Neutrinos in cosmology

TL;DR

Neutrinos exert a pivotal influence on early-universe physics, linking particle properties to cosmological observables via Big Bang Nucleosynthesis, the Cosmic Microwave Background, and structure formation. The paper surveys mass bounds, spectral distortions, and the roles of light and heavy, stable and unstable, as well as sterile and right-handed neutrinos, emphasizing how decoupling, heating, and decay histories modify N_ u and η_{10}. It presents exact and approximate treatments of neutrino kinetics, the impact of lepton asymmetry, and the potential observational signatures in CMB anisotropies, EM backgrounds, and large-scale structure, while outlining the still-uncertain regions in the (m, τ, sin^2 2θ) parameter space for nonstandard neutrino scenarios. Overall, cosmology provides powerful, often more stringent constraints than laboratory experiments, shaping our understanding of neutrino properties and their cosmological roles. The results underscore the sensitivity of nucleosynthesis and CMB observables to neutrino physics and highlight future tests (e.g., Planck-era precision) for probing relativistic degrees of freedom and lepton asymmetries.

Abstract

Cosmological implications of neutrinos are reviewed. The following subjects are discussed at a different level of scrutiny: cosmological limits on neutrino mass, neutrinos and primordial nucleosynthesis, cosmological constraints on unstable neutrinos, lepton asymmetry of the universe, impact of neutrinos on cosmic microwave radiation, neutrinos and the large scale structure of the universe, neutrino oscillations in the early universe, baryo/lepto-genesis and neutrinos, neutrinos and high energy cosmic rays, and briefly some more exotic subjects: neutrino balls, mirror neutrinos, and neutrinos from large extra dimensions.

Figures (17)

• Figure 1: Abundances of light elements $^2H$ (by number) $^4He$ (by mass), and $^7 Li$ (by number) as functions of baryon-to-photon ratio $\eta_{10} \equiv 10^{10}n_B/n_\gamma$.
• Figure 2: Evolution of non-equilibrium corrections to the distribution functions $\delta_j = (f_{\nu_j} - f_\nu^{eq})/f_\nu^{eq}$ for running inverse temperature $x$ and fixed dimensionless momentum $y=5$ for electronic (dotted curves) and muonic (tau) (solid curves) neutrinos in the cases of FD and MB statistics.
• Figure 3: The distortion of the neutrino spectra $\delta_j = (f_{\nu_j}-f_\nu^{eq})/f_\nu^{eq}$ as functions of the dimensionless momentum $y$ at the final "time" $x=60$. The dashed lines $a$ and $c$ correspond to Maxwell-Boltzmann statistics, while the solid lines $b$ and $d$ correspond to Fermi-Dirac statistics. The upper curves $a$ and $b$ are for electronic neutrinos, while the lower curves $c$ and $d$ are for muonic (tau) neutrinos. All the curves can be well approximated by a second order polynomial in $y$, $\delta = A y (y -B)$.
• Figure 4: Cosmological energy density of massive neutrinos $\Omega = \rho_{\nu_h} /\rho_c$ as a function of their mass measured in eV. The meaning of different lines is explained in the text.
• Figure 5: Mass fraction of $^4 He$ as a function of the number of massless neutrino species. Different curves correspond to different values of the baryon-to-photon ratio $\eta_{10} \equiv 10^{10}n_B/n_\gamma = 2,3,4,5,6$ in order of increasing helium abundance.