Browsing by Author "Dr. Rabia Saleem"

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A study of generlazied scound law of Themodaynamic in masssive Gravity
(2024) Muhammad Shoib ( Sp 21- BSM-024), Muhammad Awais (sp21- BSM-002); Dr. Rabia Saleem; LHR TP -9878
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 7446
In 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.
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 7634
This 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 sound
Analysis of Light Rings and Accretion Disk around a Static Kalb-Ramond Black Hole
(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2025) Afifa Naureen; SP23-RMT-002; Dr. Rabia Saleem; LHR TP 9749
The gravitational impact causes light traveling close to the black hole (BH) to be deflected, creating a dark central region known as BH shadow, which is often surrounded by a lu minous ring. The shadow of BH are closely related to the model parameters as well as the nature of accretion disk. Our primary focus is on the visible and shadow characteris tics of non-commutative (NC) Kalb-Ramond BH, which is encircled by diverse accretion profiles.Specific values were chosen for the model parameters to produce the BH shadow profile, and we came to the conclusion that changes in each parameter directly affect the size and light paths of the BH. We determine that the profile of BH includes the bright pho ton ring with its direct lensing, emissions and the dark inner area for accretion disk. Their specifics are dependent on the emissions, though; in general, direct emission contributes significantly to the overall apparent luminosity, while photon and lensing rings contribute very little, as is customary, and latter can be safely disregarded.
Baryogenesis in f(R) Theory of Gravity
(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2025) Laiba Zulfiqar (FA20-BSM-018) : Adeeba Iqbal (FA20-BSM-063); Dr. Rabia Saleem; LHR TP 9926
f (R) Theory of gravity are reviewed in the context of the so called gravitational baryoge nesis. The latter is a mechanism for generating the baryon asymmetry in the universe, and relies on the coupling between the Ricci scalar curvature R and the baryon current. Gravity Lagrangians of the form L(R) ∼ Rn, where n differs from 1 (the case of the General Rela tivity) only for tiny deviations of a few percent, are consistent with the current bounds on the observed baryon asymmetry.
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 9566
Our 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.
Dynamical Analysis of Cosmic Evolution in f (R, T ) Theory of Gravity
(Library Information Services, CUI Lahore, 2022) Naeema Khan; FA20-RMT-033; Dr. Rabia Saleem
This thesis is mainly focused to investigate the cosmological implications of f (R, T ) theory of gravity considering two distinct models that are, linear f (R, T ) = R + αT , and quadratic in T , f (R, T ) = R + αT 2. The background of ﬂat FRW space-time under perfect ﬂuid distribution is being considered to de- velop the dynamical system of equations. To check the viability of these models and to distinguish between them, we develop important cosmological param- eters including deceleration parameter, square speed of sound, stateﬁnder and Om(z)-diagnostic (in terms of redshift), using obtained solutions. These cosmo- logical parameters give compatible results related to expansion of the universe. The stateﬁnder pair (j, s) for both of these models indicates a rich behavior as it shows different DE eras; phantom, quintessence as well as Chaplygin gas, and also the results for both of these models lie in stable region as shown by graphical analysis of the square speed of sound.We also use ﬁxed points method to describe the stability of the solutions by minimizing the complexity of the ﬁeld equations. Moreover, we study the growth of matter perturbation for general f (R, T ) theory which discover that the anisotropic stress in this theory is equal to unity, which is the same result as obtained in general relativity. We ﬁnd that linear f (R, T ) model is close to ΛCDM model but quadratic f (R, T ) model exactly attains ΛCDM limit, so we conclude that the quadratic f (R, T ) is a physically viable model more than that of linear f (R, T ) model.
Gravitational Baryogenesis within f (R; T ; Q) Gravity using Isotropic and Anisotropic Universe Models
(Library Information Services, CUI Lahore, 2022) Aqsa Saleem; SP21-RMT-032; Dr. Rabia Saleem
This thesis is concerned with two parts: The main objective of the ﬁrst part is to analyze the baryogenesis in the framework of f (R; T; Q) gravity under an isotropic universe model, where R and T represent the Ricci scalar and the trace of the energy-momentum tensor (EMT), respectively, and Q R T . We ex- plore baryogenesis with non-minimal (NM) coupling between R and T . For this, we use dust and stiff-type ﬂuids with power-law and de-sitter-type solu- tions of the scale factor. In the end, we compute the baryon asymmetry factor (BAF) for some recently proposed models of this gravity, and then we com- pare our ﬁndings with observational limits on BAF. We also discuss generalized gravitational baryogenesis in detail.In the second part, we use different types of dark energy (DE) entropies such as Barrow entropy (BE), Sharma-Mittal entropy (SME), and generalized con- struction of these entropies to discuss the issue of baryogenesis. We ﬁnd con- straints on the entropies parameters to get the physical results of BAF com- pared to the standard observational value. To derive the modiﬁed Friedmann ﬁeld equations (FFEs), we use entropies and the ﬁrst law of thermodynamics. Thus, even in the presence of the typical interaction between the Ricci scalar and baryon current, this results in non-trivial modiﬁcations to the mass density and pressure content of the universe, which provide a viable mechanism allowing for baryogenesis. Finally, we show that even in the radiation-dominant era BAF is non-zero.
