Origin of cosmological neutrino mass bounds: background $textit{versus}$ perturbations
Toni Bert’olez-Mart’inez, Ivan Esteban, Rasmi Hajjar, Olga Mena, Jordi Salvado
arXiv:2411.14524v1 Announce Type: new
Abstract: The cosmological upper bound on the total neutrino mass is the dominant limit on this fundamental parameter. Recent observations-soon to be improved-have strongly tightened it, approaching the lower limit set by oscillation data. Understanding its physical origin, robustness, and model-independence becomes pressing. Here, we explicitly separate for the first time the two distinct cosmological neutrino-mass effects: the impact on background evolution, related to the energy in neutrino masses; and the “kinematic” impact on perturbations, related to neutrino free-streaming. We scrutinize how they affect CMB anisotropies, introducing two effective masses enclosing $textit{background}$ ($sum m_nu^mathrm{Backg.}$) and $textit{perturbations}$ ($sum m_nu^mathrm{Pert.}$) effects. We analyze CMB data, finding that the neutrino-mass bound is mostly a background measurement, i.e., how the neutrino energy density evolves with time. The bound on the “kinematic” variable $sum m_nu^mathrm{Pert.}$ is largely relaxed, $sum m_nu^mathrm{Pert.} arXiv:2411.14524v1 Announce Type: new
Abstract: The cosmological upper bound on the total neutrino mass is the dominant limit on this fundamental parameter. Recent observations-soon to be improved-have strongly tightened it, approaching the lower limit set by oscillation data. Understanding its physical origin, robustness, and model-independence becomes pressing. Here, we explicitly separate for the first time the two distinct cosmological neutrino-mass effects: the impact on background evolution, related to the energy in neutrino masses; and the “kinematic” impact on perturbations, related to neutrino free-streaming. We scrutinize how they affect CMB anisotropies, introducing two effective masses enclosing $textit{background}$ ($sum m_nu^mathrm{Backg.}$) and $textit{perturbations}$ ($sum m_nu^mathrm{Pert.}$) effects. We analyze CMB data, finding that the neutrino-mass bound is mostly a background measurement, i.e., how the neutrino energy density evolves with time. The bound on the “kinematic” variable $sum m_nu^mathrm{Pert.}$ is largely relaxed, $sum m_nu^mathrm{Pert.}