Speaker
Description
We investigate the formation of the large-scale cosmic structure in a scalar-tensor theory of gravity belonging to the class of the Brans-Dicke theories. The universe contains baryonic matter alone and neither dark matter nor dark energy. The two arbitrary functions of the scalar field characterizing the kinetic term and the self-interaction potential are set to W (φ) = −1 and V (φ) = −Ξφ, respectively, with Ξ a positive constant. In the weak-field limit, the theory reduces to Refracted Gravity, a non-relativistic theory whose modified Poisson equation contains the scalar field φ that provides the gravitational boost required to describe the dynamics of galaxies and galaxy clusters without dark matter. In a flat, matter-dominated, homogeneous and isotropic universe the same scalar field φ drives the accelerated expansion of the universe and describes the observed redshift evolution of the Hubble-Lemaître parameter H(z). However, in the equation of the growth factor of the linear perturbation theory, the form of V (φ) makes the coefficient of the source of the gravitational field proportional to H−1(z); therefore the gravitational field is strongly suppressed at early times and structure formation is delayed to redshift z < 1, in disagreement with the observation of formed galaxies at much larger redshifts. In addition, the form of W (φ) and a linear V (φ) imply that φ generates twice the gravitational boost on massive particles than on photons, with possible observable consequences on the gravitational lensing phenomenon. It remains to be investigated whether different choices of W (φ) and V (φ), that can still make the theory reduce to Refracted Gravity in the weak-field limit, might alleviate these
problems.