Effort to get us all on the same page (balloon analogy)

Drakkith

Our current knowledge can only get us so far. Instead of saying "this MUST be true" you would be much better off learning about what we currently know and how we know it.

alphachapmtl

Maybe galaxies are not moving away but shrinking.
The end result would be the same (redshift , etc...)

alphachapmtl

Moving away, not accelerating.

Drakkith

This is not an accepted view.

salvestrom

Doesn't it actually require the complete opposite of the bing bang: a large space with diffuse matter that slowly coalleces overtime and shrinks into a denser and denser state?

zbe

Hi everyone.

I have a question. As already mentioned the measurments tell us the universe is flat or nearly flat. It was also stated that only closed universe would be spatialy finite and both the open and flat one would be infinite. My question is: How can universe have a beginning and is spatialy infinite? It is widely accepted that our universe has a beginning so wouldn't that suggest that our universe is either closed or that flat/open universe doesn't have to be infinite, somehow? =)

Please, tell me if I am missing something.

Regards.

marcus

There are so many concepts here it makes for confusion. I'm not sure what you mean by "open" and "closed".
Let's talk about the meaning of words another time. I understand you when you say spatially finite and spatially infinite. Either could be the case. We don't know yet which is more supported by the evidence. I don't believe either. I am waiting to see more evidence.

I don't think there is any scientific evidence that our universe had a beginning. The "big bang" is not necessarily the beginning of time. It could have been preceded by a contraction. All that the evidence suggests is that the expansion that we see apparently had a beginning. We can follow time back to a very dense state. I am waiting for the models/theories to be tested, that go back further.

I don't see any contradiction. Personally I do not believe the U had a beginning because I do not see any observational evidence to support that belief. It is always possible that what we can understand and explain will go back farther in time as our science gets better. There is no fixed limit on how far back in time we can discover explanations and causes.

Understanding is a gradual step-by-step process. Perhaps it never ends. Right now our job is to understand what caused the start of expansion (the "big bang").

But maybe you know somebody who believes the U had a beginning! There are people who believe this! Let's say this person is named Bob. If Bob is able to imagine that the U had a beginning, then why can't he also imagine that the U is spatially infinite? What is the problem? Both things are hard to imagine, I admit. (And I prefer not to believe either, since there is no hard evidence for either.) But I don't see any contradiction.

If someone is able to believe that the U had a beginning, and wants to believe that, then why can't they also believe that it is spatially infinite, if they want? We should just let them think what they want.

cepheid

The contradiction would seem to be that the scale factor goes to 0 as t → 0. That seems to suggest that as you go back in time, everything collapses down to a singular point. For any finite spatial volume, that sort of almost even works, because it means that the universe contracts to zero size as you go back in time to the beginning of the expansion. I suppose you could view this as the expansion of spacetime from some "initial singularity" of infinite density, but that sounds like nonsense -- what does it even mean? So what it really seems to mean is that our current physics breaks down and is incapable of describing what happens at t = 0. This is my view, actually. For me a singularity is just a mathematical problem having no physical significance other than, "your equations don't work here."

For an infinite spatial volume, even if the scale factor goes to zero, it doesn't mean that the universe goes to zero size. It just means that what happens as t → 0 is undefined. (Even more undefined than in the finite case). So the point is, since the universe having a beginning (or at least a beginning of the expansion) seems to require the scale factor going to zero, and since it's undefined what happens to a infinite spatial volume under these circumstances, it seems that (spatially infinite universe + beginning of expansion) doesn't make sense conceptually. (Then again, neither does "initial singularity.")

Am I thinking of this along the right lines? I really could use your insight here.

marcus

I think along similar lines. GR develops a singularity. A singularity means the theory is breaking down and you have to stop trusting it. Any manmade theory will have a limited "domain of applicability" and will give a worse and worse approximation to nature as you approach where it breaks down.

I'd say you are thinking along the right lines, but I'm not an authority on this or anything really. All I can do is observe that what you say makes perfectly good sense to me. (You may know more than I do about this in fact.)

