Bounds on ultralight hidden-photon dark matter from observation of the 21 cm signal at cosmic dawn

Ultralight hidden-photon dark matter produces an oscillating electric field in the early Universe plasma, which in turn induces an electric current in its ionized component whose dissipation results in heat transfer from the dark matter to the plasma. This will affect the global 21 cm signal from the dark ages and cosmic dawn. In this work we focus on the latter, in light of the reported detection by the EDGES collaboration of an absorption signal at frequencies corresponding to redshift $z\sim 17$. By measuring the 21 cm global signal, a limit can be placed on the amount of gas heating, and thus the kinetic mixing strength $ϵ$ between the hidden and ordinary photons can be constrained. If the recent measurements are evidence of—and corroborated as—21 cm absorption in standard cosmology, one can derive a constraint on the amount of heating due to hidden-photon dark matter based on the requirement that the 21 cm brightness temperature does not exceed a given value. To demonstrate the promise of this method, without loss of generality, we calculate the constraints in the limit of infinite Lyman-$\alpha$ coupling, for fiducial measurements of $T_{21}=0\mathrm{mK}$ or $T_{21}=-100\mathrm{mK}$. The inferred bounds on $ϵ$ in the mass range ${10}^{-23}\mathrm{eV}\lesssim m_{\chi }\lesssim {10}^{-13}\mathrm{eV}$ would be the strongest yet.

Figure 1

The evolution of the baryon temperature, with and without heating due to HPDM, for different values of $m_{\chi }$, setting $ϵ$ in each case to yield a 21 cm brightness temperature $T_{21}(z=17)=-100\mathrm{mK}$. For masses $m_{\chi }\gtrsim 1.7\times {10}^{-14}\mathrm{eV}$, the heating weakens once the plasma frequency ${\omega }_p$ falls below the mass [as discussed in the text, for $m_{\chi }=1.7\times {10}^{-14}\mathrm{eV}$ this transition happens precisely at redshift $z=17$, see Eq. (2)].

Figure 2

The 21 cm brightness temperature as a function of the kinetic mixing parameter $ϵ$ and the HPDM mass $m_{\chi }$, in the limit of infinite Lyman-$\alpha$ coupling (i.e., $T_s\equiv T_b$). This map exhibits clear gradients from $T_{21}\sim -200\mathrm{mK}$ to $\sim 30\mathrm{mK}$.

Figure 3

Predominant bounds on the kinetic mixing parameter $ϵ$ for different HPDM masses $m_{\chi }$. We show constraints from ISM heating [21] (red) and from the CMB [22, 46] (blue). Our inferred 21 cm bounds from requiring that $T_{21}=-100\mathrm{mK}$ or $T_{21}=0\mathrm{mK}$ (black and dashed-black) are two orders of magnitude stronger for $m_{\chi }\lesssim {10}^{-14}\mathrm{eV}$ and the only ones to penetrate the fuzzy-DM mass range ${10}^{-23}\mathrm{eV}\lesssim m_{\chi }\lesssim {10}^{-20}\mathrm{eV}$.