Need help identifying young adult/ fantasy book from the early 2000s
I’m looking for a book I read when I was in junior high- roughly 10 years ago. I can’t remember the name of the book or the name of the author, but vaguely remember the cover being black with a dagger on the front, surrounded by a glowing ring of light. It was about a scientist camping out at a research site with his colleague, studying these rock ruins that resembled a portal. The colleague goes into the tent and the MC (I think his name was Eric?) goes back to the portal and suddenly trips trough it and is transported back in time to the age of Vikings, knights, and warriors. He’s found by warriors and taken in, where he slowly learns their language and history, becomes one of them, and I believe takes a wife. He eventually learns of some sort of issue in the mountains causing spiritual imbalance, travels to said cave and resolves the issue (I vaguely remember mentions of a river in the cave he follows). He ends up stumbling back though the portal (years have passed for him) and discovers that only 40mins have passed in the real world and I think goes back through the portal. I believe it was a trilogy and I had read the first book and about half of the second. It would’ve been published before 2010. I also remember undertones of norse mythology but I could be mistaken.