Axion-Photon Conversion In Magnetized Universe: Impact On The Global 21-cm Signal

Abstract: The reported anomalous global 21-cm signal $(T_{21})$ from the cosmic dawn era by Experiment to Detect the Global Epoch of Reionisation Signature (EDGES) could hint towards new physics beyond the standard model. The resonant conversion of the axion-like particles (ALPs) into photons in the presence of primordial magnetic fields (PMFs) could be a viable solution. However, the strength of the PMFs can change over the time as they can decay by ambipolar diffusion and turbulent decay. Consequently, PMFs can dissipate their energy into the intergalactic medium (IGM), which can alter the global 21-cm signal. We simultaneously consider both magnetic heating of IGM and resonant conversion of ALPs to derive physically motivated upper bounds on the coupling strength $(g_{a\gamma})$ and magnetic field strength $(B_n)$. Our findings report that, for $B_n= 0.1\,\rm nG$, $g_{a\gamma}B_n\lesssim (3.6\times 10^{-4}-3\times 10^{-3})$ is required to recover standard $T_{21}=-156\,\rm mK$, while a deeper absorption of $-500$ mK pushes the upper bound to $g_{a\gamma}B_n\lesssim (6.5\times 10^{-4}-5.7\times 10^{-3})$.