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The primary objective of this paper is to examine singularity-free solutions within the framework of anisotropic solutions for the Chaplygin equation of state in f (R , ϕ , X) modified gravity theory. Herein, R signifies the Ricci scalar, ϕ denotes the scalar field, and X represents the kinetic term associated with ϕ. The investigation employs the Krori–Barua metric to explore the characteristics of an anisotropic compact star. In the course of our analysis, we specifically consider observational data about three distinct stellar structures: Her X-I, LMC X-4, and 4U 1538-52. Our inquiry extends to the graphical examination of various physical parameters, encompassing energy density, pressure components, anisotropic components, equation of state parameters, equilibrium conditions, stability analysis, adiabatic index, and energy conditions. Furthermore, we delve into the intricacies of the mass-radius function, compactness factor, and surface redshift–integral features of charged compact stars. The culmination of our findings suggests that the f (R , ϕ , X) theory of gravity when subjected to observational scrutiny, manifests as realistic, stable, and devoid of singularities. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the conditions for mapping the geometric description of the kinematics of particles that probe a given Hamiltonian in phase space to a description in terms of Finsler geometry (and vice-versa). [ABSTRACT FROM AUTHOR]

In this paper, we review a recently offered notion of quasi-static evolution of the axial self-gravitating structures at large scales and the criterium to characterize the corresponding evolutionary aspects under the influence of strong curvature regimes. In doing so, we examine the axial source's dynamic and quasi-static behavior within the parameters of various modified gravity theories. We address the formalism of these notions and their possible implications in studying the dissipative and anisotropic configuration. We initiate by considering higher-order curvature gravity. The Palatini formalism of f (R) gravity is also taken into consideration to analyze the behavior of the kinematical as well as the dynamical variables of the proposed problem. The set of invariant velocities is defined to comprehend the concept of quasi-static approximation that enhances the stability of the system in contrast to the dynamic mode. It is identified that vorticity and distinct versions of the structure scalars Y I , Y II and Y KL play an important role in revealing the significant effects of a fluid's anisotropy. As another example of evolution, we check the influence of Palatini-based factors on the shearing motion of the object. A comparison-based study of the physical nature of distinct curvature factors on the propagation of the axial source is exhibited. This provides the intriguing platform to grasp the notion of quasi-static evolution together with the distinct curvature factors at current time scenario. The importance of slowly evolving axially symmetric regimes will be addressed through the distinct modified gravitational context. Finally, we share a list of queries that, we believe, deserve to be addressed in the near future. [ABSTRACT FROM AUTHOR]

This study reveals the implication of charge within the framework of f(G) gravity models by considering the Tolman–Kuchowicz space–time. The use of the Bardeen model to describe the fundamental characteristics of the external geometry is an important aspect of current findings. Moreover, for the qualitative analysis, we compare all the parameters involved in the selected metric potentials with Bardeen’s model. We apply this analysis to the stars PSR1903 + 327, PSR1937 + 21, PSRJ1614 − 2230, 4U608 − 52, VelaX − 1, LMYX − 4, CenX − 3, HERX − 1, SMXJ1808 − 3658 and 4U1820 − 30. We study all the necessary characteristics of stellar structures, such as energy density and pressure progression, equation of state parameters, equilibrium conditions, mass–radius ratio, adiabatic index, and energy conditions. A thorough graphical analysis is provided to establish the viability of the considered f(G) models. It is important to emphasize that our acquired solutions are physically acceptable. [ABSTRACT FROM AUTHOR]

The matter–anti-matter asymmetry in the early Universe’s history is explained theoretically by the baryogenesis. It is well known that modified gravity theories are highly effective in explaining the Universe’s current accelerated state. In this paper, we examine the gravitational baryogenesis for f(Q,C) gravity, where Q is the non-metricity scalar and C is the boundary term. To perform the baryogenesis, we assume linear, nonlinear and logarithmic forms of f(Q,C). We evaluate the baryon-to-entropy ratio for our models by assuming the power law form of the scale factor a(t) = a0tδ. We also investigate the generalized baryogenesis for our model by assuming the interaction term ∂μ(Q + C). We constrain the model parameters by comparing with the observational value ηB s = 9.42 × 10−11. Our derived models are physically viable. [ABSTRACT FROM AUTHOR]

This paper deals with the study of quasi-spherical gravitational collapse in the framework of f(R,T) theory. We will divide geometry of the star into two distinct portions i.e. interior portion and exterior portion. In the interior portion, we consider quasi-spherical Szekeres line element and in the exterior portion we take Vaidya line element. The interior and exterior portions will be glued by using Israel’s junction conditions [W. Israel, Il Nuovo Cimento B (1965–1970) 44, 1 (1966); W. Israel, Phys. Lett. A 24, 184 (1967)]. The field equations will be derived in the context of f(R,T) theory for a particular model. Apparent horizons and their time configuration for various desirable cases will also be discussed. [ABSTRACT FROM AUTHOR]

This paper examines the viable characteristics of anisotropic compact stars in f(ℜ,T2) theory (ℜ is the Ricci scalar and T2 = T αβTαβ). In this perspective, we use Tolman–Kuchowicz solutions (μ = wr2 + 2ln z and ν =ln(1 + xr2 + yr4)) to examine the configurations of static spherically symmetric structures. The unknown constants are found by the first fundamental form of Darmois junction conditions. We analyze the behavior of fluid parameters in the interior of 4U 1538-52, PSRJ 1614-220, EXO 1785-248 and SAXJ 1808.4-3658 compact stars correspond to different models of this theory. Furthermore, the stability of the proposed compact stellar objects is examined through sound speed and adiabatic index methods. The satisfaction of requisite conditions ensures that stable compact objects exist in this framework. [ABSTRACT FROM AUTHOR]

