Challenges for $Λ$CDM: An update

TL;DR

This review surveys a wide range of signals in cosmology and astrophysics that show tension with ΛCDM beyond the standard Hubble tension, organizing them into H0-related challenges, growth and CMB-anomaly curiosities, and additional dipoles and small-scale issues. It presents a structured comparison of observational datasets and methods (SnIa, BAO, lensing, GW standard sirens, megamasers, etc.) and documents the persistent spread in inferred $H_0$, as well as the weaker but notable discrepancies in growth ($S_8$) and lensing amplitudes. The work surveys two broad classes of theoretical responses: late-time deformations of the Hubble rate and early-time or new-physics modifications (including EDE, IDE, decaying DM, dark radiation, and modified gravity), highlighting their respective successes and tensions with other probes like growth or CMB. It emphasizes that addressing the tensions likely requires a combination of improved data, robust cross-calibration among probes, and models that can coherently fit $H(z)$, $\mu_G(z,k)$, $\Sigma(z,k)$ and possible variations of fundamental constants across cosmic time. The paper also looks forward to upcoming surveys and GW missions (e.g., Euclid, LSST, CMB-S4, LISA/DECIGO) that will greatly sharpen tests of ΛCDM and its extensions, helping to determine whether the observed tensions point to new physics or residual systematics.

Abstract

A number of challenges to the standard $Λ$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 ($2σ$ or larger) with the standard $Λ$CDM model as specified by the Cosmological Principle, General Relativity and the Planck18 parameter values. In addition to the well-studied $5σ$ challenge of $Λ$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 $α$, velocity and quasar dipoles), CMB asymmetries, BAO Ly$α$ 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.

Figures (20)

• Figure 3: The $1\sigma - 3\sigma$ confidence contours in the parametric space ($\Omega_{0m}$, $\mathcal{M}$). The blue contours correspond to the $1\sigma - 3\sigma$ full Pantheon dataset ($1048$ SnIa datapoints) best fit, while the red contours describe the $1\sigma - 3\sigma$ confidence contours of the four bins (from left to right). The black points represent the best fit of each bin, while the green dot represents the best fit value indicated by the full Pantheon dataset ($\Omega_{0m}=0.285$ and $\mathcal{M}=23.803$) Kazantzidis:2020tko.
• Figure 4: The best fit values of $\mathcal{M}$ (left panel) and $\Omega_{0m}$ (right panel) as well as the $1\sigma$ errors for the four bins, including the systematic uncertainties. This oscillating behaviour relatively improbable in the context of constant underlying $\mathcal{M}$ and $\Omega_{0m}$Kazantzidis:2020tko.
• Figure 5: The snapshots show the radial mass profile of perturbation as a function of the comoving radius of an initially point-like overdensity located at the origin for redshifts $z=6824,1440,848,478,79,10$. The time after the Big Bang are given in each snapshots. The black, blue, red, and green lines correspond to the dark matter, baryons, photons, and neutrinos (all perturbations are fractional for that species), respectively. The top snapshots are for the early time before recombination where the overdensities in photons and baryons evolve together, the middle snapshots for soon after but close to recombination where the baryons freeze at the location reached with the photons forming a thick spherical shell, and the bottom snapshots are for long after recombination where the baryon overdensities start to gravitationally grow like dark matter overdensities Eisenstein:2006nj.
• Figure 6: The Planck$18$ CMB angular power spectrum $\mathcal{D}_l^{TT}\equiv l(l+1)/(2\pi) C_l^{TT}$ (top) and residual angular power spectrum (bottom) of temperature fluctuations as a function of multipole moment $l$. The light blue line in the upper panel is the best-fitting to the Planck TT, TE, EE+lowE+lensing likelihoods assuming the base-$\Lambda$CDM cosmology. The red points correspond to the binned Planck data. The lowest multipole range ($l\leq 30$) is dominated by cosmic variance (approximated as Gaussian), while positions and amplitudes of the acoustic peaks are accurately constrained Planck:2018vyg.
• Figure 7: The signature of baryonic acoustic oscillations in galaxy two-point correlation function $\xi(s)$ as measured by Eisenstein:2005su using the luminous red galaxy samples of the Sloan Digital Sky Survey. The data show the existence of a baryonic acoustic peak in the galaxy correlation function $\xi(s)$ around the comoving separation scale $100\, h^{-1} Mpc$. The solid green, red, and blue lines correspond to model predictions with $\Omega_{0m} h^2=0.12, 0.13$ and $0.14$, respectively. All models are taken to have the same $\Omega_{0b}h^2=0.024$ and $n=0.98$. The magenta line corresponds to a model with no baryons and $\Omega_{0m}h^2=0.105$, which has no acoustic peaks Eisenstein:2005su.