Friedmann-Robertson-Walker Universe: s=-d+aaa+rd0+sn6uo） 1)k=-1 open 2)k=0 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