How likley is it that our universe is one of many others?

100%. Google or youtube "M Theory" it is part of "String Theory" and it real. To put it in a nutshell. Our entire universe is actually a vibrating membrane, the vibrations are what makes matter. You can see this in the 10th dimension. Now in the 11th dimension you will find and infinite number of vibrating membranes, some like ours some very different. All of these membranes are other universes. A coalition of two or more membranes is what caused our big bang and our universe split off to form its own membrane.

That's complicated. Many science fiction writers love making new universes, and saying they all live in a universe cluster. But what is a universe exactly? Maybe our 'universe' is really a cluster of universes and there's a division in between that we haven't seen? Maybe another universe exists where protozoans evolve and collapse in a matter of seconds? Theoretically speaking, there is only one universe, and that's ours. The reason is better understood in an example.

Suppose we've got a closed system (that means nothing goes in or out of it), and there's already a bunch of bubbles in there. However, these bubbles are much more durable and harder to pop. Now, let's say that bubble U is our universe, and U is currently expanding. This means that U's outsides will be pushing against the other bubbles, and causing them to pop, while U continues to grow bigger. At some point, U will push towards the edges of the closed system, and all the bubbles will be gone. That is possibly occurring in our universe, right? Maybe we're 'murdering' other universes by expanding into them?

Wrong! Totally incorrect, baloney! Theory suggests that during the universe's birth, there was already no 'space.' So when the universe exploded in the Big Bang, it created space. Which means that there are no other universes.

I note someone is still pleading that the meaning of a word defines reality. The fact that "universe" until now has meant all that there is doesn't have any bearing on reality. It merely means we may have to change our definition of universe. It is a bit like saying unicorns must really exist because we have a word for them! According to any of several Multiverse theories, there are other universes, though none of them state there has to be an infinite number of them. We simply have no way of knowing. However, there has been very recent work done which suggests those of us researching Multiverse possibilities may well be on the right track. Some otherwise unexplained phenomena in our own universe are accounted for, such as the apparent weakness of gravity, and the fact that gravity is a scalar field. M Theory suggests it might just be showing us the first indications of a theory of quantum gravity which in turn may lead to a Theory of Everything.

According to the multiverse-theory, there are an infinite amount of universes. If this theory is correct, then the chance of our universe being not the only one, is 100%.

Sadly enough, physics hasn't advanced to the point where we can confirm this theory.

As a reaction to gethin's post, the universe does include everything.

But, it also includes "space" and "time". This would mean that if another universe existed, it would have another "space" and another "time". So, two universes can coexists, without ever being aware of eachother, because the other universe is not within our spacetime.

It is extremely highly not likely, sorry. :) There was already an ultra dimension that existed before the universe was created but not other universes. :)

100% likely. And there would be many others...some quite similar to our own, some completely different. As to how many - well, how many variations could you think of? At this time, the number seems infinite.

It seems the key would be fourth-dimensional math.

And for fun (but good philosophical) reading, I would suggest the book "One", by Richard Bach.

There may be, or there may not be other universes, Stephen Hawking said that there might be other universes, that were created with there own big bang.

very unlikely since the universe includes everything, so if there was another universe it would not actually be another universe, it would just be a part of this universe. And plus the universe is constantly expanding :)

There's infinite universes with infinite people with infinite people replying to this question and reading this right now.

<QUOTE>How likley is it that our universe is one of many others?</QUOTE>

Just as likely as that it's the only one. That's because we don't know either way, just like you don't know whether it's morning, afternoon or night where I'm writing from.