# Press-Schechter Formalism – Definition & Detailed Explanation – Cosmology Glossary

## I. What is Press-Schechter Formalism?

Press-Schechter Formalism is a mathematical model used in cosmology to predict the distribution of dark matter halos in the universe. It was first introduced by William H. Press and Paul Schechter in 1974 as a way to estimate the mass function of collapsed objects, such as galaxies and galaxy clusters, in the early universe. The formalism is based on the theory of gravitational collapse, which states that regions of high density in the universe will eventually collapse under their own gravity to form structures like galaxies and galaxy clusters.

## II. How does Press-Schechter Formalism work?

Press-Schechter Formalism works by assuming that the initial density fluctuations in the universe are Gaussian and that they evolve over time according to the linear theory of gravitational instability. The formalism then calculates the probability that a region of a given density will collapse to form a dark matter halo of a certain mass at a specific point in time. This probability is determined by comparing the density of the region to a critical threshold value, known as the collapse threshold.

## III. What are the assumptions of Press-Schechter Formalism?

There are several key assumptions underlying Press-Schechter Formalism. Firstly, it assumes that the initial density fluctuations in the universe are Gaussian, which means that they are randomly distributed and have a bell-shaped distribution. Secondly, it assumes that the density fluctuations evolve linearly over time, meaning that they grow at a constant rate. Finally, it assumes that dark matter halos form when the density of a region exceeds a critical threshold value, which is determined by the collapse of the region under its own gravity.

## IV. What are the limitations of Press-Schechter Formalism?

While Press-Schechter Formalism has been widely used in cosmology research, it has several limitations. One major limitation is that it does not take into account the effects of baryonic matter, such as gas and stars, on the formation of dark matter halos. This can lead to inaccuracies in predicting the mass function of collapsed objects. Additionally, the formalism assumes that dark matter halos form spherically symmetric structures, which may not always be the case in reality.

## V. How is Press-Schechter Formalism used in cosmology research?

Press-Schechter Formalism is used in cosmology research to study the formation and evolution of dark matter halos in the universe. By predicting the mass function of collapsed objects, researchers can better understand the distribution of matter in the universe and how structures like galaxies and galaxy clusters form. The formalism is also used to make predictions about the abundance of dark matter halos at different points in cosmic history, which can help test theories of structure formation.

## VI. What are some criticisms of Press-Schechter Formalism?

Despite its widespread use in cosmology research, Press-Schechter Formalism has faced criticism from some researchers. One criticism is that the formalism does not accurately predict the abundance of low-mass dark matter halos, leading to discrepancies with observations. Additionally, the assumption of spherical symmetry in the formation of dark matter halos has been questioned, as recent studies have shown that halos can have more complex shapes. Overall, while Press-Schechter Formalism has been a valuable tool in studying the distribution of matter in the universe, it is important for researchers to be aware of its limitations and consider alternative models when necessary.