expanding universe

I understand that the universe is expanding, we know this because of the redshifts we observe. However I assume we are only able to see a small part of the universe. How then can we conclude when we can only see a small part that the universe as a whole is expanding.So first my question is if my assumption is correct. Even if that assumption is incorrect, and we can see most of the universe my question is if it would be correct to infer the universe is expanding even if we cant see all of it?

Hi Dino,This is another question related to the expanding universe. There is something about an expanding universe that puzzles me, what is it expanding into?If something is expanding then obviously it is growing bigger, but it can only expand if there is space for it to expand into. Therefore, if everything in existence is contained within the universe, how then can that universe expand into space that would by definition have to be inside it? Surely an expanding universe means that there is space outside of the universe for it to expand into?I hope I have explained the question clearly, as you can see from the poor construction of the question, I am not a scientist, but perhaps some of the scientists here can explain this to me, in very very simple terms LOL, if you can understand the question that is!Many thanks.Brian.

The answer, as I understand it, is that space itself is expanding.
It follows from that that the universe is not expanding INTO anything.

HI Paul,Thanks for the quick reply, but what is the space expanding into?I am going ot get a sore head over this LOL.Cheers!

You can't think of the universe as a whole as you would think of an ordinary object like a balloon. The universe IS everything. Whether it objects inside it move farther apart or stay the same distance from one another is irrelevant. Either way, the universe is everything. You can't look at it from the outside, in other words. The expansion is viewed from the inside. There doesn't have to be an outside.------------------
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HI John,Ok. let me see if I understand it now.Right we got a universe that contains everything, inside this universe we witness expansion, but this expansion is still within the universe, and when, or if, it contracts, is the universe still the same size but the objects inside it come closer together?I am genuinely having a hard time getting my head around this LOL, I must sound like a freaking creation scientist!Brian.

I always have a miserable time wrapping my brain around the concept. I usually just ignore it and hope it goes away. However, the closest I got was the balloon analogy. Consider two spots adjacent to each other when the balloon is deflated. As it expands, the dots "move away" from each other. We're more or less looking at this expansion from the "inside" - the balloon is all there is, with true "nothing" outside it (more or less). It's a pretty weak analogy, and apparently (according to some of my physicist friends who pretend to know what they're talking about) breaks down really quickly. But the balloon thingy is about the only way I've ever been able to "visualize" an expanding universe that contains the entire universe.(Did I use enough quotation marks in that last paragraph? I'm trying to hit my annual quota before the end of the fiscal year...)

Hi Quetzal, thanks for the reply,I think I am beginning to see it clearer now. The problem I am still having is that when the universe, the balloon, is deflated, what happens to the space that the expanded balloon occupied, and similarly, doesn't there need to be space there for the ballon to expand into? I think it is the concept of there being 'nothing' outside it that I have the problem with.The way I see it is that if the universe is size X and then expands to size Y, doesnt there need to be something between X and Y for it to expand through?There is probably an easy answer to this, and I am too scientifically uneducated to graspt it LOL!I sometimes wish I had the time to do a science related degree, but I simply do not have the time.Brian.

I dont want to be blunt and I too find Brian's question fascinating but the answers given didnt really answer my initial question.How do we know the universe as a whole is expanding if we are only able to see a fraction of it?

HI Dino,Just a quick apology for hijacking your topic, I just realised that my question was Off Topic.Cheers!Brian

I suppose it depends on whether the contraction was based on space contracting or simply the matter within that space reversing direction. I suspect that the former was what the physicists had in mind, but I don't know.Last I heard it was academic since the data pointed to the rate of expansion accelerating, wth no reverse in sight.

Hi Brian,Sad fact is that no-one can imagine what space is expanding "into", because our brains only evolved to cope with (or of course were intelligently designed not to be able to imagine for some good reason that only the designer(s) knows about and won't tell anyone). The balloon analogy is useful only if we imagine space to be 2d (the surface of the balloon). An ant on the surface of the balloon would see the dots on the surface moving away from one another as the 2d object expands in 3 dimensions. Whether or not this means that space is expanding into a hyperspace, I leave to the philosophers.Another way of thinking about it is to think of space as not actually expanding but the distances between any two objects are increasing with time (insert appropriate emoticon here). This site might help, or further confuse. It unhelpfully rubbishes the balloon analogy anyway.On second thoughts, sorry - that hasn't explained anything at all PEedit found a better link[This message has been edited by Primordial Egg, 07-18-2003]

Thanks PE,Your link was a great help.Brian

Hi Dino,To return to your original question, and the point of this thread:

quote:

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I understand that the universe is expanding, we know this because of the redshifts we observe. However I assume we are only able to see a small part of the universe. How then can we conclude when we can only see a small part that the universe as a whole is expanding.
So first my question is if my assumption is correct. Even if that assumption is incorrect, and we can see most of the universe my question is if it would be correct to infer the universe is expanding even if we cant see all of it?

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Two of the fundamental premises upon which most of cosmology are based are homogeneity (the universe is the same in all locations) and isotropy (the universe is the same in all directions). These are initial assumptions. We can perform experimental tests to give us comfort that these assumptions hold, but in the end, they are, as you imply, assumptions.The uniform nature of Cosmic Background radiation lends itself to the idea that the universe is the same everywhere - obviously, beyond the visible universe there could be all sorts of strange and weird things going on for which we would never have any knowledge. However, one important thing to remember is that as telescopes have become more powerful, nothing has raised one scintilla of doubt that somewhere else in the universe has different physical laws, or is not expanding. This explains it better than me, anyway.PE

It also needs to be said that the equations of Einsteins general theory of relativity predict that space is not static. It is either expanding or contracting, and no one has really found a solution that would allow for space that isn't.So the only assumption here is that general relativity is an accurate description of space and time throughout the entire universe.[This message has been edited by Beercules, 07-18-2003]