Etherington's reciprocity theorem

The Etherington's distance-duality equation is the relationship between the luminosity distance of standard candles and the angular diameter distance.^[1] The equation is as follows: ${\displaystyle d_{L}=(1+z)^{2}d_{A}}$, where ${\displaystyle z}$ is the redshift, ${\displaystyle d_{L}}$ is the luminosity distance and ${\displaystyle d_{A}}$ the angular-diameter distance.