Thermalization of large energy release in the early Universe

Understand

Spectral distortions of the cosmic microwave background (CMB) provide a unique tool for learning about the early phases of cosmic history, reaching deep into the primordial Universe.

• At redshifts $z<10^6$, thermalization processes become inefficient and existing limits from COBE/FIRAS imply that no more than $\Delta \rho/\rho<6\times 10^{-5}$ (95% c.l.) of energy could have been injected into the CMB.
• However, at higher redshifts, when thermalization is efficient, the constraint weakens and $\Delta \rho/\rho \simeq 0.01-0.1$ could in principle have occurred.
• Existing computations for the evolution of distortions commonly assume $\Delta \rho/\rho \ll 1$ and thus become inaccurate in this case.