yeah its Lux Aeturna but then right after the begining of it he changes to the Requiem movements.  Anyway, yeah bungie says this is the last of the Halo ARC, which means there could be more games with master chief.  Also about the ending of the game.  I was just thinking, since the Forerunners were humans i don't think that they would want their race destroyed, so maybe MC DOES activate The Ark...