106 research outputs found
FRW Universe Models in Conformally Flat Spacetime Coordinates. III: Universe models with positive spatial curvature
We deduce general expressions for the line element of universe models with
positive spatial curvature described by conformally flat spacetime coordinates.
Models with dust, radiation and vacuum energy are exhibited. Discussing the
existence of particle horizons we show that there is continual annihilation of
space, matter and energy in a dust and radiation dominated universe, and
continual creation in a LIVE domined universe when conformal time is used in
Friedmann-Robertson-Walker models with positive spatial curvature. A general
procedure is given for finding coordinates to be used in Penrose diagrams. We
also calculate the age and the redshift of some universe models using conformal
time.Comment: 22 pages, 9 figure
Identifying coastal archaeology using multispectral satellite imagery of the intertidal zone: A pilot study
This project is a desk-based assessment to test the value of using multispectral high-resolution data to examine intertidal archaeology and morphology. The project takes advantage of recent advances in remote-sensing technologies and GIS spatial mapping capabilities to identify archaeological features, integrate a wide range of archaeological, morphological and palaeoenvironmental data, and to reconstruct the coastal prehistory of the Solent, on the southern coast of England. The project focuses on the development and assessment of the multispectral mapping technique, which has the potential to provide a cost-effective approach for monitoring, mapping and managing the coastal zone and their related archaeological sites, particularly in regions of significant human activity
FRW Universe Models in Conformally Flat Spacetime Coordinates. II: Universe models with negative and vanishing spatial curvature
We deduce general expressions for the line element of universe models with
negative and vanishing spatial curvature described by conformally flat
spacetime coordinates. The empty Milne universe model and models with dust,
radiation and vacuum energy are exhibited. Discussing the existence of particle
horizons we show that there is continual creation of space, matter and energy
when conformal time is used in Friedmann-Robertson-Walker models with negative
spatial curvature.Comment: 25 pages, 12 figure
Dark Energy and Viscous Cosmology
Singularities in the dark energy universe are discussed, assuming that there
is a bulk viscosity in the cosmic fluid. In particular, it is shown how the
physically natural assumption of letting the bulk viscosity be proportional to
the scalar expansion in a spatially flat FRW universe can drive the fluid into
the phantom region (w -1)
in the non-viscous case.Comment: 11 pages. Printing error in eq.(23) corrected. To appear in Gen. Rel.
Gra
Accelerating Universe as Window for Extra Dimensions
Homogeneous cosmological solutions are obtained in five dimensional space
time assuming equations of state and where p
is the isotropic 3 - pressure and , that for the fifth dimension. Using
different values for the constants k and many known solutions are
rediscovered. Further the current acceleration of the universe has led us to
investigate higher dimensional gravity theory, which is able to explain
acceleration from a theoretical view point without the need of introducing dark
energy by hand. We argue that the terms containing higher dimensional metric
coefficients produce an extra negative pressure that apparently drives an
acceleration of the 3D space, tempting us to suggest that the accelerating
universe seems to act as a window to the existence of extra spatial dimensions.
Interestingly the 5D matter field remains regular while the \emph{effective}
negative pressure is responsible for the inflation. Relaxing the assumptions of
two equations of state we also present a class of solutions which provide early
deceleration followed by a late acceleration in a unified manner. Interesting
to point out that in this case our cosmology apparently mimics the well known
quintessence scenario fuelled by a generalised Chaplygin-type of fluid where a
smooth transition from a dust dominated model to a de Sitter like one takes
place.Comment: 20 pages,3 figure
FRW Universe Models in Conformally Flat Spacetime Coordinates. I: General Formalism
The 3-space of a universe model is defined at a certain simultaneity. Hence
space depends on which time is used. We find a general formula generating all
known and also some new transformations to conformally flat spacetime
coordinates. A general formula for the recession velocity is deduced.Comment: 12 page
A Comparison of the LVDP and {\Lambda}CDM Cosmological Models
We compare the cosmological kinematics obtained via our law of linearly
varying deceleration parameter (LVDP) with the kinematics obtained in the
{\Lambda}CDM model. We show that the LVDP model is almost indistinguishable
from the {\Lambda}CDM model up to the near future of our universe as far as the
current observations are concerned, though their predictions differ
tremendously into the far future.Comment: 6 pages, 5 figures, 1 table, matches the version to be published in
International Journal of Theoretical Physic
Bianchi Type I Cosmology in N=2, D=5 Supergravity
The dynamics and evolution of Bianchi type I space-times is considered in the
framework of the four-dimensional truncation of a reduced theory obtained from
the N=2,D=5 supergravity. The general solution of the gravitational field
equations can be represented in an exact parametric form. All solutions have a
singular behavior at the initial/final moment, except when the space-time
geometry reduces to the isotropic flat case. Generically the obtained
cosmological models describe an anisotropic, expanding or collapsing, singular
Universe with a non-inflationary evolution for all times.Comment: revised version to appear in PR
Magnetic Surfaces in Stationary Axisymmetric General Relativity
In this paper a new method is derived for constructing electromagnetic
surface sources for stationary axisymmetric electrovac spacetimes endowed with
non-smooth or even discontinuous
Ernst potentials. This can be viewed as a generalization of some classical
potential theory results, since lack of continuity of the potential is related
to dipole density and lack of smoothness, to monopole density. In particular
this approach is useful for constructing the dipole source for the magnetic
field. This formalism involves solving a linear elliptic differential equation
with boundary conditions at infinity. As an example, two different models of
surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page
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