Challenges for $\Lambda$CDM: An update

TL;DR

This review summarizes various observational tensions and anomalies challenging the standard $\\Lambda$CDM cosmological model, highlighting their significance, potential implications for new physics, and recent developments as of 2022.

Contribution

It provides a comprehensive, unified overview of multiple cosmological and astrophysical signals that challenge $\\Lambda$CDM$, including lesser-known anomalies and their possible theoretical explanations.

Findings

Hubble $H_0$ tension at $5\sigma$ significance

Multiple signals show $2\sigma$ or larger tension with $\\Lambda$CDM

Discussion of potential new physics explanations for observed anomalies

Abstract

A number of challenges to the standard $\Lambda$CDM model have been emerging during the past few years as the accuracy of cosmological observations improves. In this review we discuss in a unified manner many existing signals in cosmological and astrophysical data that appear to be in some tension ($2\sigma$ or larger) with the standard $\Lambda$CDM model as specified by the Cosmological Principle, General Relativity and the Planck18 parameter values. In addition to the well-studied $5\sigma$ challenge of $\Lambda$CDM (the Hubble $H_0$ tension) and other well known tensions (the growth tension, and the lensing amplitude $A_L$ anomaly), we discuss a wide range of other less discussed less-standard signals which appear at a lower statistical significance level than the $H_0$ tension some of them known as 'curiosities' in the data) which may also constitute hints towards new physics. For example such signals include cosmic dipoles (the fine structure constant $\alpha$, velocity and quasar dipoles), CMB asymmetries, BAO Ly$\alpha$ tension, age of the Universe issues, the Lithium problem, small scale curiosities like the core-cusp and missing satellite problems, quasars Hubble diagram, oscillating short range gravity signals etc. The goal of this pedagogical review is to collectively present the current status (2022 update) of these signals and their level of significance, with emphasis on the Hubble tension and refer to recent resources where more details can be found for each signal. We also briefly discuss theoretical approaches that can potentially explain some of these signals.