[Paper Review] Cold New Early Dark Energy pulls the trigger on the $H_0$ and $S_8$ tensions: a simultaneous solution to both tensions without new ingredients

Juan S. Cruz, Florian Niedermann+1 more|arXiv (Cornell University)|May 15, 2023

Cosmology and Gravitation TheoriesPhysics and Astronomy86 references8 citations

TL;DR

The paper shows that Cold NEDE, with a small trigger-field contribution to dark matter, can simultaneously alleviate the H0 and S8 tensions without introducing new ingredients, achieving a preferred NEDE fraction of about 0.134 and H0 ~ 71.7 km/s/Mpc, S8 ~ 0.793.

ABSTRACT

In this work, we show that the Cold New Early Dark Energy (Cold NEDE) model in its original form can solve both the Hubble tension and the $S_8$ tension without adding any new ingredients at the fundamental level. So far, it was assumed that the trigger field in the Cold NEDE model is completely subdominant. However, relaxing this assumption and letting the trigger field contribute a mere $0.5\%$ of the total energy density leads to a resolution of the $S_8$ tension while simultaneously improving it as a solution to the $H_0$ tension. Fitting this model to baryonic acoustic oscillations, large-scale-structure, supernovae (including a SH0ES prior), and cosmic microwave background data, we report a preferred NEDE fraction of $f_\mathrm{NEDE}= 0.134^{+0.032}_{-0.025}$ ($68\%$ C.L.), lifting its Gaussian evidence for the first time above $5σ$ (up from $4 σ$ when the trigger contribution to dark matter is negligible). At the same time, we find the new concordance values $H_0 = 71.71 \pm 0.88 \,\mathrm{km}\, \mathrm{sec}^{-1}\, \mathrm{Mpc}^{-1}$ and $S_8 = 0.793 \pm 0.018$. Excluding large-scale structure data and the SH$_0$ES prior, both Gaussian tensions are reduced below the $2 σ$ level.

Motivation & Objective

• Motivate the H0 and S8 tensions within LambdaCDM and explore NEDE as a simultaneous solution.
• Investigate whether relaxing the assumption of a negligible trigger-field energy density can resolve both tensions.
• Quantify how a small trigger-field dark matter fraction impacts cosmological observables and parameter constraints.
• Provide an updated TriggerCLASS implementation to track trigger-field perturbations after the NEDE phase transition.

Proposed method

• Extend Cold NEDE with a non-negligible trigger-field energy density Omega_phi as a free parameter.
• Model the NEDE phase transition as a first-order transition triggered by an ultralight field phi evolving to roll and induce tunneling.
• Compute linear perturbations by tracking the trigger field and its perturbations post-transition using an updated Boltzmann code TriggerCLASS.
• Use an effective fluid description for the ultra-light trigger post-transition to handle rapid oscillations.
• Fit the model to BAO, LSS, SN (including SH0ES prior), and CMB data to extract f_NEDE, H0, and S8.

Experimental results

Research questions

• RQ1 Can Cold NEDE with a non-negligible trigger-field energy density simultaneously alleviate the H0 and S8 tensions?
• RQ2 What is the preferred NEDE fractional energy density f_NEDE when Omega_phi is allowed to be non-zero?
• RQ3 How do H0 and S8 constraints change when including trigger-field perturbations in the cosmological fit?
• RQ4 What are the implications for the trigger-field mass scale and its UV completion in light of the data?
• RQ5 How robust are the NEDE predictions to modeling choices for the NEDE fluid equation of state and perturbations?

Key findings

• A non-zero trigger-field DM fraction yields f_NEDE = 0.134^{+0.032}_{-0.025} (68% CL), with Gaussian evidence rising above 5σ.
• The model favors H0 = 71.71 ± 0.88 km/s/Mpc and S8 = 0.793 ± 0.018.
• Excluding LSS data and SH0ES prior, both H0 and S8 tensions are reduced to below ~2σ.
• Including LSS and SH0ES prior maintains reduced tensions while remaining compatible with LSS.
• The approach achieves a simultaneous solution to both tensions without adding new ingredients beyond the Cold NEDE framework.