I still have my own theory about time travel. "When the ring blew it shook itself to peices, did a number on the Ark."
#1: Cortana said it might take years to be found.
#2: It could have taken years to get to Reach.
#3: They may have went through a Black Hole.
It would be a great story line with Arbitor now gone, Johnson, Miranda, all the Profits(or so we think ; ), Gravemind, plus so many more main charectors gone.