The Bifröst Bridge

The Bifröst Bridge is the way to Asgard in the Viking stories. The god Heimdall who waits there is the guardian. He can hear the grass growing and leaves falling from Yggdrasil, he can see to the end of the world and he never sleeps.

Or, it's the Aurora Borealis. Auroras are produced when the solar wind collides with the Earth's magnetic field and the energy released forms auroras around the poles. Up above is Wikipedia's Photo of the Year in 2006, taken in Alaska.

Or the Bifröst Bridge is the Milky Way. Or it's a rainbow over Bergen:

Rainbows in Scandinavia lead as often as not into the clouds, which would seem to be the right place for Asgard.