Decaying Dark Matter and the Hubble Tension

TL;DR

Decaying dark matter models can modify the universe's expansion rate around matter-radiation equality, offering a potential early-time solution to the Hubble tension, but current models only partially alleviate the discrepancy.

Contribution

This paper analyzes early-time decaying dark matter models and their impact on the Hubble tension, providing cosmological parameter estimates with current data.

Findings

Current models yield H0 ≈ 68.7 km/s/Mpc, still 2.7σ below local measurements.

Late decay models are constrained and do not resolve the tension.

Early decay models partially alleviate the Hubble tension but do not fully resolve it.

Abstract

Decaying dark matter models generically modify the equation of state around the time of dark matter decay, and this in turn modifies the expansion rate of the Universe through the Friedmann equation. Thus, a priori, these models could solve or alleviate the Hubble tension, and depending on the lifetime of the dark matter, they can be classified as belonging to either the early- or late-time solutions. Moreover, decaying dark matter models can often be realized in particle physics models relatively easily. However, the implementations of these models in Einstein--Boltzmann solver codes are non-trivial, so not all incarnations have been tested. It is well known that models with very late decay of dark matter do not alleviate the Hubble tension, and in fact, cosmological data puts severe constraints on the lifetime of such dark matter scenarios. However, models in which a fraction of the dark matter decays to dark radiation at early times hold the possibility of modifying the effective equation of state around matter-radiation equality without affecting late-time cosmology. This scenario is therefore a simple realization of a possible early-time solution to the Hubble tension, and cosmological parameter estimation with current data in these models yields a value of $H_0 = 68.73^{+0.81}_{-1.3}$ at $68\%$ C.I.. This still leads to a $2.7\sigma$ Gaussian tension with the representative local value of $H_0 = 73.2 \pm 1.3$ km s$^{-1}$ Mpc$^{-1}$. Additional work is, however, required to test more complex decay scenarios, which could potentially prefer higher values of $H_0$ and provide a better solution to the Hubble tension.