In Scientific American, I have read about the concept of multiple universes existing.  If our universe occupies a spherical or elliptical three dimensional space and another universe began several million light years away with a Big Bang, would our two universes collide?  What would happen in such a collision?  How can universes exist in other dimensions that occupy space that is different from the space in our universe?  Is there a term that encompasses ALL universes in the infinity of space?  Would universes have gravity and pull on other universes, and cause orbits and rotation like planets and solar systems and galaxies in our universe?  How far away are these other universes?  Can we even test if this theory is true?  How could physical constants be different in other universes?
211452
When two universes colide, the time difference and the size of each universe is so great that we should not feel any difference except a slight increase of radiation from other stars that were previously not in our universe. Universes should exist in the other dimensions as superstrings, which are smaller than quarks, have exibited ability to go to 16 other dimensions other than our own.
610241
The concept of universe is associated with matter expanding in space . The same space can be occupied by anti matter as well . So space and time quantum are across the boared on the universe and even far beyond that .
332634
Wow, what great questions.  Just so you know, we don't know the answer to what was before the big bang.  One idea is that our universe expands, then contracts on itself, over billions of years, and the whole big bang repeats over.  In other words countless universes preceded our own.
562458
According to String Theory (M Theory being one of them), there exist a megaverse, which contains all existing universes. There is nothing outside of this megaverse. The megaverse has many higher spatial dimensions (how many exactly? right now I think it's 10). These higher spatial dimensions (i.e. 4th or higher) can be either curled up infinitesimally small (compactified), or very large like our 3 dimensional space. Our universe is just a tiny bubble inside this megaverse. Some String Theories say that our universe is a 3-brane attached to some other high dimensional brane, and that it's the collisions between these higher dimensional branes that creates the big bang and thus our universe. But there are other String Theories that say our universe were created perhaps via nucleation (the instantaneous formation of a bubble of 3-dimensional space), and because of its compactified higher spatial dimensional configuration, it was unstable and had alot of energy, which was instantly released (aka big bag) and also expanded exponentially the 3-dimensional space of the bubble.\n\nThe fact is we currently do not know exactly how our universe, or this megaverse came to being. We have many theories, but no definitively answers. But we are certain that there was a big bang. All indication points to the existence of higher dimensions, but no definitive proof yet. One thing is for sure, we exist in a very strange place indeed. Much of its secrets lie hidden and outside of our common perception. Very soon (within the next 5 years) we will begin to see some experimental results from the LHC (large hadron collider) that will likely provide some answers regarding the existence of higher dimensions.\n\nAs for your question about time, we also don't really understand what it is. But it is perhaps meaningless to ask whether time existed "before" some event like the big bang, since time is how you define events in the first place. All we know is that space and time is the fabric of existence. To say that anything exists is to say that it occupies some point in space at a particular time. So "before" the universe (or megaverse), before there was space and time, nothing existed period. There was no existence of any kind.
175080
Outer space is not nothing. Even without the presence of matter, it is teeming with quantum foam, particle-antiparticle pairs, space, and time. For all we know, spacetime may not exist outside our universe."\nAlso, the universe may not be bounded. The only way to "exit" may be to find a "fourth direction". Ie., think of a point in an unbounded plane. The only way for the point to exit its universe is to somehow find the "third direction".
77343
The above answers are interesting but, modern physicists speak extensively about 'multiverses'.  So, does one universe end at the edge of another? or, do they 'co-exist' in space and time? OR??????
429908
I am going to put a slightly spin on this question. I DO believe that there are parallel universes. There are at least as many universes as there are people and possibly as many universes as there are living things. We each experience a different universe, but they all exist at the same time. Our "Universe" is what we experience (see, hear, smell, taste, feel, and any other undocumented sense). THAT is where parallel universes come into their own, and THAT is where relativity comes into play. My universe starts with me where I am and ends "out there somewhere". I will never get to experience the end, because wherever I am will always be the start of my universe.
