Quote:
Originally Posted by HarryT
It's everything that exists in our four-dimensional spacetime, which is all that we can perceive. However, many modern theories postulate high-dimensional spaces within which our universe is merely one of many, and each of which could have very different physical laws. Eg, a minor change in Planck's constant, or the gravitational constant, or the charge on the electron would make life as it exists in our universe impossible. But that, of course, is merely a restatement of the "anthropic principle" which answers the question "why is our universe the way it is" with the answer "because if it were different we wouldn't be here to observe it".
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I understand that, but that's talking about other possible ways this universe could have turned out. That doesn't mean there ARE other universes...
and no it's NOT limited to 4 dimensions as a definition of the universe. That is another unknown speculation at the moment, certainly String theory requires up to 12 dimensions, but is still a theory of the (i.e. this) universe.