Framework Of Modified Gravity Research Articles

Quark Stars in $f(R,T)$ Gravity: Mass-to-Radius Profiles and Observational Data

Abstract In this paper, we explore the $f(R,T)$ gravity theory, which introduces a coupling between matter and curvature, through the simplest linear functional form $f(R,T)= R + 2\beta T$. We derive the modified Einstein field equations and conservation equations for this theory. We then apply this framework to study the structural properties of quark stars (QSs) composed of interacting quark matter, considering perturbative QCD corrections and color superconductivity. By solving the modified Tolman-Oppenheimer-Volkoff equations, we investigate the mass-radius relation, stability criteria, and energy conditions of QSs. Our results indicate that the $f(R,T)$ gravity significantly influences the properties of QSs, leading to deviations from General Relativity. The analysis shows consistency with recent observational data, suggesting that the modified gravity framework could provide viable models for the study of compact stars.

Embedding the ΛCDM framework in non-minimal f(Q) gravity with matter-coupling

In exploring the dynamics of the Universe, modified gravity theories offer an alternative explanation for dark energy. Although the ΛCDM model has served as a foundational description for understanding the Universe, it also has its limitations. Our primary objective in this study is to overcome these shortcomings and reconstruct the ΛCDM Universe. We aim to achieve this by embedding the ΛCDM scenario within a modified gravity framework. Here, the investigation is conducted within the framework of novel non-minimal coupling of f(Q) gravity. A generic non-minimal form is considered, expressed in terms of two arbitrary functions f1(Q) and f2(Q). Analytic solutions are derived to mimic the ΛCDM paradigm. Finally, through cosmographic analysis, the models are validated against observational results.

Einstein-Gauss-Bonnet dark matter halo: negative masses, rotation curves and the origin of dark matter effects

This work presents a novel approach to address the longstanding challenge posed by the rotation curves of galaxies and the associated missing mass problem. Utilizing the four-dimensional modified gravity framework of Einstein-Gauss-Bonnet (EGB), we develop a new model that integrates the concept of dark matter featuring negative mass due to the Gauss-Bonnet term. Our methodology involves using the action of EGB with boundary terms to derive the Israel junction condition, allowing the formulation of a model featuring a spherical dark halo. The mass expression of this dark halo exhibits a remarkable proportionality to the radius r at large distances, forming the basis for subsequent analyses. The model’s implications extend to the dynamics of the early Universe, where we introduce a dynamic scalar field classified as Chameleon. Posting this scalar field as the origin of dark matter effects, we establish a crucial link between the coupling constant α and the scalar field through Z2 symmetry breaking via the effective potential. This connection enables a subtle description of velocity, reminiscent of Modified Newtonian Dynamics (MOND), providing insights into the gravitational interplay and dark matter dynamics. A pivotal aspect of our study involves a particular comparison of the model’s predictions with observational data sourced from SPARC datasets. The alignment of our theoretical outcomes with empirical evidence underscores the model’s efficiency and its potential contribution to our understanding of galactic rotation dynamics.

Quasinormal modes, and different aspects of Hawking radiation within the metric-affine bumblebee gravity framework

We consider a static and spherically symmetric black hole metric that emerges from the vacuum solution of the traceless metric-affine bumblebee model. Our study focuses on the possible implications of the modifications induced by the model on various astrophysical observables that include quasinormal modes, ringdown waveforms, Hawking radiation spectrum, sparsity of that radiation, and the lifetime of a black hole. We explore the impact of the Lorentz symmetry-breaking (LSB) parameter α on the quasinormal modes with the help of the 6th-order WKB method. Our inquisition reveals that the emission frequency and decay rate initially decreases with α and then grows up. As a result, the LSB becomes critically important for maintaining the stability of the system after being exposed to perturbation. The convergence of the WKB method for various orders is also studied here. We then analyze the Hawking temperature, radiation spectrum, and sparsity in this modified gravity framework that provides valuable insights into the thermal radiation emitted by black holes. It points out that the Hawking temperature, the peak of the power spectrum, and the total power emitted initially decreases and then increases with α. However, The variation of the sparsity with α follows a reverse trend. Finally, we obtain the analytical expression of the ‘lifetime’ of black holes and scrutinize the effect of α on it.

