Sometimes behind what appears to be a mere grammatical issue hides a much deeper question of meaning.
The reader can easily check, after glancing at a handful of books and articles, including here at 13.7, that the word "universe" sometimes is capitalized and sometimes not. How is that decided, exactly? And who decides it? A choice is being made every time an author (or, more realistically, an editor) refers to the cosmos as "Universe" or as "universe." Let's ponder the reasoning behind this choice.
The most common position and, in my opinion, the worst, is to simply adopt "universe" indiscriminately. [Editor's note: That's largely what we've done here on 13.7, until today.] But what universe is this, exactly?
According to modern cosmological theories, and the burgeoning field of philosophical cosmology, we must be careful when we refer to the cosmos. There are several "universes" and so a distinction is essential for clarity.
Let's anchor our discussion on the most concrete knowledge we have, our observations. This is a choice already, one that shows my own preference to stay close to the spirit of the physical sciences, that is, to what is empirically validated.
We know that the information we can obtain about the cosmos, what we can "see," is limited in two ways. First, since nothing travels faster than light, information from a far away object like a galaxy takes time to reach us. Second, the cosmos we live in is time limited, starting 13.7 billion years ago in the event we call Big Bang (capitalized or not? [Editor's Note: Not. But we can change that policy today, too!]).
Putting the two together, we conclude that, at most, we can receive information (in the form of electromagnetic radiation — light, radio waves, etc.) from objects that emitted it 13.7 billion years ago. A bit more precisely, since the first stars and galaxies appeared around 200 million years after the "bang," our limit is for objects that emitted information some 13.5 billion years ago.
What exists beyond this information boundary — called the "horizon" — is inaccessible to us. (There are different kinds of "horizon," but we will stick with this one for clarity.) So, we can talk of the "observable universe," that comprises everything that we can measure (and what we can't yet). This part of the cosmos, that we know is out there, I like calling the Universe, since it's a concrete entity that includes all that we can know. Its boundary, the horizon, sits at a distance of about 46 billion light-years from us, the distance traveled by light in 13.7 billion years. (Why not 13.7 billion light-years away? Because the ongoing cosmic expansion gives light a boost, increasing its traveled distance by roughly a factor of three.)
But the Universe doesn't necessarily end at the boundary of what we can see. Very possibly, it extends beyond the measurable.
In fact, from a different vantage point in space, the observable universe will extend into different regions, that, like partially overlapping soap bubbles, may or not include ours. This is somewhat like the horizon we see from a beach; we know the sea goes on even if we can see it directly.
This continuation of the cosmos beyond the visible into regions which, presumably, are not so different from what's within our Universe, I call the universe. I don't think it deserves the "U" because we can only infer its existence and don't really know what's there. We can speculate that what's beyond the horizon won't be very different from what we see within it, but we can't be sure. (Unless we wait a real long time ... ) According to this choice, despite the "u," the universe contains the Universe.
Is that logical? You be the judge. It's hard to say when you don't have all the facts in hand.
Regardless, we must continue on. After all, today we speculate that the universe may not be unique, but part of a vast entity called the "multiverse." The multiverse itself may contain an enormous number of universes, possibly even an infinite number of them, although infinite is not something we can ever measure. We just don't know if the multiverse exists.
Worse, it seems to be impossible to confirm its existence, given that it naturally extends well beyond our Universe. At most, as some colleagues have calculated, we may obtain information of neighboring universes, if they have collided with ours in the past. (So far, we don't have anything indicative that this has happened.) Even so, to know of a neighbor or two is not the same as knowing about a country or a continent with hundreds of millions of people, or of an infinite multiverse. Concretely, we only have our Universe, even if our ideas may fly boundless across expanses beyond what we know.
Not a bad deal, really, given that we know so little of what's going on right here within our own cosmic information bubble.