Constraints on the neutrino parameters by future cosmological 21cm line and precise CMB polarization observations

TL;DR

This work forecasts how future cosmological observations, notably the 21 cm line from SKA and precise CMB polarization measurements, can tightly constrain neutrino properties including the total mass $\Sigma m_\nu$, the effective number of neutrino species $N_\nu$, and the mass hierarchy. By employing Fisher-matrix forecasts that combine 21 cm data (SKA1/2), CMB (Planck+Polarbear-2 or Simons Array), and DESI BAO, the authors quantify how degeneracies between cosmological parameters are broken and how foreground removal impacts the results. They find that SKA data significantly improve neutrino constraints, with $\sigma(N_\nu)$ reaching $\sim 0.06$–$0.09$ (95% CL) without aggressive 21 cm foregrounds, and that SKA phase 2 enables potential 95% CL discrimination of the neutrino mass hierarchy for $\Sigma m_\nu \lesssim 0.1$ eV, especially when combined with Planck/Simons Array and DESI. Overall, the paper demonstrates a strong synergistic path to probing neutrino physics via cosmology, highlighting the critical roles of instrumental design, sky coverage, and foreground mitigation in achieving robust conclusions about the neutrino sector.

Abstract

Observations of the 21 cm line radiation coming from the epoch of reionization have a great capacity to study the cosmological growth of the Universe. Also, CMB polarization produced by gravitational lensing has a large amount of information about the growth of matter fluctuations at late time. In this paper, we investigate their sensitivities to the impact of neutrino property on the growth of density fluctuations, such as the total neutrino mass, the effective number of neutrino species (extra radiation), and the neutrino mass hierarchy. We will show that by combining a precise CMB polarization observations such as Simons Array with a 21 cm line observation such as Square kilometer Array (SKA) phase 1 and a baryon acoustic oscillation observation (Dark Energy Spectroscopic Instrument:DESI) we can measure effects of non-zero neutrino mass on the growth of density fluctuation if the total neutrino mass is larger than 0.1eV. Additionally, the combinations can strongly improve errors of the bounds on the effective number of neutrino species sigma(N_nu) ~ 0.06-0.09 at 95 % C.L.. Finally, by using SKA phase 2, we can determine the neutrino mass hierarchy at 95 % C.L. if the total neutrino mass is similar to or smaller than 0.1 eV.

Figures (13)

• Figure 1: Contours of 95% C.L. forecasts in $\Sigma m_{\nu}$-$N_{\nu}$ plane. Fiducial values of neutrino parameters, $N_{\nu}$ and $\Sigma m_{\nu}$, are taken to be $N_{\nu} = 3.046$ and $\Sigma m_{\nu} = 0.1$ eV. In the left two panels, the contours are the constraints by adopting Planck (outer dashed line), Planck combined with Polarbear-2 (PB-2) ($f_{{\rm sky}}=0.016$) (outer dotted line) or Simons Array (SA) (inner thick dashed line), Planck + BAO(DESI) + Hubble prior + Polarbear-2 ($f_{{\rm sky}}=0.016$) (inner thick dotted line) or Simons Array (thin solid line), respectively. In the right two panels, they are the constraints by adopting Planck (outer dashed line), Planck + BAO(DESI) + Hubble prior combined with Polarbear-2 ($f_{{\rm sky}}=0.016$) (dotted line) or Simons Array (outer thin solid line), Planck + BAO(DESI) + Hubble prior + Simons Array combined with SKA phase 1 ($N_{{\rm filed}}=4$) (inner thick dashed line) or phase 2 ($N_{{\rm filed}}=4$) (inner thick line), respectively.
• Figure 2: Same as Fig.\ref{['fig:Nnu01_fsky0016']}, but the fiducial values of neutrino parameters, $N_{\nu}$ and $\Sigma m_{\nu}$, are taken to be $N_{\nu} = 3.046$ and $\Sigma m_{\nu} = 0.06$ eV.
• Figure 3: Same as Fig.\ref{['fig:Nnu01_fsky0016']}, but the sky coverages of Polarbear-2 and Simons Array are $f_{{\rm sky}}=0.2$.
• Figure 4: Same as Fig.\ref{['fig:Nnu01_fsky02']}, but the fiducial values of neutrino parameters, $N_{\nu}$ and $\Sigma m_{\nu}$, are taken to be $N_{\nu} = 3.046$ and $\Sigma m_{\nu} = 0.06$ eV.
• Figure 5: Same as Fig.\ref{['fig:Nnu01_fsky0016']}, but the sky coverages of Polarbear-2 and Simons Array are $f_{{\rm sky}}=0.65$.