Search Results (82)

Recently, several studies have investigated the validity of General Relativity’s predictions. Gravitational waves provide an ideal probe for testing the theory in the strong-field regime. In this work, we consider a class of modified-gravity theories, specifically $f\left(R\right)$, and scrutinize their predictions for the gravitational-wave emission from the coalescence of two astrophysical compact objects. We also assess the impact of next-generation gravitational-wave detectors on the ability to test these extensions of General Relativity. Full article

We propose a novel method for testing gravity models using seismic waves’ velocities. By imposing observational constraints on Earth’s moment of inertia and mass, we rigorously limit the gravitational models’ parameters within a $2\sigma$ accuracy. Our method, taking the PREM model as our reference and assuming its viability, constrains the parameters governing additional terms to the General Relativity Lagrangian to the following ranges: $-2\times {10}^9\lesssim \beta \lesssim {10}^9{\mathrm{m}}^2$ for Palatini $f\left(R\right)$ gravity, $-8\times {10}^9\lesssim ϵ\lesssim 4\times {10}^9{\mathrm{m}}^2$ for Eddington-inspired Born–Infeld gravity, and $-{10}^{-3}\lesssim \mathrm{{\rm Y}}\lesssim {10}^{-3}$ for Degenerate Higher-Order Scalar–Tensor theories. We also discuss potential avenues to enhance the proposed method, aiming to impose even tighter constraints on gravity models. Full article

In this work, we examine viable models of $f(Q,T)$ gravity theories against observational data with the aim to constrain the parameter space of these models. We have analyzed four different models of $f(Q,T)$ gravity and tested them against against late-time background probes: Cosmic Chronometer (CC), Baryon Acoustic Oscillations (DESI BAO), $Pantheo{n}^{+}$ and Gravitational wave(GWTC-3) data. We put stringent constraints on the $f(Q,T)$ gravity models, $f(Q,T)=\alpha Q+\beta T$, $f(Q,T)=\alpha Q^n+\beta T$, $f(Q,T)=-\alpha Q-\beta T^2$ and $f(Q,T)=\alpha Q^{-2}{T}^2$ along with other late-time cosmological parameters such as deceleration parameter ($q_0$), equation of state parameter ($w_0$), sound horizon distance ($r_d$) and demonstrate their alignment with the $\mathrm{\Lambda }CDM$ model and the observational data. We show that these models have the capability to alleviate the Hubble tension in late time universe, by predicting the present value of the Hubble parameter close to 74 km/s/Mpc. $f(Q,T)$ gravity theory introduces alterations in the background evolution and imposes a friction term in the propagation of gravitational waves, this phenomenon has also been examined. We have shown their agreement with the Gravitational Wave (GW) luminosity distance with the Electromagnetic (EM) counter part GWTC-3 data from Advanced LIGO and Advanced VIRGO across different observing runs capturing coalescence of Binary Neutron Stars (BNS), mergers of Binary Black Holes (BBHs), and Neutron Star-Black Hole (NSBH) binaries with EM counterparts. Full article

In this research paper, we investigate a Finsler-Randers spacetime in the context of a Bianchi type-V model of universe within the framework of Lyra geometry, employing a modified $f(\mathcal{R},\mathcal{T})$ gravity theory that incorporates a cosmological constant $\mathrm{\Lambda }$. We have derived the corresponding anisotropic Friedmann equations for the Finsler–Randers Bianchi type-V model of universe with modified $f(\mathcal{R},\mathcal{T})$ gravity in Lyra geometry, including the contributions of the cosmological constant and Randers anisotropic terms $b_0\left(t\right)$ and obtained analytical solutions. Further, we have examined the behavior of various dynamical parameters, commonly used in cosmological analysis, both geometrical and graphical interpretations have been provided. Furthermore, we have derived the Raychaudhuri equation in terms of the cosmological constant as a function of the cosmic time t. Our analysis reveals that the shear scalar ${\sigma }^2$ and the scalar expansion $\theta$ decrease with cosmic time and tend to zero at late times, indicating the isotropization of the universe in the presence of the cosmological constant; however, the Hubble parameter approaches a constant value rather than vanishing, while the energy density $\rho$, pressure $\mathcal{P}$, and the Lyra gauge function $\beta$ remain finite and non-zero even at large cosmic times. Ultimately, we conclude that the universe described by this framework exhibits continuous acceleration, as indicated by the negative value of the deceleration parameter q. Full article

