This work focuses on analyzing a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe within the Brans–Dicke (BD) scalar-tensor theory of gravity, incorporating a dynamical cosmological constant and Barrow holographic dark energy (BHDE) alongside pressureless dark matter (DM). The cosmological dynamics is explored by solving the field equations using an exponential form of the scale factor, and the scalar field evolving according to a power-law relationship. In this study, the Hubble horizon is chosen as the infrared (IR) cut-off, which plays a crucial role in defining the energy density of the BHDE. Both non-interacting and interacting scenarios between dark energy (DE) and dark matter (DM) are investigated to assess the influence of these components on cosmic evolution. To gain deeper insights, the analysis is further extended using cosmographic parameters, including the jerk, snap and lerk parameters, as well as the Statefinder diagnostic pair [Formula: see text], which serve as powerful tools to characterize different phases of cosmic expansion. The findings reveal that the proposed model consistently replicates the accelerated expansion of the universe, in agreement with the current observational data. It is noteworthy that the dynamical behavior exhibited in the interacting scenario closely resembles that of the non-interacting case, thereby reinforcing consistency and the physical viability of the model.

Nonlinear scalar-tensor theories of gravity have been considered as candidates for dark matter and dark energy. Often, they possess screening mechanisms, which allow the fifth force mediated by the additional scalar degree(s) of freedom to evade detection from local experiments. Their classical behaviour is well studied, but their quantum nature is relatively unexplored. We outline a Green's function method for obtaining the leading-order quantum corrections to the classical symmetron field profile, in the vicinity of a spherically symmetric extended source, in the planar limit. For parameters that experiments had previously ruled out, our calculations indicate that the symmetron force may be weaker than the classical field suggests.

Abstract In our previous papers we have analyzed the stability of vacuum and electrovacuum static, spherically symmetric space-times in the framework of the Bergmann–Wagoner–Nordtvedt class of scalar-tensor theories (STT) of gravity. In the present paper, we continue this study by examining the stability of exceptional solutions of the Brans–Dicke theory with the coupling constant $$\omega =0$$ ω = 0 that were not covered in the previous studies. Such solutions describe neutral or charged wormholes and involve a conformal continuation: the standard conformal transformation maps the whole Einstein-frame manifold $${\mathbb {M}}_\textrm{E}$$ M E to only a part of the Jordan-frame manifold $${\mathbb {M}}_\textrm{J}$$ M J , which has to be continued beyond the emerging regular boundary S, and the new region maps to another manifold $${\mathbb {M}}_\textrm{E}$$ M E $${}_{-}$$ - . The metric in $${\mathbb {M}}_\textrm{J}$$ M J is symmetric with respect to S only if the charge q is zero. Our stability study concerns radial (monopole) perturbations, and it is shown that the wormhole is stable if $$q \ne 0$$ q ≠ 0 and unstable only in the symmetric case $$q=0$$ q = 0 .

In this paper, we study an anisotropic cosmological model in scalar tensor theory of gravitation proposed by Brans-Dicke, considering a Bianchi type- spacetime geometry filled with a macroscopic body. To solve the nonlinear Brans-Dicke field equations by using the average scale factor and the relation between metric coefficients, with a radiation universe, we obtained exact solutions for the metric functions and scalar field. We derive and analyze several key cosmological parameters as functions of redshift, including the Hubble parameter, expansion scalar, shear scalar, energy density, pressure and the Brans-Dicke scalar field. The evolution of these parameters is graphically illustrated, revealing that the model predicts an expanding universe with dynamics transitioning from an early decelerating phase to a late-time acceleration. The behavior of the deceleration parameter supports the accelerated expansion in accordance with current observational data. Furthermore, we investigate higher-order cosmological diagnostics such as the jerk and snap parameters as well as the statefinder parameters which are instrumental in distinguishing this model from the standard cosmology and other dark energy models. We also explore the Om diagnostic to characterize dark energy evolution and compare our theoretical Hubble parameter values with a compilation of 57 recent Hubble dataset points obtained from differential age (DA) and baryonic acoustic oscillation (BAO) methods. A good agreement is observed between the model predictions and observational data, as indicated by the best-fit curve with minimal root mean square error.

