"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." - Albert Einstein
"Nature is an infinite sphere of which the center is everywhere and the circumference nowhere." - Blaise Pascal (1623- 1662)
To examine the question of how large is the total universe, we first must try to decide if what we know as the visible universe is the full universe, or just possibly a very small portion of the real total. To go beyond the universe of our post-Big-Bang bubble is to journey deeply into questions that cannot be fully answered. However, multiple cosmological theories give us a playground for examining options. If an idea is within reason, it's at least worth considering.
The fundamental barrier to knowledge is the philosophical dilemma where the lesser can never fully know the greater. There are two types of logic, with inductive logic being the lesser making statements about the greater. Deductive logic, where statements are made from the greater to the lesser, is supposed to be far superior. That's what Aristotle thought. The problem is, how can you deduce from the greater to the lesser if you can never fully know the greater? Therefore, both forms of logic are equally deficient.
Current theories about what makes up so-called Dark Matter and Dark Energy are all made in the dark. It is impossible to clearly talk about 100% of the known universe while conceptualizing only 4.6% of it. What we really need is a theory where we comprehend 100% of the known universe. Even that does not take us far beyond our universal bubble, but it's a start.
Modern science has recently learned about inflation during the first microsecond of our Big Bang; and science has also learned about the accelerating expansion of our universal bubble. Nobel prizes have been given out for these discoveries, without anybody having a clue as to what is really going on! Sketchy concepts hardly bode well for answering this essay's question. It is not progress to put a label on our ignorance, naming it Dark Energy or anything else, when we truly are in the dark as to what it actually is and does.
Fortunately, the paradigm of graviton gravity (with gravitons in three different states constituting baryonic matter, dark matter, and dark energy) moves us down the road toward the big question. Still unknown is does this road have an end, or does the road itself go on forever?
If there is no multiverse, then the known universe could be all that is, with possible chaos of some sort beyond. However, the idea of omnipresent and omnidirectional flows of free gravitons in massive numbers, leads support for the idea that there are many universes beyond and near our own universe. Some call this the bubble bath scenario.
Net vectors of omnipresent graviton flows explain the paradox of gravity. Because of blocking and diverting effects associated with nearby massive bodies, gravity is revealed to be like a low pressure area which sees net positive pressures from all other directions pushing two bodies together. This is the opposite of a tractor beam model for gravity, something that would need to be shooting out of every thing in all directions, for the sole purpose of pulling in.
Dark Matter, constituting loosely stable gravitons, mostly expresses itself as a nearly circular halo around galaxies and galaxy clusters. It also expresses itself as a vast filamentous web within our universe, onto which ordinary matter accumulates. The largest such structure just discovered is some ten billion light years long. That's amazing, and hardly something that quickly formed after our Big Bang. More likely, it is a remnant from previous universes in our region of space.
The hyper intensity and ubiquity of graviton flows from all directions leads to the question of their origins. The best answer among a few options is that such flows are mostly inter-universal, where gravitons fly from universe to universe. So, let's for now entertain the idea of a multiverse, which is starting us on the way to infinite expansion.
The gravity we measure follows the math of Newton and Einstein, even though neither knew what it is. Einstein thought gravity is warped spacetime, but graviton gravity follows the exact contours of Einstein's dubious aether-based General Relativity. Graviton relativity is thus more elegant than ethereal General Relativity. Graviton gravity also works within all dimensions, which GR does not, allowing it to be a theory of everything.
The best explanation for the ubiquity and uniformity of graviton flows everywhere is produced by a multiverse model. Graviton gravity is also by far the best explanation for accelerating expansion toward the distant regions of our bubble, and increasingly closer to the blocking mass of the adjacent universal mass. We do not need to invent an unknown expansive force.
The original "infinite" question is not answered by any one multiverse. So we place our local multiverse among a vast number of other multiverses, all mutually interpenetrated by graviton flows. Eventually we must ask about the outside edge of all these theorized multiverses. All systems, and systems of systems, have a functional outside barrier. This principle applies in dimensions from quarks to quasars and beyond.
Since gravitons flow from all directions, to all directions, within our experience (and since gravitons appear to be primary particles with rapidly morphing matter/energy), we can assume that gravitons in other multiverses would be fundamentally like those in our multiverse.
Where it gets VERY interesting is at the hypothetical edge of all universes. Even if there are a billion multiverses, there will be an edge around it all. Only if there are an infinite number of universes would there be no edge. Short of that infinity, when it comes to random gravitons, what happens to free gravitons produced just inside the edge of it all, and heading outward?
We are not talking about standard black hole event horizons. They are in many ways fundamentally different from the universe of universes. The horizon, or outside boundary, of all universes is not an "event horizon."
Semantically, if a graviton "escapes" this ultimate boundary, it has not really left the totality of universes. Because gravitons essentially make up nearly all of what's inside empty space, the mere leakage of gravitons leads to an expansive colonization of empty space beyond the space we could know. In other words, gravitons colonize and create dimensionable space. Does this mean that gravitons could make inhabited space infinite? Is uninhabited space something or nothing? If it is nothing, but potentially could be something, does that make nothing equal to something, time independent?
Are we just talking about mathematical possibility, or something else? It is not enough to just say that a "god of the gaps" is mysteriously beyond. God out there as an actuality, not just an idea of faith, would also have systemic borders, beyond which would still be pure space, or the idea thereof.
This entire discussion could be meaningless from our everyday perspective, even while it is meaningful from an intellectual perspective. Anything that never does measurably influence our everyday existence and free will could be considered meaningless, even when it is otherwise very factual. For human society, we create our own meaning within the universe that we experience. What happens from the edge of it all is existentially irrelevant, except where an interacting god could be involved.
Here's an idea to put under your thinking cap: After trillions of Earth years the Second Law of Thermodynamics turns almost everything into what appears to be entropic chaos. In that scenario all appears to be free kinetic energy gravitons; but hope is not lost. Free gravitons can and will recombine into energy/matter collections that become new big bangs and new local universes. By converting kinetic energy into potential energy, gravitons are truly everything both now and into the distant future. This dialectical process involving conversions of forms of energy/matter is ongoing, not allowing ultimate chaos. The law of conservation of energy and matter ensures continuity; and gravitons are the ultimate expression of this conservation law, ensuring the persistence of energized matter in all its forms.