This study is dedicated to investigate the Noether symmetry approach in a newly introduced gravitational theory known as f(R,G,T) gravity, where f is the function of Ricci scalar R, trace of energy–momentum tensor T, and the Gauss–Bonnet invariant term G. The Noether symmetry approach plays a supportive role in generating models and then determining the exact solutions by using the conserved quantities. For this purpose, the flat Friedmann–Robertson–Walker space–time is selected to observe cosmic evolution. The set of partial differential equations is calculated in the background f(R,G,T) modified theory gravity. We further examine the four different f(R,G,T) gravity models and calculate the conserved quantities to investigate the exact solutions. The three models show the increasing behavior of the cosmic scale factor that supports the expansion of universe. In fourth model, we are able to get the corresponding conserved quantity only and it is anticipated that this model will also support the expansion of universe. [ABSTRACT FROM AUTHOR]

In the region where the gravitational field is strong, we have examined the influence of different gravities on the accretion disk formed due to spherical accretion. To achieve this, we obtain numerical solutions of the GRH equations, utilizing Schwarzschild, Kerr, Einstein–Gauss–Bonnet, and Hartle–Thorne spacetime metrics. We investigate the impact of the rotation parameter of a black hole (a∕M), the EGB coupling constant (α), and the quadrupole moment of the rotating black hole (q) on the accretion disk formed in a strong field. The formation of the disk for the slowly and rapidly rotating black hole models is separately examined, and comparisons are made. Our numerical simulations reveal that, under the specific conditions, the solution derived from Hartle–Thorne gravity converges toward solutions obtained from Kerr and other gravitational models. In the context of the slowly rotating black hole with a∕M=0.28, we observe a favorable agreement between the Hartle–Thorne result and the Kerr result within the range of 01, it becomes apparent that the Hartle–Thorne solution diverges from solutions provided by all gravitational models. Our motivation here is to utilize the Hartle–Thorne spacetime metric for the first time in the numerical solutions of the GRH equations for the black holes, compare the results with those obtained using other gravities, and identify under which conditions the Hartle–Thorne solution is compatible with known black hole spacetime metric solutions. This may allow us to provide an alternative perspective in explaining observed X-ray data. [ABSTRACT FROM AUTHOR]

This research work discusses the energy bounds in f(R,T) gravity with an anisotropic background, where R is the Ricci scalar and T is the trace of the energy–momentum tensor. For our study, we consider the locally rotationally symmetric Bianchi Type-I space-time. Moreover, we calculate the field equations and find exact solutions by considering different f(R,T) gravity models. We investigate the graphical behavior of energy density and pressure components by considering these distinct gravity models. We further use Hubble, deceleration, jerk, and snap parameters to evaluate the viability of our models. We also perform the graphical analysis to observe the behavior of energy conditions especially null energy conditions, weak energy conditions, strong energy conditions, and dominant energy conditions. It can be noticed that all considered f(R,T) gravity models support the idea of the presence of exotic matter, which justifies the phenomenon of expansion of the universe. [ABSTRACT FROM AUTHOR]

We study inflation in scalar–tensor perturbative quantum gravity driven by a one-loop effective potential. We consider effective potentials generated by three models. The first model describes a single scalar field with a nonvanishing mass. The second model describes a massless scalar field with nonminimal coupling to the Einstein tensor. The third model generalizes the Coleman–Weinberg model for the gravitational case. The first model can be consistent with the observational data for N∼70 e-foldings. The second model can be consistent with the observational data for N∼40 e-foldings. We did not find parameters that make the generalized Coleman–Weinberg model consistent with the observational data. We discuss the implications of these results and ways to improve them with other terms of effective action. [ABSTRACT FROM AUTHOR]

In order to alleviate the Hubble tension, we propose a new law for the Hubble parameter that varies periodically with time and we study the validity of the dynamics of a flat Friedmann–Robertson–Walker Universe in the context of a generalized Chern–Simons modified gravity (CSMG). We prove that for specific numerical values of the free parameters in the theory, the phantom divide line is crossed and the universe undergoes decelerating expansion and accelerating expansion alternately without singularity. The deceleration and the jerk parameters oscillate also around numerical values in agreement with astronomical BAO+Pantheon SNeIa+Cosmic Chronometers observations. [ABSTRACT FROM AUTHOR]

A modified fourth-order gravity theory namely the f(T,B) theory is considered in flat FLRW model to investigate both classical and quantum cosmology. The field equations are reduced to second-order differential equations by considering the technique of Lagrange multiplier. Noether symmetry analysis has been used both for obtaining classical solution and the wave function of the Universe in quantum cosmology. Finally the cosmological solutions are examined from observational view point. [ABSTRACT FROM AUTHOR]

The primary objective of this study is to conduct a thorough examination of relativistic structures within the framework of f(ℛ,풯 ) gravity, where ℛ is the Ricci scalar and 풯 is the trace of the energy–momentum tensor. We investigate the emerging characteristics of compact stars, focusing on the ℛ + 2ξ풯 model, where ξ is the coupling constant. In the initial sections of the paper, we concentrate on the examination of exact interior solutions involving perfect fluid. With this aim, we introduce the metric coefficient L = S(1 + 2ar2)d/2, where a and S are arbitrary constants to be computed from the matching conditions and d is a positive odd integer. It is worth noticing that we select a group of 10 significant compact stars from recent studies, namely, Vela X − 1, EXO 1785 − 248, 4U1820 − 30, SMCX − 4, HerX − 1, PSR J1614 − 2230, PSRJ1903 + 327, CenX − 3, 4U1538 − 52 and LMCX − 4. To investigate the viability of our model, we conduct various checks, including the analysis of energy density and pressure components, equilibrium, and energy conditions, stability analysis and the adiabatic index. An important part of this work provides the slowly rotating property of stars, accompanied by the derivation of the moment of inertia. The relationship of mass, energy density, radius and moment of inertia for stellar objects are observed. Our findings demonstrate that obtained results are physically acceptable and consistent with our considered model. [ABSTRACT FROM AUTHOR]