We investigate time delays of wave scatterings around black hole backgrounds in scalar-tensor effective field theories of gravity. The scalar-Gauss-Bonnet (sGB) couplings, being corrections of the lowest orders, can give rise to hairy black holes. By requiring infrared causality, we impose lower bounds on the cutoff scales of the theories. With these bounds, we further discuss the detectability of sGB gravity in gravitational waves from binary black hole mergers. Compared with the gravitational effective field theories that contain only the two tensor modes, adding extra degrees of freedom, such as adding a scalar, opens up a detectable window in the planned observations.

Objective: To get a solution for the dark energy model using a Bianchi type III model with an EoS (Equation of State) parameter filled with a perfect fluid in the scalar tensor theory of gravity. Methods: To acquire exact solutions of field equations, we considered: (i) the shear scalar is proportional to scalar expansion. (ii) a special form of scale factor, and (iii) the EoS (Skewness) parameter is proportional to the skewness parameter. Findings: We have seen that the EoS and the Skewness are functions of time. We also observed some physical and kinematic aspects of the resulting model. It is found that the resultant model is consistent with the current observations of SNeIa and CMBR. Novelty: We obtained a Bounce Bianchi type III dark energy model with an EoS parameter filled with a perfect fluid in the scalar tensor gravitational theory. Keywords: Bianchi Type III space time metric; Perfect Fluid; 𝑓(𝑅, 𝑇 ) Gravity; Dark Energy

Recently, possible hints of parity violation have been observed in the connected galaxy four-point correlation function. Although the true origin of the signal from the analysis has been debated, should they have a physical origin, they might point to primordial non-Gaussianity and would be evidence of new physics.In this work, we examine the single-field slow-roll model of inflation within chiral scalar-tensor theories of modified gravity. These theories, treated here as new Lorentz-breaking theories, extend the Chern-Simons one by including parity-violating operators containing first and second derivatives of thenon-minimally coupled scalar (inflaton) field. This model is capable of imprinting parity-violating signatures in late-time observables, such as the galaxy four-point correlation function.We perform an analysis of the graviton-mediated scalar trispectrum of the gauge-invariant curvature perturbation ζ(t,x) using one of the parity-violating operators of these theories as a case study.We estimate that for a set of parameters of the theory it is possible to produce a signal-to-noise ratio for the parity-violating part of the trispectrum of order one without introducing modifications to the single-field slow-roll setup.Even if the signal found in the analysis turns out to be spurious or if no parity violation is ever detected in the galaxy four-point correlation function, our analysis can be used to constrain the free parameters of these theories.

This paper investigates the dynamical behavior of hypersurface homogeneous spacetime cosmological models within the framework of the scalar-tensor theory of gravitation formulated by Saez and Ballester (Phys. Lett. A, 113, 467 1986) in Lyra geometry. We present two cosmological models derived from this theory by solving the field equations using: (i) Special law of variation for Hubble’s parameter and (ii) the proportional relationship between the shear scalar σ2 and scalar expansion θ as described by Collins et al. (Gen. Rel. Grav. 12, 805 1980). For each model, we evaluate key dynamical parameters, including the equation of state (EoS) parameter, the deceleration parameter, the statefinder parameter, and the total energy density parameter of dark energy. Additionally, we determine the scalar field in both models. Our findings indicate that these models describe an accelerated expansion of the universe, with theoretical results showing reasonable agreement with observational data.

We compute tidal signatures in the gravitational waves (GWs) from neutron star binary inspirals in scalar-tensor gravity, where the dominant adiabatic even-parity tidal interactions involve three types of Love numbers that depend on the matter equation of state and parameters of the gravitational theory. We calculate the modes of the GW amplitudes and the phase evolution in the time and frequency domain, working up to first order in the post-Newtonian and small finite-size approximations. We also perform several case studies to quantify the dipolar and quadrupolar tidal effects and their parameter dependencies specialized to Gaussian couplings. We show that various tidal contributions enter with different signs and scalings with frequency, which generally leads to smaller net tidal GW imprints than for the same binary system in General Relativity.

