The anomalous 21-cm absorption at high redshifts

• Main Author: Fulvio Melia
• Format: Article
• Language: English
• Published: SpringerOpen 2021-03-01
• Series: European Physical Journal C: Particles and Fields
• Online Access: https://doi.org/10.1140/epjc/s10052-021-09029-4

Abstract The EDGES collaboration has reported the detection of a global 21-cm signal with a plateau centered at 76 MHz (i.e., redshift 17.2), with an amplitude of $$500^{+200}_{-500}$$ 500 - 500 + 200  mK. This anomalous measurement does not comport with standard cosmology, which can only accommodate an amplitude $$\lesssim 230$$ ≲ 230  mK. Nevertheless, the line profile’s redshift range ( $$15\lesssim z\lesssim 20$$ 15 ≲ z ≲ 20 ) suggests a possible link to Pop III star formation and an implied evolution out of the ‘dark ages.’ Given this tension with the standard model, we here examine whether the observed 21-cm signal is instead consistent with the results of recent modeling based on the alternative Friedmann–Lemaître–Robertson–Walker cosmology known as the $$R_{\mathrm{h}}=ct$$ R h = c t universe, showing that – in this model – the CMB radiation might have been rethermalized by dust ejected into the IGM by the first-generation stars at redshift $$z\sim 16$$ z ∼ 16 . We find that the requirements for this process to have occurred would have self-consistently established an equilibrium spin temperature $$T_{\mathrm{s}}\approx 3.4$$ T s ≈ 3.4 K in the neutral hydrogen, via the irradiation of the IGM by deep penetrating X-rays emitted at the termination shocks of Pop III supernova remnants. Such a dust scenario has been strongly ruled out for the standard model, so the spin temperature ( $$\sim 3.3$$ ∼ 3.3 K) inferred from the 21-cm absorption feature appears to be much more consistent with the $$R_{\mathrm{h}}=ct$$ R h = c t profile than that implied by $$\Lambda $$ Λ CDM, for which adiabatic cooling would have established a spin temperature $$T_\mathrm{s}(z=17.2)\sim 6$$ T s ( z = 17.2 ) ∼ 6 K.