Thermalization of large energy release in the early Universe
Abstract
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 Δρ/ρ ≲ 6 × 10^{5} ( $95{{\ \rm per\ cent}}$ c.l.) of energy could have been injected into the CMB. However, at higher redshifts, when thermalization is efficient, the constraint weakens and Δρ/ρ ≃ 0.010.1 could in principle have occurred. Existing computations for the evolution of distortions commonly assume Δρ/ρ ≪ 1 and thus become inaccurate in this case. Similarly, relativistic temperature corrections become relevant for large energy release, but have previously not been modelled as carefully. Here, we study the evolution of distortions and the thermalization process after single large energy release at z ≳ 10^{5}. We show that for large distortions the thermalization efficiency is significantly reduced and that the distortion visibility is sizeable to much earlier times. This tightens spectral distortions constraints on lowmass primordial black holes with masses $M_{\rm PBH}\lesssim 2 \times 10^{11}\, {\rm g}$ . Similarly, distortion limits on the amplitude of the smallscale curvature power spectrum at wavenumbers $k\gtrsim 10^4\, {\rm Mpc}^{1}$ and shortlived decaying particles with lifetimes $t_X\lesssim 10^7\, {\rm s}$ are tightened, however, these still require a more detailed timedependent treatment. We also briefly discuss the constraints from measurements of the effective number of relativistic degrees of freedom and light element abundances and how these complement spectral distortion limits.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 October 2020
 DOI:
 10.1093/mnras/staa2131
 arXiv:
 arXiv:2005.11325
 Bibcode:
 2020MNRAS.498..959C
 Keywords:

 cosmic background radiation;
 early Universe;
 cosmology: theory;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory
 EPrint:
 22 pages, 19 figures, accepted MNRAS version