timequake

— Or you can have a revisable universe with one timeline (see Hogan's Thrice Upon a Time or Milennium^([sic]) by (I think) Joe Haldeman for the general idea) and in which your actions can "rewrite" the past from that point on. Here, if you shoot your grandfather, you generate a "timequake" in which something ugly happens. Probably you disappear, but there may be other side-effects as the universe works it out, which is, I think, what you are reffering^([sic]) to as the "great expense".