Scalar-tensor Theories Of Gravity Research Articles

Gravitational redshift constraints on the effective theory of interacting dark energy

Upcoming galaxy surveys provide the necessary sensitivity to measure gravitational redshift, a general relativistic effect that generates a dipole in galaxy clustering data when correlating two distinct populations of galaxies. Here, we study the constraining power of gravitational redshift within the framework of the effective theory of interacting dark energy. This formalism describes linear cosmological perturbations in scalar-tensor theories of gravity with a limited number of free functions, and allows each particle species to be coupled differently to the gravitational sector. In this work, we focus on Horndeski theories with a non-minimal coupling of dark matter to the scalar degree of freedom, yielding a breaking of the weak equivalence principle for this cosmic component, a scenario that is yet untested. We show that the dipole generated by gravitational redshift significantly breaks degeneracies and tightens the constraints on the parameters of the effective theory compared to the standard redshift-space distortion analysis solely based on the even multipoles in the galaxy correlation function, with an improvement of up to ∼ 50% for populations with a galaxy bias difference equal to 1. We make the Python package EF-TIGRE (Effective Field Theory of Interacting dark energy with Gravitational REdshift) developed for this work publicly available (https://github.com/Mik3M4n/EF-TIGRE).

Freund – Nambu cosmology with the massless scalar field

Within the framework of the generalization of Freund – Nambu scalar-tensor theory of gravity, a massless scalar field is considered, the source of which is the trace of its own energy-momentum tensor. For the cosmological problem, numerical solutions of field equations were obtained, with the help of which the dependencies of the Hubble parameter and the photometric distance to the observed sources on red-shift were constructed. To the consistency of the models with observational data, contours of confidence intervals for model parameters were constructed.

Phenomenology of axionic static neutron stars with masses in the mass-gap region

In this work we consider an axionic scalar-tensor theory of gravity and its effects on static neutron stars (NSs). The axionic theory is considered in the regime in which the axion oscillates around its potential minimum, which cosmologically occurs post-inflationary, when the Hubble rate is of the same order as the axion mass. We construct the Tolman–Oppenheimer–Volkoff equations for this axionic theory and for a spherically symmetric static spacetime and we solve these numerically using a quite robust double shooting LSODA based python integration method. Regarding the equations of state, we used nine mainstream and quite popular ones, namely, the WFF1, the SLy, the APR, the MS1, the AP3, the AP4, the ENG, the MPA1 and the MS1b, using the piecewise polytropic description for each. From the extracted data we calculate the Jordan frame masses and radii, and we confront the resulting phenomenology with five well-known NS constraints. As we demonstrate, the AP3, the ENG and the MPA1 equations of state yield phenomenologically viable results which are compatible with the constraints, with the MPA1 equation of state enjoying an elevated role among the three. The reason is that the MPA1 fits well the phenomenological constraints. A mentionable feature is the fact that all the viable phenomenologically equations of state produce maximum masses which are in the mass-gap region with Mmax>2.5M⊙ , but lower that the causal 3 solar masses limit. We also compare the NS phenomenology produced by the axionic scalar-tensor theory with the phenomenology produced by inflationary attractors scalar-tensor theories.

Anisotropic Bianchi Type-1 Cosmological Model Containing Dark Energy

Considering the LRS Bianchi type I metric here we studied the spatial homogeneous and anisotropic dark energy cosmological models in the frame work of B-D scalar tensor theory of gravity along with variable equation of parameter and deceleration parameter. Exact solutions of field equations are obtained with certain physical assumptions in different cases. The physical behavior of the model is also discussed in details.

Numerical computation of quasinormal modes in the first-order approach to black hole perturbations in modified gravity

We present a novel approach to the numerical computation of quasi-normal modes, based on the first-order (in radial derivative) formulation of the equations of motion and using a matrix version of the continued fraction method. This numerical method is particularly suited to the study of static black holes in modified gravity, where the traditional second-order, Schrödinger-like, form of the equations of motion is not always available. Our approach relies on the knowledge of the asymptotic behaviours of the perturbations near the black hole horizon and at spatial infinity, which can be obtained via the systematic algorithm that we have proposed recently. In this work, we first present our method for the perturbations of a Schwarzschild black hole and show that we recover the well-know frequencies of the QNMs to a very high precision. We then apply our method to the axial perturbations of an exact black hole solution in a particular scalar-tensor theory of gravity. We also cross-check the obtained QNM frequencies with other numerical methods.

