Structure of the observable universe for dummies.

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My astronomy course and my own investigations into the subject have raised a question in my mind that refuses to die out. For various reasons, I don't trust my instructor to adequately answer my question in layman's or other terms. My instructor comes off as an intelligent woman but is poorly equipped to explain scientific concepts to someone who is thoroughly right brained rather than left brained. My text does not shed any light on the subject either.

The model used by our instructor and the text for us to imagine the growth of the universe since the Big Bang is that of an inflating balloon with ants on the surface, representing galaxy clusters. As the universe (balloon) expands the ants are increasingly separated by the ever increasing surface area. My question is: keeping this layman's terms model in mind, is the universe hollow? I.e. according to the latest observations are all of the galaxies more or less on the same plane of this imaginary balloon with the center and everything below the "surface" hollow or are there "ants" / galaxies present throughout the structure and not just on the "surface"?

Obviously nothing is truly flat depending on the scale you are looking at it but when you look the grand scale, is our universe a filled sphere or a hollow bubble?

[General Zod]

Depending on what studies you prefer, the universe could very well be horn shaped.

[Surlethe]

In terms of simplistic models, think about rising bread dough with raisins inside. As the dough rises, the raisins get further and further apart.

[R. U. Serious]

My question is: keeping this layman's terms model in mind, is the universe hollow? I.e. according to the latest observations are all of the galaxies more or less on the same plane of this imaginary balloon with the center and everything below the "surface" hollow or are there "ants" / galaxies present throughout the structure and not just on the "surface"?

No, that's not part of the "intended metaphor". While the universe is sometimes called "flat", it means something else in that context, and is unrelated to the metaphor:
http://www.astro.ucla.edu/~wright/cosmo ... .html#FLAT

I think you'll find that the FAQ will answer a lot of the questions you mgiht have about an expanding universe. Your misconception about the balloon metaphor is touched upon here:
http://www.astro.ucla.edu/~wright/balloon0.html
and here:
http://www.astro.ucla.edu/~wright/cosmo ... q.html#XIN

[TheLemur]

is our universe a filled sphere or a hollow bubble?

According to general relativity, it is both possible to have a universe that is finite and unbounded (like a balloon's surface) and infinite and unbounded (like an infinite sheet of rubber). It is not possible to actually have a continuous "edge of the universe". The structure of the universe on a large scale is determined by the geometrical structure of spacetime, which is in turn determined by how much mass there is and any other basic properties of space.

I.e. according to the latest observations are all of the galaxies more or less on the same plane of this imaginary balloon with the center and everything below the "surface" hollow or are there "ants" / galaxies present throughout the structure and not just on the "surface"?

The "hollowness" of the "balloon" would only be apparent in a higher dimension; within three dimensions, all we can notice is that if you go in a straight line you return to your starting point. Essentially, the surface of the balloon is 3D, while "in" and "out" is another dimension. The "bread" model is three-dimensional; no higher dimensions are involved.

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So essentially if one were to take a cross section of the observable universe today, it would indeed be 3 dimensional on the grand scale rather than a shell and if you were to come back a few billion years later and take another cross section, it would still be 3 dimensional but the distance between galaxy clusters would have increased.

[kheegster]

So essentially if one were to take a cross section of the observable universe today, it would indeed be 3 dimensional on the grand scale rather than a shell and if you were to come back a few billion years later and take another cross section, it would still be 3 dimensional but the distance between galaxy clusters would have increased.

Yes. But you should realise that the distance between them is increasing not just because they are moving away from each other, but because space itself is stretching. Hence the idea of the homogeneous and isotropic universe.