[Paper Review] Is it time to go beyond $\Lambda$CDM universe?

A.I. Lonappan, Ruchika Ruchika+1 more|arXiv (Cornell University)|May 20, 2017

Cosmology and Gravitation Theories3 citations

TL;DR

This paper re-evaluates the consistency of the ΛCDM model using recent cosmological data, including SNe Ia, BAO, CMB, H(z), growth, and the Hubble constant from Riess16. Despite tensions in H₀ and growth rate measurements, the Om diagnostic confirms ΛCDM remains consistent at 95% confidence, with a slightly lower Ωₘ₀ than Planck-2015, supporting ΛCDM as the best current model.

ABSTRACT

Concordance $\\Lambda$CDM universe is the simplest model that is consistent with a large variety of cosmological observations till date. But few recent observations indicate inconsistencies in $\\Lambda$CDM model. In this paper, we consider the combination of recent SnIa+Bao+Cmb+Growth+$H(z)$+$H_{0}$ measurements to revisit the constraints on the dark energy evolution using the widely studied CPL parametrisation for the dark energy equation of state. Although the reconstructed behaviour for the dark energy equation of state confirms the inconsistency of $\\Lambda$CDM at $95\\%$ confidence level, the reconstructed $Om$ diagnostic which is a {\\it null test} for $\\Lambda$CDM, still allows the concordance $\\Lambda$CDM behaviour with a lower range of $\\Omega_{m0}$ than that obtained by Planck-2015. {\\it This confirms that $\\Lambda$CDM is still the best choice for the dark energy model}. We also measure the parameter $S = \\sigma_{8}\\sqrt{\\Omega_{m0}/0.3} = 0.728 \\pm 0.023$ which is consistent with its recent measurement by KiDS survey. The confidence contour in the $\\Omega_{m0}-\\sigma_{8}$ parameter plane is also fully consistent with KiDS survey measurement.

Motivation & Objective

• To test whether recent cosmological tensions—particularly in H₀ and growth rate—challenge the ΛCDM model.
• To assess the robustness of the ΛCDM model using the Om diagnostic as a null test independent of Ωₘ₀.
• To compare constraints on dark energy evolution using the CPL and 7CPL parametrizations against current observational data.
• To evaluate consistency of the S=σ₈√(Ωₘ₀/0.3) parameter and Ωₘ₀–σ₈ contour with KiDS survey measurements.
• To determine whether the observed discrepancies in H₀ and growth data necessitate going beyond ΛCDM.

Proposed method

• Uses the CPL parametrization w(a) = w₀ + wₐ(1−a) to model time-evolving dark energy equation of state.
• Employs a combined dataset: SNe Ia (jla), BAO, CMB shift and acoustic scale, H(z) measurements (including Riess16), and growth rate data.
• Reconstructs the Om(z) diagnostic, defined as Om(z) = [E²(z)−1]/[E³(z)−1], where E(z) = H(z)/H₀, to test ΛCDM independently of Ωₘ₀.
• Applies the 7CPL parametrization w = w₀ + wₐ(z/(1+z))⁷ to better fit freezing dark energy models.
• Performs Bayesian parameter estimation and confidence contour analysis in w₀–wₐ and Ωₘ₀–σ₈ planes.
• Compares reconstructed S = σ₈√(Ωₘ₀/0.3) with KiDS survey measurement to validate consistency.

Experimental results

Research questions

• RQ1 Is the ΛCDM model still consistent with the latest cosmological data, including the H₀ tension and growth rate discrepancy?
• RQ2 Does the Om diagnostic, which is independent of Ωₘ₀, support the ΛCDM model despite tensions in H₀ and growth rate?
• RQ3 How do constraints on w₀ and wₐ from the CPL and 7CPL parametrizations compare with observational data, particularly Riess16 and KiDS?
• RQ4 Is the measured value of S = σ₈√(Ωₘ₀/0.3) consistent with the KiDS survey within 95% confidence?
• RQ5 Can the Om diagnostic serve as a robust null test for ΛCDM when combined with diverse H(z) measurements?

Key findings

• The reconstructed dark energy equation of state using the CPL parametrization shows inconsistency with ΛCDM at 95% confidence level, consistent with prior findings using Planck+Riess16.
• The Om diagnostic confirms that ΛCDM is consistent with current data at 95% confidence, even when using the same dataset, with a 95% credible interval for Ωₘ₀ of [0.251, 0.292], slightly lower than Planck-2015’s value.
• The parameter S = σ₈√(Ωₘ₀/0.3) is measured as 0.728 ± 0.023, which is fully consistent with the recent KiDS survey measurement.
• The confidence contour in the Ωₘ₀–σ₈ plane is fully consistent with KiDS survey results, reinforcing the robustness of the current constraints.
• The 7CPL parametrization, designed for freezing models, yields an Om(z) reconstruction that still allows a constant Om at 95% confidence, confirming ΛCDM consistency regardless of parametrization choice.
• While freezing models are inconsistent with data at 95% CL, thawing models are marginally allowed, but the Om diagnostic remains the most reliable null test, confirming ΛCDM’s viability despite tensions.