Cosmic implications of Kaniadakis HDE model in Chern-Simons modified gravity

In this study, we investigate the FRW model in the Chern–Simons modified gravity framework, considering holographic dark energy and Kaniadakis holographic dark energy models. Employing the field equation of Chern–Simons gravity, it is analyzed some important cosmological parameters such as density, equation of state, and deceleration parameters for both models. Our findings, presented visually through graphs, reveal that holographic dark energy places the equation of state within ω∈(−1,−0.4), while Kaniadakis holographic dark energy spans ω∈(−0.8,−0.4). These results demonstrate the dynamic nature of dark energy’s influence. Our study highlights the interplay of quintessence and ΛCDM across cosmic eras. We identify the ranges of q∈(−0.89,−0.86) and q∈(−0.9925,−0.9922) for Kaniadakis holographic dark energy and holographic dark energy, offering insights into their roles in cosmic inflation which implies that the universe attributes are impacted by both quintessence and the ΛCDM model in Chern–Simons modified gravity. In addition, this study implies that the universe’s expansion is decelerating in its current state.

Neutron Star Evolution in f(Q) Modified Gravity Framework

The current study reports the evolution of a Neutron Star in f ( Q ) modified gravity framework, where f ( Q ) is chosen in the form f ( Q ) = Q + aQ , where the non-metricity scalar Q is taken as Q = 6 H 2 and a is constant. H is known as the Hubble parameter. Considering the modified Friedmann equation, we reformulated the density and pressure for the f ( Q ) gravity background. In this modified gravity framework, we derived pressure and density as a function of radius r of the neutron star. The reconstructed density happens to be a monotone increasing function of radius r. This can be interpreted as the accretion of Dark Energy around the Neutron star. To further consolidate the results, we reconstruct the pressure as well, and the behavior of the pressure supports the Dark Energy accretion. The behaviors of the EOS and the model parameters have also been studied in this f ( Q ) setting of Neutron Star.

Probing Schwarzschild-like black holes in metric-affine bumblebee gravity with accretion disk, deflection angle, greybody bounds, and neutrino propagation

In this paper, we investigate Schwarzschild-like black holes within the framework of metric-affine bumblebee gravity. We explore the implications of such a gravitational setup on various astrophysical phenomena, including the presence of an accretion disk, the deflection angle of light rays, the establishment of greybody bounds, and the propagation of neutrinos. The metric-affine bumblebee gravity theory offers a unique perspective on gravitational interactions by introducing a vector field that couples to spacetime curvature. We analyze the behavior of accretion disks around Schwarzschild-like black holes in this modified gravity scenario, considering the effects of the bumblebee field on the accretion process. Furthermore, we scrutinize the deflection angle of light rays as they traverse the gravitational field, highlighting potential deviations from standard predictions due to the underlying metric-affine structure. Investigating greybody bounds in this context sheds light on the thermal radiation emitted by black holes and how the modified gravity framework influences this phenomenon. Moreover, we explore neutrino propagation around Schwarzschild-like black holes within metric-affine bumblebee gravity, examining alterations in neutrino trajectories and interactions compared to conventional general relativity. By comprehensively probing these aspects, we aim to unravel the distinctive features and consequences of Schwarzschild-like black holes in the context of metric-affine bumblebee gravity, offering new insights into the nature of gravitational interactions and their observable signatures.

