All the data we have at this point says that the universe that is just under 13 billion years old. The universe exploded from a big bang and after a brief period of inflation, it has been expanding steadily at the speed of light. Although there are still some unanswered details, most cosmologists agree on these basic points. From here, however, there are numerous opinions and conflicting theories on the basic nature of the universe.
There are a few different mathematical models that show it’s possible that we can be living in an “open” universe which will continue to exponentially expand forever, a “flat” universe where the expansion will slow down and eventually stop at a certain size, or a “closed” universe that will reverse direction and eventually contract into a “big crunch” – presumably to explode again in another big bang (and with multiple series of “big bang/big crunch” cycles, we have an “oscillating universe”). While all of these are mathematically feasible (and yes, I’m leaving out a few other good theories), the current data strongly suggests that we are in the “open” model, where the universe will continue to expand forever, until all the stars burn through their remaining fuel and all that’s left is ashes and space dust.
Of course, there are many unanswered questions – such as, “what is the universe expanding into?”
Some models, especially with a “closed” description, suggest an actual gravitational edge to the universe, where light and energy that hits the edge of the universe will curve around and end up coming in from the other side. In this view, there is nothing else – space doesn’t go on to infinity. Euclidian geometry wraps around itself and we are trapped inside a bubble of energy and matter and there is nothing on the outside. Other models are more pragmatic: the edge of the universe is just the space that light, energy, and matter have expanded into. While the gravitational effect from the galaxies at the “edge” of this model will bend the fabric of time and space, much like a wind tunnel will bend air around a car, the fabric of time and space continues beyond that edge.
Personally, I like this model. However, it does open up the question about the possibilities of other universes. While we can speculate all we want about the existence of other universes in this scenario – just as we can speculate about the possibility of life on other planets within our own universe – getting actual data that supports an opinion either way is damn near impossible. Obviously, if the “edge” of the universe is simply defined as the area that light has expanded into, then the area outside this edge is completely devoid of energy or matter. The temperature is absolute zero and it is an absolute vacuum. Gravity, light, or any other kind of radio energy from a neighboring universe cannot reach us unless the edge of that universe has merged with ours.
If you think the vastness of empty space in between galaxies within our own universe is mind-numbing, just think of the distances that are possible between universes!
Imagine that our universe is the relative size of a basketball floating around the ether – a few billion years ago, we were the size of a soccer ball, but business is good and we’re expanding. And say there’s another nearby universe that’s about the same size – but does “nearby” mean a meter away or a kilometer away compared to our basketball sized universe? What if the neighboring universe is the relative size of a ping-pong ball? That’s easy to imagine, but how about if it’s the size of the Earth’s moon? Wow.
Are there as many universes as there are stars in our own universe? Are time and space truly infinite?
Physicists speculate about the potential for other universes to have different laws of physics. For example, general relativity and the relationships between the strong force, the weak force, electro-magnetic force, and gravity really define our physical universe – but are these relationships constant throughout other universes or is that just the way "matter" and "energy" exists? While speculation is good, and it opens us up to new ideas and alternate possibilities, we really have no concrete reason to believe that the laws of physics will be any different anywhere else. If there IS energy and matter outside our universe, it is highly likely that it will be energy and matter as we know it – especially if you subscribe to the “open” universe model we’re discussing here.
I’m confident that continued research and the gathering of scientific theories and data will eventually show, beyond a shadow of a doubt, how our universe started and how it will end. We may find the sought-after “theory of everything” which could answer the question of whether or not there may be alternate laws of physics in other universes. But coming up with actual evidence that there are other universes? That’s going to be a hard one.