Question: Multiple Universes?

[Havok]

I have wondered this for quite a while and figured I would actually ask some people that might know.

Are there any theories or thoughts on the idea that there is more than one (ours) Universe and that they simply are so vastly far apart that light from any others out there has just not reached ours yet, or I guess hasn't reached us yet. I suppose it is possible that it has, and we just haven't seen it, but that is getting away from the main question.

[Singular Intellect]

The latest information I've come across regarding our universe is that it's expanding at such a rate that in a few billion years you won't even see our neigbouring galaxies and galactic clusters, never mind other 'universes'.

The concept of 'multi-universes' sounds utterly stupid to me, quite frankly.

[Simon_Jester]

The three-dimensional space we occupy is all one universe. If you could somehow run fifty billion light years in a matter of moments, you wouldn't leave the universe, even though you'd leave the part of the universe visible from Earth far behind.* The edge of the spherical region we can see says more about how far light has traveled since the universe began than it does about the physical dimensions of the universe.

So "multiple universes" as you describe them wouldn't be a meaningful concept. Of course there are parts of the universe we can't see, but that doesn't mean they qualify as separate universes in their own right.

On the other hand, a part of the universe far enough away would be inaccessible without an FTL drive, because the expansion of the universe would add distance between you and it faster than you could reach it. So in a sense, it might as well be in another universe- but physically speaking, it isn't.

*Assuming the size of the universe is large enough to let you do this, which we can't exactly prove...
_______

If there are other universes, then they are not part of the same 3D space that we're in, which means that we can't reach them just by traveling really far, even if we have some hypothetical method for traveling faster than light.

[RedImperator]

I have wondered this for quite a while and figured I would actually ask some people that might know.

Are there any theories or thoughts on the idea that there is more than one (ours) Universe and that they simply are so vastly far apart that light from any others out there has just not reached ours yet, or I guess hasn't reached us yet. I suppose it is possible that it has, and we just haven't seen it, but that is getting away from the main question.

Well, let's say you had an Earthlike planet inhabited by astronomers that was, say, more than 26 billion light years away. That planet's visible universe and our visible universe wouldn't overlap anywhere (since the universe is only 13 billion years old, the light horizon for any given point in the universe is 13 billion light years away--beyond that, and light hasn't had enough time to reach any observers yet).

But this is taking place entirely within our own universe, just in parts of it we can't see. Entirely separate universes wouldn't be visible at all from this one. They may exist--I think some theories in cosmology require them, and I know some allow them. But they're not proven and I don't know how you'd go about doing that. Of course, there are plenty of things I don't know about cosmology, so take that for what it's worth.

[Havok]

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

The concept of 'multi-universes' sounds utterly stupid to me, quite frankly.

Care to elaborate on this?

[Lord of the Abyss]

Are there any theories or thoughts on the idea that there is more than one (ours) Universe and that they simply are so vastly far apart that light from any others out there has just not reached ours yet, or I guess hasn't reached us yet. I suppose it is possible that it has, and we just haven't seen it, but that is getting away from the main question.

I've read of scientific speculation about that, yes. For example, that our observable universe is just a small area in a much larger one, most of which is eternally expanding exponentially just as ours did in it's first few moments. And that embedded at enormous distances in this eternally inflating superuniverse are universes more or less like ours, where inflation stopped. Or, that other universes may extend into/overlap with ours, with the intersection being a vast, flat spacetime defect called a "domain wall"; since we see no such wall, if there are any they'd be beyond our range of vision. Or that the universe is basically an assemblage of bubbles, the bubble walls being those domain walls, with each bubble possibly having somewhat different values for some laws of physics.

I'm not a cosmologist, just an occasionally interested layman. So I have no idea how seriously any of these ideas are or were taken ( or how badly I've misunderstood and explained them, for that matter ). But I have run across mention of such theories.

