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 $e^{-2f(\sigma)}$) which differs from that of the standard Randall--Sundrum by the addition of a quadratic piece in the $f(\sigma)$ 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 $w=-1+\epsilon\dot{\phi^2}$, which can be expressed as a function in terms of the redshift $z$. 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 $\mathcal{A}$ measured from the SDSS and the shift parameter $\mathcal{R}$ from WMAP observations to constrain the parameters in our models.Comment: 6 pages, 2 figures; v2: accepted by IJMP