Chameleon Brans-Dicke cosmology

Abstract

We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. We assume a power law form for the potential and the coupling functions as the inputs of the model and show that acceleration of the universe can be realized for a constrained range of exponent of the potential function. We also argue that this accelerating phase is consistent with a large and positive Brans-Dicke parameter. In our analysis, the potential plays a more important role with respect to the coupling function in dynamics of the universe as the latter does not contribute to any of the relations characterizing evolution of scale factor of the universe and the scalar field. However, we will show that the coupling function is closely related to magnitude and direction of the energy transfer between matter and the scale field. We use this fact and some thermodynamic aspects of the model to put some constraints on the coupling function. In particular, we argue that the second law of thermodynamics constrains direction of the overall energy transfer.

Conservation laws in theories with universal gravity/matter coupling

“…For instance, at the classical level, equation (34) leads to a modification of the distance luminosity d L because of the energy transfer between the gravitational fields and the electromagnetic field. As an example, for a flat FRLW metric, the new equation reads 2 :…”

“…This type of universal gravity/matter coupling has been proposed in order to explain the phenomena usually associated to dark matter (without modifications of the standard model of particles) [1,[26][27][28][29][30], to mimic the cosmological constant [31], to give a reheating scenario in the context of Starobinsky inflation [32], to explain the acceleration of the expansion of the Universe [4,5,19,[33][34][35], to predict (essentially) stable WIMPs [18], or to explain the smallness of the gravitational constant G compared to the proton mass scale [36]. Moreover, it has been shown that under specific assumptions such theories can be compatible with current tight constraints on the equivalence principle(s) (see for instance [10,[37][38][39]).…”

Chameleon effect in the Jordan frame of the Brans-Dicke theory

“…Usually the chameleon effect is discussed, exclusively, in the Einstein frame formulation of the Brans-Dicke theory, where the scalar field couples directly with the matter degrees of freedom [5][6][7][8][9][10][11][12][13][14][15]. Due to the non-trivial way which the self-interaction potential enters in the KG equation, the discussion of the chameleon effect in the Jordan frame formulation of BD theory seems more obscure than in the Einstein frame.…”

“…Thanks to the chameleon effect [5][6][7][8][9][10][11][12][13][14][15], for instance, it was just recently understood that the experimental bounds on the BD coupling parameter ω bd , which were set up through experiments in the solar system, might not apply in the large cosmological scales if consider BD theory with a potential. According to the chameleon effect, the effective mass of the scalar field m φ , depends on the background energy density of a Electronic address: iquiros@fisica.ugto.mx b Electronic address: rigarcias@ipn.mx c Electronic address: tamegc72@gmail.com d Electronic address: anthort@hotmail.com the environment: In the large cosmological scales where the background energy density is of the order of the critical density ρ crit ∼ 10 −31 g/cm 3 , the effective mass is very small m φ ∼ H 0 ∼ 10 −33 eV, so that the scalar field has impact in the cosmological dynamics.…”

Cosmic strings in $$f\left( R,L_m\right) $$ f R , L m gravity

“…Geometry-matter couplings in the presence of scalar fields were discussed in [10] and the cosmological and astrophysical implications of the f (R, L m ) type gravity theories were investigated in [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. For a recent review of the f (R, L m ) model see [29].…”