Expanding space needs surroundings to expand into. T or F.

[Martin]

On average, largescale distances between unbound-together objects in our universe are EXPANDING. So we have the common way of talking where we say "space is expanding" or "the expanding universe". All this means is that distances are increasing according to a certain law (technically the Friedmann equation, which determines the changing value of the Hubble parameter).

 

Since space is expanding, do you think it's necessary for it to have a larger surrounding space to expand into? That's the poll question.

 

========================

In case anyone wants it, I'll give some clarification. If this is all familiar to you, don't bother with it, just register your viewpoint in the poll:-) .

 

An idea basic to Gen Rel is that distances between stationary objects can expand or contract----the metric is dynamic and changing. It's part of how spacetime can have curvature.

 

In cosmology, GR is specialized to the case where matter fills all of space approximately uniformly. Things look approximately the same wherever you are and in whatever direction you look. (technically called the cosmological principle of homogeneity and isotropy).

 

If you imagine space as having a physical boundary, then you are not doing cosmology as normally understood. There are various horizons defined from our Milkyway point of view, but they are not physical walls. Someone at one of those horizons would not notice anything special.

 

Space can be either infinite or finite. Distances in an infinite space (like the Euclidean 3D space called R³) can be expanding. So there is no conceptual problem with in infinite space expanding. Also space could be finite (like the sphere-analog S³) with no centerpoint and no boundary---simply a finite-volume space. And distances in THAT could be increasing. So there is no conceptual difficulty there either. Either way, space can be expanding in the sense that distances within it are increasing according to some law.

 

Observational cosmologists tend to treat the two cases (exemplified by R³ and S³) similarly. They fit the data to either case. Intuitively a large enough sphere might as well be flat:-) and the same goes for the 3D analogs.

 

So that is what is meant by "expanding universe". The poll question is, if you have an expanding universe does it necessarily have to have some larger empty surrounding space to expand into?

[Bettina]

Sorry Martin, but I'm the "blue" in your poll.

 

Bee

[iNow]

What if the size of the universe has been itself been unchanging, but everything within it has become relatively smaller?

 

 

There's my pub question for the night... you know... the one which sounded brilliant at the time, but which I will have forgotten by morning, and be glad about that lack of memory ta boot.

[Martin]

Sorry Martin, but I'm the "blue" in your poll.

 

Bee

 

what i like about you sometimes is the absolute honesty. you call it exactly as you see it.

 

the key thing being able to describe growth from the inside, without visualizing the outside

 

to be a cell inside the trunk of a tree and know that the tree is growing even if you can't look from the outside and see the bark of the tree. even if you don't know if the bark exists.

 

or to be a cell in somebody's leg-bone and aware that it is growing without having any idea of the outward shape

 

the person who invented the methods for INTERNAL GEOMETRY was Riemann in 1850. Using Riemann tools you can study the shape of something that has no exterior. Riemann geometry enabled Einstein to take the 1915 GR step.

 

Riemann tools enable one to explore curvature internally even if the outside does not exist---even if there is no outside.

 

===============

and then there are eternal inflation people and multiverse people who CHOOSE to imagine our universe as a patch in some larger quilt. I think you know about them. they aren't representative of the field but they get press attention.

 

the thing with them is they all know Riemannian geometry and GR and they know that expanding space doesn't NEED a larger space around it. they just choose to put a larger space around because it fits their inflation scenario or some other thing.

===============

 

I hope college courses are going well for you. Must be in 3rd year now right?

 

I'm fine with your being True Blue. Would you care to explain your choice?

 

What if the size of the universe has been itself been unchanging, but everything within it has become relatively smaller?

...

 

It is a good question iNow because it brings out the fact that there isn't necessarily any difference.

 

What you ultimately measure is ratios. How many steel meter rods fit between here and some galaxy? If the meter rods get smaller then (as long as everything else does, atoms, bond lengths, planck units...) it is just the same as if the galaxy got farther away.

 

the fundamental units are the Planck quantities whose size depends on G, hbar, and c. it almost doesnt make sense to talk about them changing because it wouldn't be empirically detectable. physics has pretty much succeeded in explaining why everything including iron atoms is the size it is in terms of planck units. So if you make the meter rod smaller, then all the atoms have to become smaller, and they are a fixed number of planck units, so the planck units have to become smaller, which means that G, hbar, c are changing in some way that doesnt effect anything. the solar system is getting smaller. but nobody can detect the change!

 

If I say any more you have to buy me a drink at your pub.

