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

TL;DR

The paper investigates whether resonant ALP-photon conversion in a magnetized early Universe can account for the EDGES global $21$-cm signal while accounting for energy injection from primordial magnetic fields. It develops a joint thermal-history model that includes PMF-induced heating via ambipolar diffusion and turbulent decay, and computes the ALP-photon conversion probability $P_{a\gamma}$ and the resulting nonthermal radio photons in the EDGES band. By requiring consistency with $T_{21}$ amplitudes between the standard $\Lambda$CDM value and the EDGES observation, it derives upper bounds on the product $g_{a\gamma}B_n$ (and hence on $g_{a\gamma}$ for given $B_n$) across ALP masses $m_a$ in the $10^{-14}$–$2\times10^{-10}$ eV range. The results show that magnetic heating can damp the $21$-cm absorption, tightening the required ALP-photon coupling, yet a viable parameter space remains, inviting future 21-cm experiments to test this scenario.

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 $(g_{aγ})$ and magnetic field strength $(B_n)$. Our findings report that, for $B_n= 0.1\,\rm nG$, $g_{aγ}B_n\lesssim (3.6\times 10^{-4}-3\times 10^{-3})$ is required to recover standard $T_{21}=-156\,\rm mK$, while a deeper absorption of $-500$ mK pushes the upper bound to $g_{aγ}B_n\lesssim (6.5\times 10^{-4}-5.7\times 10^{-3})$.

Figures (4)

• Figure 1: Represents the evolution of conversion probability $(P_{a\gamma})$ over redshifts for $g_{a\gamma}B_n = 10^{-12}\,\rm nG\, GeV^{-1}$ and energies $E_a = 0.414\,\mu\rm eV$ (blue) and $E_a = 0.207\,\mu\rm eV$ (orange).
• Figure 2: The ratio of energy density spectrum $(Ed\rho/dE)$ and energy density of CMB $(\rho_{\gamma})$. The blue dashed and green solid lines illustrate the ratio of present-day CMB $(Ed\rho_{\gamma}/dE)$ and ALP $(Ed\rho_a/dE)$ energy density spectrum, respectively, with respect to $\rho_{\gamma}$. The magenta band shows nonthermal photons for ALP energies $E_a$ in the range $(50-100)\,\rm MHz$. The vertical purple band shows $(50-100)\,\rm MHz$ frequency band.
• Figure 3: (a) Thermal evolution of the IGM with primordial magnetic fields. The black dashed and solid lines show the CMB temperature and $T_g$ in $\Lambda\rm CDM$ without star formation. The blue solid line includes X-ray heating. Yellow, green, and red solid lines show $T_g$ for PMFs with $n_B = -2.99$ and $B_n = 0.1, 0.3, 0.55\,\rm nG$, respectively. Cyan and grey lines correspond to $(B_n/\mathrm{nG}, n_B) = (0.1,-2.50)$ and $(0.3, -2.50)$. (b) Evolution of global 21-cm signals as a function of redshift in the presence of PMFs. Black dotted and blue solid lines represent $T_{21} = 0$ and the $\Lambda\rm CDM$ signal. Solid and dashed lines show PMF cases with different $(B_n/\mathrm{nG}, n_B)$ values.
• Figure 4: Constraint on $g_{a\gamma}B_n$ and mass of ALP $(m_a)$ in the presence of nearly scale-invariant PMFs with different strengths. The black shaded region represents constraints from spectral distortion Mirizzi:2009nq. The red shaded region represents the excluded parameter space from the absence of the $\gamma$-ray burst associated with SN1987A + CAST Payez:2014xsa. The colour-coded band presents the variations of $g_{a\gamma}$ with $m_a$ such that the nonthermal resonant photons can produce an absorption amplitude of $T_{21}$ signal at $z\sim 17.2$ in the range $(156-500)\,\rm mk$. Here $T_{21} = -156\,\rm mK$ represents the absorption amplitude in the $\Lambda\rm CDM$ framework.