34th Annual Report (2021-22)

174 Abstract: A dissipative model of the Universe based on the causal relativistic truncated Israel-Stewart theory is analysed in the context of recent accelerated expansion of the Universe. The bulk viscosity and relaxation time are taken as ξ = αρ s and τ = α ϵ γ (2− γ ) ρ s −1 respectively. For s =1/2, we found an analytical solution for the Hubble parameter of the model. We have estimated the model parameters by treating γ =1 and γ as a free parameter using the latest cosmological data. The model predicts a prior decelerated phase and an end de Sitter phase as in the standard Λ CDM model. The dynamical system analysis shows that the prior decelerated epoch is an unstable equilibrium, while the far future de Sitter epoch is stable. The age of the Universe obtained around 13.66 Gyr, which is close to the recent observations. The second law of thermodynamics is found to be satisfied throughout the evolution in this model. The feasibility of the model has been checked by contrastingwithmodels based on the full Israel-Stewart and the Eckart viscous theories. The truncated viscous model appea r s mo r e compa t i b l e wi t h astronomical observations than the Eckart and full causal viscousmodels. 2. Sarath N and Titus K Mathew, 'Decaying vacuum and evolution from early inflation to late acceleration', Mod. Phys. Lett. B36 (2021) 23, 2150160. Abstrac: Decaying vacuum models are a class of models that incorporate the vacuum energy density as a time- evolving entity that has the potential to explain the entire evolutionary history of the universe in a single framework. A general solution to the Friedmann equation can be obtained by considering vacuum energy density as a function of the Hubble parameter. We have obtained the asymptotic solution by choosing the appropriate equation of state for matter and radiation. Finite boundaries in the early and late de Sitter epoch could be defined by considering the evolution of primordial perturbation wavelength. An epoch invariant number Nc determines the number of perturbation modes that cross the Hubble radii during each epoch has been obtained. 3. Sarath N and Titus K Mathew, 'Running vacuu model versur LambdaCDM – a bayesian analysis', Mon. Not. Roy. Astron. Soc. 510 (2022) 4, 5553. Abstract: We study the running vacuum model in which the vaccum energy density depends on square of Hubble parameter in comparison with the Λ CDM model. In this work, the Bayesian inference method is employed to test against the standard Λ CDM model to appraise the relative significance of our model, using the combined data sets, Pantheon+CMB+BAO and Pantheon+CMB+BAO+Hubble data. The model parameters and the corresponding errors are estimated from the marginal likelihood function of the model parameters. Marginalizing over all model parameters with suitable prior, we have obtained the Bayes factor as the ratio of Bayesian evidence of our model and the Λ CDMmodel. The analysis based on Jeffrey's scale of bayesian inference shows that the evidence of our model against the Λ CDMmodel is weak for both data combinations. Even though the running vacuum model gives a good account of the evolution of the universe, it is not superior to the Λ CDMmodel. Department of Physics, University of Calicut during April 1, 2021 – March 2022 Coordinator: Prof. C. D. Ravikumar Areaof research ¬ Seminarsconducted. Galaxy formation and evolution, Active GalacticNuclei. 1. Online seminar on Simultaneous UV/Optical, X-ray spectral analysis of Fermi bright Blazar by Aminabi T on May 19, 2021 2. Online seminar on CIR scaling relations for early-type galaxies hosting nuclear star clusters by Sruthi K on Jun9, 2021. 3. Online seminar on Infra-red central intensity ratio study in early-type galaxies by Baheeja. C on July 7, 2021. 4. Online seminar on Study of central intensity ratio in nearby seyfert galaxies by Vinod K T on July 14, 2021 5. Online seminar on Signature of jet MHD turbulence in blazar spectra by Habeeb Rahman. C on Aug4, 2021. 6. Online seminar on Analysis and Interpretation of spectral features or blazar by Sitha K Jagan on October 20, 2021 1. 'Convex X-ray spectra of PKS 2155-304 and constraints on the minimum electron energy', 2021, Jagan, Sitha K. ; Publicationsusing ICARDfacilities Sahayanathan, S. ; Rieger, Frank M. ; Ravikumar, C. D., Monthly Notices of the Royal Astronomical Society, Volume 506, 3996-4006 (2021), doi:10.1093/mnras/stab1993.