Addressing the Hubble and S ₈ tensions with a kinetically mixed dark sector

Abstract

We present a kinetically mixed dark sector (KMIX) model to address the Hubble and S 8 tensions. Inspired from string theory, our model includes two fields: an axion, which plays a role similar to the scalar field in early dark energy models, and a dilaton. This theory differs from other axio-dilaton models aimed at the Hubble tension in that there is necessarily kinetic mixing between the two fields which allows for efficient energy transfer from the axion into the dilaton which has w ≈ 1. As a direct consequence of these dynamics, we find the model does not need to resort to a fine-tuned potential to solve the Hubble tension and naturally accommodates a standard axion potential. Furthermore, the axion will necessarily makeup a small (fuzzy) fraction of Ωcdm once it begins to oscillate at the bottom of its potential and will suppress the growth of perturbations on scales sensitive to S 8. Interestingly, the scale of the potential for the dilaton has to be small, ≲ 𝒪(10 meV)4, suggesting the possibility for a connection to dark energy. Implementing the dynamics for the background and perturbations in a modified Boltzmann code we calculate the CMB and matter power spectra for our theory. Exploring the parameter space of our model, we find regions which can accommodate a ∼ 10% increase in H 0 from the Planck inferred value and S 8 values that are consistent with large-scale structure constraints.

Ultra-light axions and the S 8 tension: joint constraints from the cosmic microwave background and galaxy clustering

“…Many proposed solutions to the H 0 tension based on new physics, however, exacerbate the discrepancy in S 8 [e.g., 99]. Models of ultra-light axions, with m a ∼ (10 −27 − 10 −26 ) eV, combined with modifications to the dynamics of the DE component [100][101][102] are invoked to alleviate simultaneously both parameter tensions. In this work, we stress the importance of assessing tension in the full parameter space.…”

A grounded perspective on new early dark energy using ACT, SPT, and BICEP/Keck

“…It has been established that New Early Dark Energy (NEDE) is a natural and promising solution to the notorious Hubble tension [20][21][22][23][24][25][26][27] and remains compatible with measurements of large-scale structure (LSS) [22,26] (for other adaptions of NEDE see also [28,29]). So far, it has shared this phenomenological success with its predecessor Early Dark Energy (EDE) [30][31][32][33][34], although both models rely on different microphysics and lead to different predictions at the more detailed level (for further EDE-type models and explorations see [28,29,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] and for reviews [50,51]). However, just like other EDE models, the NEDE model has been thought not to lead to a simultaneous alleviation of the S 8 problem.…”

Cosmological Tensions and the Transitional Planck Mass Model

“…To offset the enhanced decay of gravitational potential wells, more dark matter is needed, which results in increased clustering in the late Universe and thus larger values of S 8 . There have also been cosmological models designed to resolve the S 8 tension (see, e.g., Alexander et al 2023;Ashoorioon & Davari 2023;Poulin et al 2023), but it would be preferable to have a single model that resolves both tensions at the same time.…”