[Vehrec]

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

Not really. The way you think about the universe, like a loaf of bread rising or a balloon inflating is in almost every meaningful respect totally wrong. It's not expanding outwards into anything, it's just spatially larger, at least from the inside. Inside and outside may not even be the right terms for talking about this.

What is outside the universe is a very difficult question to answer because it's kinda hard to put any sort of instrument or observer into places that are not places and where no physical laws as we know them apply. It may very well be impossible for us to ever determine anything about what may or may not exist outside our own space-time.

[Surlethe]

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

Why? Vehrec has an explanation, but to make you think it through a little more, answer my question.

[Rye]

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

What's space?

[Singular Intellect]

The concept of 'multi-universes' sounds utterly stupid to me, quite frankly.

Care to elaborate on this?

My opinion is based upon the definition of the term universe, like the following: "The Universe comprises everything we perceive to physically exist, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them."

In other words, proposing multiple universes is the equivalent of proposing planets/stars/galaxies/whatever as seperate 'universes' if they have zero interaction with one another. "Multi-verse" only works if you butcher the term universe into something that no longer applies to "all that exists".

[Oskuro]

To make a metaphor, we could say our universe is a videogame. Within the game, there are rules for how everything behaves and contains information, outside the game, nothing makes sense, since there are no rules to define information*.

Other universes, in this context, would be other videogames, with vastly different rules, and whose information is not compatible with our videogame.

The ability for two different universes to comunicate would depend on them having similar rules so information can be meaningful, as well as a way to intiate contact (the fictional portal), or there must be some way to convert information from one ruleset to the next.

As for the parallel universes so prevalent in fiction, I would see them as very similar versions of the same game, with compatible data.

So, in essence, the definition of Universe is not about spatial separation, but about the rules (space time) that define how everything works. I don't know about serious research on the subject, but from my limited perspective I see nothing wrong about the notion that other rulesets might coexist with ours, and that even communication might be possible, the problem is that data from those rulesets will be meaningless to us, and viceversa, or they simply will be unreachable because outside of our ruleset we cease to have sense and thus cannot exist past the "boundary" of our reality.

Darn, this is sounding too metaphysical.

*Of course, for this metaphor to work, there must be a common framework (the computer system), but we can ignore this for this little exercise, as well as the pesky issue of hard drive size. Let's imagine a perfect and infinite computer for the time being.

[Starglider]

Darn, this is sounding too metaphysical.

Yes, it is. 'Meaning' is a loaded word, don't use it. Other universes would be systems that normally have no causal connection to the system we are embedded in. If a connection was created, information could travel in one direction, or both directions. Both directions requires an equivalence of timeline, and establishes a two-way causal connection. An alien system of physics just means we start the scientific method over again from first principles (or more likely not, if it is merely a variation of the laws we have). 'Universe' may not be the right word here, people who talk about domain walls separating regions of different physical laws usually still put them in the context of one 'universe'. Regardless, not being able to send stuff through does not rule out sending information and having it reconstructed into a functionally equivalent system on the other side, which may or may not require assistance from friendly natives. See Greg Egan's 'Diaspora' and 'Schild's Ladder' for quite accessible sci-fi treatments of this.

[Darth Ruinus]

I thought there was no "outside" the Universe, because the Universe means "everything" and hence there would be no "outside" because that "outside" doesn't exist.

[Havok]

The concept of 'multi-universes' sounds utterly stupid to me, quite frankly.

Care to elaborate on this?

My opinion is based upon the definition of the term universe, like the following: "The Universe comprises everything we perceive to physically exist, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them."

In other words, proposing multiple universes is the equivalent of proposing planets/stars/galaxies/whatever as seperate 'universes' if they have zero interaction with one another. "Multi-verse" only works if you butcher the term universe into something that no longer applies to "all that exists".

So you just have a problem with the use of the word to describe the idea. Gotchya.

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

Why? Vehrec has an explanation, but to make you think it through a little more, answer my question.

