So here is one natural notion of the universe: all of three-dimensional space at the present time. Thus, we could make the assumption that we can locate anything in the universe using three Cartesian coordinates: at this frozen moment in time that we call the present, every object occupies a certain x, y and z in our three-dimensional continuum. If we take this route, we may first notice that space appears to us to be three-dimensional. Leaving aside the issue of whether "now" can have a universal meaning-and the even subtler ontological question of what it means to exist-it makes sense to think of the totality of space and all of its contents at the present time, and to imagine this totality as a contiguous entity. If we follow this line of thought, the first thing we notice is that the present tense of the verb “to exist” implicitly assumes that we are referring to “everything that exists now.” In colloquial English, the word is often taken to mean “everything that exists.” So this intuitive notion of universe seems like a good place to start. The subtlety is that the word “universe” has different meanings in different contexts. But all of them are true, or at least plausible.
Statements like these appear quite frequently in popular science magazines-including Scientific American-and they seem to be in utter contradiction with one another. Or a hall of mirrors, shaped like soccer ball. The universe is 84 billion light-years wide.
The universe has nine, or ten or eleven dimensions. The universe is four-dimensional-three for space, one for time.
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