Qu'est ce que mesure vraiment le nombre effectif de neutrinos dans le modèle cosmologique standard dit de concordance ?

Réponse de blogueur et commentaires d'experts 

Encore un billet sur les neutrinos ! Oui ça commence à faire beaucoup. On en parlait dans le billet précédent et il y a plus longtemps ici ou là -  mais que voulez-vous chers lecteurs-trices se sont les seules particules détectables à ce jour dont les propriétés nous permettent de sonder la physique au delà du Modèle Standard, brefs les seuls messagers fiables  porteurs d('un)e nouvelle( phy)s(iques) à se mettre sous la dent (pour le moment bien sûr). Alors il est toujours utile d'y revenir, particulièrement lorsque la lecture d'un billet de blog spécialisé (et de ses commentaires) comme ci-dessous nous permet de découvrir clairement les hypothèses implicites des modèles explicatifs de certaines données expérimentales pas toujours très explicites :

One interesting result [from the Planck satellite] is the new improved constraint on the effective number of neutrinos, Neff in short. The way this result is presented may be confusing. We know perfectly well there are exactly 3 light active (interacting via weak force) neutrinos; this has been established in the 90s at the LEP collider, and Planck has little to add in this respect. Heavy neutrinos, whether active or sterile, would not show in this measurement at all. For light sterile neutrinos, Neff implies an upper bound on the mixing angle with the active ones. The real importance of Neff lies in that it counts any light particles (other than photons) contributing to the energy density of the universe at the time of CMB decoupling. Outside the standard model neutrinos, other theorized particles could contribute any real positive number to Neff, depending on their temperature and spin. A few years ago there have been consistent hints of Neff much larger 3, which would imply physics beyond the standard model. Alas, Planck has shot down these claims. The latest number combining Planck and Baryon Acoustic Oscillations is Neff =3.04±0.18, spot on 3.046 expected from the standard model neutrinos. This represents an important constraint on any new physics model with very light (less than eV) particles.

Planck: what's new 

Jester, Blog Résonaances,  Saturday, 13 December 2014

Anonymous comment:

Note that the standard value of N_eff = 3.046 for 3 active neutrinos relies on several assumptions:

*) 3 active neutrinos in the Standard Model

*) No partly thermalised lights species

*) Reheating happened above ~4 MeV

*) No entropy production between 1 MeV and today

*) No cooling of photons between 1 MeV and today (e.g. through Dark Sector mixing)

Only the first assumption was verified by LEP, so there were plenty of room for N_eff to be different from 3.046.

Jester reply:

I completely agree, maybe except that "Reheating happened above ~4 MeV" is independently confirmed by nucleosynthesis. I didn't mean that Neff is not useful, on the contrary. I meant that it is often presented as a measurement of the number of neutrinos, which may be misleading.

En attendant le retour du lièvre continuons à écouter se que nous raconte la tortue 

Autrement dit, approfondissons patiemment nos connaissances sur le rôle des neutrinos dans l'astrophysique des débuts de l'univers vue depuis nos modernes spectro-télescopes collecteurs-analyseurs de photons ayant voyagés plus de dix milliards d'années ... avant que les physiciens des particules ne relancent leur accélérateur géant en espérant trouver rapidement des réponses à leurs questions dans la collision-désintégration de quelques paires quark-antiquark ou gluons-gluons en une fraction de seconde (ou presque).

Physics Beyond the Standard Models (BSMs), i.e. beyond Electro-Weak Model and beyond Standard Cosmological Model (... also called λ Cold Dark Matter model) is required for the explanation of the astrophysical and cosmological observational data. Namely, the contemporary SCM, contains considerable BSMs components - the so called dark energy (DE) and dark matter (DM), both with yet unrevealed nature, alas. These constitute 96% of the universe matter today, and play a considerable role at the matter dominated epoch, i.e. at later stages of the Universe evolution! 

BSMs physics is needed also for revealing the nature and the characteristics of the inflaton (the particle/field responsible for inflationary expansion stage) and CP-violation (CPV) or/and B[aryon number]-violation (BV) mechanisms. These are expected necessary ingredients in the theories of inflation and baryon asymmetry generation, which are the most widely accepted today hypotheses providing natural explanations of numerous intriguing observational characteristics of our universe. 

The inflationary theory explains naturally and elegantly the initial conditions of the universe in the pre-Friedmann epoch, namely: the extraordinary homogeneity and isotropy at large scales of the universe at its present epoch; its unique isotropy at the Cosmic Microwave Background (CMB) formation epoch (when the universe was ∼ 380000 years old); its unique flatness and the pattern of structures it has. Besides, the inflationary early stage explains the lack of topological defects in the universe. While the baryon asymmetry generation models explain the locally observed matter-antimatter asymmetry of the universe. 

...we have been already the lucky witnesses of the experimental establishment of the BSM physics in the neutrino sector. Experimental data on neutrino oscillations firmly determined three neutrino mixing angles and three mass differences, corresponding to the existence of at least two non-zero neutrino masses. The concrete neutrino mass pattern and possible CPV mechanism are to be detected in near future. Thus, the neutrino experimental data ruled out the Standard Models assumptions about zero neutrino masses and mixing and about flavor lepton number (L) conservation. Cosmology provides complementary knowledge about neutrino and BSM physics in the neutrino sector, because neutrino had a considerable influence on the processes during different epochs of the universe evolution. At the hot early universe stage, radiation dominated (RD) stage, light neutrinos were essential ingredients of the universe density, determining the dynamics of the universe.

Neutrinos played also an essential role in different processes as for example Big Bang nucleosynthesis (BBN). In particular, electron neutrino participated in the pre-BBN neutron-proton transitions, that took place during the first seconds, and nucleons freezing, and thus they influenced considerably the primordial production of the light elements (BBN) during the first minutes of the universe. Hence, BBN is very sensitive to the number of the light neutrino types, neutrino characteristics, neutrino chemical potentials, the possible presence of sterile neutrino, etc. BBN is capable to differentiate different neutrino flavors, because νe participates into proton-neutron transitions in the pre-BBN epoch, essential for yields of the primordially produced elements, while νµ and ντ do not exert kinetic effect on BBN 

At later stages of the universe evolution (T < eV) relic neutrinos, contributing to the matter density, influenced CMB anisotropies, played a role in the formation of galaxies and their structures. CMB and Least Scattering Surface (LSS), being sensitive to the total neutrino density and provide information about the neutrino masses and number of neutrino species. Hence, although the relic neutrinos, called Cosmic Neutrino Background (CNB) are not yet directly detected, strong observational evidence for CNB and stringent cosmological constraints on relic neutrino characteristics exist from BBN, CMB and LSS data. In particular, the determinations of light elements abundances and BBN theory predictions are used to put stringent constraints on neutrino characteristics (the effective number of relativistic particles, lepton asymmetry, sterile neutrino characteristics, neutrino mass differences and mixings) while CMB and LSS data provide constraints on neutrino masses and neutrino number density corresponding to CMB and LSS formation epochs.

Neutrinos from the Early Universe and Physics Beyond Standard Models

Daniela Kirilova (Submitted on 7 Jul 2014)