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Eha, I get the impression that you are the kind of person who is asking questions and trying to understand the regular science version. There are some mental stretches to make, you'll probably find it helpful here if you keep asking, at least I hope so.

 Quote by eha ... with inside of baloon is empty which represent the past, outside is empty and represent the future, just surface area of baloon holds all [space, and all] matter, galaxies, stars and everything else. That is our universe, spherical, but inside is empty. I was thinking of what part of the surface area represent our observable universe, it should be 14Gly in radius?
The balloon model is just a 2D analogy for 3D space. To use the analogy you have to think of space as 2D and whatever stars galaxies creatures as approximately flat 2D things. Nothing inside or outside the balloon, just the pure surface.

It is possible that our 3D space is infinite and to include that possibility with the balloon analogy (it's hard but you have to) think of the balloon as really huge, but still expanding.
So huge that to a creature on the surface it seems perfectly flat, but still expanding. Effectively like an infinite flat sheet of rubber which even though it is already infinite is still expanding. I know that's hard to understand, but it is how you include the infinite space possibility in the balloon analogy.

All expansion means is that distances between stationary points increase according to Hubble law. It doesnt mean there is any "outside" that space expands out "into". Expansion is measured internally by the record of increased distances.

You are right about the observable being a round patch on the balloon. The present radius is 45 Gly. That is the current radius---what it is now. If you could freeze expansion it would take 45 billion years for light to travel it.

Because distances expand light that has come along way is always farther from its source than it could have traveled by itself in a non-expanding universe. Light now arriving as the microwave background has been traveling for 14 billion years, but the presentday distance to the source matter is 45 billion lightyears.

To get some hands-on experience with the standard model cosmos, google "wright calculator" and type in 1090 for the redshift. That is the redshift for the CMB (cosmic micrtowave background) radiation. The calculator will tell you the current distance of 45 Gly and how long the light has been traveling, 14 Gy. It helps to keep the two things straight.

 Then I heard universe should be in flat-open shape like sheet of paper according to latest critical density findings. Now i'm trying to imagine how is this flat universe expanding?what is the size of it? Hubble constant should be still valid but how does it work?
Well flat with infinite volume is one possibility consistent with the WMAP data. But all they said was nearly flat. The data allow a generous possibility that space is finite, just very large. The most recent authoritative report on this was the March 2008 WMAP report by Komatsu et al, and they said that if the universe is spatially a hypersphere (like the balloon except 3D instead of 2D) then the circumference is, with 95 percent confidence, at least 600 billion light years (that is, current presentday distance---if you could freeze expansion then light would take 600 Gy to travel all the way around.)

If you want a link to the technical report, let me know.

So if someone tells you the U is spatially flat, infinite volume, do not believe them. We don't know. The issue really is not settled yet! But the successor to the WMAP spacecraft was launched in May, its the Planck mission. It will refine the curvature estimate and may help settle the issue.