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Energy release in the early Universe (z<~ 2x10^6) should lead to some broadspectral distortion of the cosmic microwave background (CMB) radiation field,which can be characterized as y-type distortion when the injection processstarted at redshifts z<~ 5x10^4. Here we demonstrate that if energy wasreleased before the beginning of cosmological hydrogen recombination (z~1400),closed loops of bound-bound and free-bound transitions in HI and HeII lead tothe appearance of (i) characteristic multiple narrow spectral features at dmand cm wavelengths, and (ii) a prominent sub-millimeter feature consisting ofabsorption and emission parts in the far Wien tail of CMB spectrum. Theadditional spectral features are generated in the pre-recombinational epoch ofHI (z>~1800) and HeII (z>~7000), and therefore differ from those arising due tonormal cosmological recombination in the undisturbed CMB blackbody radiationfield. We present the results of numerical computations including 25 atomicshells for both HI and HeII, and discuss the contributions of severalindividual transitions in detail. As examples, we consider the case ofinstantaneous energy release (e.g. due to phase transitions) and exponentialenergy release because of long-lived decaying particles. Our computations showthat due to possible pre-recombinational atomic transitions the variability ofthe CMB spectral distortion increases when comparing with the distortionsarising in the normal recombination epoch. The existence of these narrowspectral features would open an unique way to separate y-distortions due topre-recombinational ($1400<~ z <~5x10^4) energy release from those arising inthe post-recombinational era at redshifts z<~800. (abridged)

Pre-recombinational energy release and narrow features in the CMB spectrum