1,461 research outputs found
On the weak field approximation of Brans-Dicke theory of gravity
It is shown that in the weak field approximation solutions of Brans-Dicke
equations are simply related to the solutions of General Relativity equations
for the same matter distribution. A simple method is developed which permits to
obtain Brans-Dicke solutions from Einstein solutions when both theories are
considered in their linearized forms. To illustrate the method some examples
found in the literature are discussed.Comment: 12 pages, latex, no figure
Shear and Vorticity in Inflationary Brans-Dicke Cosmology with Lambda-Term
We find a solution for exponential inflation in Brans-Dicke cosmology endowed
with a cosmological term, which includes time-varying shear and vorticity. We
find that the scalar field and the scale factor increase exponentialy while
shear, vorticity, energy density, cosmic pressure and the cosmological term
decay exponentialy for beta < 0, where beta is defined in the text.Comment: 8 pages including front one. Published by Astrophysics and Space
Scienc
Drastic effects of damping mechanisms on the third-order optical nonlinearity
We have investigated the optical response of superradiant atoms, which
undergoes three different damping mechanisms: radiative dissipation
(), dephasing (), and nonradiative dissipation
(). Whereas the roles of and are equivalent in
the linear susceptibility, the third-order nonlinear susceptibility drastically
depends on the ratio of and : When , the third-order susceptibility is essentially that of a single atom.
Contrarily, in the opposite case of , the third-order
susceptibility suffers the size-enhancement effect and becomes proportional to
the system size.Comment: 5pages, 2figure
Optically controlled spin-glasses in multi-qubit cavity systems
Recent advances in nanostructure fabrication and optical control, suggest
that it will soon be possible to prepare collections of interacting two-level
systems (i.e. qubits) within an optical cavity. Here we show theoretically that
such systems could exhibit novel phase transition phenomena involving
spin-glass phases. By contrast with traditional realizations using magnetic
solids, these phase transition phenomena are associated with both matter and
radiation subsystems. Moreover the various phase transitions should be tunable
simply by varying the matter-radiation coupling strength.Comment: 4 pages, 3 figure
Generation of scalar-tensor gravity effects in equilibrium state boson stars
Boson stars in zero-, one-, and two-node equilibrium states are modeled
numerically within the framework of Scalar-Tensor Gravity. The complex scalar
field is taken to be both massive and self-interacting. Configurations are
formed in the case of a linear gravitational scalar coupling (the Brans-Dicke
case) and a quadratic coupling which has been used previously in a cosmological
context. The coupling parameters and asymptotic value for the gravitational
scalar field are chosen so that the known observational constraints on
Scalar-Tensor Gravity are satisfied. It is found that the constraints are so
restrictive that the field equations of General Relativity and Scalar-Tensor
gravity yield virtually identical solutions. We then use catastrophe theory to
determine the dynamically stable configurations. It is found that the maximum
mass allowed for a stable state in Scalar-Tensor gravity in the present
cosmological era is essentially unchanged from that of General Relativity. We
also construct boson star configurations appropriate to earlier cosmological
eras and find that the maximum mass for stable states is smaller than that
predicted by General Relativity, and the more so for earlier eras. However, our
results also show that if the cosmological era is early enough then only states
with positive binding energy can be constructed.Comment: 20 pages, RevTeX, 11 figures, to appear in Class. Quantum Grav.,
comments added, refs update
Mach's Principle and Model for a Broken Symmetric Theory of Gravity
We investigate spontaneous symmetry breaking in a conformally invariant
gravitational model. In particular, we use a conformally invariant scalar
tensor theory as the vacuum sector of a gravitational model to examine the idea
that gravitational coupling may be the result of a spontaneous symmetry
breaking. In this model matter is taken to be coupled with a metric which is
different but conformally related to the metric appearing explicitly in the
vacuum sector. We show that after the spontaneous symmetry breaking the
resulting theory is consistent with Mach's principle in the sense that inertial
masses of particles have variable configurations in a cosmological context.
Moreover, our analysis allows to construct a mechanism in which the resulting
large vacuum energy density relaxes during evolution of the universe.Comment: 9 pages, no figure
Singularity Free (Homogeneous Isotropic) Universe in Graviton-Dilaton Models
We present a class of graviton-dilaton models in which a homogeneous
isotropic universe, such as our observed one, evolves with no singularity at
any time. Such models may stand on their own as interesting models for
singularity free cosmology, and may be studied further accordingly. They may
also arise from string theory. We discuss critically a few such possibilities.Comment: 11 pages. Latex file. Revised in response to referees' Comments.
Results remain same. To appear in Phys. Rev. Let
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