A sphere/balloon would fill the "cavity" uniformly, such that every single point of it would touch the walls of the trough at the same time, therefore, All the ("edge") points of the sphere/balloon/Universe would "stop"/"touch it" at the same time. Plus to that, you forget that a pipe is an analogy that we used arbitrarily. It is actually incorrect to use to begin with. With this analogy we wanted to translate/communicate the assumed for the argument purposes "finality" of the Universe, as if “saying” that it is finite (again, exclusively for the argument purposes).

Thus, if we pretend/assume that it is finite then it, itself, must be contained within something else, right? If we assume the Universe to have a spherical (3D) symmetry then we would normally assume that that other something it is contained within is symmetrical as well. Whether it is cylindrical/triangular, or any other rectangular type of symmetry (say, square), we don’t know. However, a pipe is not symmetrical; for, the top of the bowl is simply missing, which implies infinity, because potentially, even if the lower part would stop, the upper can go expanding, AND accelerating at it, for ever, I-n-f-i-n-i-t-y. The latter, in its turn, in itself is the proof, and implication, that the Universe, potentially, IS infinite (of course… it can go up forever and ever and ever).

So, we have a finite Universe in an infinite "something", better yet – “half” (not semi) infinite something. But we know, that infinity divided by a number IS infinity, you can do it 'till you're blue in the face it will still be Infinity. Thus, no matter what you stick the Universe into, be it a cylinder, or a cube, or another sphere, or whatever - that other will have to be either infinite, which would immediately make the Universe infinite by default, or, in case of that Something finite it, the Something, itself being finite, implies that there must be something it, Itself, is contained within, which will again bring us back to where we started. In that case, what we think is the Universe is not the Universe at all but some well defined space - the latter definition/statement depriving it of the status of being the Universe (by now it is no better than say, the local group or a cluster of galaxies, even though very… very… very, very large, but still… finite.

This denies the very existence of the subject in question which cannot be true for the Universe is an actual physical phenomenon that is clearly seen, interactive, and measurable. If we stick the Universe into an "unfinished" (<= don't want to use scientific terms, so that anyone can understand us), “Something” (like the pipe, for example, or a glass, or an open salt-shaker, whatever) that, being infinite, on one side will immediately make/convert the rest of the universe (as we had said – by default) into infinite, and remembering that for every action there is a re-action, as soon as the "bottom” of this, by now (oh, jeebus) "finite-infinite" Universe, touches the walls, these very walls will start pushing right back at the Universe, which will begin building up an internal pressure, that would increase the inner temperature, that would increase the entropy, that would be immediately seen AND detectable/readily-verifiable by the instruments of ANY more or less advanced civilization, such as ours as well as visually noticeable at night in the skies AND , at some point after a while, also during the day as a permanent phenomenon in the sky, like the Moon, for example. In other words, things would really start getting nuts all around the place and we would be able to see that in our life times (since the good Professor presents his argument as a phenomenon taking place right now).

On top of that, as soon as the pressure would start building inside the "other half" that is not confined by the limits of the pipe-bowl, that part would immediately give in and begin expanding into the infinite(!) direction of the opening of the pipe-bowl (though we know that the Universe does NOT expand INTO ANYthing). The latter, thus, would eventually become too "cramped" for the expanding, (still Infinite(!) ), Universe and would ultimately pop out like a pop-corn from a casserole, thus "releasing" itself into the Infinite(!) of the unobstructed.... well... Nothing, leaving the sphere of the Universe wobble in the “air” like a soap-bubble when you jerk your hand with the ring aside, to release it to fly. Here, again, the argument hits the wall – since the bubble/Universe does contain mater AND a “physical” “membrane”/”border” that holds all the stuff inside of it, the wobbling of the “walls” of the bubble/sphere would, again, create pressure on the matter inside the bubble, which would, again result in increased chaos… and so on and so forth..

( this, by the way, gets us to the question that is being debated right now all around the world, including our little humble corner of it, here on tribe - what is this Nothing, and what is it "made" of?)
…

The extent of Chaos in the case of an “enclosed” “Universe” caused by the increased entropy due to the walls, or any otherwise, pressure would be, with time, unimaginable, and long before we would be able to see its… well… unimaginability, the life in the Universe, at least as we know it, would seize to exist for any practical purposes (in this plane, anyway).

However, none of this is taking place right now, none what I am personally aware of at least, being a humble (AND modest, did I mention modest? ::)) ) undergrad.

Clearly, in this direction there is a paradox, therefore, we might attempt to try to think to drop this “exit“ as a plausibility.