Neutrino Physics from the Cosmic Microwave Background and Large-Scale Structure

Abstract

Executive SummaryThe cosmological background of neutrinos thermally produced in the big bang has been definitively (albeit indirectly) detected. Measurements of the cosmic microwave background (CMB) alone have led to a constraint on the effective number of neutrino species of N eff = 3.36 ± 0.34 [1], a value 10σ away from zero and consistent with expectations. Experiments planned and underway are prepared to study this background in detail via its influence on distance-redshift relations and the growth of structure. The program for the next decade described in this document, including upcoming spectroscopic surveys eBOSS and DESI and a new Stage-IV CMB polarization experiment CMB-S4, will achieve σ( m ν ) = 16 meV and σ(N eff ) = 0.020. Such a mass measurement will produce a high significance detection of non-zero m ν , whose lower bound derived from atmospheric and solar neutrino oscillation data is about 58 meV. If neutrinos have a normal mass hierarchy, this measurement will definitively rule out the inverted neutrino mass hierarchy, shedding light on one of the most puzzling aspects of the Standard Model of particle physics -the origin of mass.This precise a measurement of N eff will allow for a precision test of the standard cosmological model prediction that N eff = 3.046. The difference from three is due to the small amount of entropy from electron/positron annihilation that gets transferred to the neutrinos; N eff is by design equal to three in the idealized case that all of this entropy is transferred to photons. Finding N eff consistent with 3.046 would demonstrate that we understand very well the thermal conditions in the universe at just one second after the big bang. On the other hand, finding N eff significantly different from 3.046 would be a signature of new physics. Possibilities include sterile neutrinos produced via oscillations in the early universe, a large matter/antiarXiv:1309.5383v3 [astro-ph.CO] 30 May 2014 2 matter asymmetry in the neutrino sector, or new particles that interact even more weakly than neutrinos but that were in thermal and chemical equilibrium deep in the radiation-dominated era (e.g., [2]).The effects of neutrino properties on cosmology are precisely predicted from theory, appear in many observables in diverse ways, and are clearly observationally distinguishable from the effects of other cosmological parameters. For example, increasing N eff not only suppresses small-scale fluctuation power in the CMB, but also leads to detectable changes in the temporal phase of the acoustic oscillations, e.g., [3]. Increasing m ν in a manner that is consistent with the measured CMB power spectrum leads to very specific redshiftand scale-dependent changes to the power spectrum of both matter and the galaxy distribution [4]. These signatures will provide important consistency tests.Much of the sensitivity to m ν will come from measurements of the gravitational lensing of the CMB, measurements of the baryon acoustic oscillation (BAO) features and broadband power spectrum, and measuremen...

Cosmological signals of a mirror twin Higgs

“…The next generation of CMB experiments are projected to strengthen this constraint to ∆N eff 0.02 at the 1σ level [49].…”

First Constraints on the Complete Neutrino Mixing Matrix with a Sterile Neutrino

“…The dependencies include assuming a "standard" thermal history for the Universe [39]. Sterile neutrino thermalization can be suppressed a number of plausible ways [40][41][42][43][44][45][46][47][48].…”

Cosmological constraints on neutrinos with Planck data

“…The effects of neutrino properties on cosmology are precisely predicted from theory and leave peculiar imprints on different cosmological probes such as the Galaxy distribution, the Lyman-α forest,the 21 cm surveys and of course on the Cosmic Microwave Background (CMB) and its lensing [2]. We will show that is possible to use precise measurement of CMB anisotropies from Planck ( Fig.1) to infer neutrino absolute mass scale and neutrino energy density.…”

“…We will show that is possible to use precise measurement of CMB anisotropies from Planck ( Fig.1) to infer neutrino absolute mass scale and neutrino energy density. Future experiments are expected to improve on these results [2] definitively legitimizing cosmology as a laboratory for Neutrino Physics. …”

“…Since the masses are small their contribution as a matter component is peculiar. Their thermal velocities are nonnegligible in the early universe which leads to a smearing of over-dense regions that has a characteristic shape in the power spectrum of matter distribution [2]. Concerning the effect on the CMB it is important to remark that the shape of the angular power spectrum is mostly affected by the evolution of the primordial plasma before recombination.…”