On the cosmology of scalar-tensor-vector gravity theory

Abstract

Abstract. We consider the cosmological consequences of a special scalar-tensor-vector theory of gravity, known as MOG (for MOdified Gravity), proposed to address the dark matter problem. This theory introduces two scalar fields G(x) and µ(x), and one vector field φ α (x), in addition to the metric tensor. We set the corresponding self-interaction potentials to zero, as in the standard form of MOG. Then using the phase space analysis in the flat Friedmann-Robertson-Walker background, we show that the theory possesses a viable sequence of cosmological epochs with acceptable time dependency for the cosmic scale factor. We also investigate MOG's potential as a dark energy model and show that extra fields in MOG cannot provide a late time accelerated expansion. Furthermore, using a dynamical system approach to solve the non-linear field equations numerically, we calculate the angular size of the sound horizon, i.e. θ s , in MOG. We find that 8 × 10 −3 rad < θ s < 8.2 × 10 −3 rad which is way outside the current observational bounds. Finally, we generalize MOG to a modified form called mMOG, and we find that mMOG passes the sound-horizon constraint. However, mMOG also cannot be considered as a dark energy model un;ess one adds a cosmological constant, and more importantly, the matter dominated era is still slightly different from the standard case.

Barred spiral galaxies in modified gravity theories

“…It has been shown that the theory has a true sequence of cosmological epochs. In other words, the cosmos starts from a radiation-dominated universe and then enters a matterdominated phase, before finally evolving towards an accelerated de Sitter universe (Jamali et al 2018). One main issue which should still be addressed by MOG is cosmic structure formation.…”

Gravitational theory of cosmology, galaxies and galaxy clusters

“…One is to pursue a description without dark matter employing only baryonic matter and the baryonic density ρ b . This approach has been explored with some success [25][26][27] but requires further exploration in the future. The second approach is to promote the gravitational VG (spin 1 vector graviton) degree of freedom as a dominant density ρ φ in the early universe, so that it dominates the baryon density ρ b for the time before the reionization phase when stars and galaxies are first formed [2,22].…”

A Modified Dynamical Model of Cosmology I Theory

“…In the matter dominated phases, the growth of the scale factor is found to be as a(t) ∝ t 0.46 and a(t) ∝ t 0.52 slower than the growth in standard model and the present model reported above as a(t) ∝ t 2/3 . Although, it should be mentioned that as Jamali et al (2018) have reported, the standard MOG possesses a valid sequence of standard cosmological eras.…”