I think we are in a transitional situation where everybody realizes that classical GR has limited applicability and we need a quantum theory of the U's changing geometry. So various theories are being developed and replacements and no one stands out as favorite.
GR is a non-quantum vintage 1915 theory of dynamic geometry (how it changes and interacts with matter). It's beautiful and exquisitely accurate over its vast range of applicability. Only a few flaws and blemishes at the extreme limits.

So people are now proposing quantum geometry theories that attempt to extend the domain of applicability. Cover for classical everywhere that classical is good PLUS push the bounds of what we understand just a little bit further, to understand better around the start of expansion and the pits of black holes.

It would be nice if a quantum geometry could also explain the cosmological constant too (maybe it arises in some natural way from the quantum theory, a natural tendency for expansion to accelerate? well...) It would be nice if a quantum geometry would tell us that tiny microscopic primordial black holes don't evaporate quickly (classically or semiclassically they do so we wouldn't expect to see any, but what is "dark matter"?)

Lots of things would be nice. But the main thing is to resolve the "initial" singularity and get a testable model of what was happening around the time expansion started.
A testable model that also reproduces the beautiful classical picture, with the same fine accuracy.

We're talking attitudes/opinions at this point. I told you mine. it's similar to what you said, I think.

marcus

Wow! I just looked at your "about me". You picked really interesting things to get a PhD in!
That stuff has the potential to really open up and get increasingly significant over the next 20 years IMHO. I'm just a retired mathematician who loves cosmology. I should be listening to you, not the other way around Good career and life choices!

zbe

By "open" and "closed" (or flat) I mean the curvature of the universe. And (on page 3 or 4) it was said that only closed universe would "guarantee" finite space --- but it looks like it is infinite since we most likely live in a flat universe.

I think that all our knowledge ATM goes towards notion that the beginning of our universe (big bang) is the most acceptable truth unless I am missing something here. It could have been preceded by a contraction indeed but that would not change the contradiction.


Exactly. That is my point. How could you follow - in a finite amount of time - infinitely big universe to a finite (or infinitely small, it doesn't matter) universe.

I couldn't agree more. The only thing that bothers me is why could there even be a possibility that universe is infinitely large since it has a start - of expansion.

IMHO both things aren't hard to imagine unless they are both true for the same thing. =) Look at it that way: A 1x1x1 (cm, doesn't matter) cube is born.=) It grows. In what time does it reach infinity? Never! Or better - in infinite time. I say F*** infinity since it has nothing to do in physics.=)

Also, I would guess my point is somehow the other way around of the cepheid's one.

I strongly agree with everything known in cosmology, this is just one of the rare questions that popped in my mind while reading this thread. =)

Regards.

cepheid

Thanks for the kind words. I'm not a theorist and don't have really in-depth knowledge of General Relativity, just the basics and certain things that are applicable to cosmology. But I'm trying to learn it, over and above "building stuff" to collect more data via observations which is my main pastime, as you probably gathered.

mysearch

Hi,
I hope the extracts above and the highlighted words from previous posts do not misrepresent the original context, as they seem to capture some key issues, which I would like to raise. In the first quote, the key word appears to be whether ‘everything’ in the totality of the universe expanded from some conceptual singularity? In my own personal review of the various cosmology models, there seems to be plenty of scope to, at least, speculate that the expansion of what we often describe as the universe is only part of some larger process/universe.

I agreed that what happens as t->0 is ‘undefined’ in terms of current science, i.e. both GR & QFT, although we might be allowed to speculate that our ‘universe’ could have been triggered by some sort of quantum process within some larger definition of the universe.

In this respect, the speculative larger universe might not have any obvious ‘beginning’, although it might still be correct to say that our ‘local’ universe came into existence some 13.7 billion years ago.

