Two sides of the same coin: sterile neutrinos and dark radiation. Status and perspectives

Abstract

The presence of light sterile neutrinos is one of the unanswered questions of particle physics. The cosmological counterpart is represented by dark radiation, i.e. any form of radiation present in the early Universe besides photons and standard (active) neutrinos. This short review provides a comprehensive overview of the two problems and of their connection. We review the status of neutrino oscillation anomalies, commenting on the most recent oscillation data and their mutual tensions, and we discuss the constraints from other terrestrial probes. We show the shortcomings of translating light sterile neutrinos in cosmology as additional thermalised relativistic species, produced by neutrino oscillations, and we detail alternative solutions, specifically focusing on neutrino non standard interactions, and on their link to the Hubble constant problem. The impact of a new force leading to dark radiation -dark matter interactions is also discussed in the realm of new physics in the dark sector.

Quantifying the global "CMB tension" between the Atacama Cosmology Telescope and the Planck satellite in extended models of cosmology

“…In particular, allowing a non-standard lensing amplitude A lens = 1 in the cosmological model, we do not observe great changes in the consistency between the two experiments and estimate a global tension of about 2.6 σ, close to the ΛCDM result. On the other hand, considering the possibility of non-flat background geometries or a non-standard Dark Energy equation of state, the tension between Planck and ACT-DR4 slightly reduces to ∼ 2.3 σ. Conversely, other extensions involv-2 In the case of three active massless neutrinos, the Standard Model of particle physics predicts N eff = 3.044 [51][52][53][54][55][56] while larger (smaller) values are possible if additional (less) relativistic degrees of freedom are present in the early Universe (see, e.g., Refs [57][58][59][60][61][62][63][64][65][66] and the references therein).…”

Early-Universe Model Building

“…Interestingly, it is possible to completely change this constraint by relaxing the assumption of free-streaming. In fact, CMB data are powerful to test new secret interactions of neutrinos since these interactions affect the streaming properties of neutrinos [855][856][857][858][859][860][861]. As a striking example, Planck 2015 temperature data can fully accommodate 4 species of strongly interacting neutrinos through a contact interaction, with H 0 = 72.3 ± 1.4 km/s/Mpc [859,[862][863][864][865].…”

“…interacting DR [861,868,879], evaporating PBHs [880] -see review [822]). Additionally, models involving interacting dark radiation produced around matter-radiation equality (for instance following the decoupling of heavy particles from the thermal bath) can perform significantly better (and in fact are even favored by Planck data) than standard interacting dark radiation scenarios (present from early times), as shown recently in Ref.…”

Constraining decaying dark matter with BOSS data and the effective field theory of large-scale structures

“…[2,6,11] for discussion), these discrepancies might be the first clue about the intrinsic nature of the ΛCDM dark sector. On the one hand, the resolution of the Hubble tension most likely involves new physics in the pre-recombination era, through a decrease of the sound horizon before recombination [12][13][14][15][16][17], such as model involving dark radiation and/or new neutrino properties [18][19][20][21][22][23][24][25][26][27], early dark energy [28][29][30][31][32][33], modified gravity [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] or exotic recombination [54][55][56][57]…”