CMB spectral distortions from continuous large energy release
  Accurate computations of spectral distortions of the cosmic microwave
background (CMB) are required for constraining energy release scenarios at
redshifts $z\gtrsim 10^3$. The existing literature focuses on distortions that
are small perturbations to the background blackbody spectrum. At high redshifts
($z\gtrsim 10^6$), this assumption can be violated, and the CMB spectrum can be
significantly distorted at least during part of its cosmic evolution. In this
paper, we carry out accurate thermalization computations, evolving the
distorted CMB spectrum in a general, fully non-linear way, consistently
accounting for the time-dependence of the injection process, modifications to
the Hubble expansion rate and relativistic Compton scattering. Specifically, we
study single energy injection and decaying particle scenarios, discussing
constraints on these cases. We solve the thermalization problem using two
independent numerical approaches that are now available in {\tt CosmoTherm} as
dedicated setups for computing CMB spectral distortions in the large distortion
regime. New non-linear effects at low frequencies are furthermore highlighted,
showing that these warrant a more rigorous study. This work eliminates one of
the long-standing simplifications in CMB spectral distortion computations,
which also opens the way to more rigorous treatments of distortions induced by
high-energy particle cascade, soft photon injection and in the vicinity of
primordial black holes.
