672 research outputs found
World-line observables and clocks in General Relativity
A proposal for the issue of time and observables in any parameterized theory
such as general relativity is addressed. Introduction of a gauge potential
3-form A in the theory of relativity enables us to define a gauge-invariant
quantity which can be used by observers as a clock to measure the passage of
time. This dynamical variable increases monotonically and continuously along a
world line. Then we define world line observables to be any covariantly defined
quantity obtained from the field configurations on any such causal past with
dynamical time T.Comment: 16 page
Attractors, Statefinders and Observational Measurement for Chameleonic Brans--Dicke Cosmology
We investigate chameleonic Brans--Dicke model applied to the FRW universes. A
framework to study stability and attractor solutions in the phase space is
developed for the model. We show that depending on the matter field and
stability conditions, it is possible to realize phantom-like behavior without
introducing phantom filed in the model while the stability is fulfilled and
phantom crossing occurs. The statefinder parameters to the model for different
kinds of matter interacting with the chameleon scalar field are studied. We
also compare our model with present day observations.Comment: 28 pages, 38 figure
A 5D noncompact Kaluza -Klein cosmology in the presence of Null perfect fluid
For the description of the early inflation, and acceleration expansion of the
Universe, compatible with observational data, the 5D noncompact Kaluza--Klein
cosmology is investigated. It is proposed that the 5D space is filled with a
null perfect fluid, resulting a perfect fluid in 4D universe, plus one along
the fifth dimension. By analyzing the reduced field equations for flat FRW
model, we show the early inflationary behavior and current acceleration of the
universe.Comment: 9 page
Varying alpha and cosmic acceleration in Brans-Dicke-BSBM theory: stability analysis and observational tests
By integration of generalized BSBM and Brans-Dicke cosmological models, in
this article, we investigate the theoretical framework of fine structure
constant variation and current cosmic acceleration. We first develop a
mathematical formalism to analyse the stability of the model. By employing
observational data to constrain the model parameters, phase space is performed
and the attractor solutions of the model are detected. We then examine the
model against observational data such as observational Hubble parameter dataset
and quasar absorption spectra. The results confirms current universe
acceleration and also predicts fine structure constant variation. Furthermore,
extrapolation of the best fitted model in high redshift () illustrates a
significantly larger variation of fine structure constant in earlier epoch of
the universe.Comment: 11 pages, 5 figures, to be published in JCA
Dirac observables and the phase space of general relativity
In the canonical approach to general relativity it is customary to
parametrize the phase space by initial data on spacelike hypersurfaces.
However, if one seeks a theory dealing with observations that can be made by a
single localized observer, it is natural to use a different description of the
phase space. This results in a different set of Dirac observables from that
appearing in the conventional formulation. It also suggests a possible solution
to the problem of time, which has been one of the obstacles to the development
of a satisfactory quantum theory of gravity.Comment: Latex,18 pages,no figure
Stochastic quantisation of locally supersymmetric models
Stochastic quantisation normally involves the introduction of a fictitious
extra time parameter, which is taken to infinity so that the system evolves to
an equilibrium state.In the case of a locally supersymmetric theory, an
interesting new possibility arises due to the existence of a Nicolai map. In
this case it turns out that no additional time parameter is required, as the
existence of the Nicolai map ensures that the same job can be done by the
existing time parameter after Euclideanisation. This provides the quantum
theory with a natural probabilistic interpretation, without any reference to
the concept of an inner product or a Hilbert space structure.Comment: 19 page
Bouncing Universe and phantom crossing in Modified Gravity and its reconstruction
In this paper we consider FRW cosmology in modified gravity which contain
arbitrary functions . It is shown that the bouncing solution appears
in the model whereas the equation of state (EoS) parameter crosses the phantom
divider. The reconstruction of the model is also investigated with the aim to
reconstruct the arbitrary functions and variables of the model.Comment: 18 pages, 10 figure
The curvaton scenario in Brane cosmology: model parameters and their constraints
We have studied the curvaton scenario in brane world cosmology in an
intermediate inflationary scenario. This approach has allowed us to find some
constraints on different parameters that appear in the model.Comment: 16 pages, 4 figure
Observational constraint in FRW cosmology with a nonminimal scalar field-matter coupling
In this paper within the scope of FRW cosmology for k = 0, \pm1 we study the
dynamics of the universe for a cosmological model with a scalar field
nonminimally coupled to matter. By best- fitting the model parameters with the
observational data for the direct interaction between the dark sectors in the
model we obtain observational constraints on cosmological parameters. The
result shows that with the best fitted model parameters, only in flat universe,
the phantom crossing occurs twice in the past and once in the future, whereas
no crossing occurs for open and closed models of the universe.Comment: 16 pages, 23 figure
On Dynamics of Brans--Dicke Theory of Gravitation
We study longstanding problem of cosmological clock in the context of
Brans-Dicke theory of gravitation. We present the Hamiltonian formulation of
the theory for a class of spatially homogenous cosmological models. Then, we
show that formulation of the Brans-Dicke theory in the Einstein frame allows
how an identification of an appropriate cosmological time variable, as a
function of the scalar field in the theory, can be emerged in quantum
cosmology. The classical and quantum results are applied to the
Friedmann-Robertson-Walker cosmological models.Comment: 15 page
- …