[Paper Review] Mitigating Cosmological Tensions via Momentum-Coupled Dark Sector Model

Gang Liu, Jiaze Gao+3 more|arXiv (Cornell University)|Oct 15, 2023

Cosmology and Gravitation TheoriesPhysics and Astronomy3 citations

TL;DR

This paper proposes a momentum-coupled dark sector (MCDS) model that couples early dark energy (EDE) to cold dark matter via pure momentum exchange to alleviate cosmological tensions. The model preserves EDE’s success in resolving the Hubble tension (H₀ ≈ 72.23 km/s/Mpc) while suppressing structure growth, reducing S₈ to 0.8192—partially mitigating the large-scale structure tension, though not fully resolving it.

ABSTRACT

In this paper, we investigate the momentum coupling between early dark energy (EDE) and cold dark matter to alleviate cosmological tensions. EDE has exhibited promising efficacy in addressing the Hubble tension, but it exacerbates the large-scale structure tension. We consider the interaction between EDE and cold dark matter, introducing a pure momentum exchange between them to alleviate the large-scale structure tension introduced by the EDE model. We find that this coupling model is consistent with the EDE model, yielding a higher value for $H_0$, which can resolve the Hubble tension. Additionally, the new model exhibits a suppressive effect on structure growth, contributing to the alleviation of the large-scale structure tension. By utilizing the Markov Chain Monte Carlo method and incorporating various cosmological data, the coupling model constrains the best-fit values for $H_0$ to be $72.23$ km/s/Mpc and for $S_8$ to be 0.8192. Compared to the $Λ$CDM model, the new models have not fully resolved the large-scale structure tension. However, in contrast to the best-fit value of 0.8316 for $S_8$ obtained from the EDE model, the new model alleviates the negative impact of the EDE model.

Motivation & Objective

• To address the Hubble tension (4.8σ discrepancy between CMB-inferred and local measurements of H₀) by extending the early dark energy (EDE) model.
• To alleviate the exacerbated large-scale structure tension (S₈ discrepancy) caused by the EDE model, which increases S₈ beyond DES-Y3 observations.
• To explore a novel interaction mechanism—pure momentum exchange—between EDE and cold dark matter to suppress structure growth without energy transfer.
• To test the viability of the momentum-coupled dark sector (MCDS) model against multiple cosmological datasets, including CMB, BAO, SNIa, SH0ES, and DES-Y3 S₈.
• To evaluate model performance using both χ² and AIC criteria, assessing whether the added complexity improves fit despite potential overfitting.

Proposed method

• Formalize a momentum-coupled dark sector (MCDS) model using a generalized fluid action with pure momentum transfer, inspired by Type 3 models in pull-back formalism.
• Derive modified background and perturbation evolution equations for EDE and cold dark matter under pure momentum coupling, with a coupling constant ξ.
• Implement initial conditions for density perturbations in the MCDS framework, accounting for momentum exchange effects on structure growth.
• Constrain model parameters using Markov Chain Monte Carlo (MCMC) sampling with combined datasets: CMB (Planck 2018), BAO, SNIa, SH0ES (H₀), and DES-Y3 (S₈).
• Calculate χ²_total and Akaike Information Criterion (AIC) to compare the MCDS model with ΛCDM and EDE models, evaluating goodness-of-fit and model complexity trade-offs.
• Analyze the matter power spectrum and σ₈ evolution to quantify suppression of structure growth due to momentum coupling.

Experimental results

Research questions

• RQ1 Can a pure momentum coupling between early dark energy and cold dark matter reduce the large-scale structure tension without compromising Hubble tension resolution?
• RQ2 How does momentum exchange affect the evolution of cold dark matter density perturbations and the matter power spectrum?
• RQ3 What are the best-fit values of H₀ and S₈ in the MCDS model, and how do they compare to ΛCDM and EDE models?
• RQ4 Does the MCDS model achieve a better overall fit to cosmological data than the EDE model, considering both χ² and AIC criteria?
• RQ5 What is the significance of the coupling constant ξ, and does it favor a suppression of structure growth at small scales?

Key findings

• The MCDS model achieves a best-fit H₀ of 72.23 km/s/Mpc, resolving the Hubble tension by increasing H₀ above the ΛCDM prediction of 67.37 km/s/Mpc.
• The model reduces S₈ to 0.8192, which is lower than the EDE model’s 0.8316 and closer to the DES-Y3 measurement of 0.776, partially mitigating the large-scale structure tension.
• The coupling constant is constrained to ξ = -0.006 ± 0.014 at 68% confidence level, with a negative sign indicating suppression of structure growth on small scales.
• The MCDS model achieves the lowest χ²_total value (-12.26) relative to ΛCDM, outperforming both ΛCDM and EDE models in data fit, primarily due to improved agreement with SH0ES and DES-Y3.
• Despite better χ² performance, the AIC value for MCDS (-4.26) is worse than for EDE (-5.74), indicating that the added complexity of the coupling constant reduces model preference under AIC.
• The MCDS model preserves the Hubble tension resolution of EDE while reducing the adverse impact on S₈, demonstrating that momentum coupling can partially alleviate the large-scale structure tension.