In order to solve the mystery of the accelerating and expanding universe model first, various dark energy candidates, such as Quintessence, Tachyon, k-essence, Phantom and DBI-essence, were investigated in detail in the f(Q) modified symmetric teleparallel gravitation theory for homogeneous and isotropic FRLW universe model. The f(Q)=aQn+b model is used to obtain solutions in f(Q) theory. Also, the physical behaviors of some cosmological parameters such as pressure and density were studied. Dark energy candidates were analyzed for different values of n with various graphs. [ABSTRACT FROM AUTHOR]

The concept of wormholes is still hypothetical in physics but attractive among researchers. In this work, motivated by the important effect of f(R,T) gravity theory, we seek the existence of wormhole solutions. We consider the field equations that can be derived from two different choices of matter Lagrangian density and utilize them to find the possibility of having some wormhole geometry. This procedure is based on the specific choice of an equation-of-state that obeys a certain physical requirement on the matter stress-energy [S. Kar and D. Sahdev, Restricted class of traversable wormholes with traceless matter, Phys. Rev. D 52 (1995) 2030]. For this purpose, the functional form of the redshift function is taken into consideration, and the possible existence of wormhole solutions is proven. Finally, the work examines the energy conditions and shows that violations of null energy conditions hold throughout spacetime. [ABSTRACT FROM AUTHOR]

The ambiguity of the Weitzenböck connection is investigated from the perspective of frames. In doing so, the pure-tetrad formalism and the formalism with an affine connection are compared with each other; it is shown that the ambiguity in the affine connection is equivalent to that of the frame. The role played by the regularization procedure is discussed, and it is argued that the frame approach gives better results than any regularization procedure, including those that make use of a suitable choice of the affine connection. It is also argued that, for consistency, the Weitzenböck torsion must be either manifesting only gravitational effects or gravitational effects plus accelerations. Two possible interpretations for the torsion tensor are present, and an application to a frame that accelerates along the same direction as that of a pp-wave is used to show the consistence of these interpretations. Finally, some possible solutions to the well-known problems of the f(T) theories are proposed. [ABSTRACT FROM AUTHOR]

Observational evidence revealed that in the composition of the Universe, the dark energy is in heap proportion which is mainly responsible for the accelerated expansion of the Universe. The astrophysicists hypothesized that every astrophysical object is likely interact with the dark energy Sakti and Sulaksono [Phys. Rev. D 103 (2021) 084042]. Therefore, researchers are intended to study the astrophysical objects with dark energy interaction. In this paper, we proposed a dark energy stars (composed of with dark matter and ordinary matter) model in the Rastall theory of gravity. The Rastall field equations are obtained in the Tolman–Kuchowicz-type metric. To solve the field equations, the arbitrary constants involved in the field equations (due to Tolman–Kuchowicz ansatz) are obtained by matching the interior geometry with the exterior Schwarzschild geometry. The obtained solution is tested physically by computing some analytical results and plotting graphs of different physical parameters like pressure, density for ordinary matter, density for dark matter, mass function, compactness and surface redshift. Stability of the model is also analyzed by different tests like velocity of sound, adiabatic index and TOV equations. The influence of the Rastall parameter on the model has been studied by plotting graphs and computing numerical values of model’s physical parameters for different values of the Rastall parameter. We infer that our propounded model fulfills all the necessary requirements for a physically plausible model. [ABSTRACT FROM AUTHOR]

This paper is an investigation of dark energy cosmological models in non-coincident gauge formulation of non-metricity gravity with boundary term. We have considered an arbitrary function f(Q,C) = Q + λCm, where Q is the non-metricity scalar, C is the boundary term derived by C = R̈ − Q, and λ,m are the model parameters, to obtain the modified field equations from action. We have parameterized the Hubble function as H(a) = k0(a(t)−n + k 1) with n,k0,k1 are arbitrary constants and a(t) is the scale-factor depending upon cosmic time. We have compared this Hubble function with H(z) datasets and obtained best fit values of model parameters using likelihood analysis. Using these best fit values of model parameters, we have done our result analysis and discussion with cosmological parameters viz effective equation of state parameter, energy density, energy conditions, deceleration parameter, om diagnostic analysis and age of the universe. We have found a transit phase universe model with effective EoS parameter − 1.18 < ωeff < −0.2 over redshift − 1 < z < 3. [ABSTRACT FROM AUTHOR]