Tidal properties of neutron stars in scalar-tensor theories of gravity

Tidal properties of neutron stars in scalar-tensor theories of gravity

No logarithmic corrections to entropy in shift-symmetric Gauss-Bonnet gravity

Employing the covariant phase space formalism, we discuss black hole thermodynamics in four-dimensional scalar-tensor Einstein-Gauss-Bonnet gravity. We argue that logarithmic corrections to Wald entropy previously reported in this theory do not appear, due to the symmetry of the theory under constant shifts of the scalar field. Instead, we obtain the standard Bekenstein entropy of general relativity. Then, to satisfy the first law of black hole mechanics, the Hawking temperature must be modified. It has been proposed that such temperature modifications occur generically in scalar-tensor theories, due to different propagation speeds of gravitons and photons. We show that the temperature modifications also emerge in the Euclidean canonical ensemble approach to black hole thermodynamics. Notably, the boundary terms of the type we consider here can be considered in any scalar-tensor gravitational theories. Hence, we illustrate that adding a suitable boundary term to action may drastically affect black hole thermodynamics, changing both the entropy and the temperature.

Tidal Deformability of Neutron Stars in Scalar-tensor Theories of Gravity

Gravitational waves from compact binary coalescences are valuable for testing theories of gravity in the strong field regime. By measuring neutron star tidal deformability using gravitational waves from binary neutron stars, stringent constraints were placed on the equation of state of matter at extreme densities. Tidal Love numbers in alternative theories of gravity may differ significantly from their general relativistic counterparts. Understanding exactly how the tidal Love numbers change will enable scientists to untangle physics beyond general relativity from the uncertainty in the equation of state measurement. In this work, we explicitly calculate the fully relativistic l ≥ 2 tidal Love numbers for neutron stars in scalar-tensor theories of gravitation. We use several realistic equations of state to explore how the mass, radius, and tidal deformability relations differ from those of general relativity. We find that tidal Love numbers and tidal deformabilities can differ significantly from those in general relativity in certain regimes. The electric tidal deformability can differ by ∼200%, and the magnetic tidal deformability differs by ∼300%. These deviations occur at large compactnesses (C = M/r ≳ 0.2) and vary slightly depending on the equation of state. This difference suggests that using the tidal Love numbers from general relativity could lead to significant errors in tests of general relativity using the gravitational waves from binary neutron star and neutron star black hole mergers.

Gravitational waves during Higgs inflation from complex geometrical scalar-tensor theory of gravity

In this paper we investigate tensor fluctuations of the metric at the end of a Higgs inflationary period in the context of a recently introduced complex geometrical scalar-tensor theory of gravity. In our model the Higgs field has a geometrical origin and the affine connection is determined by the Palatini’s principle. Additionally, we consider an extra contribution to the tensor-fluctuations equation coming from the vacuum term in the energy–momentum tensor associated to the Higgs field. The Higgs potential is rescaled by the non-canonicity function of the kinetic term of the field which is modified by the symmetry group of the background geometry. We obtain a nearly scale invariant spectrum and a scalar to tensor ratio in agreement with PLANCK 2018 cosmological results.

Orbital precession and hidden symmetries in scalar-tensor theories

We revisit the connection between relativistic orbital precession, the Laplace-Runge-Lenz symmetry, and the t-channel discontinuity of scattering amplitudes. Applying this to scalar-tensor theories of gravity, we compute the conservative potential and orbital precession induced by both conformal/disformal-type couplings at second Post-Minkowskian order (𝒪(GN 2)), complementing the known third/first order Post-Newtonian results. There is a particular tuning of the conformal coupling for which the precession vanishes at leading PN order, and we show that this coincides with the emergence of a Laplace-Runge-Lenz symmetry and a corresponding soft behaviour of the amplitude. While a single scalar field inevitably breaks this symmetry at higher PN orders, certain supersymmetric extensions have recently been shown to have an exact Laplace-Runge-Lenz symmetry and therefore classical orbits do not precess at any PN order. This symmetry can be used to relate scattering amplitudes at different loop orders, and we show how this may be used to bootstrap the (classically relevant part of the) three-loop 2 → 2 scattering of charged black holes in 𝒩 = 8 supergravity from existing two-loop calculations.

