We investigate the electromagnetic cascade initiated by injection of very high energy photons in the early universe and calculate the cascade spectrum by solving a set of Boltzmann equations numerically. In the calculation we take account of Compton scattering off the background electrons and pair creation off the background nuclei as well as photon-photon processes and inverse-Compton scattering. We also apply our cascade spectrum to the big bang nucleosynthesis with photodissociation processes due to heavy unstable particles and obtain the constraint on their lifetime and abundance.
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A 1995 study studied this question.