Umm... what?

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

Not really. The way you think about the universe, like a loaf of bread rising or a balloon inflating is in almost every meaningful respect totally wrong. It's not expanding outwards into anything, it's just spatially larger, at least from the inside. Inside and outside may not even be the right terms for talking about this.

I generally get this, and this probably has to do with a serious lack of education on the subject. but say there was a planet at the 'edge' of the universe... at maximum expansion, what would they see? Would it be their own light reflecting back at them? I mean, is there a literal wall? Isn't there a point where the expansion hasn't reached? What is beyond that?

What is outside the universe is a very difficult question to answer because it's kinda hard to put any sort of instrument or observer into places that are not places and where no physical laws as we know them apply. It may very well be impossible for us to ever determine anything about what may or may not exist outside our own space-time.

It is fascinating, if mind boggling, that we can conceptualize it though.

[Havok]

Ghetto edit:
If I have my facts straight, at the point before the 'Horrendous Space Kablooie' the universe was just a small super condensed ball of matter, that in no way was the distance across that the universe now is, so what was around it at that point before the HSK? What did it explode and expand into? Isn't whatever that is still there and still around the universe?

[Starglider]

I generally get this, and this probably has to do with a serious lack of education on the subject. but say there was a planet at the 'edge' of the universe... at maximum expansion, what would they see? Would it be their own light reflecting back at them? I mean, is there a literal wall? Isn't there a point where the expansion hasn't reached? What is beyond that?

If the geometry of the universe is (hyper-)torodial, then there is no edge for the same reason that there is no 'edge' to the earth. It is continuous in three dimensions (the way the surface of the earth is continuous in two dimensions, though without the curvature) and wraps back around on itself.

What did it explode and expand into?

It didn't explode into anything; effectively, more space was created inbetween the existing particles.

[Vehrec]

OK, so then what is outside our Universe? If it is expanding, there has to be space that it is expanding into.

Not really. The way you think about the universe, like a loaf of bread rising or a balloon inflating is in almost every meaningful respect totally wrong. It's not expanding outwards into anything, it's just spatially larger, at least from the inside. Inside and outside may not even be the right terms for talking about this.

I generally get this, and this probably has to do with a serious lack of education on the subject. but say there was a planet at the 'edge' of the universe... at maximum expansion, what would they see? Would it be their own light reflecting back at them? I mean, is there a literal wall? Isn't there a point where the expansion hasn't reached? What is beyond that?

Again, edge is the wrong term. They'd see... well the other side of the universe, because as far as I know it loops around. That's partially what is meant when people talk about the curvature of the universe.

What is outside the universe is a very difficult question to answer because it's kinda hard to put any sort of instrument or observer into places that are not places and where no physical laws as we know them apply. It may very well be impossible for us to ever determine anything about what may or may not exist outside our own space-time.

It is fascinating, if mind boggling, that we can conceptualize it though. [/quote]
The problem is, are we conceptualizing it in a technically correct fashion?

Ghetto edit:
If I have my facts straight, at the point before the 'Horrendous Space Kablooie' the universe was just a small super condensed ball of matter, that in no way was the distance across that the universe now is, so what was around it at that point before the HSK? What did it explode and expand into? Isn't whatever that is still there and still around the universe?

It may in fact not so much be a case of space expanding as it was a case of space being unfolded. As much as it pains one to think of it, consider string theory. Spatial dimensions can exist on a very small or a very large scale. The HSK may have just been setting length width and height to non-zero values. The thought of the universe pushing out into pre-existing space is just wrong, plain and simple in this case.

[Kuroneko]

but say there was a planet at the 'edge' of the universe... at maximum expansion, what would they see? Would it be their own light reflecting back at them?

As you look farther away, the objects you find a traveling faster away from you. However, visually, what you see is a bit different--they get more and more redshifted, so that instead of seeing them cross the horizon ('edge'), their images hover right above it, getting closer and but not quite reaching it, growing fainter and fainter due to said redshift. They do cross the horizon, but you won't actually see them do so.