Instability of the cosmological DBI-Galileon in the non-relativistic limit

The DBI-Galileon model is a scalar-tensor theory of gravity which is defined as the most general theory of the dynamics of a 4D brane embedded in a 5D bulk. It is of particular interest as it introduces only a few free parameters, all with a clear physical meaning, such as the brane tension which is related to the cosmological constant. From the tight constraints on the gravitational waves speed, we are naturally led to consider the non-relativistic limit of the model where the kinetic energy of the brane is small compared to its tension, that we study in the context of late-time cosmology. We show that the Dirac–Born–Infeld-Galileon in the non-relativistic limit is an expansion around General Relativity which can be expressed as a shift-symmetric Horndeski theory. We develop the description of this theory at the background and perturbation level. However, by studying the scalar and tensor perturbations around a flat FLRW background, we find that they contain a ghost degree of freedom leading to a fatal instability of the vacuum for every combination of the free parameters. In order to avoid it in the more general cases of shift-symmetric Horndeski theories, we emphasize which of the Horndeski terms are in competition to produce this instability.

Noether symmetry analysis in scalar tensor cosmology: a study of classical and quantum cosmology

The present work deals with a complex scalar field in scalar tensor gravity theory in the background of spatially flat Friedmann–Lema{hat{i}}tre–Robertson–Walker (FLRW) geometry. Noether symmetry analysis has been used to determine the classical cosmological solution of a scalar field in scalar–tensor theory with the scalar field as a nonminimally coupled complex field. Noether symmetry analysis is not only used to find a symmetry vector and potential but also it helps in finding an appropriate transformation (a,~phi ,~theta )rightarrow (u,~v,~theta ) in the augmented space so that one of the new variables becomes cyclic. In quantum cosmology, the Wheeler–DeWitt (WD) equation has been formed in the minisuperspace and its solution i.e. the wave function of the universe has been evaluated by using the operator version of the conserved (Noether) charge. Finally, the nature of the classical solution has been discussed from the observational point of view and the cosmological singularity has been examined both classically and quantum mechanically.

CMB power spectrum for emergent scenario and slow expansion in scalar–tensor theory of gravity

We analyze the stability of the Einstein static universe in scalar-tensor theory of gravity, and find it can be stable against both scalar and tensor perturbations under certain conditions. By assuming the emergent scenario originating from an Einstein static state, followed by an instantaneous transition to an inflationary phase, we study and obtain the analytical approximations of the primordial power spectrum for the emergent scenario. Then, we plot the primordial power spectrum and CMB TT-spectrum of the emergent scenario and the slow expansion scenario. These figures show that both of these spectra for the slow expansion scenario are the same as that for $\Lambda$CDM, and the spectra of the emergent scenario are lower than that for $\Lambda$CDM at large scales.

Constraining Dark Energy from the Local Group Dynamics

This Letter develops a method to constrain the cosmological constant Λ from binary galaxies, focusing on the Milky Way and Andromeda galaxies. We provide an analytical solution to the two-body problem with Λ and show that the ratio between the Keplerian period and controls the importance of effects from the cosmological constant. The Andromeda–Milky Way orbit has a period of ∼17 Gyr, and so dark energy has to be taken into account. Using the current best mass estimates of the Milky Way and Andromeda galaxies, we find the cosmological constant value has an upper bound that is 5.44 times the value obtained by Planck. With future astrometric measurements, the bound on the cosmological constant can be reduced to . Our results offer the prospects of constraints on Λ over very different scales than previously. The Local Group provides also a completely novel platform to test alternative theories of gravity. We illustrate this by deriving bounds on scalar-tensor theories of gravity over megaparsec scales.

Testing generalized scalar-tensor theories of gravity with clusters of galaxies

Testing generalized scalar-tensor theories of gravity with clusters of galaxies

Scalar fluid in scalar-tensor gravity from an equivalent picture of thermodynamic and fluid descriptions of gravitational dynamics

We revisit the thermodynamic description of fluid, represented by scalar field in scalar-tensor gravity theory through a general approach to study the thermodynamics of relativistic fluids. In order to identify the fluid energy-momentum tensor, contrary to the existing way, we use equivalent description of gravitational dynamics both in Jordan and Einstein frames as thermodynamical identity and fluid equation on a generic null surface. Such an approach provides the energy-momentum tensors for scalar fluid as that of an ideal fluid in both the frames. We then mention few issues in the existing way of using Eckart's formalism for an ideal fluid. Our investigation suggests that the Eckart's frame may not be suitable to consider ideal fluid. Consequently a possible alternative description, valid in other than Eckart's frame, is being suggested for a general ideal fluid. Based on this we obtain a unified thermodynamic description in both Jordan and Einstein frames from our identified energy-momentum tensors. Finally, the relations between thermodynamic entities on different frames are being put forwarded. Thus, the description in either of the frames is enough to provide the complete picture.

