Implications of the possible 21-cm line excess at cosmic dawn on dynamics of interacting dark energy

TL;DR

The paper investigates whether background cosmic evolution, via interacting dark energy (IDE) and an optional early dark energy (EDE) stage, can explain the strong 21-cm absorption signal reported around cosmic dawn. It shows that, for linear IDE models, the dominant effect on the global 21-cm signal arises from changes to the optical depth $\tau$ rather than the Compton heating decoupling time, and derives an evolved $H(z)$ around $z\sim 17$ to enhance absorption. By adding an EDE-dominated phase around $z\sim 100$, the authors demonstrate a route to further modify the expansion history, but these scenarios remain in tension with CMB, BAO, and Hubble constraints, and cannot fully reproduce the EDGES amplitude. The work clarifies the link between early-universe background dynamics and 21-cm observables and outlines the observational requirements needed to robustly test such models in the near future.

Abstract

In this paper we study implications of the possible excess of 21-cm line global signal at the epoch of cosmic dawn on the evolutions of a class of dynamically interacting dark energy (IDE) models. We firstly summarize two dynamical mechanisms in which different background evolutions can exert considerable effects on the 21-cm line global signal. One is the decoupling time of Compton scattering heating, the other stems from the direct change of optical depth due to the different expansion rate of the Universe. After that, we investigate the IDE models to illustrate the tension between the results of Experiment to Detect the Global Epoch of reionization Signature (EDGES) and other experiments. To apply the analyses of these two mechanisms to IDE models, we find that only the optical depth can be significantly changed. Accordingly, in order to relieve the tension by including the effects of the decoupling time of Compton scattering heating, we deduce a possible evolution form for the Hubble parameter within IDE that begins at an early stage around $z\sim 100$ and then smoothly evolves to a value at $z\sim 17$ which is smaller than that obtained in the standard paradigm. Eventually, we fulfill this scenario by adding an early dark energy dominated stage to the cosmological paradigm described by IDE models, which can alleviate the tension between EDGES and other cosmological observations but cannot completely solve it.

Figures (5)

• Figure 1: Comparison between EDGES and other experiments for different IDE models with different EoS parameters $\omega$. The values of $\lambda$ and $\Omega_ch^2$ picked up by the lines yield the upper limit of EDGES's results $T_{21}=-0.3K$. The parameter spaces that lie right and below the lines can lead to a stronger 21-cm brightness temperature signal. The error bars are derived from the constraints on $\lambda$ and $\Omega_ch^2$ and their colours correspond to different IDE models as has been explained in the plot.
• Figure 2: The Hubble parameter and Compton-heating rate for different interacting strengths and different IDE models. The decoupling of the gas temperature from the radiation temperature occurs when $H(z)\approx 1/t_C(z)$ for a given model and a given $\lambda$, i.e. the intersection of the lines with the same color.
• Figure 3: The change of the 21-cm line signal $\Delta T_{21}$ from different factors (Compton-heating, cosmic expansion and taking both of them into consideration.) for different models. And we use the red line to label the upper limit of EDGES's results $\Delta T_{21}=-0.1K$. The areas surrounded by dashed lines and $\Delta T_{21}=0$ measure the degree of influence of different factors. Note that the parameter range of $\lambda$ is slightly different among different models, since we need to take the singularity of the models into our consideration.
• Figure 4: The Hubble parameter and Compton-heating rate for $\Lambda$CDM, the IDE model $\lambda=0.15$ and the interacting plus early dark energy model $\lambda=0.15$, $\Omega_{ee}=0.5\times 10^{-5}$, $z_c=100$.
• Figure 5: The 21-cm line brightness temperature given by different early dark energy parameters $\Omega_{ee}$ and $z_c$ for different IDE models. The parameter spaces above the black line are excluded by the measurements of $\theta_*$ and $H_0$.