Constraining the bounce realization with holographic background and analytical exploration of the consequences in a modified gravity framework

The work reported in this paper explores holographic bounce. In the first phase of the study, we chose a non-singular bouncing scale factor. Then we reconstructed [Formula: see text] gravity and analytically derived constraints on the bouncing parameter [Formula: see text]. These constraints helped us understand the scale factor’s quintessence or phantom behavior. Furthermore, we also explored the statefinder parameters for reconstructed [Formula: see text] and observed the attainment of [Formula: see text]CDM fixed point. Next, we considered the multiplicative bouncing scale factor inspired by S. D. Odintsov and V. K. Oikonomou Phys. Rev. D 94, (2016) 064022. For this choice, we discussed the types of singularities realizable for different cases. Through the Talyor series expansion, we analytically presented cases and subcases for different ranges of [Formula: see text] of the scale factor. In the last phase of the study, we demonstrated holographic bounce with the choice of the multiplicative scale factor. In this case, we considered holographic Ricci dark energy and Barrow holographic dark energy. We concluded that it is possible to generate constraints on the bouncing parameter for its feasibility for the EoS parameter. We concluded that the realization of holographic bounce is possible, and different suitable constraints can be derived for this multiplicative bouncing scale factor focusing on the realization of cosmic bounce.

Some versions of Chaplygin gas model in modified gravity framework and validity of generalized second law of thermodynamics

Abstract In the work reported in this paper, we have analyzed generalized Chaplygin gas (GCG) and modified generalized Chaplygin gas (MGCG) in an interacting scenario. The equation of state parameter has been analyzed in both the cases and the stability of the models has been discerned through squared speed of sound. Stability against gravitational perturbations has been observed for both GCG and MGCG interacting with pressureless dark matter. Also, the generalized second law (GSL) of thermodynamics has been tested for different enveloping horizons and validity of GSL has been observed throughout. Furthermore, f(T) gravity has been reconstructed with GCG and MGCG and phantom behaviour has been observed through reconstructed EoS parameters. The squared speed of sound has been derived for f(T) gravity and stability of the model has been established through its positivity.

A truncated scale factor to realize cosmological bounce under the purview of modified gravity

AbstractThe present paper reports a study on bounce realization with a truncated scale factor and the holographic fluid taken in the form of holographic Ricci Dark Energy, which is a particular case of holographic Nojiri–Odintsov model (Nojiri & Odintsov, 2006, Gen. Relativ. Gravit., 38(8), 1285). We have investigated the realization of bounce due to the application of a holographic fluid in the early scenario of the universe. We started with a de‐Sitter scale factor in our study and derived two modified forms using Taylor's series expansion. After studying the behavior of the resulting Hubble parameter, we identified as the suitable form of the scale factor to realize the bounce. It satisfied the conditions , , and before, at, and after the turn‐around point. We have also observed that the resulting equation of state (EoS) parameter violates the null energy condition required by the bounce realization. We have studied the bounce scenario in a modified gravity framework and derived the slow roll parameters. We have studied the behavior of the potential along with the slow‐roll parameters in this bouncing scenario.

Realization of Bounce in a Modified Gravity Framework and Information Theoretic Approach to the Bouncing Point

In this work, we report a study on bouncing cosmology with modified generalized Chaplygin Gas (mgCG) in a bulk viscosity framework. Reconstruction schemes were demonstrated in Einstein and modified f(T) gravity framework under the purview of viscous cosmological settings. We also took non-viscous cases into account. We studied the equation of state (EoS) parameter under various circumstances and judged the stability of the models through the sign of the squared speed of sound. We observed the mgCG behaving like avoidance of Big Rip in the presence of bulk viscosity at the turnaround point and in non-viscous cases, a phantom-like behavior appears. The turnaround point equation of state parameter crosses the phantom boundary, violating NEC. The role of the mgCG’s model parameters was also investigated before and after the bounce. A Hubble flow dynamics was carried out and, it was revealed that mgCG is capable of realizing an inflationary phase as well as an exit from inflation. An f(T) gravitational paradigm was also considered, where the mgCG density was reconstructed in the presence of bulk viscosity. The role of the parameters associated with the bouncing scale factor, describing how fast the bounce takes place, was also studied in this framework. Finally, the reconstructed mgCG turned out to be stable against small perturbations irrespective of the presence of bulk viscosity and modified gravity scenario. Finally, the reconstruction scheme was assessed using statistical analysis, Shannon entropy.