The general relativity unification and quantum theory is a significant open problem in theoretical physics. This problem arises from the fact that these two fundamental theories, which describe gravity and the behavior of particles at the microscopic level, respectively, are currently incompatible. The unification of these theories is crucial for a complete comprehension of the fundamental forces and the nature of the universe. In this regard, the quantum properties of a Black Hole result in fundamental importance. By analyzing such properties in quantum field theory, in the first step, the gravity enters as a classical background. In semi-classical approximation, Black Holes will emit Hawking radiation with an almost thermal spectrum, while Black Hole's entropy is proportional to the Black Hole's horizon. Besides, Hawking's temperature and Black Hole entropy should follow the first law of Black Hole thermodynamics. Also, Jacobson [Thermodynamics of spacetime: The Einstein equation of state, Phys. Rev. Lett.75 (1995) 1260, doi:10.1103/PhysRevLett.75.1260] showed shown that there is a connection between Black Hole thermodynamics and Einstein's equation that opens the root of a potential thermodynamic nature of gravity. This issue opened a new impressive research framework in which the Einstein field equation can be expressed as a form of the first law of thermodynamics and vice versa. In this study, it is assumed that the universe has a fractal structure. Accordingly, modified Friedmann's equations and the Black Holes thermodynamics in a fractal universe have been examined. The fractal framework shows what features and changes occur in the description of the universe, particularly in studying the thermodynamics of a Black Hole. However, the paper strategy is organized as follows: in the beginning, we consider the first thermodynamic law in a fractal universe. Then, we investigate the Friedmann equation of the fractal universe in the form of the entropy balance, this means d Q = T h d S h , where d Q and T h are the thermal energy and horizon temperature. We consider the entropy S h have two terms; (1) obeys the usual area law and (2) the entropy production term due to the non-equilibrium thermodynamics of a fractal universe. Therefore, in a fractal universe, a term with non-equilibrium thermodynamics of spacetime may be needed. Also, we study the generalized second law of thermodynamics in a fractal universe. When the temperature of the apparent horizon and the temperature of the matter fields inside the horizon are equal, i.e. T = T h , the second law of generalized thermodynamics can be obtained according to the state parameter range equation, which is consistent with the recent observations. Finally, in Sec. 6, based on the mathematical calculations, we study the various cosmological parameters such as the Hubble parameter, scale factor, deceleration parameter and equation of state parameter. [ABSTRACT FROM AUTHOR]

A generalized Robertson–Walker spacetime is not, in general, a perfect fluid spacetime and the converse is not, in general, true. In this paper, we show that if a perfect fluid spacetime admits an η – ρ -Einstein soliton, then the integral curves generated by the velocity vector field u are geodesics and the acceleration vector vanishes. Also, we show that if a perfect fluid spacetime with Killing velocity vector field admits a gradient η – ρ -Einstein soliton, then the spacetime represents either dark matter era or, the acceleration vector vanishes. Next, we show that if a generalized Robertson–Walker spacetime admits an η – ρ -Einstein soliton, then it becomes a perfect fluid spacetime. Finally, we prove that in a dark matter fluid with vanishing vorticity satisfying gradient η – ρ -Einstein soliton in f (ℛ) -gravity with constant Ricci scalar, either the energy density and isotropic pressure are constant or, the potential function ψ remains invariant under the velocity vector u. Also, for two models f (ℛ) = e α ℛ + α log ℛ and f (ℛ) = ℛ n (α = constant and ℛ is the Ricci scalar of the spacetime), various energy conditions in terms of the Ricci scalar are examined and state that the Universe is in an accelerating phase and satisfies the weak, null and dominant energy conditions, but violate the strong energy condition. [ABSTRACT FROM AUTHOR]

In this paper, we investigate in this paper the Type IV singular bouncing in the framework of f (R ,) theory of gravity where R and mean the curvature scalar and the Gauss–Bonnet invariant, respectively. Cosmological f (R ,) models constrained by the slow-roll evolution is reconstructed and their explicit forms are provided near the bounce and far away from it. One obtains finally two models whose stability is numerically analyzed in this work. Our results show that the stability of the reconstructed models is very affected by their parameters. The model far from the singularity recovers stability quickly than the model near the singularity. [ABSTRACT FROM AUTHOR]

Differentiation of the scalar Feynman propagator with respect to the spacetime coordinates yields the metric on the background spacetime that the scalar particle propagates in. Now Feynman propagators can be modified in order to include quantum-gravity corrections as induced by a zero-point length L > 0. These corrections cause the length element s 2 to be replaced with s 2 + 4 L 2 within the Feynman propagator. In this paper, we compute the metrics derived from both the quantum-gravity free propagators and from their quantum-gravity corrected counterparts. We verify that the latter propagators yield the same spacetime metrics as the former, provided one measures distances greater than the quantum of length L. We perform this analysis in the case of the background spacetime ℝ D in the Euclidean sector. [ABSTRACT FROM AUTHOR]

In this paper, we propose a specialized parameterization for the Hubble parameter, inspired by Λ CDM cosmology, to investigate the cosmic expansion history of the Universe. This parameterization is employed to analyze the universe's late-time behavior within the context of Q n gravity, where Q represents non-metricity. By using data from 57 Hubble data points, 1048 supernova (SNe) data points, and six Baryon Acoustic Oscillation (BAO) data points, we determine the optimal values for the model parameters. Additionally, we explore three distinct cosmological models based on the parameter n , specifically when it takes on the values of 0. 5 5 , 1. 5 and 2. 0. The results of our analysis indicate that our proposed parameterization, along with the associated models for different values of n , predicts an accelerated cosmic expansion phase. [ABSTRACT FROM AUTHOR]

In this paper, the coupled system nonlinear (2 + 1)-dim Broer–Kaup–Kupershmit (BKK) equation under consideration is based on extension of modified rational expansion method. The various kinds of multiple soliton solutions named anti-kink soliton, traveling wave solutions, periodic solution, dark soliton, kink soliton, bright soliton, anti-kink bright solitons, kink bright solitons for nonlinear BKK system are successfully constructed under symbolic computation Mathematica. The calculated results are very interesting, different and novel which have not been investigated in past literature. The graphical demonstration of constructed solutions show by contour, 2D and 3D shape by using computer software Mathematica. The constructed multiple soliton solutions prove that EMRE approach is very straightforward, efficient, reliable and powerful for the investigation of other nonlinear partial differential equations. The investigated results will be described and play a keen role in the study of nonlinear physical phenomena in the area of physics and engineering. [ABSTRACT FROM AUTHOR]

