Wherein Redalien seeks to dispel some myths
The science bit
Many worlds interpretation
This is a scientific theory that seeks to describe how quantum physics can function. There is also the Copenhagen Interpretation, the Hidden Variables interpretation and many others (mostly variations on the three main ideas). Many worlds is the idea that every quantum event splits the whole universe so that each possibility is played out in a daughter universe. This would be transparent to people in the universe but raises some interesting ideas.
It is this idea that gives us the idea that conscious decisions or other macro events create universes, whereas there are so many universes being created that there are a lot for each decision. It's safe to assume that there are a relatively high number of almost identical universes though (i.e. down to the position of a single photon in a single double-slit experiment 50 years ago).
Ok, imagine the universe is infinitely big and contains a lot more energy than we suspect (specifically infinite energy). As information is bound by the universal speed limit of the speed of light we can't know about anything above a certain threshold away. It's possible that the universe consists of localised centres of mass that we call the observable universe. As it's got infinite energy, there are an infinite number of these centres, so an infinite number of observable universes. As the probability of you existing is not infinitesimal there are likely to be infinite copies of you with varying amounts of congruence to your life. But then, if there's even a small difference are you the same person? Twins are different people, so arguably you're different and just very similar.
Sounds familiar, right? Two options, both with seemingly infinite universes. Except under many-worlds the number is growing, in the latter it is static. Whether many-worlds is an infinite number of worlds or merely a growing finite number depends on whether all quantum events have a finite number of outcomes or not. The granularity of spacetime seems to be the big issue here. No matter what you do you cannot split your universe. Also, you cannot visit another universe without being able to exceed c. All probabilities still play out, as there are an infinite number of worlds currently identical to yours that will later diverge.
God I hate infinite numbers.
The pop-culture bit
As the above explanation of many-worlds, if you can leave your own universe you'll have a very difficult time getting back as there'll be more than one version of you and more than one home-world. Not only might there be a difference in numbers, but getting exactly one version of you per universe would be a logistical nightmare.
Star Trek style
Everyone knows this plot line, there's an alternate universe with all the same characters who have completely different personal histories, not to mention the huge political differences in the whole galaxy. If our whole civilisation was changed hundreds of years ago we wouldn't be here. People would be, but they'd have different ancestors. Even if we did have the same ancestors, which has such a small chance it's not worth considering, unless the exact same sperm/egg pair met we'd have significant differences. Just look at non-identical twins.
So, how does quantum physics work then?
Well, nobody knows! Personally I lean towards the hidden variables idea ("God doesn't play dice" and all) but Copenhagen seems more likely than many-worlds. If you really believe in many-worlds you can prove it to yourself, but I really wouldn't recommend it. Basically, hook up a gun to a quantum random number generator and run it until there's a million-to-one chance of surviving. If you experience life for much longer chances are you're in one of a very small number of worlds in a many-worlds universe.
If you want to talk about multiple universes, feel free, but remember than a huge number of scientists prefer the Copenhagen interpretation which doesn't automatically means there are multiple universes.
Maybe I shouldn't have watched that episode of sliders today.