 

it is simpler to think of it as the galaxy getting farther away and it amounts to exactly the same thing.

[YT2095]

your poll options don`t match the question.

 

I would answer True to your Question, but not tick True in your poll, that says something Entirely different again.

 

in fact False in your poll would be the one I would tick, as Void Space by it`s nature isn`t Anything at all.

 

it`s incongruent, I can`t vote in this.

[iNow]

It is a good question iNow because it brings out the fact that there isn't necessarily any difference.

 

What you ultimately measure is ratios.

What I like about the idea is that it seems to be equivalent, but concurrently erases the problem which arises from the need for "something to expand into."

 

 

If I say any more you have to buy me a drink at your pub.

 

Any time, mate. Thanks also for the very helpful information above. Cheers.

[Bettina]

Martin,

 

I'm second year college which is why I don't have any time to talk to you.

 

I can't answer your question because it's not accepted science. I just don't believe in the "nothing". I just don't believe in finality. "There is nothing outside our universe"... "There was nothing before our universe".... "The big bang started it all".... etc, etc.

 

Ever since I was little and first heard of the BB, I always looked through my binoculars in the opposite direction that scientists were looking because I wanted to know what it was like 2 seconds before the BB. So, yes, I believe our universe is expanding inside something greater, along with all the other universes.

 

I could ramble on..... but I'm off to the mall.

 

Bee

[Martin]

I could ramble on..... but I'm off to the mall.

 

Bee

 

Fortunately before taking off for the mall, Bee posted this rather lovely image of the bubble, on another thread. It was in response to a helpful and sharply focused post by Edtharan

 

No. Where on the surface of the balloon is the centre?

 

Yes, the Universe has a 4+D centre, we call that the Singularity at the Big Bang. But as a 3D position (that is the surface of the balloon) there is no centre...

 

 

Hi Edtharan, thanks for the reply and explanation The 4+D center is what I was trying to get at and I understand the center on the surface does not exist.

 

I always imagined the universe being a sphere and further imagined a child dipping a plastic wand into a bottle of "Super Bubble Solution" and gently blowing a bubble.

 

As she continues to blow into the wand, you would see an inflating sphere that has nothing inside, nothing outside, but yet contains colorful swirls on it's surface all moving away from each other as the bubble expands.

 

Sometimes, if the bubble is large and a breeze exists, the bubble would be grossly mishaped, and although the surface and swirls remain intact, the center would be difficult to determine.

 

I also remember that WMAP produced omega numbers that showed the universe had a curvature.

 

Bee

[Martin]

I'd like to propose an attitude we could adopt about different cosmology pictures people have.

 

Cosmology is only gradually becoming an observational science and there is a lot of room for different opinions. So everybody is free to picture the universe any way they want. But there's a but.

 

But I would like to encourage everybody to understand big bang basics as a common ground. Then if you choose to deviate, you know what you are deviating from.

The term "big bang" is misleading. I'm told it was originally a term of contempt invented by Fred Hoyle who was scoffing at the expansion picture. Hoyle was promoting a nonexpansion alternative model, which turned out not to fit the data, and is seldom mentioned any more. But he named the model he didn't like, and the name stuck.

 

The words "big bang" immediately give the wrong idea. But that is the phrase most often used. By "big bang basics" what I mean is the basics of standard expansion cosmology. The standard LCDM model that is homebase for cosmologists. This provides some common references for anchoring terminology and getting one's bearings, which makes for a better chance to communicate.

 

Anyway, in the standard LCDM model space is represented by a mathematical construct called a manifold---continuum is another term. There's a distance measuring tool defined on it, called a metric. The metric can tell you distances, areas, volumes, angles---it defines the geometry of space.

 

(This setup is just a slice of a 4D continuum equipped with a 4D metric----the 4D manifold is spacetime. For simplicity I'm just talking about space.)

 

I have to go do some other things, but what i'm driving it is some things like

 

1. the manifold is all the space there is

 

2. it has no walls or boundaries. there is no surrounding void (that would just be extra---no benefit to defining it)

 

3. expansion is something that happens to the metric----distances defined by the metric change. in our case they mostly increase.

 

4. relativity teaches us not to expect distances to be constant (except when anchored to something material like rock or metal or part of a bound system where forces operate to prevent change)

 

The reason that expanding space doesnt need surroundings is that all "expanding space" means is that the metric is changing. So of course there doesnt have to be any empty void around the manifold. It is not even clear what that would mean---since the manifold has no limits or walls or boundary or edges of any kind.

