My question is the same as the title 'Is there a boundary present for our universe?'
That's still an open question. People talk about the boundary of the observable universe, a boundary that any light starting its journey beyond it, can't reach us because the expansion of the universe becomes faster than light there. But whether there is actually some boundary to the universe, we still don't know. Different cosmological models make different assumptions and so we can just work with different answers and see what will come out!
I've moved this thread into the Cosmology section. There's already an FAQ there: https://www.physicsforums.com/threads/is-the-universe-finite-or-is-it-infinite.506986/
Are there actually any models that use a boundary? (and to be clear, I don't mean being bounded)
Actually I was using "model with a boundary" interchangeably with "a model suggesting a bounded universe".
I'm not that much into cosmology and can't name different models but I know there are models saying universe is infinite and others saying its finite. So I was referring to this. I thought having a boundary is the same as being bounded. I mean, its the definition of boundary to bound something, right?(or wrong?)
My own "expertise" is of the shoddy variety so seek confirmation elsewhere, but from what I understand the terms are not interchangeable. Being bounded means that all points in the set lie within a finite distance from one another - i.e., in 3D space you can draw a sphere of a finite radius that will encompass all of space; which would be true if the space loops around on itself like the surface of a higher-dimensional object. Having boundary means that you can name points that lie outside the set - i.e., in 3D space you go one way until you reach the "end" of the universe, beyond which there is something else.
Wouldn't that contradict the notion of universality? Set containing all elements? What would a point outside of the universe even be? Multiverse stuff?
That's why I find it hard to believe there are any models that have a universe with a boundary.
Correct me if I'm wrong, but couldn't you have an unbounded universe that has a boundary, similar to a balloon (has boundary, but if you go straight, you'll just end back up at the starting point)?
I'm not sure of the various models, but I thought I've seen that mentioned somewhere on PF.
You're mixing the dimensions.
A balloon's surface (a 2D space) is without a boundary and bounded. It's volume (3D) has a boundary and is bounded. Infinite 3D space minus the volume of the balloon is unbounded and has got a boundary.
When used as an analogy to visualise the expansion of the universe, only the 2D surface of a balloon is being considered.
D'oh! You're absolutely correct. I completely spaced on that.
As a follow up, would it be possible the curvature of space time at the boundary appear that it is boundless like the surface of a balloon, or would that still be considered a bounded system?
Apologies, I can't quite follow your question. I think there's still some confusion, and not unexpectedly so, considering the nomenclature.
Bounded means basically the same as "finite". Unbounded is then infinite. As in, we don't know whether the universe is bounded or not = we don't know if it's finite or infinite.
Boundary means the same as "edge". E.g., the observable universe is bounded has a boundary = it's got an edge (beyond which we can't see), but the Universe as a whole doesn't.
Perhaps we should stick to "in/finite" and "edge" as they are less prone to getting confused with each other.
Could you try and reword your question using those terms?
(mind you, this is all talking about space, not space-time)
Your post answered my question with clearing up my confusion with bounded and unbounded, as well as with clarifying that it's space and not spacetime. Thanks!
All evidence suggests the observable universe is temporally bounded, but, there is no evidence to suggest it is [or is not] spatially bounded. There is strong observational evidence favoring that it has a finite age of about 13.8 billion [+/- some change] years. Given the finite speed of light, light can only travel a finite distance in a finite period of time. In that sense the universe must also have an observationally finite volume, but unlike the age of the universe, we have no evidence [or logic] that constrains the volume of space beyond the observable volume. Any such volume cannot, by definition, be in causal contact with the observable universe. Unlike the age of the universe, we don't even know how to test the hypothesis the universe is [or is not] spatially bounded. We have looked [without success] for 'circles in the sky'. That would be logical if we lived in a pacman [wrap around] universe. But, the universe could easily be, and probably is much too large for light to have had time to complete a round trip. It has been suggested the observable universe may merely be a 'tiny bubble' embedded in a vast, perhaps infinite sea of other 'tiny bubbles'. But, that only further complicates matters because it forces you to concede even the age of the universe is uncertain. The correct answer is the 'true' age and size of the 'universe' is unknown [and probably unknowable]. Most scientists resist abandoning our hard fought knowledge of the age of the universe, and are content to merely concede our ignorance is limited to its spatial extent.
That is, it has a start and an end? I was of the impression time got way complicated in early-universe stuff so that the question of "before that" becomes too difficult or even pointless.
Sort of off-topic or even off-science, the idea of observability is interesting in this context. If things beyond the observable universe can't actually, you know, have any influence on anything that we may ever know, does it even exist at all, existing meaning it can influence other existing things? Along those lines, if the expansion on the universe accelerates quickly enough, so that no particle can ever reach and assert influence upon another, does that make the universe unexist the same way? (It's beyond me, but I'm assuming the particles couldn't influence eachother because of the Speed Of Gravity?)
We don't know if it will end, or suffer some other fate.Regions of the universe that are causally disconnected are generally held to have always been unobservable. A region that is causally connected cannot break that connection. Most galaxies we can currently detect will never become unobservable in theory, despite the fact the light they are currently emitting will never reach us. They will eventually just redshift beyond detectable limits, not become unobservable in theory. Its like a stationary observer watching another observer free fall into a black hole. The infalling observer will never appear to lose causal contact with the stationary observer, her redshift merely appears to approach infinity as her distance from the event horizon approaches zero. See 'Expanding Confusion' by Davis and Lineweaver for a more rigorous discussion.
But if information cannot be sent faster than the expansion of space between the two interacting objects, doesn't that causally disconnect them? Or am I being ignorant and misunderstanding that answer?
No. Causal disconnection forbids even observation of that region of the universe. So long as we detect a photon signal, the event that resulted in photon emission is causally connected. Otherwise the photons would not be here NOW to affect our signal detectors [e.g., cameras]. If I chucked a a peach at you 3 seconds ago to get your attention, and it just now struck you, we would both agree the two events were causally connected even though my action ended before you detected it.
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