The
accelerating expansion of the universe is the observation that the
universe appears to be
expanding at an increasing rate. In formal terms, this means that the
cosmic scale factor ![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=4131)
has a positive
second derivative, so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.