642 research outputs found
Noncompactified Kaluza--Klein theories and Anisotropic Kantowski-Sachs Universe
We provide an overview of noncompactified Kaluza-Klein theories. The
space-time-matter theory (or induced-matter theory) and the modified
Brans-Dicke theory are discussed. Finally, an extended version of the
Kantowski-Sachs anisotropic model is investigated as a cosmological application
of the latter.Comment: 11 pages, see https://a.co/d/fF2fCk
Extended anisotropic models in noncompact Kaluza-Klein theory
In this paper, new exact solutions for locally rotational symmetric (LRS)
space-times are obtained within the modified Brans-Dicke theory (MBDT) (Rasouli
et al 2014 Class. Quantum Grav. 31 115002). Specifically, extended
five-dimensional (5D) versions of Kantowski-Sachs, LRS Bianchi type I and
Bianchi type III are investigated in the context of the standard Brans-Dicke
theory. We subsequently extract their corresponding dynamics on a 4D
hypersurface. Our results are discussed regarding others obtained in the
standard Brans-Dicke theory, induced-matter theory and general relativity.
Moreover, we comment on the evolution of the scale factor of the extra spatial
dimension, which is of interest in Kaluza-Klein frameworks.info:eu-repo/semantics/publishedVersio
Gravitational Collapse of a Homogeneous Scalar Field in Deformed Phase Space
We study the gravitational collapse of a homogeneous scalar field, minimally
coupled to gravity, in the presence of a particular type of dynamical
deformation between the canonical momenta of the scale factor and of the scalar
field. In the absence of such a deformation, a class of solutions can be found
in the literature [R. Goswami and P. S. Joshi, arXiv:gr-qc/0410144],
%\cite{JG04}, whereby a curvature singularity occurs at the collapse end state,
which can be either hidden behind a horizon or be visible to external
observers. However, when the phase-space is deformed, as implemented herein
this paper, we find that the singularity may be either removed or instead,
attained faster. More precisely, for negative values of the deformation
parameter, we identify the emergence of a negative pressure term, which slows
down the collapse so that the singularity is replaced with a bounce. In this
respect, the formation of a dynamical horizon can be avoided depending on the
suitable choice of the boundary surface of the star. Whereas for positive
values, the pressure that originates from the deformation effects assists the
collapse toward the singularity formation. In this case, since the collapse
speed is unbounded, the condition on the horizon formation is always satisfied
and furthermore the dynamical horizon develops earlier than when the
phase-space deformations are absent. These results are obtained by means of a
thoroughly numerical discussion.Comment: 17 pages, 17 figure
Modified Saez-Ballester scalar-tensor theory from 5D space-time
In this paper, we bring together the five-dimensional Saez-Ballester~(SB)
scalar-tensor theory [1] and the induced-matter-theory~(IMT) setting [2], to
obtain a modified SB theory (MSBT) in four dimensions. Specifically, by using
an intrinsic dimensional reduction procedure into the SB field equations in
five-dimensions, a MSBT is obtained onto a hypersurface orthogonal to the extra
dimension. This four-dimensional MSBT is shown to bear distinctive new features
in contrast to the usual corresponding SB theory as well as to IMT and the
Modified Brans-Dicke Theory (MBDT)~\cite{RFM14}. It should be emphasized that
the herein appealing solutions can emerge solely from the geometrical
reductional process, from presence also of extra dimension(s) and not from any
ad-hoc matter either in the bulk or on the hypersurface.
Subsequently, we apply the herein MSBT to cosmology and consider an extended
spatially flat FLRW geometry in a five-dimensional vacuum space-time. After
obtaining the exact solutions in the bulk, we proceed to construct, by means of
the MSBT setting, the corresponding dynamic, on the four-dimensional
hypersurface. More precisely, we obtain the (SB) components of the induced
matter, including the induced scalar potential terms. We retrieve two different
classes of solutions. Concerning the first class, we show that the MSBT yields
a barotropic equation of state for the induced perfect fluid. We then
investigate vacuum, dust, radiation, stiff fluid and false vacuum cosmologies
for this scenario and contrast the results with those obtained in the standard
SB theory, IMT and BD theory. Regarding the second class solutions, we show
that the scale factor behaves similar to a de Sitter (DeS) model. However, in
our MSBT setting, this behavior is assisted by non-vanishing induced matter
instead, without any a priori cosmological constant.info:eu-repo/semantics/publishedVersio
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