We test whether $f\left(Q\right)$ symmetric teleparallel gravity theories can solve the Hubble tension consistently with DESI DR2 BAO. We consider three $f\left(Q\right)$ functional forms: logarithmic, exponential, and hyperbolic tangent. We extend these models by allowing a cosmological constant, and compare to phenomenological models with a flexible exponential, hyperbolic secant, and polynomial decay addition to the standard $\Lambda$CDM $H\left(z\right)$. We test these models against DESI DR2 BAO, CMB (Planck 2018 + SPT-3G + ACT DR6), local $H_0$, and Cosmic Chronometer data. The logarithmic and hyperbolic tangent $f\left(Q\right)$ models do not provide an adequate solution, but the exponential model does. Furthermore, it slightly reduces the $({\Omega }_m,H_0{r}_d)$ parameter space tension between CMB and BAO datasets to $2.56\sigma$, down from $2.65\sigma$ for $\Lambda$CDM. Although $\Lambda$CDM faces only $1.66\sigma$ tension in DESI data space, the $1\sigma$ higher tension in parameter space suggests a real anomaly. The models assisted by the cosmological constant perform slightly better still, at the cost of undermined theoretical motivation. They also perform poorly once local $H_0$ measurements are included. The phenomenological models fit all data reasonably well, yet the best-fitting models predict isotropically averaged BAO distances exceeding the DESI DR2 measurements at all redshifts. This highlights the difficulties of finding a theoretically motivated solution to the Hubble tension while remaining consistent with BAO data. Full article

We investigate primordial black hole (PBH) production via the collapse of supercritical domain walls in a quadratic $f\left(R\right)$-gravity model with tensor extensions. The effective field theory for an extra space’s scalar curvature provides a foundation for the formation of these dense walls. In our work, domain walls are found to be supercritical. Their properties were extensively studied in the literature, where it was demonstrated that they create wormholes and escape into baby universes through them. Closure of the wormhole leads to black hole creation, providing a mechanism for the production of primordial black holes in our model. We calculate the mass spectrum of such black holes and mass distribution within clusters of them. When accretion is accounted for, the black holes produced under this mechanism present viable dark matter candidates. Full article

In this paper, we investigate the stability and feasibility of an anisotropic stellar model under $f(R,T)$ gravity that embraces the Karmarkar condition. In order to develop the $f(R,T)$ gravity model, the functional form of $f(R,T)$ is taken into consideration as the linear function of the trace of the energy-momentum tensor T and the Ricci scalar R, respectively. This study proposes a well-known form of the radial metric function and finds another metric function by employing the Karmakar condition, which provides the exact solution to the field equation. The expression of the model parameters is derived by matching the obtained interior solutions with the Schwarzschild exterior metric over the bounding surface of a celestial object, along with the requirement that the radial pressure vanish at the boundary. The current estimated data of the star, pulsar 4U1608-52, is used to graphically explore the model. The physical attributes of the celestial object are thoroughly examined within the framework of the present model. Adjusting the model parameter, a detailed analysis of the stability criterion is presented that involves the adiabatic index, the Herrera cracking technique, and the causality condition. Furthermore, the Tolman–Oppenheimer–Volkhoff equation is used to analyze the stellar model’s equilibrium state. In order to maintain the stability condition of the anisotropic stellar structure, a suitable range for the model parameter is determined by the graphical analysis of the present model in this study. In addition, the numerical values of the physical parameters related to the compact stars Her X-1, LMC X-4, Cen X-3 and KS1731-207 are used to examine the model solution within the desired range of the model parameter. Full article

