QUOTE (danielost @ Feb 16 2008, 12:12 AM)

By our current thinking yes.
But what I am saying is there is Just one universe. What we call the universe, lets make it simple, think of the milky way has a solar system and the universe(what we call the universe) as a galaxy. That would then mean that there are billions of these super galaxies in the single universe. Even at the speed of light it would be impossible to travel between them.
You do hit on an interesting discussion. Notwithstanding the semantics of 'universe' (which should really encapsulate all of existence, if there is more than what we currently see as 'the universe'), there are various 'directions' in which we might find more universes.
As you point out, one possibility is that this universe we're in is much, much bigger than we currently think. So in fact the detectable universe as far as we're concerned would be just a tiny corner of a vast, perhaps infinite, universe. Trouble is with that idea, is that it requires much of conventional scientific theory to be wrong (perhaps not terribly, maybe just a bit, but it does go against current data).
Within the theory of quantum mechanics, the Everett interpretation (also called the 'many worlds' interpretation) of quantum mechanics may imply that there is another universe for every possible state of every sub atomic particle in this universe: it is known that subatomic particles exist in a state of superposition which follows the shape of a probability wave until measured. Conventional theory is that when measured, the particle/wave breaks down and all other 'ghost particles' which made up the wave cease to exist; but the Everett interpretation of the same data postulates that each of the ghost particles becomes a real particle in another universe, which would be identical to our own but for the position of that one particle.
Or it could be that there have been many universes similar to our own before ours, and perhaps after too (though 'before' and 'after' are highly questionable concepts in that context) - one formulation of this is the idea of the 'big bounce', in which the universe begins with a big bang, expands, decelerates and finally collapses in on itself in a 'big crunch', and then bounces back into another big bang, in an infinite cycle.
Another idea is the 'black holes and baby universes' postulation, which would imply that 'inside' every black hole is a godawful mess which has ripped space a new timehole, in which is born a new 'baby universe' from a big bang caused by the stresses of the formation of the singularity.
Yet another is the thought that this universe exists in some sort of 'metaverse', as some effect of the phyics contained in there. An example of this type of thinking is M-theory, which formulates this universe as simply a random and insignificant bubble caused by the collision of two infinite membranes ('branes', to use the jargon) in a vast ocean of similar branes. So there would be as many other universes as there are bubbles, all contained within one main reality.
The trouble is with all of these ideas is that they are completely untestable by today's technology, and so remain speculative (one or two are not testable even in principle). But there are plus-sides to the theories too. For instance, to help explain why we find ourselves in a universe which is capable of sustaining life. Any one of these ideas could imply the existence of an infinite number of other universes, each of which has a different set of laws. The vast majority of other universes would be cold, empty and lifeless, but we would find ourselves, of course, in one of the ones which can sustain life.