Cosmological bounds on masses of neutrinos and other thermal relics
Steen Hannestad
TL;DR
This paper reviews cosmological bounds on neutrino masses and other thermally produced relics, highlighting how precision cosmology constrains the absolute neutrino mass scale beyond laboratory limits. It derives neutrino decoupling physics from the Boltzmann equation and the comparison of the interaction rate to the Hubble expansion, obtaining a decoupling temperature around a few MeV and the relic neutrino temperature relation $T_\nu/T_\gamma = (4/11)^{1/3}$ with small corrections. It then explains how massive neutrinos imprint a scale-dependent suppression of structure growth through their free-streaming behavior, characterized by a finite $\lambda_{\rm FS}$ and a corresponding transfer-function signature in a flat $\Lambda$CDM framework, quantified by $\Omega_\nu h^2$ and $N_\nu$. The analysis of WMAP, SDSS, and Lyman-$\alpha$ data yields stringent upper bounds on the sum of neutrino masses (e.g., $\sum m_\nu \lesssim 0.65$ eV at 95% C.L.) and extends to constraining other light thermal relics, such as Majorana fermions decoupling near the electroweak transition (for which $m < 5$ eV).
Abstract
With the advent of precision data, cosmology has become an extremely powerful tool for probing particle physics. The prime example of this is the cosmological bound on light neutrino masses. Here I review the current status of cosmological neutrino mass bounds as well as the various uncertainties involved in deriving them. From WMAP, SDSS, and Lyman-alpha forest data an upper bound on the sum of neutrino masses of 0.65 eV (95% C.L.) can be derived with any assumptions about bias. I also present new limits on other light, thermally produced particles. For example, a hypothetical new Majorana fermion decoupling around the electroweak phase transition must have m < 5 eV.