That's what you see when you look outward. What do they see? Exactly the same kinds of things, because in every point in the universe has a horizon some large distance away from it.

For example, if you see a young galaxy far out and near the horizon (and it must be young due to the finite speed of light, so that looking looking at distant objects also looks into the distant past), getting smudged on the horizon, a hypothetical civilization looking in the direction of the Milky Way would see a young Milky Way getting smudged on the horizon.

This is actually easier to imagine for an infinite universe. Picture an infinite line of galaxies equidistant from each other (not really accurate, but close enough on average; not to scale):

Code: Select all

<-*--*--*--*--*--*--*--*--*->
<-*---*---*---*---*---*---*->
<---*----*----*----*----*--->
<-*-----*-----*-----*-----*->

As time goes on, space expands and the distances between galaxies increase. It's easy to imagine the center galaxy as 'special', with every other moving away from it, but it is not correct--the distance from any galaxy to every other galaxy is increasing in the same way. There is no 'central galaxy'. There is also nothing that the universe is 'expanding into'; it's just expanding. A finite universe is similar (e.g., spherical), but harder to imagine because of a natural human instinct to imagine spheres as embedded in some higher space.

I mean, is there a literal wall? Isn't there a point where the expansion hasn't reached? What is beyond that?

No. Just as around us there is a horizon beyond which we cannot observe, the same holds of every other point. Yes, that means according to some other galaxy, we're near their 'edge', getting smudged and redshifted on their horizon.

As to what's beyond the cosmological horizon, naturally we cannot know since it's unobservable, but cosmological models typically suppose that space is homogeneous on the large scale, meaning the answer to that is 'more of the same, on average'.

Note that this makes it kind of strange to call regions inside the horizon a 'universe', because, well, such 'universes' overlap.

[Havok]

OK, if I am following, there is no 'center' of the universe and what appears to be the 'edge' for us is actually someone else's center and we appear to be at their 'edge'?

So I guess the fundamental way I look at 'expansion' (as Vehrec initially pointed out) as far as it comes to the universe is incorrect.

I'm still having a hard time wrapping my head around how there can be nothing outside of the universe if it is spherical or how it can be just be infinite, but I guess we can save that for another thread.

Thanks for the info guys and if you want to add anything I am more than interested.

[Kuroneko]

OK, if I am following, there is no 'center' of the universe and what appears to be the 'edge' for us is actually someone else's center and we appear to be at their 'edge'?

Yes.
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You know that old Asteroids arcade game, where your spaceship crossing the top of the screen appears on the bottom at the same vertical level and the same velocity, and similarly for left and right? That's a finite universe shaped like a two-dimensional torus. It's fairly easy to picture why: take out a piece of paper, fold it into a cylinder, and then it is obvious that one can make a torus by folding it again.

If you've played that game, it probably didn't occur to you that the arena was that of a torus, and you just thought that it was a screen with a funny wrap-around property. And that's not only perfectly fine, but rather the way you should think about it if you were living in this universe. The only important parts are the intrinsic properties--how points within it relate to each other--and it's toroidal just in the sense that those relationships are 'like' that of the points on a torus. There's no need to imagine any higher-dimensional space for your universe to fit into. It would be a space that wraps around on itself in a particular way, and that's it. (Note also that if imagine yourself to be on the spaceship, there's nothing special about the edges of the screen, in the sense that you wouldn't see anything interesting happen there. It wouldn't be 'physically real' to you.)

A two-dimensional spherical universe is similar to that, except it would look a bit stranger. One way to represent it in terms of Asteroids with a square screen would be like this: an object crossing the top of the screen would come out from the right same distance from top-left, moving perpendicular to its old direction, while an object crossing the bottom would come out from the bottom, again perpendicular to its old direction and identical distance from bottom-right. But notice that we still don't really need to imagine a higher-dimensional space to contain such a universe; we can just imagine it as a space that wraps around on itself (but in a somewhat different way than the torus).

