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

Energy release in the early Universe (z less than or similar to 2 x 10(6)) should produce a broad spectral distortion of the cosmic microwave background (CMB) radiation field, which can be characterized as y-type distortion when the injection process started at redshifts z less than or similar to 5 x 10(4). Here we demonstrate that if energy was released before the beginning of cosmological hydrogen recombination (z similar to 1400), closed loops of bound-bound and free-bound transitions in HI and HeII lead to the appearance of (i) characteristic multiple narrow spectral features at dm and cm wavelengths; and (ii) a prominent sub-millimeter feature consisting of absorption and emission parts in the far Wien tail of CMB spectrum. The additional spectral features are generated in the pre-recombinational epoch of Hi ( z greater than or similar to 1800) and He ii (z greater than or similar to 7000), and therefore differ from those arising due to normal cosmological recombination in the undisturbed CMB blackbody radiation field. We present the results of numerical computations including 25 atomic shells for both H i and He ii, and discuss the contributions of several individual transitions in detail. As examples, we consider the case of instantaneous energy release ( e. g., due to phase transitions), and exponential energy release (e.g., because of long-lived decaying particles). Our computations show that because of possible pre-recombinational atomic transitions the variability in the CMB spectral distortion increases when comparing with the distortions arising in the normal recombination epoch. The amplitude of the spectral features, both at low and high frequencies, directly depends on the value of the y-parameter, which describes the intrinsic CMB spectral distortion resulting from the energy release. The time-dependence of the injection process also plays an important role, for example leading to non-trivial shifts in the quasi-periodic pattern at low frequencies along the frequency axis. The existence of these narrow spectral features would provide an unique way to separate y-distortions caused by pre-recombinational (1400 less than or similar to z less than or similar to 5 x 10(4)) energy release from those arising in the post-recombinational era at redshifts z less than or similar to 800.