/our actual physical universe in its entirety.
This is the universe of cosmology and physics in general. It is not separate from humans - we are part of it. In a multiverse, this is our universe. Probably synonymous with "reality". There are no connotations concerning restrictions to outerspace, large distance scales, etc. This is the entire physical system of which we definitely know (as of A.D. 2016) but with the true governing laws of nature. It may also include rules of logic and mathematics. It is as objective as can be demonstrated, but I do not wish to guarantee such a philosophy.