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Register a new userA Cosmological Exact Solution of Complex Jordan-Brans-Dicke Theory and its Phenomenological Implications
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When Brans-Dicke Theory is formulated in terms of the Jordan scalar field phi, dark energy is related to the mass of this field. We show that if phi is taken to be a complex scalar field then an exact solution of the vacuum equations shows that Friedmann equation possesses a term, proportional to the inverse sixth power of the scale factor, as well as a constant term. Possible interpretations and phenomenological implications of this result are discussed.
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We investigate a Jordan-Brans-Dicke (JBD) scalar field, $Phi$, with power-law potential in the presence of a second scalar field, $phi$, with an exponential potential, in both the Jordan and the Einstein frames. We present the relation of our model with the induced gravity model with power-law potential and the integrability of this kind of models is discussed when the quintessence field $phi$ is massless, and has a small velocity. We prove that in JBD theory, the de Sitter solution is not a natural attractor but an intermediate accelerated solution of the form $a(t)simeq e^{alpha_1 t^{p_1}}$, as $trightarrow infty$ where $alpha_1>0$ and $0<p_1<1$, for a wide range of parameters. Furthermore, in the Einstein frame we get that the attractor is also an intermediate accelerated solution of the form $mathfrak{a}(mathfrak{t})simeq e^{alpha_2 mathfrak{t}^{p_2}}$ as $mathfrak{t}rightarrow infty$ where $alpha_2>0$ and $0<p_2<1$, for the same conditions on the parameters as in the Jordan frame. In the special case of a quadratic potential in the Jordan frame, or for a constant potential in the Einsteins frame, these solutions are of saddle type. Finally, we present a specific elaboration of our extension of the induced gravity model in the Jordan frame, which corresponds to a linear potential of $Phi$. The dynamical system is then reduced to a two dimensional one, and the late-time attractor is linked with the exact solution found for the induced gravity model. In this example the intermediate accelerated solution does not exist, and the attractor solution has an asymptotic de Sitter-like evolution law for the scale factor. Apart from some fine-tuned examples such as the linear, and quadratic potential ${U}(Phi)$ in the Jordan frame, it is true that intermediate accelerated solutions are generic late-time attractors in a modified JBD theory.
Since the evidence for an accelerated universe and the gap of 70% in the total energy, collected by WMAP, search for alternatives for the general relativity is an important issue, for this theory is not suited for these new phenomena. A particular alternative is the Brans-Dicke theory which has being allowing inspiring results, for example, concerning k-essence type fields in 4 dimensions. However, this theory is almost unexplored in the context of the dimensional reduction of the theory in 3 dimensions. In this work, we address some problems in this dimensional reduction, namely, evaluation of the deceleration parameter of the universe described by the 3 dimensional Brans-Dicke with and without matter. In both cases, we see that it is not possible to consider the theory as a model of k-essence descrybing the dark energy, but it can be considered as descrybing the dark matter.
Vacuum Brans-Dicke theory can be self-consistently described in two frames, the Jordan frame (JF) and the conformally rescaled Einstein frame (EF), the transformations providing an easy passage from one frame to the other at the level of actions and solutions. Despite this, the conformal frames are inequivalent describing different geometries. It is shown that the predictions of the weak field lensing (WFL) observables in the EF are different from those recently obtained in the JF for the vacuum Brans-Dicke class 1 solution. The value of the Brans-Dicke coupling parameter $omega$ from the Cassini spacecraft experiment reveals the degree of accuracy needed to experimentally distinguish the WFL measurements including the total magnification factor in the two frames.
The purpose of this study is to investigate observational features of Brans-Dicke wormholes in a case if they exist in our Universe. The energy flux from accretion onto a Brans-Dicke wormhole and the so-called maximum impact parameter are studied (the last one might allow to observe light sources through a wormhole throat). The computed values were compared with the corresponding ones for GR-wormholes and Schwarzschild black holes. We shown that Brans-Dicke wormholes are quasi-Schwarzschild objects and should differ from GR wormholes by about one order of magnitude in the accretion energy flux.
In the context of generalised Brans-Dicke cosmology we use the Killing tensors of the minisuperspace in order to determine the unspecified potential of a scalar-tensor gravity theory. Specifically, based on the existence of contact symmetries of the field equations, we find four types of potentials which provide exactly integrable dynamical systems. We investigate the dynamical properties of these potentials by using a critical point analysis and we find solutions which lead to cosmic acceleration and under specific conditions we can have de-Sitter points as stable late-time attractors.
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