A Welcome Note from the Editor

We are delighted and proud to welcome you to the second issue of Volume 2. Each article received and accepted is an important contribution to the already existing knowledge in the field of Biomedical Sciences. All the editorial team is excited about the progress of PBMJ as an international journal. As editors, we would like to express our heartiest congratulation to the team and welcome to the authors and readers. We are also grateful to the advisory board and managing editors. We hope that PBMJ can promote the academic and applicable research and improve the research activities and collaborations. We are aware of the bumps along the way, but we are determined to keep pursuing the research goals to meet the high quality standards and move forward with great courage. If you have any suggestions to improve, you may write to us as a reader. In the age of technology, we can actively conversate with the readers and get their feedback to improve the quality with their valuable input. PBMJ will continue to serve the Biomedical Sciences as an outlet for high-quality research. This is an exciting time for the journal and we look forward to working with authors, the Editorial board and the team to make PBMJ as a leading source for work in the space.

Study of Cosmic Issues in Modified Gauss-Bonnet Theories

This thesis is devoted to study some interesting cosmic issues in the context of modi- ¯ed Gauss-Bonnet theories. Firstly, we explore the instability ranges of a spherically symmetric anisotropic collapsing °uid under expansion-free condition in f(G) grav- ity. We apply the ¯rst order perturbation scheme to the metric components as well as °uid variables and construct the corresponding ¯eld equations for both static as well as perturbed con¯gurations using viable power-law f(G) model. We establish dynamical equations using contracted Bianchi identities to discuss the dynamical in- stability in both Newtonian and post-Newtonian regimes. It is found that instability ranges depend on energy density, anisotropic pressures and Gauss-Bonnet terms but independent of adiabatic index for expansion-free collapsing °uid. Secondly, we generalize f(G) gravity by introducing non-minimal coupling be- tween Gauss-Bonnet invariant and trace of the energy-momentum tensor named as f(G; T) gravity and explore energy conditions for two reconstructed models in the background of homogeneous and isotropic universe. It is found that the massive test particles move along geodesic trajectories due to the presence of extra force originated from non-zero divergence of the energy-momentum tensor. The energy bounds are expressed in terms of deceleration, jerk and snap cosmological parameters. We study energy conditions for reconstructed models corresponding to de Sitter and power-law cosmological background using pressureless °uid and obtain feasible constraints on free parameters. Thirdly, we discuss stability of the Einstein static universe against homogeneous as well as inhomogeneous scalar perturbations in f(G; T) gravity. We investigate sta- bility regions for particular f(G; T) models corresponding to zero as well as non-zero covariant divergence of the energy-momentum tensor. The graphical analysis shows that stable Einstein universe exists for both spatially closed as well as open universe xi xii models against homogeneous and inhomogeneous perturbations for appropriate choice of parameters. Finally, we analyze stability of some cosmic evolutionary models against linear per- turbations in Hubble parameter and energy density of matter distribution in f(G; T) gravity. We establish the ¯eld equations for both general and particular f(G; T) forms in the context of FRW universe model. We apply the reconstruction technique and found that this theory describes the de Sitter universe, power-law solutions as well as phantom/non-phantom eras cosmological backgrounds. We also discuss stability of de Sitter and power-law reconstructed f(G; T) models and ¯nd stable results against linear perturbations.