A spatially homogeneous and anisotropic Bianchi type-VI cosmological model in the framework of f (Q , T) gravity has been studied by considering three different functional forms of f (Q , T) i.e. f (Q , T) = a Q + b T , f (Q , T) = a Q k + b T and f (Q , T) = − a Q − b T 2 . The field equation of f (Q , T) gravity has been solved with the help of the Bianchi type-VI line element. The cosmological parameters of the models are evaluated, and the dynamical aspects of the model have been discussed. The result agrees with the observations of accelerated expansion of the universe. [ABSTRACT FROM AUTHOR]

General theory of relativity can be equivalently formulated on a flat spacetime associating a torsion-free affine connection of non-vanishing non-metricity scalar Q. In this paper, we present an extension of this, viz., the f (Q) theory of gravity, and explore the early evolution of the universe in the background of anisotropic Bianchi-I model. The f (Q) theory in the current setting through its geometric modification is quite successful in explaining the late time accelerated expansion. Here, we note that it accommodates latest released constraints on the inflationary parameters by Planck's collaboration group with excellent precession, but fails to produce a viable decelerated expansion in the radiation dominated era. [ABSTRACT FROM AUTHOR]

While two highly intensive laser beams collide, they create a region where the refractive index varies so quickly that photons are created. The variance of the refractive index is analog to the universe scale factor variance. Therefore, this laser system can be an analog to the expansion of the universe. We find that several hundreds of photons can be created under feasible conditions. This system can demonstrate the particle creation during inflation or other similar periods. [ABSTRACT FROM AUTHOR]

This study investigated the formation and evolution of a strange star known as SAX.J1808.4–3658 in the Krori–Barua Rainbow spacetime, resulting from the collapse of string fluid. The study examined the dynamical variables derived from the field equations, taking into consideration the influence of the particle’s energy on the mass density, pressure, and string tension. Additionally, various techniques were employed to analyze the physical properties, including gradients, energy conditions, anisotropy, stability, the Tolman–Oppenheimer–Volkoff equation, mass function, compactness, and red-shift. The study’s findings revealed that the strange star SAX.J1808.4–3658 satisfies all the conditions necessary for its evolution from an anisotropic string fluid. This discovery suggests that strange stars might have emerged during the string-oriented quark era of the Universe. The researchers presented all the physical quantities within the frameworks of both rainbow gravity and general relativity, while also utilizing graphical representations to aid in comprehending the study’s findings. The energy conditions and anisotropy were found to be fulfilled, indicating the stability of the strange star. Furthermore, the Tolman–Oppenheimer–Volkoff equation was employed to determine the maximum mass of the strange star, which was found to align with observational data. [ABSTRACT FROM AUTHOR]

Wormholes are considered both from the Wheeler deWitt equation, as well as from the field equations in the Euclidean background of Robertson Walker mini-superspace in R2 gravity. Quantum wormhole satisfies Hawking Page wormhole boundary condition in the Euclidean background of mini-superspace, however, in the Lorentzian background wave functional turns to the usual oscillatory function. The Euclidean field equations for κ = 0,±1 lead to the wormhole configuration, as well as oscillating universe in Euclidean time τ. The oscillating universe in Euclidean time τ transforms to an expanding universe only for κ = 1 in Lorentz time t under analytic continuation τ = it and asymptotically leads to an exponential solution. An Euclidean wormhole in the very early era evolves to an oscillating universe in τ, thereafter crossing deSitter radius transition to an inflationary era is evident at later epoch only for κ = 1. [ABSTRACT FROM AUTHOR]

This research investigates the impact of modified gravity on cosmic scales, focusing on f(Q) cosmology. By applying energy conditions, the study reconstructs various f(Q) models, considering an accelerating Universe, quintessence, and a cosmological constant Λ. Using up-to-date observational data, including the Supernova Pantheon sample and cosmic chronometer data, Hubble constants H0 are estimated as 70.37−0.92+0.84 km/s/Mpc (from H(z) data) and 70.02−0.25+0.44 km/s/Mpc (from pantheon compilation of SN Ia data). The matter energy density parameter (Ω0m) is calculated as 0.26−0.0100.015 (OHD) and 0.27−0.0140.025 (SN Ia). Furthermore, as a function of red-shift z, explicit expressions of f(Q) and the EOS parameter ω are produced, and their graphical analysis describes the late time acceleration of the Universe without the usage of dark energy. [ABSTRACT FROM AUTHOR]

In this paper, we introduce and characterize a pseudo-generalized Ricci-recurrent spacetimes and produce an example to verify the existence of such a spacetime. Then we demonstrate that a conformally flat generalized Ricci-recurrent spacetime with certain condition is a pseudo quasi-Einstein spacetime. Besides, it is proved that a pseudo-generalized Ricci-recurrent generalized Robertson–Walker spacetime represents a perfect fluid spacetime. Lastly, we study the impact of this spacetime under f (ℛ , T 2) and f (ℛ ∗) gravity scenario and deduce several energy conditions. [ABSTRACT FROM AUTHOR]