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 Saleem
The 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.
Inflationary Dynamics within f(Q) Gravity using Non-Abelian Gauge Fields
(Library Information Services COMSATS University Lahore Campus, 2023-03-13) Rukhsana Batool; SP22-RMT-038; Dr. Rabia Saleem; LHR TP 8737
Within the framework of the f(Q) theory of gravity, we investigated warm inflation using non-Abelian gauge fields. We propose this requirement as the mechanism that terminates the gauge-flation model. For this model, slow-roll parameters and perturbation parameters are presented. We discover the fundamental conditions needed for the slow-roll approxima tion of this model to be realizable. Additionally, we build our model within the context of logamediate and intermediate cases, which, in the Einstein hypothesis, are exact solutions to the inflationary field equation. In terms of the inflaton field, general expressions of the slow-roll parameters, the tensor-scalar ratio, and the scalar spectral index are offered for these two cases. Graphical portrays show that our results are compatible with Planck 2018 data.
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 Saleem
In 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, SAX􀀀J 1808.4-3658, 4U 1820-30, V ela􀀀X-12 and PSR􀀀J1614-2230, 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. ix
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 7932
In 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.
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 Saleem
The 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.
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 10072
In 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.
Optical Appearance of Kerr Black Hole Surrounded by Cold Dark Matter
(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2025) Sadia Aslam (/FA23-RMT-047); Dr. Rabia Saleem; LHR TP 9790
In this work, we provide graphical representation of shadow images of Kerr-like black hole (BH) in the presence of cold dark matter (CDM) halo illuminated with two different accre- tion ﬂow models. The impact of spin parameter a, critical density ρc and the scale radius Rs on the observed images of BHs is carefully addressed. The results indicate that as a in- creases, the circular orbits shifted towards the right, while the larger values of both ρc and Rs caused to enhance the radius of circular orbits of BH shadow. In the case of background light source, the impact of parameter ρc on shadow distortion is extremely negligible, but this inﬂuence is relatively small and becomes appreciable when the parameter Rs has larger values. Next, we expand the interior region of accretion disk to the BH event horizon, and consider the motion of particles inside and outside the regions of inner stable circular or- bit (ISCO). The results exhibit that the intensity as well as the radius of BH shadow are gradually increasing with the increase of both ρc and Rs. On the other hand, in the case of retrograde ﬂow, the intensity of the shadow images signiﬁcantly decreases and a crescent moon emerges in the top of the observer’s frame. Subsequently, we discuss the distinctive features of red-shift conﬁgurations for both prograde and retrograde ﬂows. The obtained results interpret that the conﬁguration of red-shifts and the visual signatures is associated to the behaviour of accreting ﬂow as well as with relevant parameters. Finally, using the recent observational data of EHT results, we found the shadow radius of Sgr A∗ provides the best-ﬁt parameter constraints as compared to M87∗.
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 9359
The 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.
Shadow of Charged Rotating Wormholes
(Library Information Services COMSATS University Lahore Campus, 2023-03-13) Shokaib Shahid; SP22-RMT-032; Dr. Rabia Saleem; LHR TP 8733
In this work, we investigate the optical appearance of qualitatively new observational fea tures of accretion disk images around the charged rotating traversable wormhole (TWH) space-time for different spin, throat, and charge values. To accomplish this, we first con sider the Hamilton-Jacobi method to derive the geodesic equations for the motion of pho tons and study the effects of parameters on the photon orbit in the observer’s sky. We found that each parameter affects the size and shape of the wormhole (WH) shadow and flatness is observed in the shadow because of spin and other parameters. To produce shadow im ages of sufficient visual quality but within manageable computational times, we adopt the ray-trace procedure and characterize the significant features of light trajectories on the ob server’s screen, depending on the interaction between the space-time structure and the ac cretion disk. In addition, we consider the static spherically symmetric accretion flow model to observe the specific intensity around the TWH space-time geometry. It is found that the intensity and positions of the photon ring vary with respect to the involved parameters. In future observation, this type of study may provide a fertile playground to test the nature of compact objects, specifically the WH in the strong-field regime.
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 7396
This 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.