M.Phil / MS
Permanent URI for this collectionhttps://repository.cuilahore.edu.pk/handle/123456789/52
This collection archives the complete set of theses produced by students of the COMSATS University Islamabad, Lahore Campus.
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Item Optical and Thermal Properties of Quantum Corrected AdS Black Hole in Kiselev Spacetime(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2025) Husnain Hafeez; CIIT/SP24-MSMATH-010/LHR; Dr. Rabia Saleem; LHR TP 10072In this work, we investigate the optical and thermodynamic properties of a quantum-corrected AdS black hole (BH) in Kiselev spacetime. The study focuses on the effects of key param-eters, including the quantum correction parameter a, the cosmological fluid parameter λ. the electric charge Q, and the entropy correction parameter ẞ, on the Hawking temperature, entropy, specific heat, Gibbs free energy, and BH shadow. The Hawking temperature gen-erally increases with the horizon radius, while the quantum and cosmological parameters enhance the corrected entropy and specific heat, indicating regions of local thermodynamic stability. Analysis of the Gibbs free energy reveals that small black holes (BHs) exhibit global instability, which transitions to stability for larger horizons. The BH shadow and photon sphere are found to expand with increasing a and A, and the infalling matter shows characteristic Doppler-induced darkening. These results highlight how quantum correction parameter a and surrounding cosmological fluid parameter A influence both the thermal behavior and observational features of AdS black holes, providing new insights beyond classical treatments.Item Accelerating Expansion of the Universe within Rastall Gravity Via Cosmography of Some Dark Energy Models(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Shan Ali; CIIT/SP20-RMT-029/LHR; Dr. Rabia Saleem; LHR TP 7634This thesis is mainly focused to investigate the cosmological implications of Rastall gravity with the help of two dark energy models, i.e., new Tsallis agegraphic dark energy (NTADE) and Sharma-Mittal holographic dark energy (SMHDE) for interacting and non-interacting cases. The background of flat FLRW space-time is being considered to develop the dynamical system of equa- tions. To check the viability of these models and to distinguish between them, we develop important cosmological parameters including equation of state pa- rameter, deceleration parameter, square speed of sound, statefinder and Om(z) diagnostic for both cases. By constraining the involved model parameters, it is observed that equation of state parameter for NTADE model lies in phantom re- gion for both interacting and non-interacting cases. Whereas for non-interacting SMHDE, it shows quintom as well as quintessence regions depending on the values of model parameter δ, and it indicates only the phantom region for inter- acting case. The ωD − ω′ D pair shows freezing and thawing regions for non-interacting NTADE while only freezing region for interacting NTADE and non-interacting SMHDE. For interacting SMHDE case, it shows freezing and thawing regions as well as region transition from freezing to thawing. The statefinder pair (j, s) for NTADE and SMHDE indicates a rich behavior as it shows different DE eras; phantom, quintessence as well as Chaplygin gas for both cases. We conclude that non-interacting SMHDE is more physically viable model than NTADE model because it approaches to ΛCDM limit and also the results for this model lie in stable region as shown by graphical analysis of the square speed of soundItem Warm Viscous Inflation Inspired by Modified Chaplygin Gas within Rastall Theory of Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Abdul Wahab; CIIT/FA19-RMT-107/LHR; Dr. Rabia Saleem; LHR TP 7396This thesis is a contribution to study the effectiveness of Rastall gravity (RG) to discuss warm viscous inflation inspired by modified Chaplygin gas. The mat- ter contents of our cosmos are taken to be standard scalar field (ϕ) and radiation within flat Friedman-Robertson-Walker (FRW) geometry. We establish the cur- rent model using intermediate form of scale factor and calculate the analytic solution of inflaton, effective potential V (ϕ) necessary to inflate the universe and entropy density S under slow-roll limit. The further work in this paper is restricted to high dissipative regime, taking constant and variable dissipa- tive (Γ) and viscous coefficients (ξ). The evaluated exact solutions are the main source in developing the inflationary observable like scalar/tensor power spec- tra, scalar/tensor spectral indices, tensor to scalar ratio and running of scalar spectral index for our model. The model parameters as well as additional the- ory parameter λ are constrained to check the compatibility of RG with Planck 2018 data via R − ns and αs − ns trajectories. We conclude that RG along