Indium tin oxide (ITO) ceramics with different SnO2 contents were fabricated using the conventional solid-state reaction. The phase structure, microstructure, electrical conductivity, heating, and mechanical properties were investigated in detail. The results showed that pure ITO ceramics with high relative density were obtained successfully. The ITO ceramics were cut into regular rectangles to serve as electrical heaters. Additionally, their saturation temperatures and thermal response times were tested at different voltages. The results showed that ITO ceramics with 10% SnO2 had the fastest thermal response rate and reached a saturation temperature of 376∘C at 1.5 V voltage, the highest temperature reported in the literature for In2O3 ceramics. Moreover, In2O3 with 10% SnO2 showed good mechanical properties. Hence, ITO ceramics, as electrical heaters with low driving voltage and high response speed, are expected to have potential heating applications such as domestic heating and industrial production. [ABSTRACT FROM AUTHOR]

This paper is devoted to observe the physical attributes of static spherically symmetric isotropic compact stellar candidates in the context of Rastall theory of gravity. In order to inspect the structural composition of compact objects, the Heintzmann ansatz is taken into account. The unknown parameters associated with Heintzmann ansatz are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition, stellar equation and adiabatic index for several choices of Rastall parameter. It is observed that the stars under consideration manifest stable structures corresponding to Heintzmann ansatz in this framework. Moreover, it is shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved. [ABSTRACT FROM AUTHOR]

In this study, our main focus is to investigate the mass–radius relation and several important properties of massive neutron stars to realize the nature, behavior and evolution of these kinds of compact objects at present time. Also, we want to understand the equation-of-state of the core nuclear matter precisely with their stable equilibrium configuration. We have chosen a few massive binary pulsars and investigated on maximum attainable mass and lowest possible radius of them. We have considered Einstein–Hilbert action as f(R,T) = R + αT, where R is the Ricci scalar and T, the trace of energy–momentum tensor with α as the coupling parameter and also used modified TOV equations. The interior space-time of the spherical neutron star is matched to the exterior Schwarzschild line element at the surface of the star. The M–R curve can predict the maximum achievable mass is about 3.51M⊙ with lowest possible radius of around 10.5 km for the massive compact stars under stable equilibrium. We have obtained a clear picture of structural evolution of massive neutron stars through accelerating space-time and can put some constraints on several quantities related to them. It is depicted from our present investigation that all the derived outcomes are compatible with physically adopted regimes which reveal the viability of our current model in the context of f(R,T) modified gravity. [ABSTRACT FROM AUTHOR]

In general, a perfect fluid spacetime is not a generalized Robertson–Walker spacetime and the converse is also not true. In this paper, it is shown that if a perfect fluid spacetime satisfies the critical point equation, then either the spacetime becomes a generalized Robertson–Walker spacetime and represents dark era or the vorticity of the fluid vanishes as well as the spacetime is expansion free. Besides, we prove that if a generalized Robertson–Walker spacetime with constant scalar curvature satisfies the critical point equation, then the spacetime becomes a perfect fluid spacetime. Next, the existence of critical point equation is established by a non-trivial example. Finally, we discuss the critical point equation in f (r) -gravity. For the model f (r) = r − α (1 − e − r α ) (α = constant and r is the scalar curvature of the spacetime), various energy conditions in terms of the scalar curvature are examined and state that the Universe is in an accelerating phase and satisfies the weak, null, dominant, and strong energy conditions. [ABSTRACT FROM AUTHOR]

In this study, the energy–momentum localization problem is addressed within the framework of rainbow gravity. Considering the black string black hole, one of the cylindrical black hole models, energy–momentum distributions are obtained for Einstein, Bergmann–Thomson, and Landau–Liftshitz prescriptions in rainbow gravity. All prescriptions' energy densities are non-zero and depend on the rainbow functions, while the momentum distributions are zero. It has been observed that space-time energy does not depend on the energy of the test particle for some particular choices of rainbow functions. The results obtained for all special cases are given in a table. [ABSTRACT FROM AUTHOR]

The f (R , T) theory of gravity investigated by Harko et al. [T. Harko, F. S. N. Lobo, S. Nojiri and S. D. Odintsov, Phys. Rev. D 84 (2011) 024020; T. Harko, Phys. Rev. D 90(4) (2014) 044067] serves as the inspiration for the homogeneous and isotropic cosmological model presented in generalized f (R , T ϕ) theories connected with a scalar field. Assuming that T ϕ is the trace of the energy-momentum tensor, f (R , T ϕ) gravity can be understood as f (R , T) gravity with a self-interacting scalar field ϕ. To address this, we provide a novel model-independent method for the parametrization of the Hubble parameter, and we apply it to the Friedmann equations in the FLRW Universe. The Markov Chain Monte Carlo (MCMC) approach is then used to estimate the best-fit values of the model parameters using a combination of the updated H (z) dataset, which comprises 57 points, and the Pantheon dataset, which contains 1048 points. The evolution of the deceleration parameter shows a transition from the universe's deceleration phase to its acceleration phase. We also investigate the behavior of the statefinder and the Om diagnostic parameter. Finally, a thorough discussion of the model's physical attributes has been carried out. [ABSTRACT FROM AUTHOR]

This paper is devoted to observe the physical attributes of isotropic relativistic spherical objects in the context of Rastall theory of gravity. In order to inspect the structural composition of spherical objects, the Tolman IV solution is taken into account. The unknown parameters associated with Tolman IV solution are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition and stellar equation for several choices of Rastall parameter. It is observed that the considered solution does not admit singularities in the matter variables. Moreover, in accordance with the graphical analysis, the stellar models satisfy all physical requirements anticipated in a realistic star. It is also shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved. [ABSTRACT FROM AUTHOR]

