AR-2019-2020

Surajit Chattopadhyay Cosmology of a generalized version of holographic dark energy in presence of bulk viscosity and its inflationary dynamics through slow roll parameters In this work done jointly with Gargee Chakraborty, we aimed at reconstructing ρ Λ through H in non-interacting and interacting scenario and holo- graphic background evolution. The bulk viscous pressure has been taken as Π = − 3 Hξ , where ξ = ξ 0 + ξ 1 H + ξ 2 ( ˙ H + H 2 ). In the reconstruc- tion scheme reported here, firstly we choose vis- cous scenario neglecting the contribution of dark matter and without any choice of scale-factor. A dark energy (DE) model with higher order deriva- tive of Hubble parameter, which is a particular case of Nojiri-Odintsov holographic DE, that uni- fies phantom inflation with the acceleration of the universe on late-time. The reconstruction has been carried out in the presence of bulk-viscosity, where the bulk-viscous pressure has been taken as a func- tion of Hubble parameter. Ranges of cosmic time t have been derived for quintessence, cosmological constant and phantom behaviour of the equation of state (EoS) parameter. In the viscous scenario, the reconstruction has been carried out in an inter- acting and non-interacting situations and in both the cases, stability against small perturbations has been observed. Finally, the slow roll parameters have been studied and a scope of exit from infla- tion, has been observed. Also, availability of quasi- exponential expansion has been demonstrated for interacting viscous scenario and a study through tensor to scalar ratio has ensured consistency of the model with the observational bound by Planck. Along with primordial fluctuations, the interacting scenario has been found to generate strong dissipa- tive regime. A study of the bulk viscous pressure in scalar fields and holographic ricci dark energy considered in the modified gravity framework This work is rigorous study of the reconstruction of the modified gravity in the framework of the scalar field models of dark energy and holographic Ricci dark energy, a generalized version of the holo- graphic dark energy presented in S. Nojiri and S. D. Odintsov [ Gen. Rel. Grav. 38 , 1285 (2006)]. The tachyon and quintessence scalar fields have been considered and the cosmology associated with the presence of bulk viscosity has been studied. In the first part of the study, we have demonstrated the behaviuor of the bulk viscosity coefficient in the framework of the reconstructed tachyon scalar field model of dark energy. The scale factor is chosen in the form a ( t ) = a 0 t β , where β > 0. Two scalar field models, namely, tachyon and quintessence have been considered in the framework of the modi- fied field equations through incorporation of the bulk viscous pressure. The reconstructed density and pressure of the scalar field models have been explored for the cosmological consequences in the presence of bulk viscosity. The behaviour of the effective equation of state parameters has been in- vestigated. Finally, we have reconstructed f ( T ) gravity in the presence of holographic Ricci dark energy and a transition of the effective equation of state parameter from quintessence to phantom has been observed. This work has been carried out in collaboration with Sthiti Chakrabarti, and Irina Radinchi. Bhag Chand Chauhan Investigating the sterile neutrino parameters with QLC in 3 + 1 scenario In the scenario with four generation quarks and leptons and using a 3 + 1 neutrino model having one sterile and the three standard active neutri- nos with a 4 × 4 unitary transformation matrix, U PMNS 4 , we perform a model-based analysis us- ing the latest global data and determine bounds on the sterile neutrino parameters, i.e., the neu- trino mixing angles. Motivated by our previous results, where in a quark-lepton complementarity (QLC) model, we predicted the values of θ PMNS 13 = (9 +1 − 2 ) ◦ and θ PMNS 23 = (40 . 60 +0 . 1 − 0 . 3 ) ◦ . In the QLC model, the non-trivial correlation between CKM 4 and PMNS 4 mixing matrix is given by the cor- relation matrix V c 4 . Monte Carlo simulations are performed to estimate the texture of V c 4 followed by the calculation of PMNS 4 using the equation, U PMNS 4 = ( U CKM 4 .ψ 4 ) − 1 .V c 4 , where ψ 4 is a di- agonal phase matrix. The sterile neutrino mixing angles, θ PMNS 14 , θ PMNS 24 and θ PMNS 34 are assumed to be freely varying between (0 − π/ 4), and ob- tained results which are consistent with the data available from various experiments, like No ν A, MI- NOS, SuperK, Ice Cube-DeepCore. In further in- vestigation, we analytically obtain approximately similar ranges for various neutrino mixing param- eters | U μ 4 | 2 and | U τ 4 | 2 . This work has been