Neutrino cosmology and Planck

TL;DR

The paper addresses how relic neutrinos shape the early and late-time evolution of the Universe and how cosmological data, especially Planck observations, constrain neutrino properties such as the total mass $M_\nu$ and the effective number of relativistic species $N_{\rm eff}$.It combines theoretical modelling of the cosmic neutrino background, neutrino free-streaming, and their imprints on the CMB and large-scale structure with current observational bounds and future projections.A key finding is that Planck data, when combined with BAO, yields a tight bound $M_\nu<0.23$ eV (95% CL), while lensing and cluster data show potential hints of nonzero masses depending on systematics; future surveys promise near-minimal mass sensitivity and prospects for distinguishing mass hierarchies.Overall, cosmology provides a powerful, complementary probe of neutrino properties that can reach into the sub-eV regime and probe new physics in the neutrino sector.

Abstract

Relic neutrinos play an important role in the evolution of the Universe, modifying some of the cosmological observables. We summarize the main aspects of cosmological neutrinos and describe how the precision of present cosmological data can be used to learn about neutrino properties. In particular, we discuss how cosmology provides information on the absolute scale of neutrino masses, complementary to beta decay and neutrinoless double-beta decay experiments. We explain why the combination of Planck temperature data with measurements of the baryon acoustic oscillation angular scale provides a strong bound on the sum of neutrino masses, 0.23 eV at the 95% confidence level, while the lensing potential spectrum and the cluster mass function measured by Planck are compatible with larger values. We also review the constraints from current data on other neutrino properties. Finally, we describe the very good perspectives from future cosmological measurements, which are expected to be sensitive to neutrino masses close the minimum values guaranteed by flavour oscillations.

Figures (4)

• Figure 1: Evolution of the background energy densities in terms of the fractions $\Omega_i$, from $T_{\nu}=1$ MeV until now, for each component of a flat Universe with $h=0.7$ and current density fractions $\Omega_{\Lambda}=0.70$, $\Omega_{\rm b}=0.05$ and $\Omega_{\rm cdm}=1-\Omega_{\Lambda}-\Omega_{\rm b} -\Omega_{\nu}$. The three neutrino masses are $m_1=0$, $m_2 = 0.009$ eV and $m_3 = 0.05$ eV.
• Figure 2: Allowed regions by oscillation data at the 3$\sigma$ level (eq. (\ref{['20-oscpardef']})) of the three main observables sensitive to the absolute scale of neutrino masses. We show the regions in the planes $m_{\beta}-\Sigma$ and $m_{\beta\beta}-\Sigma$, where $\Sigma$ is the sum of neutrino masses. Blue dotted (red solid) regions correspond to normal (inverted) hierarchy.
• Figure 3: Ratio of the matter power spectrum including three degenerate massive neutrinos with density fraction $f_{\nu}$ to that with three massless neutrinos. The parameters $(\omega_{\rm m}, \, \Omega_{\Lambda})=(0.147,0.70)$ are kept fixed, and from top to bottom the curves correspond to $f_{\nu}=0.01, 0.02, 0.03,\ldots,0.10$. The individual masses $m_{\nu}$ range from $0.046$ to $0.46$ eV, and the scale $k_{\rm nr}$ from $2.1\times10^{-3}h\,$Mpc$^{-1}$ to $6.7\times10^{-3}h\,$Mpc$^{-1}$ as shown on the top of the figure. From 20-Lesgourgues:2006nd.
• Figure 4: CMB temperature spectrum with different neutrino masses. Some of the parameters of the $\Lambda$MDM model have been varied together with $M_\nu$ in order to keep fixed the redshift of equality and the angular diameter distance to last scattering.