Snorri STuurlsun wrote about the Wordl Ash, or Yggrasil in his Prose Edda in the 13th century.  By this point  it was already a part of Norse Mythology.   What he described was a great tree whose branches reached into the heavens, and whose roots decendended to the underworld.  It was a very beautiful and inspired description of a long cherished myth.   The orginal visionaries who glimpsed Yggrasil  interpreted what they had seen in ways they could understand.    As such, its a flawed understanding.
  What these unknown prophets saw was the spaces between universes, the cracks in creation.   The pathways  between the worlds could be looked on as a tree branchs, roots, stems, and things sticking out in all directions. 
   Scientists today are starting to undestand the spaces between spaces.   Harold Van Irye AKA the Quidnunk is the top expect in interdimensiol physics.   On Prince Drakker Island he created an engine powered by cold fusion that and physically send someone to other worlds, other branches in the World Ash.    Van Iry says the idea of Nine Worlds is preposterous, that the number may be infinite.

Recently,  the alternate world of Aearth was discovered.