Brans-Dicke Model Research Articles

Nonlinear curvature Lagrangians and extended inflation in the early universe

Theories with Lagrangians nonlinear in the curvature scalar can be reduced by appropriate transformations to a form similar to the Brans-Dicke model. This may enable us to have extended inflation in the early universe without the difficulties of introducing scalar fields.

Chaotic inflation driven by a minimally coupled scalar field in Brans-Dicke models.

We consider inflation driven by the energy density of a conventional scalar field minimally coupled to gravity, which has an initial vacuum expectation value which is large compared with the value at the minimum of its potential (chaotic inflation), in the context of Brans-Dicke-type models with a time-dependent Newton's constant. The equations of motion for the scalar field driving inflation and the Brans-Dicke scalar are solved in the slow-rolling approximation for the case of a lambda-sigma-4 potential driving inflation, and the magnitude and spectrum of density perturbations produced during inflation are calculated. Sufficient inflation to account for the observed homogeneity and isotropy of the Universe is found to occur only if the coupling epsilon between the Brans-Dicke scalar and the Ricci scalar is smaller than 1. It is shown that only in the case where the initial value of the scalar field driving inflation is large compared with the initial value of the Brans-Dicke scalar can epsilon be larger than 10(-3), and that in this case the self-coupling lambda is constrained to be much smaller than in the case of conventional chaotic inflation models. It is shown also that the spectrum of density perturbations is sufficiently flat to explain galaxy formation only if epsilon <0.04.

Brans-Dicke models with time-dependent cosmological term

More general solutions than those presented by Bertolami are deduced in the Brans-Dicke cosmology, endowed with a time-dependent cosmological term, for a Robertson-Walker metric and a perfect fluid obeying the perfect gas law of state.

Density perturbations in a Brans-Dicke cosmological model

A very general flat solution for Brans-Dicke cosmology with a perfect-fluid, Robertson-Walker metric and a perfect gas law of state is examined regarding density perturbations. The model has growing instabilities, but not of exponential character.

Extended inflationary universes

We investigate the behavior of isotropic models in the general scalar-tensor theory of gravitation. We derive the criterion for inflation to occur in a range of such theories, including that of Brans and Dicke. The solutions used to model “extended” inflation in earlier investigations are shown to be special cases which do not follow the behaviour of the general Brans-Dicke model at early times. The conformal relationship between scalar-tensor theories and general relativity with a self-interacting scalar field is explored and used to find members of a new class of inflationary universes. These “intermediate” inflationary universes have expansion scale factors which increase as exp( t p ), 0 < p < 1. The possibility of successfully of realizing the ◀old” inflationary universe scenario in these models, with the present time variation of the gravitation “constant” being acceptably small, is illustrated with a specific example.

Nonsingular cosmological models in Brans-Dicke theory

An attempt has been made to construct nonsingular cosmological models in Brans-Dicke theory by taking vacuum field as the long-range interacting scalar field which tends to avert the occurrence of initial singularity in a homogeneous and isotropic cosmological model. Such a model will possess an extremely small value of coupling parameter ω, “as such they will be characteristically different from the usual Brans-Dicke models with ω ⩾ 6.” A justification for the estimated value of coupling parameter in the present paper has been given with particular reference to the helium problem. The model satisfies the initial conditions as proposed by Salam, Sinha, etc. However, it predicts a larger value of the energy density at the present epoch.

Linear density perturbation in relativistic and Brans-Dicke cosmologies

Linear density perturbation equation for general relativistic cosmologies using the homogeneous isotropic Robertson-Walker metric and the anisotropic Bianchi-I metric for the unperturbed model has been derived in a simple and rather general manner. The method has then been applied to consideration of the fate of density perturbation in the Brans-Dicke model of the universe. Two coupled differential equations connecting variations in the density rho and the scalar field phi of the model have been obtained. An approximate solution has been sought and a power law growth of the fluctuation density, delta rho , with time has been obtained.