Spinning particles in scalar-tensor gravity

We develop a new model of a spinning particle in Brans-Dicke spacetime using a metric-compatible connection with torsion. The particle's spin vector is shown to be Fermi-parallel (by the Levi-Civita connection) along its worldline (an autoparallel of the metric-compatible connection) when neglecting spin-curvature coupling.     

Black holes in scalar-tensor gravity

Hawking has proven that black holes which are stationary as the end point of gravitational collapse in Brans-Dicke theory (without a potential) are no different than in general relativity. We extend this proof to the much more general class of scalar-tensor and f(R) gravity theories, without assuming any symmetries apart from stationarity.     

Inflation and reheating in scale-invariant scalar-tensor gravity

We consider the scale-invariant inflationary model studied in Rinaldi and Vanzo (Phys Rev D 94: 024009, 2016). The Lagrangian includes all the scale-invariant operators that can be built with combinations of \(R, R^{2}\) and one scalar field. The equations of motion show that the symmetry is spontaneously broken after an arbitrarily long inflationary period and a fundamental mass scale is generated. Upon symmetry breaking, and in the Jordan frame, both Hubble function and the scalar field undergo damped oscillations that can eventually amplify Standard Model fields and reheat the Universe. In the present work, we study in detail inflation and the reheating mechanism of this model in the Einstein frame and we compare some of the results with the latest observational data.     

Neutrino oscillation phase dynamically induced by scalar-tensor gravity

We show that the shift of quantum mechanical phase can depend on the nonminimal coupling of scalar-tensor gravity. This fact could constitute a further test to discriminate among the various relativistic theories of gravity. Consequences on atmospheric, solar and astrophysical neutrinos are discussed.     

Charged scalar-tensor spheres: General solutions

The charged static and spherically symmetric vacuum field in the general Bergmann-Wagoner-Nordtvedt (BWN) theory of gravity is studied. By a conformal transformation, the field equations are brought into a standard form, which permits a decoupling of the differential equations for the relevant quantities. Using the electrostatic potential as an independent variable results in a particular Ricatti equation, the general solutions of which can be explicitly written down, for positive and for negative scalar energy densities. In distinction with the Reissner-Nordström solutions, all nontrivial positive energy BWN theories are shown to possess naked timelike singularities, whereas the negative energy theories admit a peculiar nonsingular solution, which may be interpreted as a field theoretical model of a charged particle. It is argued that the absence of an event horizon in the other solutions is not a consequence of the assumption of spherical symmetry: event horizons are absent in any static geometry which is a nontrivial solution of the (charged or uncharged) BWN equations with positive definite scalar density.     

Higgs field and a new scalar-tensor theory of gravity

The combination of Brans and Dicke's idea of a variable gravitational constant with the Higgs-field mechanism of elementary particle physics results in a new theory of gravity. Einstein's theory is realized after symmetry breaking in the neighborhood of the Higgs-fleld ground state.     

Cylindrical wave solutions of a scalar-tensor theory

A class of exact solutions of the cylindrically symmetric space-time with two degrees of freedom corresponding to the vacuum field equations of a scalar-tensor theory proposed by Dunn is obtained. The solutions possess wavelike character.     

Axially symmetric solutions in general scalar-tensor theory

We generalise Ernst's derivation of the axially symmetric solutions of Einstein's field equations to the general scalar-tensor theory proposed by Nordtvedt. The solution of the Nordtvedt theory differs by a conformai transformation from the Brans-Dicke solution. The Kerr-like solution of the Nordtvedt theory is obtained as an example.     

Exact solutions for the early Friedmann-Robertson-Walker universe in general scalar-tensor theories

Homogeneous and isotropic cosmologies with trace-free energy-momentum tensors are studied in general scalar-tensor theories. A method is presented which allows one to construct exact solutions for theories with arbitrary coupling functionω(φ). Particular attention is paid to Schwinger's theory.     

Self-dual solutions to euclidean gravity

Recent work on Euclidean self-dual gravitational fields is reviewed. We discuss various solutions to the Einstein equations and treat asymptotically locally Euclidean self-dual metrics in detail. These latter solutions have vanishing classical action and nontrivial topological invariants, and so may play a role in quantum gravity resembling that of the Yang-Mills instantons.     

Thermodynamics of event horizon with modified Hawking temperature in scalar-tensor gravity

In recent past, Hawking temperature has been modified for the validity of thermodynamical laws at the event horizon in general relativity context. This lead to the introduction of modified Hawking temperature and it has been found that the modified Hawking temperature is more realistic on the event horizon. With this motivation, here we investigate the thermodynamical consistency of scalar-tensor gravity based models by examining the validity of the generalized second law of thermodynamics (GSLT) and thermodynamical equilibrium (TE) at the event horizon. In order to attain our goal, we consider a spatially flat Friedman–Robertson–Walker Universe filled with ordinary matter and the boundary of the Universe bounded by the event horizon that is in thermal equilibrium with modified Hawking temperature. Next, we calculate the general expressions for the GSLT and TE using modified Hawking temperature in the context of the more general action of scalar-tensor gravity where there is a non-minimally coupling between the scalar field and matter Lagrangian (as the chameleon field). From the general expression of GSLT, we find that the null energy condition must hold for a viable scalar-tensor model of the Universe dominated by a perfect fluid. Furthermore, in order to better understand these complicated general expressions of GSLT and TE, we explore the validity of the GSLT and TE for two viable models of scalar-tensor gravity namely Brans–Dicke gravity with a self-interacting potential and Chameleon gravity at the event horizon using special cosmological solutions. Finally, some graphical representation of the GSLT and TE have been presented. From the graphical analysis, we found that the power-law forms of the scale factor and scalar field is much favourable for the study of universal thermodynamics as compared to other choices of the scalar field and the analytic function.     

Reconstruction of a scalar-tensor theory of gravity in an accelerating universe

The present acceleration of the Universe strongly indicated by recent observational data can be modeled in the scope of a scalar-tensor theory of gravity. We show that it is possible to determine the structure of this theory along with the present density of dustlike matter from two observable cosmological functions: the luminosity distance and the linear density perturbation in the dustlike matter component as functions of redshift. Explicit results are presented in the first order in the small inverse Brans-Dicke parameter omega(-1).