331443
By definition, the universe is a singular entity and no other exists except in theory (ex. Inter-dimensional universes, sub atomic universes and anti-matter universes). If we consider the universe as finite instead of infinite, then we can roughly conclude that it is spherical in shape, and the outermost bodies are roughly equidistant from the center. \n  In Einstein’s theory of the Time and Space continuum, Time and Space are cylinders and the intersection of two cylinders is a sphere. as the intersection of the cylinders change, so does the location of that sphere, It can be see as moving through time, or moving through space, or moving through time and space simultaneously.\n    Not all possible points of time and space are contained with that sphere at one time, but the sphere can pass through them and we move through time and space. Once a point has passed though our sphere it is gone.\n\n   UNLESS the cylinders are actually rings. The intersection of the two rings is also a sphere. In this case, a point can pass through our sphere several times as we are in constant any yet synchronous motion with the other ring. So if our universe is actually a sphere, and the intersection of circular cylinders (rings), galaxy will always be a galaxy, but it can come into existence, die and be reborn all within it's own space it passes through our sphere of existence over and over again.\n\nSorry...this was probably more than you wanted. It's possible, yes, but not likely in this sphere of reality.
425614
You have gotten some great answers up to this point.  I'd like to add yet another one...the pre big bang scenario re string/M theory.\n\nOne spin off of string/M theory is that parallel null universes are feasible.  Brian Greene pictures them as slices of bread in a cosmic, N-dimensional loaf of bread. [See source.]\n\nThese are "null" universes because, as far as we are concerned, from our myopic four dimensional world (the metric, observable universe one answerer talked about), these universes exist only in higher dimensions and cannot be observed in 4D.\n\nAccording to one hypothesis, based on string/M theory, two of these slices of bread collided.  That collision point is called the singularity where the big bang originated.  The energy such a collision could generate is one way to account for all the mass-energy postulated for our own known universe.\n\nOf course, as pointed out by other answerers, time and other physics phenomena in null universes may not exist as we know them in metric space.  Null universes are beyond our ken to understand and explain...now.  But who knows, with people like Hawkings, Witten, and others attacking the cosmos and its mysteries, we might just yet unlock the nature and existence of null universes and our own.\n\nPS:  Not all explosions are chemical explosions requiring air (oxygen) to work.  The atomic bomb, for example, does a really nice explosion by simply splitting atoms.   The sun makes spectacular explosions by fusing (putting them together) atoms.  Bottom line, the big bang is not a chemical explosion requiring oxygen.
6992
There have been alternative theories throughout history.  I believe most astronomers now accept the Big Bang theory as best fitting the known facts. Your question about a multiplicity of Big Bangfs is a good one: perhaps the biggest cosmological debate about the nature of the universe is whether the universe will expand forever, or whether it will ultimately contract again into a state similar to just prior to the Big Bang.  Of those who think it will contract, many believe that it then will repeat a Big Bang process, so that the universe will proceed on a Bang, expansion, contraction, Bang  . . . pattern forever.\n\nI've included a link to one website that provides answers to basic astronomical questions, in a range from beginning to more advanced.
659685
This "nothing" is what we perceive without the laws of physics. Our universe is limited by the knowledge we have about it. Recent theories (the "M" theory) suggest that endless universes exist, and sometimes they even collide one into other. Probably, out there, beyond the boundaries of 12 billions light years, others dimensions, other plan of existence exist.