 

The two main flavors of LCDM are finite case and infinite case.

[YT2095]

ok, you shove a deflated balloon in a steel box and then blow the balloon up, you`ll find it will only go so far (within the confines of the steel box to be exact).

 

but if there is Nothing outside the balloon, then it will expand all it likes

[dichotomy]

On average, largescale distances between unbound-together objects in our universe are EXPANDING.

 

Yes, this is possible.

 

If the question is - distances between galaxies and other cosmic bodies can increasing/decrease, I'd say true. If it's - galaxies can move about within space, I'd say true. If the question is - does visible space, in total, expand like a balloon without the need for an outside area, I'd say False.

[YT2095]

 

2. it has no walls or boundaries. there is no surrounding void (that would just be extra---no benefit to defining it)

 

I strongly disagree, I think Void Space very Much needs defining.

 

for a start it`s not in Anyway subject to any of Our universal rules or constants, incl that of light speed, the Antithesis of Void Space would be something like a Black-Hole.

 

it in itself, begs 2 questions though.

 

1) as we expand, is this Void being repelled (pushed away)?

2) as we expand, are we moving Into it and making it part of our Universe?

 

 

edited to add that I`m also with Bettina when she said "I believe our universe is expanding inside something greater, along with all the other universes."

I expect there probably IS other universes within it also.

[gcol]

There was a time, some years ago, when having only classical notions of infinity, I would have voted no. But now I have been converted. Hallelujah.

 

I prefer to think of space not being a separate entity but being, like time, merely a workspace within which things exist and events happen. Space and time may not have a priori existence

[Martin]

gcol, thanks for the encouraging words. (It's a big boost to see people thinking about these things and occasionally changing opinion and understanding of stuff)

 

Actually there is an Occam razor argument hidden in this tangle of issues. The poll question was: T or F,

Space needs a larger surrounding space to expand into. That's why (if you use the conventional cosmo model) the answer is F. It might conceivably have (highly improbable, I think, but might) however it does not need.

 

You or I or Bettina, Panic, YT or anybody can actually BELIEVE if we want (altho there's no evidence I know of) that space has some kind of boundary or wall and that outside that there is a different space perhaps with different physical laws or kinds of matterfields we don't know about, perhaps portions of it undergoing inflation like that conjectured to have happened here. Here be dragons and sea monsters! Like the old guys used to decorate blank parts of the map.

 

And it could even turn out to be true! Fifty years from now someone might find evidence of a wall, and an outside.

 

but for now most cosmologists ignore that because it is an unnecessary complication that doesnt lead to any new predictions.

 

our model does not NEED an outside, or a wall---and adding that to the picture doesn't contribute anything (to the conventional cosmology model)---so the overwhelming majority of people just ignore the idea.

It doesn't play a role.

 

Occam razor says don't add features to the model that you don't need. Like putting decorative dragons and seamonsters on the map.

 

If the day comes when observations require it, walls and outside will be added. Models like this can normally be modified and extended as required.

But don't hold your breath.

 

So I would not say that Bee, for instance, is wrong if she chooses to believe in some different kind of space outside ours. Some future science might actually discover such and prove her right! But the question was does it NEED an outside.

 

I think it pretty clearly does not need. Leastways, the practitioners haven't noticed need for a wall or an outside. In fact the basic assumption ("the Cosmological Principle") underlying the whole field is space is everywhere pretty much the same with matter distributed roughly evenly throughout---directly contradicting the idea of a wall.

[dichotomy]

1) as we expand, is this Void being repelled (pushed away)?

2) as we expand, are we moving Into it and making it part of our Universe?

 

Or

3) as we expand, is this unknown space being completely consumed as energy for the expansion?

 

If there is no expansion involved then space is just still infinite space, with bits within it that expand and move about, that give the impression of expansion.

 

I do take Martin's point here as far as being practical and getting results goes: "developing our model does not NEED an outside, or a wall---and adding that to the picture doesn't contribute anything (to the conventional cosmology model)---so the overwhelming majority of people just ignore the idea. It doesn't play a role."

 

Although, like the seamen of old, we would have to be wearing blinkers to not identify that there may be unknown "SEAS" out there. And that it is simply not within our current knowledge to identify it.

[bascule]

I'm of the opinion that there's nothing outside the universe

[Bettina]

I'm of the opinion that there's nothing outside the universe

 

Aww... don't build walls that high. Read some Michio Kaku articles.