with modification of Chaplygin gas model can lead to significant changes in the early universe.Item Interior Solutions of Charged Anisotropic Gravastar in f (R, T ) Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2022) Muhammad Taimoor Murtaza; CIIT/FA20-RMT-028/LHR; Dr. Rabia Saleem; LHR TP 7932In 1998, the Nobel Prize-winning discovery of the accelerating cosmic expan- sion demands physically consistent models to describe the origin of this current acceleration. As a result, during the last two decades, there have been several theoretical and experimental efforts into improving our understanding of the dynamics of the Universe. To support the lines, we consider the imperfect fluid distribution with in gravastar in the framework of f(R, T ) gravity and evaluate the major matter contents in the quintessence regime via conformal symmetry. After considering the well-known matching conditions with the exterior one, we investigate the various realistic features of thin shell gravastars with radial perturbation along with an electromagnetic field through a particular equation of state (EoS). The matter configuration around the black hole (BH) geometry is examined through various physical properties and checks the stability of gravastar for various values of involved model parameters. It is worth men- tioning that the realistic features of gravastars such as Proper Length, Entropy, and Thin Shell Energy satisfy the linear relationship between the thickness of the shell. Moreover, the speed of sound parameter η(a), is utilized to examine the stable configuration of the thin shell which supports the acceptability of our considered model.Item Accelerating Expansion in Curved FLRW Space-Time for Interaction/Non-interacting DE-DM within Rastall Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Shahnila; CIIT/FA19-RMT-086/LHR; Dr. Rabia Saleem; LHR TP 7446In this thesis, we explore the phenomenon of cosmic evolution using curved FLRW space-time bounded by apparent horizon with a specific holographic cut-off. To this end, we use the framework of Rastall gravity and universe is assumed to be consists of interacting/non-interacting dark energy and dark matter. In both scenarios, we evaluate exact solutions of the dynamical equa- tions and constraint the holographic parameter c2(z), assuming a slowly vary- ing function of red-shift during non-interacting model. For interacting model, we consider c2 as a constant function of red-shift. Moreover, we analyze nature of the obtained results via deceleration parameter (q), statefinder pair (j, s) and Om(z)-diagnostic by constraining the involved model parameters using latest observational data. The graphical analysis showed that interacting model is very close to ΛCDM model as compared to non-interacting case. We conclude that this holographic proposal is enough to describe the cosmic evolution at an accelerating rate.Item Development of Cosmic Inflationary Mechanism in Massive Gravity(Library Information Services COMSATS University Lahore Campus, 2024-03-18) Nadia Khan Nergis; SP23-RMT-023; Dr. Rabia Saleem; LHR TP 9566Our goal in this thesis is to examine cosmic warm inflation within the context of mas sive gravity (MG). The late-time cosmic acceleration can be satisfactorily explained by the dRGT gravity, but inflation cannot be supported by it. However, in the framework of General Relativity (GR) and Modified Gravity, the interest in inflation and attempts to use Lagrangians to characterize inflation and dark energy (DE) have recently undergone en hancements and may include poles. We demonstrate how this type of Lagrangian could support inflation within the context of dRGT (de Rham Gabadadze Tolley) gravity theory. We will also compute the fundamental inflationary parameters (slow-roll parameters, spec tral index, e-foldings number, and tensor to scalar ratio). The Planck data’s restrictions on inflationary observables are used to determine the range of the free parameters. The find ings of this study, which concentrate on the power and exponential potentials, appear to be consistent with the Planck 2018 data.Item Phase Space Analysis in f(Q) Gravity with Bulk Viscous Fluid(Library Information Services COMSATS University Lahore Campus, 2024-03-17) Nadir Saeed; FA22-RMT-003; Dr. Rabia Saleem; LHR TP 9359The standard formulation of general relativity needs to be revised in explaining several re cent phenomena observed in the universe. It motivates us to go beyond the standard idea of gravity. The f(Q) gravity theory is a fascinating modified theory of gravity in which the non-metricity Q determines the gravitational interaction. The vanishing affine connections have been utilized only in the application of the modified f(Q) gravity theory to cosmol ogy. As a result, researchers were unable to compete with the current understanding of f(τ) theory earlier. In this work, we investigate a