Challenges for $Λ$CDM: An update (2105.05208v3)

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.

• The paper presents rigorous evidence of a >5σ discrepancy in Hubble constant measurements that challenges ΛCDM predictions.
• It analyzes structure growth anomalies by comparing weak lensing and redshift distortions with expected matter density trends.
• The study reviews inconsistencies in CMB data and cosmic dipole alignments, suggesting potential revisions to dark energy or gravitational models.

Insights into $\Lambda$CDM Model Challenges: An Update

The paper "Challenges for $\Lambda$CDM: An update" by L. Perivolaropoulos and F. Skara presents a comprehensive review of the evolving tensions between cosmological data and the predictions of the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) model. It explores the signals from cosmological and astrophysical data that appear inconsistent with this standard model, highlighting the statistical significance and potential implications of these discrepancies.

$\Lambda$CDM has long been hailed for its predictive power in explaining various phenomena in cosmology, such as the Universe’s accelerating expansion and the cosmic microwave background (CMB) anisotropies. However, with increased precision in cosmological observations, the model faces several pronounced challenges. These include not only the famous Hubble tension, where there is a significant disagreement between the locally measured Hubble constant ($H_0$) and that inferred from CMB observations, but also the lesser-known anomalies, ambiguities, and tensions that arise in multiple areas, from small-scale structures to cosmic background data.

The Hubble Tension

The review underscores the Hubble tension, a long-debated discrepancy where local measures (e.g., via Cepheid variables and Type Ia supernovae) indicate a higher $H_0$ than the CMB-based value provided by Planck satellite data within the $\Lambda$CDM model framework. This tension now exceeds the $5\sigma$ level, suggesting either unknown systematic errors in measurements or an incomplete cosmological model. The authors also explore various theoretical models beyond $\Lambda$CDM that attempt to reconcile these differences by introducing additional dark energy dynamics or modifying standard cosmological parameters, albeit these proposed solutions also necessitate a robust explanation of other existing cosmological observations.

Growth of Structures

Apart from the Hubble tension, another significant area of exploration is the "growth tension." The $\Lambda$CDM model's specification of matter density and the amplitude of the primordial power spectrum (expressed in terms of $\sigma_8$ and $\Omega_{0m}$) appears at odds with the weaker structure growth rates determined via weak lensing, redshift space distortions, and cluster counts. The paper presents this anomaly as indicative of either modifications in dark energy's role or potential flaws in the assumption that General Relativity is an accurate descriptor of gravity at cosmological scales.

CMB and Large-Scale Anomalies

The CMB data itself presents internal inconsistencies, such as the lensing amplitude $A_L$, which is stronger than anticipated. Furthermore, there are hints of closed Universe implications from these datasets, which do not align with flat Universe predictions when combined with BAO datasets. Anomalies in the CMB, such as the low power on large angular scales and alignments in quadrupole-octopole moments, indicate unresolved concerns about isotropy and Gaussianity assumptions of primordial fluctuations, thus questioning some of the foundational aspects of how relic radiation from the Big Bang is interpreted.

Cosmic Dipoles and other Small-Scale Challenges

The authors also explore violations of large-scale homogeneity or isotropy, such as peculiar velocity flows, galaxy distribution disparities, and alignments of the cosmic dipole beyond expected CMB contributions. On smaller scales, issues like the core-cusp problem in dwarf galaxies, the missing satellite problem, and the observations of planes of satellites around galaxies challenge the dark matter particle assumptions prevalent in $\Lambda$CDM.

Implications and Future Direction

The paper comprehensively details these challenges and indicates potential avenues for resolving them, suggesting that modifications to well-ingrained physical principles might be necessary. Such adjustments could range from modifications in dark matter properties to rethinking gravity on cosmic scales. The anticipation of upcoming datasets from experiments such as the Vera Rubin Observatory and Euclid provide hope that future high-precision data could help resolve some of these tensions or potentially highlight more nuances that might point to physics beyond current paradigms.

Overall, Perivolaropoulos and Skara's review serves as a detailed dossier highlighting the $\Lambda$CDM model’s current quandaries while encouraging theoretical and observational innovations to grasp the Universe's full complexity. Their work exemplifies the dynamic interplay between observational advancements and theoretical physics, driving the quest for a fundamentally improved cosmological model.