A Derivation of the Etherington’s Distance-Duality Equation

The Etherington's distance-duality equation is the relationship between the luminosity distance of standard candles and the angular-diameter distance. This relationship has been validated from astronomical observations based on the X-ray surface brightness and the Sunyaev-Zel'dovich effect of galaxy clusters. In the present study, we propose a derivation of the Etherington's reciprocity relation in the dichotomous cosmology.