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 Cosmological and Dynamical Instability Analysis in Some Modified Theories of Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Tanzeela Nawaz; CIIT/FA13-PMATH-007/LHR; Dr. Shamaila Rani; LHR TP 7732The present thesis comprises the study of three dynamical phenomena such as evolution of the universe through cosmological parameters, thermodynamics via the generalized second law of thermodynamics and dynamical instability of collapsing star taking into account adiabatic index. Firstly, we consider a new holographic dark energy model in the framework of dynamical Chern-Simons modified theory of gravity to explore various cosmological parameters (like equation of state, squared speed of sound, Om-diagnostic) and cosmological planes ( d d ' where d ' is the evolutionary equation of state parameter, statefinders). We examine their graphical results and also comprise with the different observational data like Planck, 0 H , SNLS and WMAP. Secondly, the thermodynamics through generalized second law of thermodynamics in the framework of dynamical Chern-Simons theory of gravity by taking interaction between dark energy and dark matter is discussed. In the first part, we discuss the scalar field phenomenon of dark energy model having a non-canonical kinetic term in the flat universe and investigate the stability of the model through squared speed of sound. The generalized second law of thermodynamics by taking usual entropy and its correction forms like logarithmic and power law at apparent horizon is studied. In second part, this law is investigated by taking the interaction of dark matter and dark energy with holographic dark energy model and different cutoffs like Hubble horizon, event horizon, their combination, Ricci scalar and its generalized form. The generalized form of second law of thermodynamics in terms of coincidence parameter is being evolved. Next, we choose the framework of modified Brans-Dicke gravity to study generalized second law of thermodynamics by developing the expressions tot S ' for usual entropy as well as its corrected forms, logarithmic and power law corrected on the apparent and event horizons. Also, the power law forms of scalar field and scale factor are utilized in order to analyze the clear view of thermodynamic law. Finally, we find the instability ranges of spherical and cylindrical symmetric structures of a collapsing object in FT, theory where, T denotes the torsion scalar and is the trace of energy-momentum tensor. The perturbation method is used upto first order in the modified field equations to construct dynamical collapse equations. These equations with adiabatic index are utilized to check the instability ranges in Newtonian and post-Newtonian limits. It is found that the instability ranges depend on the dynamical variables of collapsing object.Item Effects of Modified Entropies on Geometric Thermodynamics of Black Holes(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Aqsa Tanveer; CIIT/SP20-RMT-011/LHR; Dr. Shamaila Rani; LHR TP 7647In this thesis, we analyze the thermodynamics of five-dimensional Schwarz- schild AdS black hole in AdS5 × S5 spacetime in the presence of Tsallis entropy. Since the cosmological constant Λ is considered as thermodynam- ical pressure with volume as its conjugate, but this explanation cannot be employed in AdS/CFT correspondence. In this study, we associate cosmo- logical constant Λ in boundary Gauge theory with the number of colors N and chemical potential is taken to be its thermodynamic conjugate. The two geometric parameters in the AdS black hole, r and L are substituted for two thermodynamic parameters in the micro-canonical ensemble, which are considered to be entropy S and N2. Moreover, we evaluate several thermodynamical geometry formulations, including Weinhold, Ruppeiner, and Quevedo and derive associated scalar curvatures for five-dimensional Schwarzschild AdS black holeItem Cosmological Consequences of Equation of State Parameterizations in Mimetic Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2022) Muhammad Rizwan Azhar; CIIT/FA20-RMT-032/LHR; Dr. Shamaila Rani; LHR TP 7935This thesis is devoted to study the cosmic aspects of the evolving uni- verse by taking into account mimetic gravity. Mimetic gravity is a Weyl- symmetric expansion of general relativity linked by a single disformal trans- formation, in which the presence of a dust-like perfect fluid may be used to imitate cold dark matter on a cosmic scale. We consider three well-knows equation of state parameterizations in terms of redshift function. Using mimetic gravity energy density, we calculate the potential, coincidence pa- rameter and deceleration parameter for all parametrization models with the help of energy conservation equation, Lagrange multiplier and normal- ized Hubble parameter. Considering the observational values of parameters in parametrization models, we get some useful results for the underlying scenario.Item Squared Speed of