Abstract
In the inhomogeneous Universe, the cosmological conversion of dark photons into ordinary photons (and vice versa) may happen at a great number of resonance redshifts. This alters the CMB observed energy spectrum and degree of small-scale anisotropies. We utilize results from the EAGLE simulation to obtain the conversion probability along random line-of-sights to quantify these effects. We then apply our results to the case where dark photons are sourced by dark matter decay and their high-redshift conversion into ordinary photons modify the global 21 cm signal expected from the cosmic dawn era. Concretely, we show that a significant portion of the parameter space for which a converted population of photons in the Rayleigh-Jeans tail of the CMB explains the absorption strength observed by EDGES, is ruled out from the brightness temperature measurements of COBE/FIRAS and the CMB anisotropy measurements of Planck and SPT.
Highlights
The observation of a global absorption feature in the sky, centered at νedg = 78 MHz in frequency [1], might mark the start of a new era in radio-astronomy
RH is a normalization factor fixed by the requirement that the model explains the EDGES-observed absorption strength, RDM is the ratio of dark matter (DM) densities at redshift zdec(ω) and the average DM density at the same redshift, and Rconv is the ratio of conversion probabilities with an overall energy dependence factored out
In this work we take the first step towards a realistic modeling of dark photon conversion in the low-redshift inhomogeneous Universe
Summary
During this period, the average plasma frequency scans through those values (green box) and the electron number density varies in the range ne,res ∈ [3.5 · 10−7, 910] cm−3 It is the purpose of this letter to point out that similar values of electron density reoccur in the late universe at z 6, after reionization and when cosmological structures have formed. This gives way to additional sources of creating extra photons relative to the CMB. MA (z) becomes highly non-monotonic at low redshift and, there may be many points along a typical LOS where the resonant condition is satisfied (even if the cosmological average value of ne is very different from the resonant value ne,res)
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