Interesting
Linguistically, the Scandanavian language (and some linguists insist that Norwegian, Swedish and Danish are all just dialects of each other) are part of the Germanic language family -- the Germanics only really come into history around 100 BC with some ROman/Greek's writing about them as barbaric peoples beyond the Celts
Even as late as the 300s AD, the Goths, Geats, Vandals, Alammani, etc. speak a mutually intelligible language, so the split was probably relatively recent with it solidifying only perhaps about 900 AD. Perhaps around 200 BC the Germanics were all peoples around the southern coasts of Norway, Sweden and the northern coasts of Germany, the Netherlands and Denmark and probably the north-western coasts of Poland (Wolin etc)
The Celts seem to have exploded around 300 BC from the Hallstadt culture, invading Rome, invading Anatolia and probably Iberia, maybe ireland at this time. What prompted this? And what prompted the later Germanic movement south?