I have a theory. I think that Nero didn't just travel through time, but to an alternate universe that already existed. His meddling didn't 'create' this tangent universe, it was already there. Furthermore, I believe that even though we weren't aware of it, Enterprise, the last Trek TV show, took place in this same alternate reality. That's the only way IMO to explain how advanced the technology is in Enterprise and the new film. The USS Kelvin was far FAR beyond anything we saw in the original Trek. With the way they explained it in the film, it seems like they are saying everything was exactly the same until Nero attacked the Kelvin, changing history and creating an alternate reality.