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Injection of high energy electromagnetic particles around the recombinationepoch can modify the standard recombination history and therefore the CMBanisotropy power spectrum. Previous studies have put strong constraints on theamount of electromagnetic energy injection around the recombination era(redshifts $z\lesssim 4500$). However, energy injected in the form of energetic($>$ keV) visible standard model particles is not deposited instantaneously.The considerable delay between the time of energy injection and the time whenall energy is deposited to background baryonic gas and CMB photons, togetherwith the extraordinary precision with which the CMB anisotropies have beenmeasured, means that CMB anisotropies are sensitive to energy that was injectedmuch before the epoch of recombination. We show that the CMB anisotropy powerspectrum is sensitive to energy injection even at $z = 10000$, giving strongerconstraints compared to big bang nucleosynthesis and CMB spectral distortions.We derive, using Planck CMB data, the constraints on long-lived unstableparticles decaying at redshifts $z\lesssim 10000$ (lifetime $\tau_X\gtrsim10^{11}$s) by explicitly evolving the electromagnetic cascades in the expandingUniverse, thus extending previous constraints to lower particle lifetimes. Wealso revisit the BBN constraints and show that the delayed injection of energyis important for BBN constraints. We find that the constraints can be weaker bya factor of few to almost an order of magnitude, depending on the energy, whenwe relax the quasi-static or on-the-spot assumptions.

CMB anisotropy and BBN constraints on pre-recombination decay of dark matter to visible particles