Primordial nucleosynthesis constraints on high-z energy releases

TL;DR

This paper uses primordial nucleosynthesis data to set constraints on energy releases in the early universe, extending previous limits derived from CMB spectrum observations.

Contribution

It demonstrates that energy releases after nucleosynthesis are limited to 7.8% of the current CMB energy density, refining constraints on early universe thermal history.

Findings

No more than 7.8% of CMB energy density was released post-nucleosynthesis

Nucleosynthesis constraints extend CMB spectrum limits by two orders of magnitude in redshift

Thermalization processes at high redshift erase imprints of large energy releases

Abstract

The cosmic microwave background (CMB) spectrum provides tight constraints on the thermal history of the universe up to $z \sim 2\times 10^6$. At higher redshifts thermalization processes become very efficient so that even large energy releases do not leave visible imprints in the CMB spectrum. In this paper we show that the consistency between the accurate determinations of the specific entropy at primordial nucleosynthesis and at the electron-photon decoupling implies that no more than 7.8% of the present day CMB energy density could have been released in the post-nucleosynthesis era. As pointed out by previous studies, primordial nucleosynthesis complements model independent constraints provided by the CMB spectrum, extending them by two orders of magnitude in redshift.