Plotting the expansion of the Universe Question? 
Hi, I'm a first year physics student. Today we were given a picture like this showing space expanding vs time. Now, i have just added onto this picture the path of where light could have come from that was hitting an object on the centre line ( i.e. one where it is stationary and everything is expanding away from it).
You can see how, in this picture, if one were to draw the path of where light, that is hitting the centre object at the present moment, could have come from, it would eventually hit the edge, but that would mean we would be able to see light from objects at the edge of the universe although from sometime in the past. Obviously, I know this is not what actually happens as there is a limit to the observable universe.
How does it work that we cannot see the edge ( specifically if you could relate it to how this way of picturing it is wrong would be useful)? Thanks,
 A: You make a few assumptions that need to be addressed:
1) You assume that the universe has an edge to begin with. This is not necessarily true; current measurements actually suggest the universe is indistinguishable from being infinite. To see why this is still compatible with the universe expanding, picture an infinite sheet of squares, like a sheet of graph paper. As time passes, all of the squares on the sheet get bigger, so that the distance between any of their corners increases. This is essentially what would happen in an infinite expanding universe. Likewise, as you run time in reverse, the squares shrink; the universe is thought to have started from a state where the squares had essentially zero area (again, since there are an infinite number of them, the sheet is still infinite, just with a much higher density of squares), or at least an area close enough to zero that we cannot with current measurements or theory distinguish it from zero.
2) Supposing that the universe is finite, you assume that light would "catch up" eventually to the edge of the universe. This may not necessarily happen; if space expands faster than light can cover the distance to the edge, then light will never "catch up" with the edge. Considering that current measurements indicate that the expansion of the universe is accelerating, and we have not yet found any kind of edge, it seems at the moment quite likely that the universe is bigger than the 13.7-billion-lightyear sphere that we can currently see, and may be bigger forever.