We compute the angular momentum flux from a noncircular nonspinning binary system of compact objects in massless scalar-tensor theories up to one and a half post-Newtonian (1.5PN) order using multipole moments. The angular momentum flux in scalar-tensor theories involves both a tensorial and a scalar contribution, which can be further decomposed as instantaneous, tail, and nonlinear memory effects up to 1.5PN order. We compute the explicit expressions of tail effects using the Fourier decomposition of tensorial and scalar multipole moments, and nonlinear memory effects using the Newtonian order quasi-Keplerian representation of elliptic orbits in scalar-tensor theories. This work is important to construct the radiation-reaction force and hence the waveform templates for eccentric binaries in scalar-tensor theories of gravity. Published by the American Physical Society 2025

We investigate anisotropic and homogeneous cosmological models in the scalar-tensor theory of gravity with non-minimal kinetic coupling of a scalar field to the curvature given by the function η·(ϕ/2)· Gμν ∇ μ ∇ νϕ in the Lagrangian. We assume that the space-times are filled a global unidirectional magnetic field that minimally interacts with the scalar field. The matter sector is not included, since the model is studied in relation to the early times of the Universe evolution. We limit ourselves to the period before and during primary inflation. The Horndeski theory allows anisotropy to grow over time. The question arises about isotropization. In the theory under consideration, a zero scalar charge imposes a condition on the anisotropy level, namely its dynamics develops in a limited region. This condition uniquely determines a viable branch of solutions of the field equations. The magnetic energy density that corresponds to this branch is a bounded function of time. The sign of parameter l = 1+εηΛ/μ determines the properties of cosmological models, where Λ is the cosmological constant, μ = M 2 PL is the Planck mass squared. The sign ε = ±1 defines the canonical scalar field and the phantom field, respectively.An inequality ε/η > 0 is a necessary condition for isotropization of models, but not sufficient. The model with l > 0 has the necessary properties: isotropization during expansion, rapid transition to inflationary expansion (a(t) ∝ e √(ε/3η)· t), absence of ghost and Laplace instabilities. In other cases l ≯ 0, the model has various disadvantages. Constraints on the tensor-to-scalar ratio, the conditions for avoidance of ghost and Laplacian instabilities lead to the inequalities: Λ > 0, η > 0, ε = 1, l < Λη/μ < 1.049.

In this paper, we have discussed the exact solution of Einstein’s field equations for LRS Bianchi type- cosmological model in the framework of the scalar-tensor theory of gravitation given by Brans-Dickeand Lyra geometry. In order to obtain a determinant solution, special law of variation for Hubble’s parameter proposed by Berman (1983) has been considered. The relationship between holographic dark energy model with the quintessence dark energy has been established. Phantom potential and dynamics of the Quintessence scalar field are reconstructed, which describes the accelerated phase of the expanding universe. Some physical and geometrical properties of the model are also discussed. Keywords: Cosmological model; LRS Bianchi type- ; Brans-Dicketheory.

In any scalar-tensor theory of gravity exhibiting a screening mechanism, the fifth force mediated by the scalar field is dynamically suppressed at sub-Solar system scales, allowing it to pass existing tests of gravity. As a result, a major research effort has been carried out over the past decades to ``outsmart'' screened scalars in this game of hide-and-seek. While most of such tests rely on fifth force effects, one should keep in mind that the latter are by no means the only physical feature of scalar-tensor gravity. In particular, this article investigates the possibility of testing screened scalar-tensor models by means of gravitational redshift measurements performed with atomic clocks. Upon deriving the expression for the redshift in this framework, we propose a thought experiment for testing the chameleon model by clock comparisons, which guides us toward more realistic experimental setups, in the laboratory and in space. We find that currently unconstrained regions of the chameleon parameter space could be ruled out by future redshift experiments.

We study scalar–tensor gravitational theories using on-shell amplitude methods. We focus on theories with gravity coupled to a massless scalar via the Gauss–Bonnet and Chern–Simons terms. In this framework, we calculate the waveforms for classical scalar radiation emitted in scattering of macroscopic objects, including spin effects. To this end, we use the Kosower–Maybee–O’Connell formalism, with the 5-particle amplitude for scalar emission in matter scattering calculated at tree level using the unitarity-factorization bootstrap techniques. We also discuss in detail the dependence of that amplitude on the contact terms of the intermediate 4-particle scalar-graviton-matter amplitude. Finally, we discuss the conditions for resolvability of classical scalar radiation.

