How to affix a wooden branch to the ceiling as a chandelier?
I'd suggest three supports as a minimum - Any less and the branch will wobble about when touched or with any sort of draught/breeze.  Also consider rigging the fixtures not in a row, also to minimise swing.

20 kilograms (42 pounds) is also a lot of weight to hang up in the air from jute rope, which does deteriorate over time.   I'd strongly recommend some braided steel rope/cable, perhaps with a thin sheathe of cosmetic jute weaving over the top.

Knotting your rope might work, but over time the rope will sag and thin, putting stress on the power wire.



I'd recommend putting three stainless steel eyebolts into the top surface of the branch and three more into the ceiling.  Make sure you hit something solid and load-bearing above the ceiling!  

Then use a swage tool to permanently secure three braided stainless steel cables to the log eyebolts, and attach three positively-locking hooks to the other end, to clip to the ceiling eyebolts.  This will allow removal of the log without cutting the wire rope.

  cable eye and a swage closure.

 An uncrimped swage.