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Spectral distortions and anisotropies of the CMB provide independent and complementary probes to study energy injection processes in the early universe. Here we discuss the synergy between these observables, and show the promising future of spectral distortion missions to constrain both exotic and non-exotic energy injections. We show that conventional probes such as Big Bang Nucleosynthesis and CMB anisotropies can benefit from and even be surpassed by future spectral distortion experiments. For this, we have implemented a unified framework within the Boltzmann code class to consistently treat the thermal evolution of photons and baryons. Furthermore, we give an extensive and pedagogical introduction into the topic of spectral distortions and energy injections throughout the thermal history of the universe, highlighting some of their unique features and potential as a novel probe for cosmology and particle physics.1 This quantity should not be confused with the actual photon temperature Tγ , which may have a more complicated evolution. The precise normalization of Tz is arbitrary, but T0 ≡ Tz(0) will be chosen close to the actual temperature today, Tγ(0) = (2.7255 ± 0.0005)K [65,66], in order to have Tγ Tz at least in the late universe.2 Note that with perturbations in the PPSD this is not true anymore as shown in [51].

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