Since 1998, overwhelming evidence has been accumulating that distant type Ia supernovae appear systematically dimmer than one would expect from their redshifts in a universe whose expansion is slowing down. One infers the distance of such a supernova from its apparent brightness, and its redshift is a direct measure of the total expansion of the cosmos since the light was emitted. The relation between redshift and distance over a large range of redshifts traces the history of cosmic expansion (see the article by Saul Perlmutter in PHYSICSTODAY, April 2003, page 53.)