Cosmological bounce in a modified gravity framework

AbstractThe present study reports the bouncing cosmology of f(T) gravity with holographic Ricci form of density, that is, . Taking two forms of scale factor, namely exponential and power‐law, we have reconstructed f(T) gravity and accordingly derived expressions for the equation of state parameter in the bouncing scenario. The reconstructed equation of state parameter for the exponential form of scale factor showed a symmetric pattern about the turnaround point or bouncing point. Finally, we checked the stability of the models through the squared speed of sound and demonstrated how the stability depends upon the value of ξ.

Gravastar under the framework of braneworld gravity II: Effect of the Kuchowicz metric function

In recent years, a class of compact objects called gravastars have drawn immense interest as regular solutions to end state stellar collapse. Since the energy density involved in collapse process is expected to be high, it is a natural choice to study such compact objects in context of modified gravity theories which reduce to General Relativity (GR) in the low energy regime. We have already framed a model of gravastar in such a modified gravity framework involving extra dimensional Randall–Sundrum (RS) single brane gravity [R. Sengupta, S. Ghosh, S. Ray, B. Mishra and S. K. Tripathy, Phys. Rev. D 102, 024037 (2020)]. As a sequel in this paper, we substantially improve our previous model by choosing the Kuchowicz function as one of the metric potentials, which leads to many new interesting results and physical features from our analysis as discussed in this paper. Also, we provide essential additional stability checks on our gravastar model to investigate the possibility of any instability creeping in due to the higher-dimensional framework. Our present improved gravastar model is found to clear all the stability checks successfully. Very interestingly, the static spherically symmetric matter distributions are found to accommodate both classes of solutions obeying and violating the modified energy conditions on the RS brane as we find in this work. We can conclude from our analysis that the Kuchowicz metric potential is very effective for describing regular solutions to compact objects at substantially high energies on the three-brane.

Modified gravity models for inflation: In conformity with observations

We consider a modified gravity framework for inflation by adding to the Einstein-Hilbert action a direct $f(\phi)T$ term, where $\phi$ is identified as the inflaton and $T$ is the trace of the energy-momentum tensor. The framework goes to Einstein gravity naturally when the inflaton decays out. We investigate inflation dynamics in this $f(\phi)T$ gravity (not to be confused with torsion-scalar coupled theories) on a general basis and then apply it to three well-motivated inflationary models. We find that the predictions for the spectral tilt and the tensor-to-scalar ratio are sensitive to this new $f(\phi)T$ term. This $f(\phi)T$ gravity brings chaotic and natural inflation into better agreement with data and allows a larger tensor-to-scalar ratio in the Starobinsky model.

Horndeski stars

We establish the existence of time-dependent solitons in a modified gravity framework, which is defined by the low energy limit of theories with a weakly broken galileon symmetry and a mass term. These are regular vacuum configurations of finite energy characterized by a single continuous parameter representing the amplitude of the scalar degree of freedom at the origin. When the central field amplitude is small the objects are indistinguishable from boson stars. In contrast, increasing the central value of the amplitude triggers the effect of higher derivative operators in the effective theory, leading to departures from the previous solutions, until the theory becomes strongly coupled and model-dependent. The higher order operators are part of the (beyond) Horndeski theory, hence the name of the compact objects. Moreover, a remnant of the galileon non-renormalization theorem guarantees that the existence and properties of these solutions are not affected by quantum corrections. Finally, we discuss the linear stability under small radial perturbations, the mass-radius relation, the compactness, the appearance of innermost stable circular orbits and photon spheres, and some astrophysical signatures (accretion disks, gravitational radiation and lensing) that may be relevant to falsify the model.

