Energy injection in pre-recombination era and EDGES detection

Abstract

We study the possibility of explaining the recent EDGES detection by an energy injection in the pre-recombination era. Our aim is to show that the residue of this energy injection could give the resultant increase in the energy density at frequencies $x_e \equiv h\nu/(kT) \simeq 10^{-3}$, which is needed to explain the EDGES result. We consider two models of energy injection: Gaussian profile with a fixed redshift of injection and radiative decay of a non-relativistic particle. We show that the energy injection should occur in the redshift range $z\lesssim 4 \times 10^3$ to prevent free-free processes from thermalizing the injected energy. The injected energy should be nearly 200--1000 times the CMB intensity at $x_e \simeq 10^{-3}$ to obtain the requisite residue. A large fraction of the injected energy gets thermalized and therefore distorts the CMB spectrum. We compute CMB spectral distortion for both the models and show that the fractional change in CMB energy density, $\Delta \rho_{\rm \scriptscriptstyle CMB}/\rho_{\rm \scriptscriptstyle CMB} \simeq 10^{-6}$, which might be detectable with the proposed experiment PIXIE. We also outline the implication of our proposed scenario for CMB anisotropies.