In any diffeomorphism invariant theory of gravity, one can define a Noether charge arising from the invariance of the Lagrangian under diffeomorphisms. We have determined the Noether charge for scalar-tensor theories of gravity, in which case the gravity is mediated by the metric tensor as well as by a scalar degree of freedom. In particular, we demonstrate that the total Noether charge within an appropriate spatial volume can be related to the heat content of the boundary surface. For static spacetimes, in these theories, there exist an “equipartition” between properly defined bulk and surface degrees of freedom. While the dynamical evolution of spacetime in these theories of scalar-tensor gravity arises due to the departure from the equipartition regime. These results demonstrate that thermodynamical interpretations for gravitational theories transcend Einstein and Lovelock theories of gravity, holding true for theories with additional scalar degrees of freedom as well. Moreover, they hold in both the Jordan and the Einstein frames. However, it turns out that there are two dynamically equivalent representations of the scalar-tensor theory in the Jordan frame, differing by total derivatives in the action, which are thermodynamically inequivalent. This depicts the importance of having a thermodynamic description, which can be used in distinguishing various dynamically equivalent representations of gravity theories beyond Einstein.

This paper investigates the dynamical behavior of hypersurface homogeneous spacetime cosmological models within the framework of the scalar-tensor theory of gravitation formulated by Saez and Ballester (Phys. Lett. A113, 467:1986). We present two cosmological models derived from this theory by solving the field equations using: (i) constant deceleration parameter and (ii) the proportional relationship between the shear scalar σ2 and scalar expansion θ as described by Collins et al. (Gen. Rel. Grav. 12, 805-823:1980). For each model, we evaluate key dynamical parameters, including the equation of state (EoS ) parameter, the deceleration parameter, the statefinder parameter, and the total energy density parameter of dark energy. Additionally, we determine the scalar field in both models. Our findings indicate that these models describe an accelerated expansion of the universe, with theoretical results showing reasonable agreement with observational data.

The paper studies the possible interplay between matter and geometry in scalar tensor theories of gravitation where the energy–momentum tensor is directly coupled with the Einstein tensor. After obtaining the scalar tensor representation of the f(R,GμνTμν) gravity, the analysis continue with an approach based on the thermodynamics of irreversible processes in open systems. To this regard, various thermodynamic properties are directly obtained in this manner, like the matter creation (annihilation) rate and the corresponding creation (annihilation) pressure. In the case of the Roberson–Walker metric several analytic and numerical solutions are found in the asymptotic regime. In the last part of the manuscript a specific parametrization for the Hubble rate is constrained using the Markov Chain Monte Carlo algorithms in the case of cosmic chronometers (CC) and BAO observations, obtaining an approximate numerical solution which can describe the cosmological model. For this model, we have obtained by fine-tuning some numerical solutions which exhibit creation mechanisms in different specific regimes.

We investigate the properties of postmerger remnants of binary neutron star mergers in the framework of Damour–Esposito-Farese-type scalar-tensor theory of gravity with a massive scalar field by numerical relativity simulation. It is found that the threshold mass for prompt collapse is raised in the presence of the excited scalar field. Our simulation results also suggest the existence of a long-lived ϕ mode in hypermassive neutron stars due to the presence of the massive scalar field that enhances the quasiradial oscillation in the remnant. We investigate the descalarization condition in hypermassive neutron stars and discover a distinctive signature in postmerger gravitational waves. Published by the American Physical Society 2024

This work examines the dark energy phenomenon by studying the Renyi Holographic Dark Energy (RHDE) and pressure-less Dark Matter (DM) within the frame-work of Saez-Ballester (SB) scalar-tensor theory of gravitation(Phys. Lett. A113, 467:1986). To achieve a solution, we consider the viable deceleration parameter (DP), which contributes to the average scale factor a=e(1/γ)[ √ (2γt+c1)], where γ, and c1 are respectively arbitrary, and integration constants. We have derived the field equations of SB scalar-tensor theory of gravity with the help of Kaluza-Klein FRW Universe. We have investigated cosmological parameters namely, DP (q), energy densities (ρM) and (ρR) of DM and RHDE, scalar field (ϕ), and equation of state parameter (ωR). The physical debate of these cosmological parameters are investigated through graphical presentation. Moreover, the stability of the model are studied through squared sound speed (vs2) and the well-known cosmological plane ωR - ω'R and all energy conditions and also, density parameters are analyzed through graphical representation for our model.

In this paper, we have discussed the exact solution of Einstein's field equations for anisotropic Bianchi type VI0 cosmological model in the framework of the scalar-tensor theory of gravitation given by Sáez–Ballester for barotropic fluid distribution. To obtain an exact solution, we have assumed that expansion (θ) is proportional to shear (σ) which leads to A =Bk, where k is a constant A and B are metric potentials and also assumed barotropic condition p = γρ; (0 ≤ γ ≤ 1) where p being isotropic pressure and ρ is matter density. Some physical and geometrical properties of the model are also discussed.