The production of light elements in big bang nucleosynthesis (BBN), cosmic microwave background (CMB) spectrum, and CMB temperature and polarization anisotropies are very sensitive to injection of energetic particles in the early Universe between the  BBN epoch when the age of the Universe is about 3 minutes until after the cosmological recombination when the age of the Universe is about half a million years. Injection of energetic standard model particles from new physics such as decay of dark matter, evaporation of primordial black holes and decay of superconducting strings triggers particle showers in the primordial plasma. I will present new calculations of evolution of these particle cascades in the early Universe which predict new kinds of CMB spectral distortions. I will also present new constraints on new physics from CMB observations as well as measurements of light elements in old stars.