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Universe:

  • Is there any limit or size of the universe?

  • Or you can say is there any edge of the universe?

  • How we can define the universe limit (either in some units or compare like)?

  • Assume that universe has a boundary & it's like a square. Alright! We all including all planets and galaxies are inside this Square. Then What is outside?

This question arises whenever i think about the universe, i mean

  • Where is the edge of Universe?

  • Or What is outside of the Universe?

  • Or either there is multi-universes (if there is, then what object hold all the universes).

Can anyone just simply imagine the limit of Universe or can define the limit in just words, i'm curious...

Your thoughts would be helpful for the topic or for the people who want to know abut the Size, Edge, Limit or the Object who hold the Universe's.

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The way I like to approach this is as follows: In the subjects of sets, the universe is defined as the set that contains all possible elements (it doesn't contain all possible sets, because then that becomes paradoxical, just the elements). When I apply that definition to cosmology, it is obvious that everything that exists would, by definition, exist within the universe. We can say the universe is that which contains all that exists. This naturally answers the question of what is outside the universe. Nothing is outside of it because if there was something outside of it, then that something would be a thing that exists, which by definition would have to exist in the universe.

The boundaries of the universe are determined by the boundaries of the dimensions. In Cartesian coordinates, the universe goes as far as x, y, and z does and exists at every moment to which a value for time can be ascribed. So if there are limits to the universe, that must be because there are limits to x, y, and z. So there can't be a "beyond the edge of the universe" because that would mean there's more points on, say, the x-axis, which is clearly contrary to the premise of there being an edge.

But let's examine the existence or possibility of existence of an edge. There's this thing called the observable universe. This represents the small corner of the universe centered on us and contains literally everything we can observe (that which contains everything that exists and that we can observe; it's an appropriate name). Because we can't observe anything outside the observable universe, we can't know anything about that stuff (okay, there are some things that used to be in the observable universe that aren't anymore, so we can know, at least, that some stuff exists outside of what we can observe, which is a remarkable amount of knowledge about that stuff, when you think about it). As a result, we rarely discuss anything outside our observable boundaries and you frequently hear cosmologists refer to the observable universe as simply "the universe" (sure, it's a bit self-centered, but technically so is the observable universe, so I feel okay doing it too). With that in mind, we can easily determine the limits of "the universe". There's a few different horizons that you could use to define the relevant limits of the universe, but the most often cited number is that the radius of our universe is on the order of $45$ billion light years, which is about $\frac{1}{\sqrt 2}$ times Avogadro's Number in kilometers. That's slightly farther than Usain Bolt can run in an hour.

There's a number of cosmological theories that have different flavours of multiverses (mmmmmm..... vanilla universes). Some of them have the multiverses co-existing with our own, where there isn't physically a time or spatial separation, but another dimension they define to separate them. That makes things mathematically simple, but conceptually tricky. Some define "regions" like our universe that exist alongside similar regions all within larger regions of "not like our universe". These are also a bit varied in how they define "alongside", but usually it's a definition that ends up hurting your head the first time you read it because (here comes the head hurting part) it allows them to be in physically different times/places but still have the possibility of the dimensions of each universe region extend infinitely. I know, but you said you wanted words and that's the best I can do without lots of math and Richard Feynman. But, as the old woman said to her husband in bed, "Where's your point?". My point is to answer your fairly broad question about multiverses. Some theories don't require a grand container for them all, others will simply apply the term "universe" to regions like that we live in (which I personally see as a misuse of the term, but I'm not the labeling police, so they're probably fine) and then define a container for them all. For the latter cases, you'll find terms from as grandiose as "Cosmic landscape" down to as mundane as "background universe". But this has been a confusing and unhelpful sojourn into the world of multiverse theories. In all cases, you can still choose to define "universe" as that which contains all that exists. And in all cases, it doesn't answer the "What's the limit of the universe?" question, all it does is shift it to "What's the limit of the multiverse?". So, let's move on.

