Friedmann-Robertson-Walker Universe: ds2 =-dt2+a"(t)( dr2 h+rd0+r产sn2euo 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