To bring you up to speed, this is a theory I developed to cater to the individuals who believe the expanding universe has some variety of walls or boundaries and possibly explain the origin and collapse of a limited limited universe. Most people believe the universe to be infinite, but without any genuine proof of this, there is no reason to rule out the possibility of limited universal space.
The first part of this theory (A) embodies the belief in spherical bodies in space and (Newton's) gravity, but the second part (B) has been revised to cover the possibility of flat objects in space and an absence of gravity. Both theories suggest multiple "big bangs".
Article A : Limited Universal Expansion with Traditional Gravity
figure 1:A - Following the "big bang", all undeveloped matter, used to forge the universe, is thrown outwards to gradually take shape. With the absence of friction, the contents will continue to move outwards, spreading out over a projected period of time. The universe itself is not expanding, but its contents are moving further and further way from the hypothetical nucleus; the illusion of expansion is created.
figure 2:A - The contents of the universe collide, at maximum velocity, with the outer walls of the universe, literally bouncing off and being forced back to the hypothetical nucleus.
figure 3:A - Matter, reflected from collision with the outer walls, collapses on itself, being reduced to a singularity and attracting an unmeasurable gravitational force.
figure 4:A - The process begins, once again, with an other "big bang" and the creation of a new universe.
Article B : Limited Universal Expansion with a Flat Earth
figure 1:B - Perhaps the universe looks like this? A universe containing a flat earth would have no need to be spherical, as all matter appears to be moving upwards. In such a case, all planets and bodies in space may have, at one time, originated from a single plate on the floor of the universe, broken apart and thrown upwards by what we know as the big bang.
figure 2:B - The contents of the trapezoid universe crash into the outer wall, fusing together in preparation for the creation of a new universe. The process repeats infinitely as the indestructible energy and subatomic matter majestically volleys back and forth.
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We will never truly know whether or not the universe is infinite, but if there are walls, they will eventually be reached.
Food for thought.