However, there still appears to be the troublesome issue of infinities, which I am not sure that physics or the maths can ultimately avoid. If the common description is taken at face value, then you have to explain a universe, which is 13.7 billion years old, created from a singularity of near ‘infinite’ density that would conceptually occupy a near infinitely small volume, outside of which ‘absolute’ nothing exists or has ever existed. In this context, the issue of ‘creation’ from absolute nothing always seemed a bit metaphysical for my taste. Of course, the speculative model suggested cannot really avoid infinities, as the idea of an extended universe suggests a possibly infinite size and infinite age. Maybe Marcus, as a mathematician, and Cephid, as an astrophysicists, might like to comment further on such issue. Thanks

marcus

To be honest, I'd appreciate it if anyone who wants to talk about these more philosophical matters would start a separate discussion thread. The original topic here is the most basic idea in cosmology: the pattern of expanding distances between observers at rest relative to the ancient light.
It's very much for beginners.
The idea is well conveyed by Ned Wright's short computer animation plus discussion of quantitative basics such as universe time, Hubble law, proper distance.

To get the brief movie diagramming expansion, google "wright balloon model". In a diagrammatic 2D analogy it shows photons moving at constant speed THRU space while the galaxies remain approximately at rest.

mysearch

Marcus, my apologises, I will happy comply with the original intention of this thread. However, it might be argued that the issues raised were not intended to be philosophical as they would seem to define the boundary conditions of any cosmological model. Equally, many of the questions within this thread appear to stem from certain assumptions about the implied ‘start’ of the Big Bang, the very name of which seems to be the source of much confusion. Again, my apologises.

salvestrom

I see no reason, so far, not to ponder if t=0 implies the moment at which spacetime was "begun" or created. Just as matter was created via reheat, perhaps the very mechanism that causes space expansion today and since t=planck time, also created the first space. I know I'm not alone in suggesting that what we think of as t=0 may not be the beginning of time. I go further and suggest it may only be the beginning of the spatial universe.

I sometimes feel there's too big a push to "get rid of it" and dismiss the singularity as a breakdown. I understand why, but feel intriguing possibilities are being overlooked.

EDiT: Having said all that, and in reading Marcus's request to not discuss philosophy, I guess I just want to make sure that people understand there isn't a consensus on "the singularity". Only opinion.

GeorgeDishman

Returning to the original question, I think we have to be careful in pushing the balloon analogy too far. It is easy for beginners to get the impression that it represents the current model when it actually gives the wrong impression in a number of ways. One has already been mentioned, the surface of the balloon is a finite area and while that is good for explaining how a closed universe can be finite but unbounded, it is not applicable to an open universe. This page from the WMAP site gives a balanced view on this:

http://map.gsfc.nasa.gov/universe/uni_shape.html

The second problem is that it is easy in the ballon model to see time as the radius of the sphere with the big bang singularity at the centre. This raises complex questions of a preferred direction of time and the difference between GR in general and specific solutions. An alternative is to still use the spherical shape of the closed model on the WMAP page but treat the big bang as the "south pole" and the big crunch as the north pole. A small area at the equator can then be likened to an x-t spacetime diagram in SR, treating the path of a photon as always being at 45 degrees to a vertical line (of "longitude"). Space is then a horizontal slice, i.e. a circle so a 1-D analog rather than the 2-D usual interpretation of the ballon model. I've heard of this being described as the "American Football" model. Of course that still only applies to the closed solution so the first objection remains valid.

The third problem is that dark energy means expansion is accelerating. Taking the football model and opening the top to eliminate the big crunch and make an inverted bell solves that (although again a horizontal slice remains a circle hence it still models a finite universe) so the "timeline" graphic is IMHO a better representation:

http://map.gsfc.nasa.gov/media/060915/index.html

If the boundaries of the bell are identified with the observable portion, the idea of expansion with infinite extent might be more accessible. The best explanation I have seen of that though is perhaps in Ned Wright's tutorial near the bottom of this page just after the Mercator illustrations:

http://www.astro.ucla.edu/~wright/cosmo_03.htm

"Also remember that the Ωo = 1 spacetime is infinite in extent so the conformal space-time diagram can go on far beyond our past lightcone".

That can also be extended to illustrate non-ovelapping Hubble volumes as in Figure 2 in this paper:

http://arxiv.org/abs/gr-qc/0102010

Sorry if some of this has been covered before, I'm new here and it's long thread but the Balloon analogy was intended to illustrate a specific concept (finite but unbounded) and IMHO is quite misleading in terms of modern cosmology.