The size of the universe... scaled

[xelasnave]

The term Big Bang is misleading.
It's implies explosion but the parallel is inappropriate and borders on useless.
We have a starting point for time and space which we can not describe and deal with the evolution of the Universe which inflated at at rate many times the rate of an explosion .
As Steven said inflation and expansion are separate matters. One is too grand to be called explosion and the other too humble to be called an explosion.
I think it is wise to remember we may not be describing reality but what we do describe is scientific theories which provide a very reasonable footing.
I personally find Inflation impossible to accept but I don't have an alternative theory.
My point is if one wishes to offer alternative cosmology one must offer a better theory.
As cosmology turns on general relativity one needs to offer something that takes us further than general relativity.
That won't be easy.

[xelasnave]

The term Big Bang is misleading.
It's implies explosion but the parallel is inappropriate and borders on useless.
We have a starting point for time and space which we can not describe and deal with the evolution of the Universe which inflated at at rate many times the rate of an explosion .
As Steven said inflation and expansion are separate matters. One is too grand to be called explosion and the other too humble to be called an explosion.
I think it is wise to remember we may not be describing reality but what we do describe is scientific theories which provide a very reasonable footing.
I personally find Inflation impossible to accept but I don't have an alternative theory.
My point is if one wishes to offer alternative cosmology one must offer a better theory.
As cosmology turns on general relativity one needs to offer something that takes us further than general relativity.
That won't be easy.

[Eratosthenes (Peter)]

Originally Posted by xelasnave

The term Big Bang is misleading.
It's implies explosion but the parallel is inappropriate and borders on useless.
We have a starting point for time and space which we can not describe and deal with the evolution of the Universe which inflated at at rate many times the rate of an explosion .
As Steven said inflation and expansion are separate matters. One is too grand to be called explosion and the other too humble to be called an explosion.
I think it is wise to remember we may not be describing reality but what we do describe is scientific theories which provide a very reasonable footing.
I personally find Inflation impossible to accept but I don't have an alternative theory.
My point is if one wishes to offer alternative cosmology one must offer a better theory.
As cosmology turns on general relativity one needs to offer something that takes us further than general relativity.
That won't be easy.

Indeed Xela,

it took several centuries before Classical Newtonian mechanics was augmented with Special Relativity (inertial/non accelerating frames of reference) and General Relativity (accelerating frames of reference).

And one needs to acknowledge that the Special Relativity and General Relativity theories are astonishingly accurate in explaining physical phenomena in the Universe (likewise Newtonian laws). However they all break down as some point (e.g singularities or phenomena that behave with an infinite curvature of space-time). In addition, the problem of unifying the Quantum world with gravity and large scale phenomena hasn't been resolved yet. This is god indication that both Quantum Mechanics and General Relativity may undergo revision or even a revolution. This of course isn't certain, because it may well turn out that resolving the microscopic domain with large scale effects could be beyond mathematics/science or inherently unresolvable. (especially with deterministic laws which are obviously contrary to the stochastic nature of Quantum mechanics).

Quantum theory is extremely accurate and reproducible, even though its description of the microscopic world is counter intuitive and insane (I will add that Quantum mechanics is not an exclusive theory of the microscopic world, but applies to the Universe as a whole and to large objects such as planets and stars and galaxies - the quantum effects are not noticeable at these scales and are swamped by other forces and more dominant effects).

The quest continues ladies and gentlemen - onward we go riding upon our grand silver horses like those that came before us.

[speach (Simon)]

Originally Posted by sjastro

Nice strawman argument.
Exactly where in this discussion did I suggest that the Universe was expanding into existing space?

Stop beating around the bush and answer my question.
How do you conclude a universe that is expanding all the time cannot have a finite dimension.

The part of the universe that has expanded can and does have a dimension, but it's expanding so this dimension is continually changing. Lets take the analogy, that cosmologists are so fond of, a balloon. As the balloon expands it does so into nothing, but nothing is already part of the whole to allow it to be expanded into. So the conclusion must be that the emptiness it's expanding into is part of the universe.
to the confusing world of mind experiments

[sjastro]

Originally Posted by speach

The part of the universe that has expanded can and does have a dimension, but it's expanding so this dimension is continually changing. Lets take the analogy, that cosmologists are so fond of, a balloon. As the balloon expands it does so into nothing, but nothing is already part of the whole to allow it to be expanded into. So the conclusion must be that the emptiness it's expanding into is part of the universe.
to the confusing world of mind experiments

This is wrong on many counts.

Firstly it is incorrect to assert that only part of the Universe has expanded.
The entire Universe both observable and unobservable expands.
A distant galaxy that passes over the particle horizon from the observable to unobservable Universe doesn't stop receding.

Secondly you do not understand the meaning of dimension in the context of space. Dimension isn't some measured quantity but represents the degrees of freedom an object can move in space.
Since we live in what is essentially 3 dimensional Euclidean space, you are confined to 3 degrees of freedom in the "up-down", "front-back" and "sideways" directions.
Expansion changes the scale factor of space not its dimension.

Thirdly analogies are limited in their descriptions of Cosmology.
The flaw in your example is that you seem to assume this is a 3 dimensional Universe. The surface of the balloon is the Universe in this case and the observers are two dimensional whose degree of freedom is confined to the surface. The observers do not perceive movement in the third dimension namely the radial expansion of the balloon.
Hence it is meaningless to refer to the expansion of the balloon into this "nothingness".

Fourthly the idea that "nothing is already part of the whole to allow it to be expanded into" as a condition for the entire Universe being infinite is not only illogical but fails mathematically. As has already been alluded to there is nothing preventing the entire Universe being bounded and finite.
While cosmologists have measured the local curvature of the observable Universe to be zero and therefore flat, the global curvature of the entire Universe could very well be a torus!!
A torus shaped Universe is topologically flat, bounded and finite.

[Eratosthenes (Peter)]

How is a finite torus model of the Universe, bounded?

Unboundedness implies there is "no end" (or no edge)

The surface of a sphere is a finite but unbounded 2D surface. It is unbounded because it does not have an 'edge'. Traveling along the surface goes on forever. You will pass over your starting point over and over again.

One model of the universe is a 'finite but unbounded' 4-dimensional space-time geometry

[sjastro]

Originally Posted by Eratosthenes

How is a finite torus model of the Universe, bounded?

Unboundedness implies there is "no end" (or no edge)

The surface of a sphere is a finite but unbounded 2D surface. It is unbounded because it does not have an 'edge'. Traveling along the surface goes on forever. You will pass over your starting point over and over again.

One model of the universe is a 'finite but unbounded' 4-dimensional space-time geometry

Yes you are correct the surface of a torus is unbounded but a 3-torus is a closed manifold and therefore forms a bounded metric space ie. a finite Universe.

Steven