429127
The answer is that it really isn't expanding into anything.  If you're confused, it's a kind of confusing idea.  Also, remember that almost by definition the universe is everything that exists, so to some extent it's meaningless to talk about what's "outside" the universe (of course, that stops no one from wondering... ;) ).\n\nOf course, that doesn't really help you much, so there's two examples, one of which is a really common one which leads to a misleading answer and one is the one I use when thinking of this issue which gives you a better idea.  The first and common one is to imagine/pretend that the universe is two-dimensional (e.g. you can move left or right or forward or backward, but up and down have no meaning), and specifically that the universe is the surface of a balloon.  Now, if put little stickers with galaxies on them on the balloon, and measure the distance between these stickers with a tape measure stretched along the surface of the balloon (the tape measure can't go through the balloon because the area inside the balloon isn't a part of our 2D universe) you have a universe.  Now blow into the balloon, causing it to expand.  The stickers on the balloons surface will stay attached to the points they were attached to, and the part of the balloon between all the stickers will expand, so that if you again take out your tape measure and measure the distances between the stickers, the distances will have increased.  This gives you an idea of how the universe (the 2D surface of the balloon) can be expanding.  However, in this case you can go, "Ah-ha!, but the balloon is expanding into the air", which makes you think that the real universe must be expanding into something.  This may even be part of why you asked the question.  But, as I said first, the universe basically by definition contains everything, so we have a contradiction.\n\nThis brings me to my second example, and the one I use.  Imagine that the universe is a two dimensional surface made of square bricks.  This surface is either infinite or has some chape such that it has no edges (it might be a sphere or what not).  Now, define the width of one of these bricks as some unit of length (a mile, a light-year, a million light-years, an inch, whatever).  Let's say one brick has a width of one light-year for the purpose of this discussion (but also note that you could make it correspond to, say, one billionth the width of an atomic nucleus or any other very, very small distance).  Now, let me again stress that a brick is one light-year wide (and hence has a square area of one square light-year or one light-year squared) *by definition*, and thus a brick in this example is *always* this size.\n\nNow, with that in mind, you could imagine putting galaxy stickers on random bricks (yes, I know a galaxy is thousands of light-years wide, but bear with me).  If you pick two such stickers, and count the number of bricks between them you get their distance, because each brick is one light-year wide, the distance between them.  Now, you let time pass, and as time passes rows and columns of bricks are inserted between existing rows and columns (and everytime you insert a row you also must insert a column and vice versa) in some mechanical, predicatable fashion.  Now, after some amount of time of this, you go back to the two stickers you picked and count the number of bricks between them, and you'll have found that the number has gone up, so that the distance between them has increased (because each block is defined as being one light year wide).  Now the stcikers never moved, they were always on the same brick, it's just that there are, as time passes, more bricks separating them.\n\nIf this seems like a hokey example, remember that we could make the bricks any size we like, including smaller than any particle or atom or whatever.  So, we change the example to make the bricks of a really, really small si
153473
from what scientists gather, the universe appears to be symmetrical on all sides when viewed from earth. this conclusion was obtained when they found out that cosmic radiation was equal and never varied more than one in ten million parts when measured from any direction in space. \n\nAlso, the distances to the stars in all directions appear to be more or less equal. So that makes the universe symmetrical from earth's perspective.\n\nI dont think we'll ever reach the edge of the universe, becoz u would hav to start travelling there at the start of the big bang, and nothing travels faster than light.\n\nnow try to answer my question on time.
114017
The universe is said to be expanding,at least in the sense\nthat all of the galaxies are supposed to be drifting farther\naway from each other.That would seem to contradict your notion of a tremendously big circle as there's no gravitational pull binding them together.\n Also,if space is finite...then what's outside the boundaries?
327338
well, spacetime does not seem to end, and nothing is "holding up" spacetime.\n\nthe universe is not infinite. the universe is finite yet has no edge and no center. it may be something like the two dimensional surface of a sphere. nothing inside or outside the sphere exists for the two dimensional surface.\n\nalso, i have something for you to think about. the radius of observable universe is about 46 billion lightyears, yet the universe is 13.7 billion years old.
17930
Even though big bang theory suggests the origin of universe from singularity, there are many questions unanswered. There are theories that suggest that big bang is not the starting point but part of a cyclical process of several big bangs and collapes happening from time to time. The concept and understanding of infinity, multiverse and timelessness are beyond our comprehension and so looks complicated. I believe in multiverse with its own space-time. String theory explains it more. I would think of universe as being a big bubble with its own space and time and when collided with another bubble(another universe) would create chaos (or may be not) and eventually form a new universe. And this might be taking place with several universes at different levels(dimensions). This might or might not be true, but just wanted to throw my thought. Additional comments/thoughts are always welcome.
71073
The whole theory of the multiverse is based on the premise that every possible outcome lies in a different universe...so some actions may be replicated to a point but the outcomes would all be different by definition.