More on Infinity: Why Does Balloon Analogy Not Describe Universe?

[Arch2008]

Well. you know her better then I. I think she probably just confused the infinite universe explanation from the search for omega with a universe that is actually infinite at present. I read a lot of posts where someone has done this and even some magazines. No big deal really.:)

P.S. I also found this:
Is the Universe really infinite or just really big?
We have observations that say that the radius of curvature of the Universe is bigger than 70 billion light years. But the observations allow for either a positive or negative curvature, and this range includes the flat Universe with infinite radius of curvature. The negatively curved space is also infinite in volume even though it is curved. So we know empirically that the volume of the Universe is more than 20 times bigger than volume of the observable Universe. Since we can only look at small piece of an object that has a large radius of curvature, it looks flat. The simplest mathematical model for computing the observed properties of the Universe is then flat Euclidean space. This model is infinite, but what we know about the Universe is that it is really big.
http://www.astro.ucla.edu/~wright/cosmology_faq.html#RB

 

[ThomasT]

You're not helping your case ...

I'm not making a case per se. Just threw a few more or less commonsense ideas out there. The original poster seemed to like at least one of them. Of course, I'm still at the "exposion good, balloon bad" stage in exploring this. But, isn't the path to eventual mathematical models sometimes paved with the spitballs of mechanical intuition, analogies from our recollections of sensory experience, and the logic of ordinary language?

... you've just spouted a big pile of incorrect ideas that one could get by pushing the explosion too far.

Only if our universe didn't begin with an explosion. However, what we can see of it looks to me like the remnant of some sort of explosion. Are there no professional cosmologists who think of it this way?

Does this notion challenge the current standard LCDM cosmological model? Not by itself. But there are some things which seem to follow logically from the explosion assumption. Hence, my big pile of incorrect ideas.

1. GR does not involve any extradimensional physics -- all the dynamics of GR are purely intrinsic. Even if we take a viewpoint where we embed the observable universe in a higher-dimensional space-time, whatever happens in those extra dimensions is completely irrelevant to the goings-on in the observable universe.

OK.

2. In GR, the big crunch is not a thermodynamic impossibility -- if the bulk composition of the universe is right (e.g. overall matter density, cosmological constant), it is something certain to happen. (Although I don't know if it's merely thermodynamically certain, or absolutely certain)

Calculational systems (mathematical models of physical systems) sometimes produce symbolic behavior which doesn't correspond to anything in the physical world. Is this the case with GR? Is it the case with electrodynamics?

Statistical thermodynamics allows some things which would be precluded by a deeper mechanical understanding of the systems involved.

Given the assumption that the kinetic energy of the expansion is the dominant energy in our universe, then a big crunch is an absolute impossibility.

 

[Hurkyl]

Just threw a few more or less commonsense ideas out there.

Commonsense is irrelevant here; we're dealing with things well outside of the realm of 'common' experience. (And even in the realm of common experience, commonsense is often wrong)

But, isn't the path to eventual mathematical models sometimes paved with the spitballs of mechanical intuition, analogies from our recollections of sensory experience, and the logic of ordinary language?

Intuition does not supercede fact.

Calculational systems (mathematical models of physical systems) sometimes produce symbolic behavior which doesn't correspond to anything in the physical world.

And sometimes, they produce symbolic behavior which does correspond to something in the physical world. But you're still firmly planted in the realm of the hypothetical -- GR is the best we've got, and the evidence says that GR doesn't make mistakes in this domain.

Given the assumption that the kinetic energy of the expansion is the dominant energy in our universe, then a big crunch is an absolute impossibility.

Given the assumption that the universe is a big rubber band, then a big crunch is a (nearly) absolute certainty. We can invent all sorts of crazy assumptions to support whatever worldview we like -- but none of that changes the fact that the science says that a big crunch is a possibility, and would be the expected outcome if the bulk statistics of the universe were slightly different.

 

[Pjpic]

The open, closed, and flat universes still seem to have boundries at(before?) zero and actual infinity. So, assuming we don't reside in a special area of space, I'm still not sure why the balloon analogy breaks down.

 

[ThomasT]

Commonsense is irrelevant here; we're dealing with things well outside of the realm of 'common' experience.

Ok, poor choice of words on my part. Making and dealing with the conjectures or assumptions I presented for consideration requires a bit more than an ordinary commonsense knowledge of cosmological observations and theories. I have an uncommon, but still only superficial, knowledge of both.

But please treat my statements as questions -- even if I present them as arrogant assertions. Ultimately I'm here to learn, and I do appreciate your (and others') replies.

Intuition does not supercede fact.

I think a more appropriate answer to my question would have been yes or no accompanied by examples (if yes).

And sometimes, they produce symbolic behavior which does correspond to something in the physical world.

I'd say they quite often produce symbolic behavior which corresponds to something in the physical world -- following suitable interpretation. Otherwise they'd be mostly meaningless.

But you're still firmly planted in the realm of the hypothetical ...

Isn't this where cosmology is planted?

... GR is the best we've got ...

Agreed.

... and the evidence says that GR doesn't make mistakes in this domain.

What domain? Cosmology? Are you absolutely sure about that?

Given the assumption that the universe is a big rubber band, then a big crunch is a (nearly) absolute certainty.

Do you really think that my statement (Given the assumption that the kinetic energy of the expansion is the dominant energy in our universe, then a big crunch is an absolute impossibility.) is that silly? If so, why? Is it that you don't think that the energy of the expansion could possibly be the dominant energy, or is it that you don't see how this precludes a big crunch, or what? The kinetic energy of the expansion is manifested in every different sort of energy and behavior.

From the kinematics of the universe, we're used to thinking in terms of attraction and repulsion. We think of gravity as an attractive force and dark energy as a repulsive force. But if you think of it in terms of the isotropic expansion of the universe, then there's only one direction -- the direction of the expansion, which is any and every possible direction.

The thermodynamics of the universe is what it is precisely because the universe is expanding. There's a radiative arrow of time because the universe is expanding. Advanced waves are impossible because the universe is expanding. Travelling backward in time is impossible because the universe is expanding. Solar, galactic, and intergalactic structures are what they are because the universe is expanding.

There's not going to be a big crunch because the universe is expanding. As it expands, its condensed structures become more widely dispersed (reducing the probability of interaction), and the energy required for the expansion (and everything else that's happening) is dissipating the finite amount of energy imparted via the origin event.

We can invent all sorts of crazy assumptions to support whatever worldview we like ...

Does anything I've written seem crazy to you? If so, what and why?

... but none of that changes the fact that the science says that a big crunch is a possibility, and would be the expected outcome if the bulk statistics of the universe were slightly different.

Theories aren't, by themselves, science. Science doesn't say that a big crunch is possible. A combination of certain mathematical-geometrical models says that it's possible. The interpretive view of gravitational behavior via this approach says that it's due to the bending in of spacetime around and toward massive objects. And thus, the deep nature of gravitational attraction is explained.

I think this is possibly a wrongheaded approach to understanding gravitational behavior (notwithstanding the usefulness of GR as a calculational tool) and has lead to some silly predictions (like a possible big crunch).