woz said:
I have read (I don't remember where) that the expansion of the universe is not as an explosion, what happens is that the space itself is expanding.
Can someone please make this clear to me and explain exactly who this expansion works?
Probably not. I'm not sure the human mind can visualize an expanding infinity with no external reference frame.
But what we do see is the galaxies in the universe getting farther apart from each other...not in any preferred direction and not just because of their own proper motion. Instead, more space is appearing in between these galaxies (which is why its said space is stretching/expanding...even though that wording has the incorrect connotation of an expansion into something...our common experience just doesn't cover the weird situation of the universe).
There are some analogies which can help. They're imperfect, but they help.
One is the "balloon analogy". Imagine yourself as a 2D (flat) creature on the surface of a balloon. As a 2D creature, you can't see up or down...only left or right in space. You can travel in any direction and never find a center or edge to the balloon. The balloon material represents 3D space of the universe. As the balloon expands, you see points on the balloon getting farther apart...and no particular point is at the center. One of the imperfections in this analogy is that you immediately picture a 3D balloon expanding into the air of the larger room its in. But you need to force yourself to understand there is no outside or inside to the balloon.
The other is the "raison bread analogy". Imagine a rising loaf of raison bread. The bread represents 3D space and the raisons represent galaxies. As it rises, the raisons move further apart from each other with no preferred center or edge. This analogy has a similar imperfection as the other. Here, you need to imagine that the bread is infinitely large (with nothing exterior to it).
Where is all this extra space coming from? Good question. Maybe someone else can take a stab at that. But since the universe is made of "spacetime" (not just "space")...it may be that the full extent is already in existence in spacetime and we're just observing it's current phase as we move through spacetime one second at a time (although we can move freely in 3D space, we're limited to a one-way timeline).
Of course, there are speculations about other dimensionalities to the cosmos which does allow for our universe to expand relative to something else...but there's no well supported theory based on the available evidence.
