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 Physics Today, Physics Today 0031-9228 564200353 https://doi.org/10.1063/1.1580050 April 2003, page 53 .)

The supernova data—bolstered by an imposing variety of other, less direct evidence—have led to an evolving consensus called the concordance model: It asserts that the cosmos is currently in an epoch of accelerating expansion driven by a pervasive dark vacuum energy dense enough to overcome the gravitational braking...

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