The last thing I want to touch on is what opinions you'll generally find. A large number of cosmologists like to say there is more evidence pointing to the universe being flat, and most of them would also say it is infinite in extent. So that would mean the universe doesn't have limits. It just extends outwards forever. You'll certainly find some that say there is a finite but unmeasurable volume to the universe (I very much doubt you'll find reputable cosmologists who seriously say the volume of the entire universe is measurable), but most would agree that we really can't be perfectly sure about that. Now, having a finite volume does not mean there are edges or boundaries. Think of the game Asteroids; the screen has a finite area, but there isn't any edge or boundary that you can fly your triangle into or go beyond (note that I'm not saying you can keep traveling one direction in the universe and wind up back where you started, I'm just giving an example of a finite region with no boundaries). I'll bet some physicists also claim the universe has boundaries or edges. I've never met one, but I'm willing to bet they exist. Still, the predominant thought is that the universe is infinite in spatial extent, which answers the question of whether or not it has limits. The answer is a resounding "No".

To sum up, the universe isn't thought of, for the most part, as having any limits or edges. You can speak of the observable universe, which has a boundary most often cited as $45$ billion light years away. There is no "outside the universe" to speak of, according to most experts. In the case of multiverses, the theories have different stances; some have background universes to contain them all. The way that's most helpful for me to think about the universe is to define it as "that which contains all things that exist", which naturally answers many of these types of questions.

Hope that helps and doesn't confuse further

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  • $\begingroup$ thanks for the detailed answer i appreciate. As you said "In Cartesian coordinates, the universe goes as far as x, y, and z does..... So if there are limits to the universe, that must be because there are limits to x, y, and z", it means there's at some point an edge or limit to universe but we can't measure or even imagine. In maths we say "limit tends to infiniy" i think there's a limit at some point but that point we can't imagine or measure. $\endgroup$ – Imran Khan Dec 2 '16 at 7:14
  • $\begingroup$ That's why i post this question to find some clue or someone just define the infinity in words or give some shape to it. As we know in our own world there's a limit to everything but something we can't measure like the sand in desert or drops of water but we KNOW they are in limit. :) $\endgroup$ – Imran Khan Dec 2 '16 at 7:16
  • $\begingroup$ @ImranKhan Utilizing intuition or comparing things to normal human experiences is a sound strategy for most things; however you will run into serious problems when comparing concepts about the universe to things we know and patterns we see in the world around us. The world around us is so limited itself and that limits our intuition. Just because everything in our finite region of space is limited is, by no means, a valid reason for believing that the universe as a whole must have limits. "There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy". $\endgroup$ – Jim Dec 2 '16 at 13:25
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    $\begingroup$ @ImranKhan You should allow yourself to accept, at least for the moment, that it is possible the universe could have no limits whatsoever; because there is no logically valid reason yet that says it must be true that it has limits. The gist of my answer was that it is not really known for sure and that experts allow for pretty much any possibility, but popular opinion is that it is limitless $\endgroup$ – Jim Dec 2 '16 at 13:28
  • $\begingroup$ Jim, If there actually is an end to space-time metric at a point beyond some horizon, would that have any observable effects on our laws of physics? I picture this as qm vacuum (which has some energy) expanding into pure nothingness (absolute zero) $\endgroup$ – Will Graham Apr 17 '17 at 2:24
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Is there any limit or size of the universe?

No, since the curvature is flat it is very likely to be infinitely big

Or you can say is there any edge of the universe?

There are horizons like the particle horizon, the event horizon, the hubble radius and so on but behind this horizons we assume everything to be the same like in front of them.

How we can define the universe limit (either in some units or compare like)?

The event horizon and the hubble radius will converge to a distance of approximately 16 Gigalightyears in proper distances and the particle horizon to a little less than 65 Gigalightyears in comoving coordinates, for the evolution of the horizons see this plot.

Assume that universe has a boundary & it's like a square. Alright! We all including all planets and galaxies are inside this Square. Then What is outside?

The universe's curvature can be flat or sattles shaped, then it's infinite. It also can be positively curved, then it's finite (like the surface of the earth). But there is no way for space to be squared and finite.

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  • $\begingroup$ " behind this horizons we assume everything to be the same like in front of them." If there actually is an end to space-time metric at this point, would that have any observable effects on our laws of physics? I picture this as qm vacuum (which has some energy) expanding into pure nothingness (absolute zero) $\endgroup$ – Will Graham Apr 17 '17 at 2:14
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There is no hard edge to the universe. Watch this animation by "Zogg of Betelgeuse" which makes some sense of this. There are several additional episodes of this.

If spacetime has some edge or boundary it would mean there is a region that imposes additional information on the universe and which also would violate Poincare symmetry of spacetime.

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protected by Qmechanic Dec 1 '16 at 15:27

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