Consider the well-established fact that our universe is expanding. Because of this expansion, every point in space is surrounded by a cosmic horizon, beyond which one could never travel—even in principle. We will never be able to observe or interact with anything that lies beyond our horizon, and someone beyond our horizon could never observe or interact with us. As far as I’m concerned, a place that one could absolutely never observe, interact with, or travel to is not in the same universe. Although there are many different definitions of this word, I think of a universe as a region of space
...more