It's not that a higher-dimensional space can't "be there" in some absolute metaphysical sense. It's just that counting unnecessary, unobservable things as unreal is good policy. I'm sorry if this Asteroids analogy seems contrived, but I thought I might as well make use of the previous video game discussion (plus Starglider's mention of tori).

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It may be noted that we don't know whether the universe actually loops around on itself. It would be true if the it has positive curvature, but as far as we can measure, the universe has zero curvature. Due to measurement error, it could be positive, zero, or negative; all we can say is that it's so small that it's indistinguishable from zero by our current instruments.

[Simon_Jester]

Einstein wanted to believe the curvature was exactly zero; we still have no reason to believe this is true, but have ruled out a large percentage of the possible alternatives.

On a less irrelevant note:
If the universe has positive curvature, as Kuroneko says it will loop back on itself. If it has zero curvature, it is "flat", in the sense that it just keeps going indefinitely. Space need not be finite, because (as others say) the "leftmost" particle in the universe does not need further empty space to the "left" to spread out into. The dimension of "right/left" may simply stop, or may continue forever, with an endless series of objects dotting it.

Except as a lie-to-children, it is not correct to picture the universe as a bunch of little fragments flying away from each other like shrapnel from an explosion. This model is useful, but almost entirely inaccurate; its only virtue is that it is easier to understand and explain. I'm not sure if others have made this fully clear yet, but what's actually happening is that new space is appearing in between the existing objects, out of nowhere. Take two points that are a mile apart today, and they will be infinitesimally more than a mile apart tomorrow. This is not because the points are flying away from some central point that blew them both outwards; it is because there is honestly more than a mile between points that were only a mile apart yesterday.

You're not looking at moving objects on a fixed background; you're looking at fixed (or what might as well be fixed) objects on a moving background. Which is why talk of edges to the universe isn't all that useful: while objects can only move if there is empty space for them to move into, there is no such constraint on the creation of more empty space from nothing in the regions between objects.

[Shinova]

You can go with the membrane, or brane theory, or whatever it's called theory of multiple universes. The idea is that our universe can be thought of as a multi-dimensional sheet of paper, and there are other sheets of paper above and below our universal 'membrane'.

There's a following idea that the big bang came about because two brane-spaces somehow fluctuated and collided together at a single point, essentially a collision between parallel universes. From that collision was born another universe, for example ours.

[madd0ct0r]

You're not looking at moving objects on a fixed background; you're looking at fixed (or what might as well be fixed) objects on a moving background. Which is why talk of edges to the universe isn't all that useful: while objects can only move if there is empty space for them to move into, there is no such constraint on the creation of more empty space from nothing in the regions between objects.

this is where the balloon analogy becomes useful again.

Imagine lots of little dots drawn on a balloon, each of these dots can represent a galxy.

The 'universe' is the surface of the ballon, perfectly 2D. When the balloon is blown up, the dots appear to move away from each other, but its the 'background material' that is expanding.
(Also, from any one of the dots, it looks as though all the others are rushing away from YOU. Hence both the idea that the Earth was at the centre of the Big Bang, and it's subsequent disproval.


Whichever direction you travel on the balloon, you end up back where you were, just like the Asteroids game.
(Except for slight differneces in the exact path to get back, Kuroneko has already covered this better then I can)

[Simon_Jester]

I'm quite familiar with the balloon analogy; and I feel that it is indeed helpful, with one minor difference: on the balloon, the dots expand, in the Robertson-Walker metric they don't. Individual particles (and molecules, and planets) are the same size as they were ten billion years ago; all the extra empty space is appearing between them.

[Havok]

But shouldn't the dots expand, as are galaxies are expanding as well? Or, I should say getting more space in between them?