Visible universe only a small part of the whole universe

The rapid expansion idea comes from the fact that what we see of the Universe appears fairly homogenous or uniform. Thus the theory goes that sometime in the initial phase of the Big Bang there was an intermediate phase of rapid expansion which took a relatively small portion of the big bang substance and expanded this by an exteremly large factor ( I dont remember what the factor is but certainly its greater than 10 to the power of 20 also this was not in the first few seconds but sometime like during the first ten to the power of minus 10 or less- some one please correct my figures).

Apart from the 'Visible Horizon' idea, it is used to explain the idea that if we look in one direction and see a particular composition in a large area of the Universe, then looking in the 'opposite' direction we see a broadly similar composition. In effect this means that any non-homogenity at the two extremes of our visible Universe has had 'time' to communicate and with the other extreme and effectively level out any imbalance. Thus it is possible that there are additional parts beyond our visible Universe that exist and MAY be different to what we can acually observe.

The Visible Horizon idea links from our general ideas of Cosmic expansion based as you know on observations correlating Red Shift and the periods of Cephid Varibles ( basically the further away you are from the point of observation the greater the apparent relative exapnsion you observe).

Hence there is a limiting distance from here to there which occurs when the relative velocity of observed expansion is 'c' the speed of light - which comes from Special Relativity. Thus the 'Boundary' of what we see from here is limited to a distance based the age of the Universe times the speed of light ( in light years). This means that we can in theory see back to that phase of the Big Bang when matter condensed from light. It also means that due to the Length Contraction effect from Special Relativity the apparent thickness of the boundary of what we see is infinitessmly small ie a shell of zero thickness.

In reply to pmgisme,

Yes it is a puzzle.

If you start with a blank graph. Take the verticle axis to be 'Observed Properties' and the horizontal to be 'Time'.

Then add two points 'Origin' at 0,0 and another point in the first
quadrant representing 'Now'.

There are obviously an infinite number of different ways to draw a path joining the two points - in fact there are infinitely more paths which can be drawn by travelling into that portion of the graph which exists in the future and then travels backwards to arrive eventually at Now.
But ignore these and concentrate on those paths limited by the rectangle whose width is just Origin to Now on the x axis, but whose height in the y axis is then 0 to Infinite.

Now the simplest path in this rectangle is just a straight line joining Origin to Now. The next would be a simple curve following an Exponential curve next a curve following a quadratic or higher order path. The main properties being that these are continuous in the allowed region and they are 'easy' to define and for someone like Euclid would be 'pure'. However there are also an infinitely greater number of paths with discontinuities in the same rectangle, indeed these include paths which are simply discrete points chosen at random in the rectangle. Then the balance of probability is that development of the Universe may have contained some discontinous parts ( equivalent to expansion) than those which were continous curves.

The above is just my own concept - I am amazed at the BS which I can conjure up sometimes!