[Paper Review] Does the fraction of dark matter diminish with early dark energy?

Hao Wang, Yun-Song Piao|arXiv (Cornell University)|Sep 20, 2022

Cosmology and Gravitation Theories121 references4 citations

TL;DR

This paper investigates whether a fraction of dark matter could have 'faded' (i.e., lost mass) due to coupling with early dark energy (EDE), inspired by the swampland distance conjecture. Using MCMC analyses with Planck CMB, BAO, Pantheon, SH0ES, and DES-Y1 data, it finds that a small fraction of dark matter (f* ≲ 0.03) coupled to EDE can alleviate the S₈ tension without worsening the H₀ tension, though c ≈ 0 remains consistent with data.

ABSTRACT

In pre-recombination early dark energy (EDE) resolutions of the Hubble tension, the rise of Hubble constant value $H_0$ is usually accompanied with the exacerbation of so-called $S_8$ tension. Inspired by the swampland conjecture, we investigate what if a fraction $f_*$ of dark matter is coupled to EDE, $m_{cdm}\sim \exp{(-c{|Δϕ_{ede}|\over M_{pl}})}$ with $c\sim {\cal O}(1)$. We perform the MCMC analysis for the relevant EDE models with PlanckCMB, BAO, Pantheon and SH0ES dataset, as well as DES-Y1 data, and find that such a fraction helps to alleviate the $S_8$ tension. However, though $c\gtrsim 0.1$ is allowed for a very small $f_*$, which suggests that a small fraction of dark matter has ever faded with EDE, $c\sim0$ is also consistent.

Motivation & Objective

• To investigate whether a fraction of dark matter could have dynamically lost mass (faded) due to coupling with early dark energy (EDE), motivated by the swampland distance conjecture.
• To assess whether such a coupling can alleviate the S₈ tension, which persists in standard EDE models despite resolving the H₀ tension.
• To test the viability of this model using cosmological datasets including Planck CMB, BAO, Pantheon, SH0ES, and DES-Y1.
• To determine whether the fading mechanism is consistent with current observational constraints, particularly on the coupling strength c and the fraction f* of dark matter involved.

Proposed method

• Proposes a mass-dependent dark matter coupling to EDE: m_cdm ∝ exp(–c|Δφ_ede|/M_pl), where c ≈ O(1) and φ_ede is the EDE scalar field.
• Performs Markov Chain Monte Carlo (MCMC) analyses using Planck CMB, BAO, Pantheon, SH0ES, and DES-Y1 datasets to constrain model parameters.
• Considers two EDE models: axion-like EDE and AdS-EDE, both allowing for a fraction f* of dark matter to couple to EDE.
• Imposes the swampland distance conjecture (SDC) to constrain the field excursion and ensure UV-completeness of the effective field theory.
• Analyzes the evolution of the scalar field φ_ede, the EDE energy density f_ede, and the effective dark matter mass m_cdm(φ_ede) over time.
• Compares model predictions for S₈ and H₀ against local measurements (SH0ES, KiDS+DES-Y1) to assess tension alleviation.

Experimental results

Research questions

• RQ1 Can a fractional coupling between dark matter and early dark energy alleviate the S₈ tension without worsening the H₀ tension?
• RQ2 What is the maximum allowed coupling strength c for a small fraction f* of dark matter to have faded due to EDE, consistent with current cosmological data?
• RQ3 Is the fading mechanism of dark matter via EDE coupling compatible with the swampland distance conjecture?
• RQ4 How do the inclusion of DES-Y1 data and different EDE models (axion-like vs. AdS-EDE) affect the constraints on f* and c?
• RQ5 Does the model prefer a non-zero c, or is c ≈ 0 still viable, particularly in light of the S₈ and H₀ measurements?

Key findings

• The inclusion of a small fraction f* of dark matter coupled to EDE helps alleviate the S₈ tension, with best-fit f* ≈ 0.03 in AdS-EDE and f* ≈ 0.5 in axion-like EDE.
• For both EDE models, c ≳ 0.1 is allowed at 1σ if f* is small, suggesting a small fraction of dark matter may have faded with EDE.
• Despite the potential for fading, c ≈ 0 remains within the 1σ confidence region, particularly in axion-like EDE where the best-fit c is negative.
• The baseline+DES-Y1 dataset disfavors significant fading, as c ∼ 0 is preferred, especially in axion-like EDE with smaller field excursion.
• The model maintains a scale-invariant primordial spectrum (n_s ≈ 1) even when including DES-Y1 data, consistent with H₀ ≈ 73 km/s/Mpc.
• The S₈ tension is reduced in both models: best-fit S₈ ≈ 0.82 (axion-like EDE) and S₈ ≈ 0.8433 (AdS-EDE), closer to local measurements than in standard EDE.