Board Thread:Speculation, Theories, and Rumors/@comment-25400375-20150330223218/@comment-679780-20150401165554

The multiverse plot element is a common element in most comics and cartoons. Even Archie comic did a cross over with Sonic X and SOnic Underground a few times.

Sol Dimension could be considered a neighbouring alternate universe of Sonic's Earth, though not prooven if they share the same Special Stages as Sonic Rush Adventure does not use the Special Zone for it's stages.