In a balloon there's a nozzle, but for the universe the balloon is a perfect sphere, so no not really, it didn't occur from a single point in space/time. Space/time IS the universe.
evman can probably explain it better than me.
Yeah I meant the limits on the speed one can travel.. and putting that aside might not be that easy, even in the context of a mental exercise. The speed limit is intrinsic to the structure of the universe so if we leave it out then we are changing the universe we're trying to explore with that mental exercise.. sometimes things can be simplified for use in a mental exercise, but speed feels like something we couldn't.
I'll clarify why I say that in a second.
When they talk about the radius of the universe, really they usually mean the radius of the "observable" universe. So the 78 billion light year radius thing just means that the light from the beginning of the universe that is reaching us now in combination with the expansion of the universe since the beginning works out to 78 billion light years.
The question about if you were able to travel more than the diameter of the universe, that's a question about the actual geometry of the universe.
If the universe were a sphere (rather a 3-sphere or sphere with higher dimensions, or whatever, I don't know what the right terminology is), then like on the surface of the balloon if you started out in one direction you would eventually end up back where you started (or if you had a big enough telescope you could peer up and see the earth as it was long ago).
Measurements from the cosmic background radiation (radiation from just after the big bang) seem to indicate this isn't the case. So either the universe is infinite (in which case you would just keep going) or has some other shape. Or it could still be a sphere, but the expansion is fast enough that the speed limit prevents us from observing that it is a sphere.
To bring back in the speed limit thing, because the speed of light and the expansion of the universe is intrinsic to the universe, by definition we can't go to the "edge" and then keep going to see what happens, so the question doesn't have a meaning (just like the question of what came before the universe; causality is part of the universe and doesn't have any meaning outside of it).
That last part might be way off though from current knowledge of cosmology, evman150 will hopefully correct me or help explain it better.
