What is outside the boundary of the expanding universe?

[beatlemaniacj]

For something to expand, It must have tangible boundaries that are growing. The ever expanding universe, thus must have boundaries. What is outside these bounds?

 

[Mark M]

For something to expand, It must have tangible boundaries that are growing. The ever expanding universe, thus must have boundaries. What is outside these bounds?

This is a common misconception about the universe, it doesn't actually expand into anything. Firstly, the universe is unbounded, whether is finite, or infinite. If it is infinite, then matters are very simple - the universe expands from every point, at every point, end of story. If the universe is finite, however, you must understand it does not have a boundary. Imagine the two dimensional surface on the surface of a sphere. Two dimensional beings will say that their universe repeats after a certain amount of time. You can now imagine this sphere grows, and the beings see space as expanding - the distance between every point in space increases. You may be confused by the fact that there is also a 3rd dimension in this example, and that this means the universe must be embedded in a 4-dimensional space. But it doesn't. Remember, the universe is the totality of all spacetime, it does not exist within a space, it is space.

Also, the universe is isotropic and homogeneous on large scales. If you looked in all directions, everything looks essentially the same. This is because the big bang was not an explosion, it occurred everywhere.

To further clear up any misconceptions, you should read this article:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf