My boundaries……
World: a concept representing that of which the totality of possible experience has access, given current natural law;
Universe: a concept representing that of which the totality of possible experience has access, given any sufficient natural law.
I suppose you realize that this is not straightforward. You’re assuming or importing a conception of what it means for something to exist. For instance, are the laws of physics part of the universe? They don’t exist in the sense that they were part of the material universe created at the Big Bang. For that matter, numbers have an existence that is, to say the least, questionable. So when you say “EVERYTHING that exists,” are you including abstracta and relata? It’s fine if you are, but it needs to be made clear.
I think that would create two subcategories of both of the above definitions. Definitions that include abstractions, and definitions that don’t.
I’m not too focused on my own definition here, I’m trying to get a guage of what other people mean.
Do you think most people include abstractions? Do you include them?
It is informative and interesting. Etymologically it means what you said initially, all that exists, but it seems to have evolved, at least with a significant fraction of the population, into referring specifically to this cosmos.
That kind of makes sense. After all, this cosmos may actually be all that exists, we don’t know that any other ones do. We can speculate (and in fact I do speculate - I do think there are alternative existences, separate from our own, with their own laws of physics, or even worlds that don’t have anything we would recognise as “physics” at all).
I would wager that the majority of people who don’t presume that this is the only cosmos, also don’t use “universe” to really mean ALL there is, but instead to mean specifically this cosmos we’re in.
I don’t know how to respond to your comment. AFAICT we’re stuck in the same spot. I offered you multiple interpretations of “universe”; others have obliged as well. Yes, there are 2 ways you can take the universe. 1, as “this cosmos we populate” and 2, as everything that exists. Some might argue that 1 = 2; I suspect you disagree.
I was asking about YOUR interpretation though. I asked you a yes or no question in order to try to clarify your interpretation. Part of your writing implies one interpretation, part of it implies the other one, so I asked a clarifying question that I think you can answer if you try.
Both definitions seem equally important. I can’t choose. Being familiar with theism, I’d say universe means this cosmos, all the galaxies and the space in which they exist.
The definition is quite context dependent.
In a naive classical context, it might be ‘all we can see’ or ‘the visible universe’, references to regions of space of radius 5.8 GLY and 48 GLY respectively.
One can also say ‘all of space’, which has no size since space is probably infinite.
I notice almost all the responders refer to space, and not spacetime. In a more formal context (talking cosmology), I’d probably say ‘all of our spacetime’, which is anything that one can specify with 4 coordinates, counterfactual or not.
MWI suggests that other states are visible, but are part of one universal wave function, so one universe consisting of multiple worlds, each of which is a point of view of that with which the observer is entangled.
If the context includes terms like ‘multiverse’, the implication is that ‘universe’ is one of many. Tegmark listed 4 kinds of multiverse. Brian Greene listed I think 15 kinds.
An alternative to that is to suggest that ‘universe’ means ‘all that exists’ (whatever that means), which renders ‘multiverse’ meaningless since the word implies that there’s more.
This seems kind of self contradictory since it precludes anything where QM and/or gravity is meaningless. Your implication seems to be an assertion sans evidence that those things are nonexistent.
@FlannelJesus seems to have had to play 20 questions to attempt clarification.
Yea, why did they do that? Just to say they did it? It’s kind of like a group hiking to the end of the trail, and one person being careful to go 1 step further that the rest just to say he hiked the furthest. Kind of like jumping at the summit of Everest just to be a few cm higher than others.
To add a tiny bit more, they took quite a slow wide turn (thousands of km) around the far side, not skimming close to the surface (100 km) like most Apollo missions did.
Ouroboros? I’m not that science literate but the implication seems to be that at some point in the history of the universe big and small converged; hence quantum gravity. By the way is there a theory of quantum gravity? My science files are a bit outdated.
I suppose that a multiverse could consist of several universes that emerge from a shared background of aspatial quantum fluctuations. Space doesn’t exist until it emerges from some state of fluctuations, and in our universe space emerged with a big bang. Thus it started to expand, and eventually the released energy began to form atoms and matter. Since the background is aspatial, it is unlikely that universes collide.