In this work, we explore a class of spherically symmetric black hole (BH) solutions within the framework of modified gravity, focusing on a non-ghost-free $f(R,G)$ theory coupled to a scalar field. We present a novel black hole geometry that arises as a deformation of the Schwarzschild solution and analyze its physical and thermodynamic properties. Our results show that the model satisfies stability conditions, with the Ricci scalar R, as well as its first and second derivatives, remaining positive throughout the spacetime. The solution admits multiple horizons and exhibits strong curvature singularities compared to those in general relativity. Furthermore, it supports a non-trivial scalar field potential. A comprehensive thermodynamic analysis is performed, including evaluations of the entropy, temperature, heat capacity, and quasi-local energy. We find that the black hole exhibits thermodynamic stability within certain ranges of model parameters. In addition, we investigate geodesic deviation and derive the conditions necessary for stability within the $f(R,G)$ gravitational framework. Full article

Recent observations suggest that General Relativity (GR) faces challenges in fully explaining phenomena in regimes of strong gravitational fields. A promising alternative is the $f\left(R\right)$ theory of gravity, where R denotes the Ricci scalar. This modified theory aims to address the limitations observed in standard GR. In this study, we derive a black hole (BH) solution without introducing nonlinear electromagnetic fields or imposing specific constraints on R or the functional form of $f\left(R\right)$ gravity. The BH solution obtained here is different from the classical Schwarzschild solution in GR and, under certain conditions, reduces to the Schwarzschild (A)dS solution. This BH is characterized by the gravitational mass of the system and an additional parameter, which distinguishes it from GR BHs, particularly in the asymptotic regime. We show that the curvature invariants of this solution remain well defined at both small and large values of r. Furthermore, we analyze their thermodynamic properties, demonstrating consistency with established principles such as Hawking radiation, entropy, and quasi-local energy. This analysis supports their viability as alternative models to classical GR BHs. Full article

We investigated the properties of Ricci semi-symmetric Robertson–Walker spacetimes within the framework of $f(\mathcal{R})$-gravity theory. Initially, we established that Ricci semi-symmetric Robertson–Walker spacetimes are locally isometric to either Minkowski or de Sitter spacetimes. We then focused on the 4-dimensional formulation of these spacetimes in $f(\mathcal{R})$-gravity, deriving expressions for the isotropic pressure p and energy density $\sigma$. To further develop our understanding, we explored various energy conditions to constrain the functional form of $f(\mathcal{R})$. We analyzed several models, namely $f(\mathcal{R})=\mathcal{R}-\alpha (1-e^{-{\displaystyle \frac{\mathcal{R}}{\alpha }}})$, $f(\mathcal{R})=\mathcal{R}-\beta tanh\mathcal{R}$, and $f(\mathcal{R})=\mathcal{R}-log(m\mathcal{R})$, where $\alpha$, $\beta$, and m are constants. Our findings suggest that the equations of state parameters for these models are compatible with the universe’s accelerating expansion, indicating an equation of state parameter $\omega =-1$. Moreover, while these models satisfy the null, weak, and dominant energy conditions reflective of the observed accelerated expansion, our analysis reveals that they violate the strong energy condition. Full article

In the present work, we study cosmology in dilatonic $f(R,T)$ gravity to address the inflationary phase of the early universe. As usual, in dilatonic gravity, the scalar potential assumes the exponential form. However, this potential is not good enough to be in accordance with the Planck 2018 data. More strikingly, the generalized $\beta$-exponential cannot take this into account either. It is just only presence of the dilatonic sector, in the intermediate coupling regime, that can help the theory to be in full accordance with the observational data. Full article

