Challenges for $Λ$CDM: An update

Abstract

A number of challenges of the standard ΛCDM model has 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 ΛCDM model as defined by the Planck18 parameter values. In addition to the major well studied 5σ challenge of ΛCDM (the Hubble H0 crisis) and other well known tensions (the growth tension and the lensing amplitude AL anomaly), we discuss a wide range of other less discussed less-standard signals which appear at a lower statistical significance level than the H0 tension (also 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 Lyα 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 of these signals and their level of significance, with emphasis to the Hubble crisis and refer to recent resources where more details can be found for each signal. We also briefly discuss possible theoretical approaches that can potentially explain the non-standard nature of some of these signals.

Easing the Hubble constant tension?

“…Maybe the most important prediction is that w B (t B 0 ) ≈ −1.2. This is actually in agreement with certain fits to data (see for instance [5], section II.3 for a recent review), but is difficult to explain from physical healthy theories based on small modifications of the ΛCDM model, and usually w(t) < −1 is associated with the presence of so-called ghost fields. However, such ghost fields are not needed in our model and w B (t) → −1 for t → ∞.…”

An extended analysis for a generalized Chaplygin gas model

“…We follow the analysis presented in [56] where we are able to write the algebraic solution for the field equations ( 10)- (12). Indeed, in the new change of variables dt = exp F (χ) 2 dχ with χ = 6 ln a, the background space reads…”

“…Despite the great success of ΛCDM model, the cosmological constant suffers from the fine tuning problem [10] and the cosmic coincidence problem [11]. For a recent discussion of the ΛCDM model we refer the reader to [12]. To overcome these issues, various DE models were explored in the literature, which include quintessence, K-essence, phantom, tachyon and so on (for review see [6] and the references therein).…”

A model-independent test of speed of light variability with cosmological observations

“…Any statistically significant departure from its predictions would hint at new physics and lead to a complete reformulation of its scenario. Recent works confirmed the validity of both FLRW and GR at large cosmological scales, yet some results may state otherwise -see [19] for a recent review on unsolved cosmological puzzles.…”