Parallel Universe Theory?

[ZeaLitY]

Yeah, it's still enshrouded in mystery. Chronopolis suggests that there are already infinite universes, yet somehow, the planet is conscious of certain ones (such as the Reptite Dimension, at least), and Belthasar has figured out how to create a new one through branching off a certain event (and merge them back together with the Chrono Cross!). The mechanics of all this are all unknown.

[Radox Redux]

So I think we can take it for granted that there are parallel universes, but they rarely are created unless a paradox occurs somewhere, in which case the only way for the universe to not implode or be caught in an infinite loop is for it to branch out into another universe while the old continues to exist,

Hmmm. This is what happens in the film, Donnie Darko, I expect that the time travel mechanisms are similar in both of the medias.

[Mr. Molecule]

My theory is this:

There exist infinite dimensions. However, the vast majority of them exist only in theory--they exist as possiblities. Chronopolis ghost says,

"Everything is purely a
   problem of possibilities,
   and the world is only
   stabilized by the viewpoint
   of the one who observes it."

Who does the observing? I posit that it's the entity/planet. That is, there're infinite universes, but they only exist when observed/interacted with by a planet's spirit. Since the entities are are the closest thing the Chronoverse has to gods, this makes a certain amount of sense. That might explain why everyone in the know can observe both dimensions--they're connected by a single entity's observation. That's also how Dinopolis came into existence--the  planet, in a mighty effort, observed the Reptite dimension briefly and thus Dinopolis sprang into existence. The RD journal could just be an easter egg, or it could be an image from a defunct dimension, or even one animated by a seperate entity.

This also explains why time reflows--the Enitity watches Chrono & Co.'s journeys--and I'd imagine the Entity to be protected by Time Error, either by a Pocket Dimension or because dude, it's the Entity. It's powerful like that.