I think that the word ‘universe’ can be used for each of the spatial universes that contain atoms or whatever they can contain (including social facts and fictions). This proposed cosmology, however, is a ‘multiverse’.
In science/physics, the universe is literally the physical structure and all it’s objects and physical laws we’re in. Even in the ordinary ways we talk about the planets, moons, and starts, we say the universe.
But note in philosophy: Dasein (being-in-the-world); Schopenhauer’s The World as Will and Representation; Goethe’s Welt, etc.
I daresay there’s a technical reason in addition to the bragging rights. Probably something to do with the physics of sling-shotting around the Moon.
Looked it up and got some weird and some likely answers.
There’s a free return trajectory available no matter where the moon is. The window they want is at Apogee mostly because it makes the trip longer.
A big part of the mission is to test life support for a long period (at least a fair fraction of the longer time it would be out there if they inserted into lunar orbit).
It also needs max time to test radiation effects, to see if shielding is adequate. Hate to tell the crew afterwards that the test failed and they all got a lethal dose of radiation. 4 months to live. Sorry…
Dubious: Lighting geometry that keeps the ship thermally balanced. This seems nonsense since apogee happens every 4 weeks, corresponding to the cycle of launch windows, and each of those has different lighting conditions of the moon. The ship gets the same light regardless unless it’s in the shadow of something. It’s not like there’s nighttime in space.
If ‘universe’ is all that exists, then there’s no history of it since it implies everything being contained by the same ‘time’. Some structures have for instance more than one dimension of time.
From your comment, I gather a different definition that you don’t realize you’re using. Only that which you see or know about ‘exists’. You also consider our universe in time instead of the other way around. Anything not in our space does not exist by definition. Our space is the one existing because it’s the the one you observe.
By the way is there a theory of quantum gravity? My science files are a bit outdated.
Still not yet. They’re working on it, and I suspect they’ll never get it. They don’t have much more time to figure it out.
Big and small have never been separate in reality. Only in our models are they separate. In reality, big things are composed of many small things.
Gravity has existed for small things just as long as it has existed for big things.
Could gravity exist at the scale of virtual particles, or before there is such a thing as scale? Say, if loop quantum gravity / cosmology is true, and there a pre-spatial state of the universe in which gravity doesn’t express. Also if we imagine the large scale, and that there is a maximum state to which a universe can expand: too diluted to sustain itself. Gravity collapses or disappears, and what’s left is the pre-spatial state of affairs from which spatial universes can emerge.
I read the ouroboros analogy in a popular science book. I think it’s got to do with looking past the 10^{-34} seconds of the 13.8 billion years. Hence my query about quantum gravity.
That’s reasonable in my book and so 
Of course there are theoretical model based predictions one has to be mindful of, yes? I believe dark matter falls in this category (there’s not enough matter in the galaxies to account for ?)
Everything, that is dependent on the distinctions of Time, i.e everything under Time.
Very VAGUE though
Does it disappear? Or does it just become too dilute to do anything interesting? I think it’s more of the latter.
If mass distorts spacetime, and that’s what gravity truly is, then when things spread out enough towards the heat death of the universe, it’s not so much that gravity doesn’t exist, it’s more that every chunk of mass is small enough not to gravitationally distort spacetime enough to affect any other thing because everything is too spread out. It’s still there, just… not “useful” so to speak.
Ah, I wasn’t expecting a solipsist answer! Interesting. Things you can’t see don’t exist. 
First of all, my own definitions were stated in the top of post 28. You quote post 33 where I deduce how Delirium uses the word, which isn’t a solipsistic one at all, just the one used by almost everybody in the world.
There is but the one world, which is equated with ‘all of reality’ or the universe. It’s anthropocentrism (not solipsism) if this world was made for us, and otherwise just a sort of naive presumption of there being but the one space and we (humans) just happen to be here by chance.
Horses exist but unicorns don’t, per empirical evidence. Most don’t consider that to be a solipsistic stance.
As I said, beliefs like that are probably held by the majority of humans, but not necessarily by most of those that know their cosmology, which Delirium admits he does not.
Dark matter is like magnetism. You can’t see it, but we know it’s there by what it does.