In this paper, we explore the existence of wormholes in the context of f (R , T) gravity. The f (R , T) theory is a curvature-matter coupled modified gravity that depends on an arbitrary function of the Ricci scalar R and the trace of the stress-energy tensor T. In this work, we adopt two different choices for the matter Lagrangian density ( ℒ m = and ℒ m = p r ) and investigate the impact of each one on wormhole structure. By adequately specifying the redshift function and the shape function, we found a variety of exact wormhole solutions in the theory. Our finding indicates that for both classes of wormholes, the energy density is always positive throughout the spacetime, while the radial pressure is negative. This means exotic matter is necessary for the existence of wormholes in f (R , T) gravity. [ABSTRACT FROM AUTHOR]

Two singularity theorems can be proven if one attempts to let a Lorentzian cobordism interpolate between two topologically distinct manifolds. On the other hand, Cartier and DeWitt-Morette have given a rigorous definition for quantum field theories (QFTs) by means of path integrals. This paper uses their results to study whether QFTs can be made compatible with topology changes. We show that path integrals over metrics need a finite norm for the latter and for degenerate metrics, this problem can sometimes be resolved with tetrads. We prove that already in the neighborhood of some cuspidal singularities, difficulties can arise to define certain QFTs. On the other hand, we show that simple QFTs can be defined around conical singularities that result from a topology change in a simple setup. We argue that the ground state of many theories of quantum gravity will imply a small cosmological constant and, during the expansion of the universe, will cause frequent topology changes. Unfortunately, it is difficult to describe the transition amplitudes consistently due to the aforementioned problems. We argue that one needs to describe QFTs by stochastic differential equations, and in the case of gravity, by Regge calculus in order to resolve this problem. [ABSTRACT FROM AUTHOR]

In this computational study, we investigate the gravity and pressure-driven electroosmotic multiphase flow of third-grade fluid through steep microchannel by using the semi-analytical technique. A two-dimensional mathematical model is proposed here for the supercritical bi-phase flow with the suspension of the nanometallic particles. A special type of non-Newtonian fluid (third-grade) is selected as a base fluid. The governing equations for the current flow problem are obtained through Poisson–Boltzmann equations, continuity equations, and Cauchy's momentum equations. The Poisson–Boltzmann equation is simplified by using the Debye–Hückel approximation. The symbolic software MATHEMATICATM is used to obtain the closed-form expression of electric potential, fluid and particle velocities, pressure gradient, and volumetric flow rate, respectively. The computational results are discussed qualitatively under the impact of the physical parameters embedded in the study. The graphical plots of fluid and particle-phase velocities are generated against the different parameters used in this study by adopting the same software as discussed earlier. The electrokinetic forces are responsible for electroosmotic flow which is modeled through the Laplace and Poisson–Boltzmann equations. The externally applied potential and electrical double-layer potential is governed by the Laplace and Poisson–Boltzmann equations, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we have provided a discussion regarding the structural properties of a spherical compact stellar object within the background of f (R , G) modified gravity. We have considered that the interior region of the compact stellar body is filled by a composition of anisotropic dark energy and isotropic normal matter which are assumed to be non-interacting. To relate the two stated fluids, we have considered the dark energy density directly proportional to the matter density. In particular, Krori–Barua (K–B) matric potentials are used to explore the solutions of modified field equations corresponding to f (R , G) = R + β R 2 + G m . Next, we have delivered a detailed 3-dimensional graphical analysis to design physically well-behavior and stability of different parameters like energy density, individual pressure components for both dark energy and normal matter and anisotropy. Also, we have explored the stability and physical acceptability criterion by causality conditions, adiabatic index, TOV equation, and Harrison–Zeldovich–Novikov criteria. Consequently, we have discussed the violation of energy conditions for dark energy and hold the same for the normal matter. Here is an important note we have taken for three compact objects like, SAX JI808.4-3658, PSR J1614-2230, and 4U1820-30, for graphical analysis. Furthermore, we have calculated the predicted values of central density, central pressure, and surface pressure for both dark and normal matter. Finally, we concluded that our proposed dark energy stellar configuration maintains all required stability and physical acceptability criteria. [ABSTRACT FROM AUTHOR]

We obtained the solutions of Einstein's Field Equations (EFEs) for locally rotationally symmetric (LRS) Bianchi type-I perfect fluid spacetimes through the concircular vector fields (CCVFs) in f (T) gravity. It is shown that such metrics admit CCVFs of 4, 5, 6, 7, 8 and 15 dimensions. We also calculated the energy density, fluid pressure, torsion scalar T and the form of the function f (T). We did not specify the form of f (T) for the solution of EFEs but found a particular form of f (T) during the process of finding the CCVFs. It is observed that energy density and fluid pressure of some solutions are related as p = − ρ , which means that the universe represented by these spacetime metrics behaves like dark energy models. For other solutions, it is observed that the fluid pressure and energy density can remain positive for particular choices of the constants involved. Thus, in these cases the rate of expansion can slow down due to positive attractive effect. We also calculated the Hubble's parameter (H), scalar expansion (), shear scalar ( σ 2 ), anisotropy parameter ( A m ) and the deceleration parameter (q) associated with each of the obtained exact LRS Bianchi type-I solutions. It is observed that the physical behavior of the spacetime changes with the number of admitting CCVFs. A constant anisotropic behavior for 15-dimensional CCVFs is observed to change to isotropic nature for seven-dimensional CCVFs. [ABSTRACT FROM AUTHOR]