Spontaneous scalarization in proto-neutron stars

Proto-neutron stars are born when a highly evolved and massive star collapses under gravity. In this paper, we investigate the spontaneous scalarization in proto-neutron stars. Based on the scalar tensor theory of gravity as well as the physical conditions in proto-neutron star, we examine the structure of proto-neutron star. To describe the fluid in proto-neutron star, we utilize SU(2) chiral sigma model and the finite temperature extension of the Brueckner–Bethe–Goldstone quantum many-body theory in the Brueckner–Hartree–Fock approximation. Here, we apply the equation of state of proto-neutron stars considering different cases i.e. hot pure neutron matter and hot beta -stable neutron star matter without neutrino trapping as well as with neutrino trapping. The effects of temperature and entropy of proto-neutron stars on the star structure are also studied. Our results confirm that the spontaneous scalarization is affected by different physical conditions in proto-neutron stars.

Scale-dependent gravitational couplings in Parameterised Post-Newtonian Cosmology

Parameterised Post-Newtonian Cosmology (PPNC) is a theory-agnostic framework for testing gravity in cosmology, which connects gravitational physics on small and large scales in the Universe. It is a direct extension of the Parameterised Post-Newtonian (PPN) approach to testing gravity in isolated astrophysical systems, and therefore allows constraints on gravity from vastly different physical regimes to be compared and combined. We investigate the application of this framework to a class of example scalar-tensor theories of gravity in order to verify theoretical predictions, and to investigate for the first time the scale-dependence of the gravitational couplings that appear within its perturbation equations. In doing so, we evaluate the performance of some simple interpolating functions in the transition region between small and large cosmological scales, as well as the uncertainties that using such functions would introduce into the calculation of observables. We find that all theoretical predictions of the PPNC framework are verified to high accuracy in the relevant regimes, and that simple interpolating functions perform well (but not perfectly) between these regimes. This study is an important step towards being able to use the PPNC framework to analyse cosmological datasets, and to thereby test if/how the gravitational interaction has changed as the Universe has evolved.

Phantom scalar-tensor models and cosmological tensions

We study three different extended scalar-tensor theories of gravity by also allowing a negative sign for the kinetic term for the scalar field in the Jordan frame. Our scope is to understand how the observational constraints for these models cope with the volume of the parameter space in which the theory is healthy. Models with a negative kinetic term lead to decreasing effective gravitational constant with redshift and behave as an effective relativistic component with a negative energy density as opposite to their corresponding version with a standard kinetic term. As a consequence, we find that the extended branch with a negative sign for the kinetic term correspond in general to lower H 0 and σ 8 compared to ΛCDM. We find that in all the cases with a negative sign for the kinetic term studied here, cosmological observations constrain these models around GR and prefer a volume of the parameter space in which the theory is not healthy since the scalar field behave as a ghost also in the related Einstein frame. We show that also in the phantom branch early modify gravity with a quartic coupling can substantially reduce the H 0 tension fitting the combination of cosmic microwave background data from Planck, baryon acoustic oscillations from BOSS and eBOSS, and Supernovae from the Pantheon sample with calibration information by SH0ES.

The study of hypersurface-homogeneous space-time in Renyi holographic dark energy

The investigation of this work is carried on the hypersurface-homogeneous space-time in the presence of two fluids, one being the pressureless matter and the other being the Renyi holographic dark energy (RHDE). Eventually, this work solves the cosmological model with Renyi HDE by taking the Hubble horizon as an infrared (IR) cutoff ([Formula: see text]). The geometrical and matter parts of space-time are solved within the Saez–Ballester scalar-tensor theory of gravitation. Interestingly, this study obtains a time-varying deceleration parameter ([Formula: see text]) that exhibits a transition from deceleration to acceleration phase. For the configurations of parameter [Formula: see text] emerging in space-time, three physically plausible cosmological hypotheses of the cosmos are outlined in this work. The kinematical properties were obtained and discussed for the three values of [Formula: see text]. The study of cosmic expansion in the accelerated phase of this work is done through various cosmological parameters like EoS, deceleration parameter, statefinder parameter, etc. The Hubble parameter is derived in terms of cosmic time and redshift both. The present value of this parameter is estimated by taking [Formula: see text] point data sets of observational Hubble data (OHD). The stability of the model is verified through the analysis of squared speed of sound ([Formula: see text]) parameter. In this work, the obtained results match with recent observational data.