Sound Parameterizations in Non- Zero Torsion Cosmology(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Iram Ashraf; CIIT/SP20-RMT-022/LHR; Dr. Shamaila Rani; LHR TP 7652In this thesis, we use some squared speed of sound models in the frame- work of the non-zero torsion theory for flat FRW spacetime. These squared speed of sound models depend on equation of state parameter. We dis- cuss the accelerated expansion of the universe through some cosmological parameters like Hubble parameter, deceleration parameter and statefinder parameters. For deceleration parameter, we get some constraints to be fulfilled by model parameters for the sake of accelerated expansion of the universe. The statefinder parameters show the ΛCDM limit as well as some consistent behavior for each model. We also check the validity of general- ized second law of thermodynamics and thermal equilibrium for each model by taking into account Bekenstein entropy. In terms of equation of state parameter having range of quintessence, cosmological constant as well as phantom era, we get some consistent results for particular values of model constants.Item Impact of alternative theories of gravity on current cosmic scenarios(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2022) Nadeem Azhar; CIIT/FA17-PMT-013/LHR; Dr. Shamaila Rani; LHR TP 7861This thesis incorporates the study of some current cosmic scenarios in the realm of alternative theories of gravity (curvature and torsion based theories). These scenarios include gravitational baryogenesis phenomenon, cosmic accelerating expansion of the universe and thermodynamic analysis. Firstly, we examine the matter-antimatter asymmetry through the gravitational baryogenesis and its generalized mechanisms for some well-known curvature and torsion-based models. The absence of intense radiation from matter/anti-matter annihilation, cosmic microwave background and big bang nucleosynthesis strongly suggested that there exists more matter than anti-matter in our universe. Observational constraints verified that the baryon number density 𝜂𝐵 to entropy 𝑆 ratio is approximately 𝜂𝐵 𝑆 ∼ 9.42 × 10−11. We compute the baryon to entropy ratio for all models and get some consistent results. Secondly, we investigate the evolution of some cosmic parameters and planes in the framework of DGP braneworld model. We assume interaction between Barrow holographic dark energy model (whose infrared cutoffs scale are set by Hubble and event horizons) and pressureless dark matter. We check the behavior of different cosmological parameters like Hubble, equation of state, deceleration, squared speed of sound, statefinder plane and 𝜔𝜗 − 𝜔𝜗 ′ plane. Also, the thermodynamic nature of the underlying model through Barrow entropy as horizon entropy is investigated. Finally, we consider Tsallis holographic dark energy model with event and apparent horizons as infrared cutoff in the framework of dynamical Chern-Simons modified gravity. The impact of non-flat FRW universe is being taken into account to explore some cosmological parameters like Hubble, equation of state, deceleration and squared speed of sound. Finally, we investigate the generalized second law of thermodynamics with Tsallis entropy for both horizons and find some interesting outcomes.Item Cosmic Expansion Phenomenon in Specific Scalar Field Theory(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Farwa Khurshid; CIIT/FA19-RMT-105/LHR; Dr. Shamaila Rani; LHR TP 7430The interaction of newly proposed holographic dark energy model, Bar- row holographic dark energy model is being used to analyze the cosmic expansion phenomenon. We consider the interaction between dark energy and dark matter in the realm of scalar field theory. For the power law, quadratic and quartic (positive and negative) potentials, we investigate the different cosmological implications. This includes the equation of state pa- rameter ωD which identifies different eras of the expanding universe, cosmic plane ωD − ω0 D (where ω0 D shows the evolution of equation of state para- meter ωD where prime shows derivative with respect to ln a) and squared speed of sound parameter v2 s for the stability analysis of the models.Item Barrow Holographic Dark Energy in Extended Teleparallel Theories of Gravity(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Ghulam Murtaza; CIIT/FA19-RMT-085/LHR; Dr. Shamaila Rani; LHR TP 7424In this thesis, we reconstruct some solutions in the context of telepar- allel based extended theories coupled with trace of energy-momentum ten- sor, boundary term and Gauss-Bonnet term. For this purpose, we ob- tain the effective energy density and pressure for these theories for flat FRW universe model. The reconstruction is made through the compari- son of these effective energy densities with the energy density of Barrow holographic dark energy model. In this regards, we use scale factor in terms of power-law form and find some gravitational Lagrangian functions as f(T ) = T +g(T ), f(T, τ ) = T +γh(τ ), f (T, τ ) = βτ +g(T ), f(T, TG) = TGg(T ), f(T, TG) = −T + TGg(T ), f(T, TG) = TG + g(T ), f(T, TG) = T + j(TG), f(T, B) = T + σy(B) and f(T, B) = B + ξg(T ). In order to discuss the cosmic analysis related to these models, we plot equation of state parameter for each model. We investigate the squared speed of sound parameter for these models to determine the stability of the models. We obtain some favorable results related to specific values of model parameters.Item Thermodynamics Quantities and Heat Engines of Various Black Holes(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Misbah Tasleem; CIIT/FA19-RMT-019/LHR; Dr. Shamaila Rani; LHR TP 7425According to Barrow, the impact of quantum-gravitation changes the standard relation of entropy S = A 4 to S = A1+∆ 2 , where A is actual area and ∆ represents the quantum gravitational deformation effects. Keeping in mind the importance of entropy in the thermodynamics of black hole, we study the impact of Barrow entropy on the thermodynamics of charged AdS black hole with global monopole. We derive and plot many thermo- dynamical quantities of charged AdS black hole like temperature, entropy, pressure, Helmholtz free energy, Gibbs free energy, heat capacity of the system at constant pressure and volume. We study the critical behavior, phase transition and thermal stability and find out that more useful work can be extracted from black hole by using Barrow entropy. We also observe that Barrow entropy rapidly increases the Gibbs free energy of black hole which leads to more stable behavior as compare to standard relation of en- tropy. We also discuss charged black hole as heat engine and find out the efficiency of the black hole. In addition, we explore the thermal properties of torus-like black hole and find useful results regarding stability.Item Analyzing Various Aspects of Black Holes in Modified Gravity(Library Information Services COMSATS University Lahore Campus, 2024-03-18) Muhammad Arslan Nasir; SP23-RMT-019; Dr. Shamaila Rani; LHR TP 9575In this thesis, we investigate the thermodynamic properties of the Bardeen-Kiselev black hole solution with a cosmological constant, focusing on stability, phase transitions, and the compressibility factor using generalized entropy corrections. By incorporating Bar row, Renyi, Sharma-Mittal, Kaniadakis, and loop quantum gravity entropy corrections, we ´ explore their impact on black hole stability and phase transitions through specific heat ca pacity, pressure, compressibility, and Helmholtz free energy. The analysis reveals that spe cific heat capacity exhibits distinct behaviors across entropy models, including regions of stability (positive specific heat), instability (negative specific heat), and divergence points indicating phase transitions. For instance, Barrow entropy shows stability for small black holes (BHs), phase transitions for intermediate BHs, and restabilization for larger BHs. Loop quantum gravity entropy highlights significant phase transitions with unique stability patterns at higher horizon radii. Pressure-volume graphs illustrate how entropy corrections influence the black hole’s equation of state. Smaller BHs often exhibit instability at low volumes, transitioning to stable phases as volume increases. Notably, the Barrow and Sharma-Mittal entropy models show smooth transitions, whereas Kaniadakis and loop quantum gravity entropy correc tions reveal more abrupt changes. The compressibility factor analyses further underscore the non-ideal thermodynamic behavior of black holes under these entropy corrections. While Barrow entropy maintains compressibility near zero, Kaniadakis and loop quan tum gravity corrections exhibit exponential divergences, highlighting significant deviations from classical thermodynamics. Finally, Helmholtz free energy graphs demonstrate entropy-corrected phase transitions ix and stability trends. Smaller BHs exhibit consistent phase shifts across all models, with larger BHs progressively stabilizing under increasing pressure. Comparative analysis indi cates that Barrow entropy provides higher stability for larger BHs, while Sharma-Mittal and loop quantum gravity corrections lead to more dynamic transitions. This comprehensive study enriches the understanding of black hole thermodynamics by bridging classical and quantum gravity perspectives. By interpreting the cosmological constant as pressure and incorporating entropy corrections, this work offers insights into black hole stability, phase transitions, and their connections to statistical mechanics and quantum gravity frameworks. These findings contribute to the broader understanding of black hole behavior in general relativity and beyond.Item Insights of Modified Entropy on Therodynamics of Black Holes in Einstein Maxwell Scalar Theory(Library Information Services COMSATS University Lahore Campus, 2024-03-17) Ayesha Siddiqa; FA22-RMT-050; Dr. Shamaila Rani; LHR TP 9383Insights of Modified Entropy on Therodynamics of Black Holes in Einstein Maxwell Scalar Theory