Solving Cosmological Tensions with Neutrino Physics

Abstract

The ΛCDM model is the most successful model of cosmology to fit most of the cosmological observations, yet a few discrepancies exist. There is a 4σ to 5σ mismatch in determining the Universe’s current expansion rate from CMB observations using ΛCDM Model and the direct estimates. There is a milder tension in measuring the growth of structures from CMB and several weak lensing surveys. The solution of these tensions might hint towards new physics in the Universe. In this thesis, we explore the possibilities of the answer coming from the neutrino sector. We showed that ’S8’ Tension might be due to the existence of non-thermal sterile (neutrino-like) particles produced from moduli decay. These non-thermal sterile neutrinos contribute fractionally to overall dark matter density, suppressing matter power spectra at small scales. The cosmological effect of such neutrino-like species can be parameterized by its effective mass meffsp and relativistic degree of freedomΔNeff . The tension can be reduced below 2σ from Planck data only, but it does not favor a non-zero {meffsp ,ΔNeff}. With the inclusion of the measurement of S8 from KIDS1000+BOSS+2dfLenS, the S8-tension would hint at the presence of nonthermal neutrinos with parameters (meffsp ≃ 0.67+0.26−0.48eV,ΔNeff ≃ 0.06 ± 0.05). Furthermore, inclusion of Pantheon and BOSS BAO/fσ8 data gives (meffsp ≃ 0.48+0.17−0.36,ΔNeff ≃ 0.046+0.004−0.031). Once these two parameters are matched, present linear cosmological observations can’t differentiate between the two models. Using this equivalence, We transferred our non-thermal model parameters to other hidden sector hot dark matter models, such as the Dodelson-Widrow and thermal models with a temperature different than the standard sector. These might have interesting implications from a particle physics point of view. The scales affected by these Light but massive relics (LiMRs) are nonlinear. Also, linear cosmological observables cannot differentiate between two non-thermal models. In the next part, we initiate a systemic study of the effects of LiMRs on smaller (non-linear) scales employing N-body simulations. We mainly focused on the nonthermal LiMRs produced from moduli decay. However, the approaches proposed here are easily generalizable to a wide range of LiMR models — for instance, we took the Dodelson-Widrow model. In broad terms, we find that small-scale signatures of LiMRs are different from the ΛCDM model, even if the σ8 value matches between the models. We demonstrated that in future surveys, weak lensing observations around massive clusters, between ∼ [0.1, 10] h−1Mpc, will have enough signal-to-noise ratio to differentiate between LiMR models and ΛCDM model, which are fitting both CMB data and large (linear) scale structure data at late times. Furthermore, we find that the LiMR models, which were indistinguishable using linear cosmological observations, may be distinguished by using these nonlinear probes. Therefore, combined large- and small-scale analyses of CMB and late-time structure formation data are the most effective ways to evaluate and restrict LiMR models. We also consider the possibility that if the produced particles are kev range and account for the entire dark matter budget in the Universe. We derived constraints from small-scale structures. We show that linear power spectra (and transfer functions) corresponding to non-thermal WDM produced from moduli decay can be mapped to effective thermal-relic warm dark matter models. This production mechanism is, therefore, subject to warm dark matter constraints from small-scale structures as probed by the abundances of the ultra-faint dwarf galaxies and strong gravitational lensing flux ratio statistics. We use the correspondence to thermal-relic models to derive a lower bound on the non-thermal particle mass of 107 keV at 95% confidence. Next, we studied the Hubble tension, which might hint towards the modification in the early Universe. Early dark energy(EDE) is one of the most favored candidates to address the Hubble tension. We show that neutrinos might interact with the scalar field in the early Universe and act as early dark energy. This interaction naturally takes place around matter radiation equality (depending on neutrino mass) and solves the problem of fine-tuning, which the conventional EDE models suffer. Further, We explore the possibility that Early Dark Energy (EDE) is dynamical and study its effect on cosmological observables. We find that the present data have a mild preference for non-cc early dark energy (wi = −0.78) using Planck+BAO+Pantheon+SH0ES data sets, leading to Δχ2min improvement of -2.5 at the expense of one more parameter. However, wi is weakly constrained, with wi < −0.56 at 1σ. We argue that allowing for wi ̸= −1 can decrease the σ8 parameter. Yet, in practice, the decrease is only ∼ 0.4σ, and σ8 is still larger than weak lensing measurements. We conclude that while promising, a dynamical EDE cannot simultaneously resolve both H0 and σ8 tensions. In the last chapter of the thesis, we study an extended dark energy model (comprising four free parameters governing the dark energy equation of state) in light of cosmological tensions. We found that present cosmological observations can not constrain all four parameters simultaneously. We also report that the model favors a non-zero value for the neutrino mass parameter at the most at ∼ 1σ level (Σmν = 0.1847+0.0698−0.165 eV) with Planck+BAO+SN1a+MB+S8. This model also brings down the Hubble tension to ∼ 2.5σ level and the S8 tension to ∼ 1.5σ level. The present value of the equation ofstate for dark energy is better constrained in this model and consistent of cosmological constant.