Small scale problems of the $\Lambda$CDM model: a short review

TL;DR

This review discusses small-scale issues in the $\\Lambda$CDM cosmological model, exploring whether these problems indicate fundamental limits or gaps in our understanding of dark matter, gravity, and baryonic physics.

Contribution

It provides a comprehensive overview of small-scale problems in $\\Lambda$CDM and evaluates proposed solutions to determine their effectiveness.

Findings

Identifies key small-scale problems in $\\Lambda$CDM.

Reviews proposed solutions and their limitations.

Highlights need for better understanding of dark matter and baryon physics.

Abstract

The $\Lambda$CDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem , the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. $\Lambda$CDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the $\Lambda$DCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the $\Lambda$CDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe.