HarrisonZeldovichs
Harrison–Zel’dovich approximation, often referred to as the Harrison–Zel’dovich formalism, is a theoretical framework in cosmology used to describe the evolution of small perturbations in the early universe. Developed independently by American physicist Harrison and Soviet physicist Zel’dovich in the 1970s, this approach provides a simplified yet powerful way to analyze the growth of density fluctuations, which are fundamental to understanding the formation of large-scale structures like galaxies and galaxy clusters.
The approximation assumes that the universe is dominated by a homogeneous and isotropic background, with small
A central concept in the Harrison–Zel’dovich framework is the "top-hat" model, where perturbations are approximated as
The Harrison–Zel’dovich formalism has been instrumental in predicting the primordial power spectrum of density fluctuations, which