Beyond $\Lambda$CDM: Problems, solutions, and the road ahead

TL;DR

This paper reviews the limitations of the standard $$CDM cosmological model, discusses potential problems and solutions, and explores promising alternative theories in light of current and future observational and theoretical developments.

Contribution

It provides a comprehensive overview of the issues with $$CDM and evaluates various alternative cosmological models and their prospects for addressing these problems.

Findings

Identification of key problems in $$CDM

Assessment of promising alternative theories

Highlighting future observational tests

Abstract

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, $\Lambda$CDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of $\Lambda$CDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.