newly suggested approach to deal with this issue by studying the proposed construction of f(Q) theory utilizing a non-vanishing affine connection in the spatially flat FLRW space-time. We use a few variables and di mensionless parameters to formulate the appropriate equations for the dynamical system approach. The conservation of the energy-momentum tensor leads to a constraint equation that establishes a relationship between the dynamical variables. The objective of this study is to investigate the cosmological models that include the effect of bulk viscosity in the cosmic fluid using the framework of f(Q) gravity. We used three models of f(Q) under bulk viscous fluids, revealing the existence of both unstable and stable de-sitter solutions for rapid expansions at early and late-times. Furthermore, it determines the existence of unstable points for matter eras in the context of every f(Q) function, so illustrating that the presence of non-zero affine connections might be responsible for a true sequence of cosmic eras.Item Interior Solutions of Anisotropic Strange Stars in f(T, T ) Gravity using Modified Chaplygin Gas(Library Information Services COMSATS University Islamabad Lahore Campus, 2021) Muhammad Israr Aslam; FA19-RMT-095; LHR TP 7355; Dr. Rabia SaleemIn this thesis, we use an extended f(T) gravity namely f (T, T ) gravity, rep- resenting the coupling of torsion scalar T and the trace of energy-momentum tensor T . In this framework, we find the exact interior anisotropic solutions of compact stars considering Krori-Barua space-time under metric potentials, a(r) = Br2 + Cr3, b(r) = Ar3 (A, B, C are the unknown parameters), and ap- plying a well-known model f(T, T ) = α1Tn(r) + βT (r) + ϕ, n > 1. To close the system of equations, we utilize an equation of state for modified Chaplygin gas model. By the well-known matching conditions of exterior and interior space- time, we evaluate the unknown model parameters. For four different strange stars, SAXJ 1808.4-3658, 4U 1820-30, V elaX-12 and PSRJ1614-2230, we made complete physical analysis by plotting trajectories for energy conditions, square speed of sound, mass function, compactness factor and surface redshift. It is observed that our results satisfy all the necessary and sufficient physical conditions, hence physically viable. ixItem Non-Commutative Wormholes Solutions in Rastall Like Teleparallel Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2020) Komal Rasool; SP19-RMT-020; LHR TP 6476; Dr. Rabia SaleemThe main focus of this thesis is to explore the phenomena of the traversable wormhole (TWH) solutions with non-commutative (NC) geometry using con formal symmetries in the framework of Rastall like teleparallel gravity (RTG). Togetthis, wederivethebackgroundfieldequationsforwormhole(WH)geom etry within RTG. We calculate the solution of the corresponding field equations using Gaussian and Lorentzian matter-energy distributions under conformal motion. Using both these distributions, we gain analytic WH solutions in terms of few special math functions such as gamma, exponential and hypergeometric functions. Further, we check the viability of weak, strong, and null energy conditions (ECs) for the considered fluid distribution at the WH throat and interpret them graphically. We are interested in finding the impact of an additional Rastall parameteronNCWHgeometriesascomparedtoteleparallelgravity. Moreover, weexplainthepossible constraints for the positivity of active gravitational mass in bothdistributions. We also explore the stability of gained WH solutions using the modified equilibrium condition in terms of four different forces in RTG. It is concluded that the constructed solutions are physically viable and stable.Item Inflationary and Accelerating Cosmic Solutions in Rastall Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Muhammad Waheed Zafar; SP19-RMT-011; LHR TP 6471; Dr. Rabia SaleemThe main focus of this thesis is to investigate accelerating expansion of our universe in the framework of Rastall theory of gravity (RTG) using non-linear inhomogeneousequationofstate(EoS)forFriedmann-Robertson-Walker(FRW) model. First we findthesolution of Hubble parameter, energy density and scale factor investigating the transition from non-phantom to phantom era. We study transition towards superaccelerating phase, i.e., the case when third derivative of the scale factor is positive. We find present and future singularity for our model. Second to check the thermodynamical evolution of cosmos, we find the product of temperature and entropy (TdS), and observe it graphically. To check the nature of cosmos, we find some important cosmological parameters using obtained solutions including deceleration, jerk, variable jerk, gradually varying jerk and snap parameters. Through detailed graphical analysis, we found that RTGis a good candidate to represent accelerating cosmic expansion.