Cosmological neutrino bounds for non-cosmologists

TL;DR

This work surveys cosmological bounds on neutrino masses and the underlying gravitational physics that make structure growth a sensitive probe. It explains how massive neutrinos suppress small-scale power in the matter power spectrum via free-streaming, and summarizes current 95% upper limits on the mass sum $M_ u$, notably $<0.42\,6{\mathrm{eV}}$ when including Lyman-$\alpha$ data. The article highlights that upcoming weak lensing and lensing tomography could push sensitivity to $\sim 0.03\,\mathrm{eV}$, risking a potential detection given the atmospheric lower bound $\sim 0.05\,\mathrm{eV}$. It also discusses how standard freezeout and the assumption of three neutrino flavors translate cosmological measurements into mass constraints, while noting challenges and the possibility of testing nonstandard neutrino scenarios through independent checks of mass density and velocity dispersion.

Abstract

I briefly review cosmological bounds on neutrino masses and the underlying gravitational physics at a level appropriate for readers outside the field of cosmology. For the case of three massive neutrinos with standard model freezeout, the current 95% upper limit on the sum of their masses is 0.42 eV. I summarize the basic physical mechanism making matter clustering such a sensitive probe of massive neutrinos. I discuss the prospects of doing still better in coming years using tools such as lensing tomography, approaching a sensitivity around 0.03 eV. Since the lower bound from atmospheric neutrino oscillations is around 0.05 eV, upcoming cosmological measurements should detect neutrino mass if the technical and fiscal challenges can be met.

Figures (2)

• Figure 1: Cosmological constraints on the current matter power spectrum $P(k)$ reprinted from sdsspower. See sdsspower for details about the modeling assumptions underlying this figure. The solid curve shows the theoretical prediction for a "vanilla" flat scalar scale-invariant model with matter density $\Omega_m=0.28$, Hubble parameter $h=0.72$ and baryon fraction $\Omega_b/\Omega_m=0.16$. The dashed curve shows that replacing 7% of the cold dark matter by neutrinos, corresponding to a neutrino mass sum $M_\nu=1$ eV, suppresses small-scale power by about a factor of two.
• Figure 2: 95% constraints in the $(\omega_d,f_\nu)$ plane, reprinted from sdsspars and sdsslyaf. The shaded red/dark grey region is ruled out by WMAP CMB observations alone. The shaded orange/grey region is ruled out when adding SDSS galaxy clustering information sdsspars and the yellow/light grey region is ruled out when including SDSS Lyman $\alpha$ Forest information as well sdsslyaf. The five curves correspond to $M_\nu$, the sum of the neutrino masses, equaling 1, 2, 3, 4 and 5 eV, respectively --- barring sterile neutrinos, no neutrino can have a mass exceeding $\sim M_\nu/3.$