This paper presents a study of brane formation in six-dimensional space. There is no a priori assumption of the existence of brane(s). However, an analysis of the generalized Einstein equations shows that there is a set of metrics describing two static branes even in the absence of matter fields. At the same time, no one-brane configurations were found. The trapping of massive particles on branes is a consequence of the metric structure, which prevents these particles from moving between branes. It is shown that communication between charged particles on different branes is provided by photons. Such positron–electron annihilation could be studied experimentally at the LHC collider. The Higgs field is distributed between the branes in such a way that it can serve as a Higgs portal connecting two worlds located on different branes. The values of the 4D physical parameters depend on the extra metric structure near the branes. We also found a non-trivial effect of the decompactification of extra space during the Hubble parameter variation. Full article

Schwarzschild and Reissner–Nordstrøm black hole solutions are found in the context of a non-minimal matter–curvature coupling with Weyl connection both in vacuum and in the presence of a cosmological constant-like matter content. This model has the advantage of an extra force term which can mimic dark matter and dark energy, and simultaneously following Weyl’s idea of unifying gravity and electromagnetism. In fact, vacuum Schwarzschild solutions differ from the ones in a constant curvature scenario in $f\left(R\right)$ theories, with the appearance of a coefficient in the term that is linear in r and a corrected “cosmological constant”. Non-vacuum Schwarzschild solutions formally have the same solutions as in the previous case, with the exception being the physical interpretation of a cosmological constant as the source of the matter Lagrangian and not a simple reparameterization of the $f\left(R\right)$ description. Reissner–Nordstrøm solutions cannot be found in a vacuum, only in the presence of matter fields, with the result that the solutions also differ from the constant curvature scenario in $f\left(R\right)$ theories by the term being linear in r, the corrected/dressed charge, and the cosmological constant. These results have bearings on future numerical simulations for black holes and gravitational waves in next-generation wavelet templates. Full article

We investigate the cosmological evolution of the universe for a spatially flat FLRW background space within the context of $f(T,B)$ gravity, which is a recently formulated teleparallel theory that connects both $f\left(T\right)$ and $f\left(R\right)$ gravity under suitable limits. The analysis focuses on four different $f(T,B)$ cosmological models corresponding to various choices of scale factor, namely, emergent, logamediate, and intermediate. In addition to this, we assume a power law-like function of $f(T,B)$ gravity. The reconstruction of $f(T,B)$ gravity considers the Holographic Ricci Dark Energy (HRDE) as the background fluid. We analyze the equation of state parameters and the squared speed of sound for the reconstructed models. Finally, we conduct a thermodynamical analysis for each reconstructed model. The generalized second law of thermodynamics (GSLT) is valid for the four different $f(T,B)$ cosmological models. Full article

In this work, we present a review of Energy-Momentum Squared Gravity (EMSG)—more specifically, $f(R,T_{\mu \nu }{T}^{\mu \nu })$ gravity, where R represents the Ricci scalar and $T_{\mu \nu }$ denotes the energy-momentum tensor. The inclusion of quadratic contributions from the energy-momentum components has intriguing cosmological implications, particularly during the Universe’s early epochs. These effects dominate under high-energy conditions, enabling EMSG to potentially address unresolved issues in General Relativity (GR), such as the initial singularity and aspects of big-bang nucleosynthesis in certain models. The theory’s explicit non-minimal coupling between matter and geometry leads to the non-conservation of the energy-momentum tensor, which prompts the investigation of cosmological scenarios through the framework of irreversible thermodynamics of open systems. By employing this formalism, we interpret the energy-balance equations within EMSG from a thermodynamic perspective, viewing them as descriptions of irreversible matter creation processes. Since EMSG converges to GR in a vacuum and differences emerge only in the presence of an energy-momentum distribution, these distinctions become significant in high-curvature regions. Therefore, deviations from GR are expected to be pronounced in the dense cores of compact objects. This review delves into these facets of EMSG, highlighting its potential to shed light on some of the fundamental questions in modern cosmology and gravitational theory. Full article