Dawn of the dark: unified dark sectors and the EDGES Cosmic Dawn 21-cm signal

TL;DR

This work tests unified dark sector scenarios, namely generalized and modified Chaplygin gas, as single-fluid descriptions of dark matter and dark energy, by combining Planck CMB data with the EDGES 21-cm absorption signal from Cosmic Dawn. The global 21-cm constraint acts as a high-redshift chronometer that modifies the background expansion $H(z)$ and helps break degeneracies in the Chaplygin gas parameter space. Across both GCG and MCG, EDGES tightens key parameters by factors of roughly $1.5$–$10$ and can alleviate the $H_0$ tension in the GCG case, illustrating that 21-cm cosmology offers a powerful probe of nonstandard dark sector physics. The study underscores the potential of 21-cm data to complement CMB and low-redshift probes in constraining unified dark sector models and guiding future theoretical and observational efforts.

Abstract

While the origin and composition of dark matter and dark energy remains unknown, it is possible that they might represent two manifestations of a single entity, as occurring in unified dark sector models. On the other hand, advances in our understanding of the dark sector of the Universe might arise from Cosmic Dawn, the epoch when the first stars formed. In particular, the first detection of the global 21-cm absorption signal at Cosmic Dawn from the EDGES experiment opens up a new arena wherein to test models of dark matter and dark energy. Here, we consider generalized and modified Chaplygin gas models as candidate unified dark sector models. We first constrain these models against Cosmic Microwave Background data from the \textit{Planck} satellite, before exploring how the inclusion of the global 21-cm signal measured by EDGES can improve limits on the model parameters, finding that the uncertainties on the parameters of the Chaplygin gas models can be reduced by a factor between $1.5$ and $10$. We also find that within the generalized Chaplygin gas model, the tension between the CMB and local determinations of the Hubble constant $H_0$ is reduced from $\approx 4σ$ to $\approx 1.3σ$. In conclusion, we find that the global 21-cm signal at Cosmic Dawn can provide an extraordinary window onto the physics of unified dark sectors.

Figures (2)

• Figure 1: Triangle plot showing the 2D joint and 1D marginalized posterior distributions for $B_s$, $\alpha$, and $H_0$ within the GCG model, obtained using only the CMB dataset (grey contours and curves), or the combined CMB+EDGES dataset (purple contours and curves).
• Figure 2: As in Fig. \ref{['fig:mcg']} but for the modified Chaplygin gas model.