88,931 research outputs found
Proper Matter Collineations of Plane Symmetric Spacetimes
We investigate matter collineations of plane symmetric spacetimes when the
energy-momentum tensor is degenerate. There exists three interesting cases
where the group of matter collineations is finite-dimensional. The matter
collineations in these cases are either four, six or ten in which four are
isometries and the rest are proper.Comment: 10 pages, LaTex, accepted for publication in Modern Physics Letters
Vacuumless cosmic strings in Brans-Dicke theory
The gravitational fields of vacuumless global and gauge strings have been
studied in Brans-Dicke theory under the weak field assumption of the field
equations. It has been shown that both global and gauge string can have
repulsive as well as attractive gravitational effect in Brans-Dicke theory
which is not so in General Relativity.Comment: 10 pages, Latex, some errors are corrected, new conclusions and
references adde
Stable two--brane models with bulk tachyon matter
We explore the possibility of constructing stable, warped two--brane models
which solve the hierarchy problem, with a bulk non--canonical scalar field
(tachyon matter) as the source term in the action. Among our examples are two
models--one with a warp factor (denoted as ) which differs
from that of the standard Randall--Sundrum by the addition of a quadratic piece
in the and another, where the warping is super-exponential. We
investigate the issue of resolution of hierarchy and perform a stability
analysis by obtaining the effective inter-brane potentials, in each case. Our
analysis reveals that there does exist stable values of the modulus consistent
with hierarchy resolution in both the models. Thus, these models, in which the
bulk scalar field generates the geometry and also ensures stability, provide
viable alternatives to the standard Randall--Sundrum two-brane scenario.Comment: Final version published in Int. Jr. Mod. Phys
Time and Tachyon
Recent analysis suggests that the classical dynamics of a tachyon on an
unstable D-brane is described by a scalar Born-Infeld type action with a
runaway potential. The classical configurations in this theory at late time are
in one to one correspondence with the configuration of a system of
non-interacting (incoherent), non-rotating dust. We discuss some aspects of
canonical quantization of this field theory coupled to gravity, and explore,
following earlier work on this subject, the possibility of using the scalar
field (tachyon) as the definition of time in quantum cosmology. At late `time'
we can identify a subsector in which the scalar field decouples from gravity
and we recover the usual Wheeler - de Witt equation of quantum gravity.Comment: LaTeX file, 24 page
Tachyon field inspired dark energy and supernovae constraints
The tachyon field in cosmology is studied by applying the generating function
method to obtain exact solutions. The equation of state parameter of the
tachyon field is , which can be expressed as a
function in terms of the redshift . Based on these solutions, we propose
some tachyon-inspired dark energy models to explore the properties of the
corresponding cosmological evolution. The explicit relations between Hubble
parameter and redshift enable us to test the models with SNe Ia data sets
easily. In the current work we employ the SNe Ia data with the parameter
measured from the SDSS and the shift parameter from
WMAP observations to constrain the parameters in our models.Comment: 6 pages, 2 figures; v2: accepted by IJMP
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