More about scalar gravity

Abstract

We discuss a class of models for gravity based on a scalar field. The models include and generalize the old approach by Nordström which predated and in some way inspired General Relativity. The class include also a model that we have recently introduced and discussed in its cosmological aspects (GSG). We present here a complete characterisation of the Schwarschild geometry as a vacuum solution of GSG and sketch a discussion of the first Post-Newtonian approximation.

Hybrid theory of gravity

“…On the other hand, GSG predicts scalar gravitational waves [18,26], which may conflict with recent direct observations of gravitational waves from a black hole binary [27]. Further, more scalar degrees of freedom may be needed in order to explain the gravitational field around a spinning source [23,28]. Therefore, the simplest GSG model has practical difficulty in describing astrophysical processes.…”

“…Novello and his collaborators found a specific form of the scalar field potential from which the Schwarzschild spacetime is derived as an exact solution. These researchers also discussed the (exotic) cosmology based on GSG [18][19][20][21][22][23]. The novel behavior of the scale factor in the GSG cosmology provides a very interesting supplementary perspective on the issue of the initial singularity [21].…”

“…In 2014, Novello and collaborators presented a new theory of gravity, called Geometric Scalar Gravity (GSG) [18][19][20][21][22][23]. 3 In this theory, the dynamics of gravity is described by a single scalar field.…”

On variations of G in the geometric scalar theory of gravity

“…The Geometric Scalar Theory of Gravity (GSG), recently proposed by Mario Novello and co-authors [11], makes a step in the opposite direction and shows that some opportunities of interest are not yet exhausted in attempts to simplify the description of gravity instead of adding its complexity. As said in [12], "Here we propose that it may be interesting to take a huge step backward and explore models in which the gravitational degrees of freedom are just described by the field . "…”

Non-Gaussianity in multi-sound-speed disformally coupled inflation

“…However, the most general form of a relationĝ µν =ĝ µν (φ, g µν ) which still complies with physical requirements such as causality, is a so called disformal relation, [4] g µν = C(φ, X)g µν + D(φ, X)φ ,µ φ ,ν , (1.1) that may involve also the derivatives φ ,µ of the scalar field, and even nonlinear dependence upon the metric through the kinetic term X = −g µν φ ,µ φ ,ν /2. Their extended range of phenomenology [5][6][7][8][9][10][11][12][13] and universality in scalar-tensor theories [14][15][16][17][18] have made disformal transformations rather popular in cosmology and modified gravity in recent years [19][20][21][22][23][24] (whilst they have also found novel applications in theory [25][26][27][28][29]).…”