 

EDIT: (added the following)

 

....But the question was does it NEED an outside.

 

Yes it does. Without an outside void, hyperspace, arena, or the "nothing that is something", there could not have been an expansion of our universe. A person would not be able to blow up a childs balloon if there were no surrounding space for it.

 

I had to give an answer.

 

Bee

[hermanntrude]

I think the universe can do what the hell it likes. It's bigger than me

[lucaspa]

Yes it does. Without an outside void, hyperspace, arena, or the "nothing that is something", there could not have been an expansion of our universe. A person would not be able to blow up a childs balloon if there were no surrounding space for it.

 

Bettina, that is our limited thinking because we are in spacetime. Our experience is that we never see expansion of something that does not expand into the space that we occupy.

 

But our experience is not always a good guide for understanding what the universe is really like. After all, we evolved in this spacetime and thus, in order to survive, we are really good at understanding what happens around us on our scale. But we didn't need to evolve senses or understanding of the very small or the very large. So our "common sense" doesn't work there.

 

Spacetime (the 3 dimensions of space and 1 of time that we occupy) is what we know. The expansion of the universe is like blowing up a child's balloon. That is an attempt to take something outside our experience and make it like something we really do experience. But analogies are not identical to what is being described. Yes, a balloon needs to expand into an existing spacetime, but spacetime itself doesn't need to expand into antything. It just expands.

[Nick_Spanich]

I'm of the opinion that there's nothing outside the universe

 

SO you believe the universe is a house. But what about outside of those walls. There has to be something. Space is never ending. Even if the there is no matter there is still something there "lifeless" space

[iNow]

SO you believe the universe is a house. But what about outside of those walls. There has to be something. Space is never ending. Even if the there is no matter there is still something there "lifeless" space

 

No. It sure seems like there "should" be, but there's not. Be sure to check out that Scientific American article everyone keeps talking about. It's a bit above 7th grade level, but that doesn't mean you can't learn something from it.

 

 

Start with the links here:

http://www.scienceforums.net/forum/showthread.php?p=384716#post384716

[Nick_Spanich]

No. It sure seems like there "should" be, but there's not. Be sure to check out that Scientific American article everyone keeps talking about. It's a bit above 7th grade level, but that doesn't mean you can't learn something from it.

 

 

Start with the links here:

http://www.scienceforums.net/forum/showthread.php?p=384716#post384716

 

 

Please elaborate.

[iNow]

Please read the links contained in the link I shared with you above in post #21, then come back and ask clarifying questions where needed. Better?

[Nick_Spanich]

Please read the links contained in the link I shared with you above in post #21, then come back and ask clarifying questions where needed. Better?

 

"No. It sure seems like there "should" be, but there's not"

 

Elaborate what you mean there please. I read some of the articles but not all yet

[iNow]

"No. It sure seems like there "should" be, but there's not"

Oh. Okay. I see where you wanted me to clarify. I didn't catch that the first time.

 

Let me try to summarize.

 

We humans evolved on earth and our senses and our perception have evolved to deal with a limited set of circumstances. Not falling off of cliffs, avoiding tigers, hunting for food... all things which happen on relatively small areas... like a few square miles at a time... and all things that impacted our survival and the survival of our ancestors here on earth.

 

Our senses and our minds are setup to be better hunters and survivers in the conditions in which humans have lived on Earth.

 

So, we try to associate everything with that which we know, things we know from the conditions we experience on earth. We think of things like houses, and like trees, and like cars and supermarkets. ... And all of those things have "an outside." We're just used to things having an "outside," because all of the things we experience on earth have an outside.

 

 

You can go "outside" of your house. You can go "outside" of your car. You can go "outside" of the supermarket.

 

 

But our senses didn't evolve to understand the universe implicitly. The universe is EVERYTHING. There is nothing "beyond" the universe, because the universe itself, by it's very definition, is absolutely everything in existence.

 

It's so large and so vast that we cannot even comprehend how big it truly is.

 

 

My point is, the only reason that you think there "must be" an outside of the universe is because that's what you're used to. That's what your senses have evolved to know and understand. There is an outside of the house and the car, so you logically conclude that there must be an outside of the universe.

 

 

But... there's not. There is no outside of the universe, since the universe is everything. It doesn't make intuitive sense, but it's true.

 

The universe is everything, so the concept of an "outside" is meaningless.

 

 

Asking the question "what's outside of the universe" is logically equivalent to asking "how loud is the direction down?" It just doesn't make any sense.

 

 

Does this help?