In this paper, we analyze energy density inhomogeneities for charged fluid configuration in the background of f (T) theory and recognize its prime features as computed in GR. The dynamical equations are composed employing Bianchi identities for the standard, f (T) extra terms, and energy-momentum tensor for the electromagnetic field. We evaluate various mathematical models of dissipative and anisotropic fluid distributions in-plane symmetry under f (T) gravity. To proceed with the investigation, we design the f (T) field equations, kinematical quantities, and mass function. We analyzed dynamical variables and Ellis equations in terms of our considered theory. To examine the associated inhomogeneity factors, specific scenarios are illustrated alongside and without dissipation. Within a non-radiating situation, we analyze dust and isotropic and anisotropic matter in the state of electric charge. We examine the inhomogeneity factor of a dissipative fluid via a charged dust haze. We derive that the electromagnetic field fosters matter inhomogeneity, but additional curvature factors make the entire structure more homogenous as time passes. [ABSTRACT FROM AUTHOR]

In this work, we reconsider the study of black holes and black strings in the compactification of M-theory on a Calabi–Yau three-fold, considered as a complete intersection of hypersurfaces in a product of weighted projective spaces given by ℙ 4 (ω , 1 , 1 , 1 , 1) × ℙ 1 . Using the N = 2 supergravity formalism in five dimensions, we examine the BPS and non-BPSsolutions by wrapping M-branes on appropriate cycles in such a Calabi–Yau geometry. For the black hole case, we compute certain thermodynamical quantities. In particular, we calculate the entropy taking a maximal value corresponding to the ordinary projective space ℙ 4 with ω = 1. Using extended black hole entropies, we evaluate the temperature involving a minimal value for ℙ 4 . Then, we approach the stability of the non-BPS black holes via the recombination factor. In the allowed electric charge regions, we show that such states are unstable. For the black string solutions, we calculate the tension taking a minimal value corresponding to ℙ 4 . Computing the recombination factor, we show that the associated non-BPS black string states are stable in the allowed magnetic charge regions of the moduli space. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the bouncing universe in a modified gravity model f (Q , T). The bouncing universe theory posits that the universe goes through periodic expansions and contractions, with a "bounce" occurring at the end of each contraction that leads to a new expansion. This theory suggests that our universe didn't emerge on its own out of nothing, but is instead the latest in a series of universes that have been expanding and contracting for eternity. It has gained renewed interest in recent years as a potential alternative to the Big Bang theory, which suggests that our universe began with a massive expansion from a singularity. According to the above explanations, first, we review of f (Q , T) gravity model, then the bouncing universe scenario according to the f (Q , T) gravity model in some special functions is analyzed in detail. We find that in which condition of parameters space, bouncing occurs. [ABSTRACT FROM AUTHOR]

In the recently suggested modified non-metricity gravity theory with boundary term in a flat FLRW spacetime universe, dark energy scenarios of cosmological models are examined in this study. An arbitrary function, f (Q , C) = Q + α C 2 , has been taken into consideration, where Q is the non-metricity scalar, C is the boundary term denoted by C = R ̈ − Q , and α is the model parameter, for the action that is quadratic in C. The Hubble function H (z) = H 0 [ c 1 (1 + z) n + c 2 ] 1 2 , where H 0 is the current value of the Hubble constant and n , c 1 and c 2 are arbitrary parameters with c 1 + c 2 = 1 , has been used to examine the dark energy characteristics of the model. We discovered a transit phase expanding universe model that is both decelerated in the past and accelerated in the present, and we discovered that the dark energy equation of state (EoS) ω (d e) behaves as (− 1 ≤ ω (d e) < 2). The Om diagnostic analysis reveals the quintessence behavior in the present and the cosmological constant scenario in the late-time universe. Finally, we calculated the universe's current age, which was found to be quite similar to recent data. [ABSTRACT FROM AUTHOR]

In this paper, we employ parametrization techniques within the framework of f (T) gravity to investigate the deceleration parameter (DP), a key quantity characterizing the universe's expansion dynamics. By analyzing the DP, we gain valuable insights into the nature of cosmic constituents and their impact on the universe's evolution. We utilize a combination of observational data, including 31 Cosmic Chronometers (CC) measurements, 1048 Type Ia Supernovae (SNIa), 17 Baryon Acoustic Oscillation (BAO) measurements, Cosmic Microwave Background (CMB), 162 Gamma Ray Bursts (GRB), and 24 observations of compact radio quasars (Q). We employ the Markov chain Monte Carlo (MCMC) sampling technique to estimate the best-fit range of model parameters. Cosmological and cosmographic parameters are investigated, and their implications in the context of cosmology have been discussed. We also analyze the statefinder and Om diagnostic to gain deeper insights into the universe's behavior. Furthermore, we conduct statistical analyses to compare our model with the standard Λ CDM model. Our investigation also includes the study of physical parameters, providing comprehensive insights into the cosmological behavior within the f (T) gravity framework. The results and comparisons presented in this work contribute to a deeper understanding of the universe's dynamics and provide valuable implications for the cosmological model under consideration. [ABSTRACT FROM AUTHOR]

In this study, the structures of stars are examined using the Karmarkar condition (KC) to assess the metric components. The study also takes into account the anisotropic source of the matter distribution in the context of Modified Teleparallel Rastall Gravity (MTRG). Various values of the model parameter η are tested by assuming different metric coefficients for the embedding spacetime. To calculate the involved model parameters, the Schwarzschild black hole solution is employed as an exterior solution. The stability and admissibility of the solutions obtained are demonstrated by analyzing some core properties of stellar objects, using observed data from established stellar models such as "PSRJ1416-2230". It is worth noting that the results obtained in this study are physically feasible and acceptable. [ABSTRACT FROM AUTHOR]