Friedmann-Robertson-Walker Universe: ds2=-dt2+a2(t) (dsined) 1)k=-1 open 2)k=0 → flat 3)k=1 closedFriedmann-Robertson-Walker Universe: 2 2 2 2 2 2 2 2 2 2 ( )( sin ) 1 dr ds dt a t r d r d kr          1) k = -1 open 2) k = 0 flat 3) k =1 closed