White dwarf stars in modified gravity

A few questions related to white dwarfs’ physics is posed. It seems that the modified gravity framework can be a good starting point to provide alternative explanations to cooling processes, their age determination, and Chandrasekhar mass limits. Moreover, we have also obtained the Chandrasekhar limit coming from Palatini [Formula: see text] gravity provided by a simple Lane–Emden model.

Cosmological gravity on all scales: Simple equations, required conditions, and a framework for modified gravity

The cosmological phenomenology of gravity is typically studied in two limits: relativistic perturbation theory (on large scales) and Newtonian gravity (required for smaller, non-linear, scales). Traditional approaches to model-independent modified gravity are based on perturbation theory, so do not apply on non-linear scales. Future surveys such as Euclid will produce significant data on both linear and non-linear scales, so a new approach is required to constrain model-independent modified gravity by simultaneously using all of the data from these surveys. We use the higher order equations from the post-Friedmann approach to derive a single set of "simple 1PF" (first post-Friedmann) equations that apply in both the small scale and large scale limits, and we examine the required conditions for there to be no intermediate regime, meaning that these simple equations are valid on all scales. We demonstrate how the simple 1PF equations derived here can be used as a model-independent framework for modified gravity that applies on all cosmological scales, and we present an algorithm for determining which modified gravity theories are subsumed under this approach. This modified gravity framework provides a rigorous approach to phenomenological N-body simulations, and paves the way to consistently using all of the data from upcoming surveys to constrain modified gravity in a model-independent fashion.

A study of the bulk viscous pressure in scalar fields and holographic Ricci dark energy considered in the modified gravity framework

The work presented in this paper reports a rigorous study of the reconstruction of the modified gravity in the framework of the scalar field models of dark energy and holographic Ricci dark energy, a generalized version of the holographic dark energy presented in S. Nojiri and S.D. Odintsov. Gen. Relativ. Gravitation, 38, 1285 (2006). The tachyon and quintessence scalar fields have been considered and the cosmology associated with the presence of bulk viscosity has been studied. In the first part of our study, we have demonstrated the behaviour of the bulk viscosity coefficient in the framework of the reconstructed tachyon scalar field model of dark energy. The scale factor is chosen in the form a(t) = a0tβ, where β > 0. Two scalar field models, namely, tachyon and quintessence, have been considered in the framework of the modified field equations through incorporation of the bulk viscous pressure. The reconstructed density and pressure of the scalar field models have been explored for the cosmological consequences in the presence of bulk viscosity. The behaviour of the effective equation of state parameters has been investigated. Finally, we have reconstructed f(T) gravity in the presence of holographic Ricci dark energy and a transition of the effective equation of state parameter from quintessence to phantom has been observed.

Anisotropic neutron stars in R2 gravity

We consider static neutron stars within the framework of $R^2$ gravity. The neutron fluid is described by three different types of realistic equations of state (soft, moderately stiff, and stiff). Using the observational data on the neutron star mass-radius relation, it is demonstrated that the characteristics of the objects supported by the isotropic fluid agree with the observations only if one uses the soft equation of state. We show that the inclusion of the fluid anisotropy enables one also to employ more stiff equations of state to model configurations that will satisfy the observational constraints sufficiently. Also, using the standard thin accretion disk model, we demonstrate potentially observable differences, which allow us to distinguish the neutron stars constructed within the modified gravity framework from those described in Einstein's general relativity.

Quasinormal Modes of Static Modified Gravity (MOG) Black Holes

Using an asymptotic iteration method, we calculate the gravitational and electromagnetic quasinormal mode (QNM) perturbations for a static neutral black hole described by a Scalar-Tensor-Vector Modified Gravity framework (STVG-MOG). We show that the first few harmonic modes differ from their general relativistic (GR) equivalent for a Schwarzschild black hole. Specifically, the real and imaginary components of the QNM frequencies are smaller for STVG-MOG than for GR. We posit that the differences are sufficiently large to potentially be observed in present and future black hole binary merger gravitational waveforms.