264 research outputs found
Rolling tachyon in anti-de Sitter space-time
We study the decay of the unstable D-particle in three-dimensional anti-de
Sitter space-time using worldsheet boundary conformal field theory methods. We
test the open string completeness conjecture in a background for which the
phase space available is only field-theoretic. This could present a serious
challenge to the claim. We compute the emission of closed strings in the AdS(3)
x S^3 x T^4 background from the knowledge of the exact corresponding boundary
state we construct. We show that the energy stored in the brane is mainly
converted into very excited long strings. The energy stored in short strings
and in open string pair production is much smaller and finite for any value of
the string coupling. We find no "missing energy" problem. We compare our
results to those obtained for a decay in flat space-time and to a background in
the presence of a linear dilaton. Some remarks on holographic aspects of the
problem are made.Comment: JHEP style, 45 pages, one figure; v2: typos corrected, references
added, version to appear in JHE
Boundary States for the Rolling D-branes in NS5 Background
In this paper we construct the time dependent boundary states describing the
``rolling D-brane solutions'' in the NS5 background discovered recently by
Kutasov by means of the classical DBI analysis. We first survey some aspects of
non-compact branes in the NS5 background based on known boundary states in the
N=2 Liouville theory. We consider two types of non-compact branes, one of which
is BPS and the other is non-BPS but stable. Then we clarify how to Wick-rotate
the non-BPS one appropriately. We show that the Wick-rotated boundary state
realizes the correct trajectory of rolling D-brane in the classical limit, and
leads to well behaved spectral densities of open strings due to the existence
of non-trivial damping factors of energy. We further study the cylinder
amplitudes and the emission rates of massive closed string modes.Comment: 25 pages, 2 figures, v2: typos corrected, reference added, v3:
emission rates of closed strings correcte
D-branes on a Deformation of SU(2)
We discuss D-branes on a line of conformal field theories connected by an
exact marginal deformation. The line contains an SU(2) WZW model and two
mutually T-dual SU(2)/U(1) cosets times a free boson. We find the D-branes
preserving a U(1) isometry, an F-flux quantization condition and conformal
invariance. Away from the SU(2) point a U(1) times U(1) symmetry is broken to
U(1) times Z_k, i.e. continuous rotations of branes are accompanied by
rotations along the branes. Requiring decoupling of the cosets from the free
boson at the endpoints of the deformation breaks the continuous rotation of
branes to Z_k. At the SU(2) point the full U(1) times U(1) symmetry is
restored. This suggests the occurrence of phase transitions for branes at
angles in the coset model, at a semiclassical level. We also discuss briefly
the orientifold planes along the deformation line.Comment: 19 pages, latex, 5 figures, references adde
Interacting fermions in self-similar potentials
We consider interacting spinless fermions in one dimension embedded in
self-similar quasiperiodic potentials. We examine generalizations of the
Fibonacci potential known as precious mean potentials. Using a bosonization
technique and a renormalization group analysis, we study the low-energy physics
of the system. We show that it undergoes a metal-insulator transition for any
filling factor, with a critical interaction that strongly depends on the
position of the Fermi level in the Fourier spectrum of the potential. For some
positions of the Fermi level the metal-insulator transition occurs at the non
interacting point. The repulsive side is an insulator with a gapped spectrum
whereas in the attractive side the spectrum is gapless and the properties of
the system are described by a Luttinger liquid. We compute the transport
properties and give the characteristic exponents associated to the frequency
and temperature dependence of the conductivity.Comment: 18 pages, 10 EPS figure
Bulk viscosity driving the acceleration of the Universe
The possibility that the present acceleration of the universe is driven by a
kind of viscous fluid is exploited. At background level this model is similar
to the generalized Chaplygin gas model (GCGM). But, at perturbative level, the
viscous fluid exhibits interesting properties. In particular the oscillations
in the power spectrum that plagues the GCGM are not present. Possible
fundamental descriptions for this viscous dark energy are discussed.Comment: Latex file, 8 pages, 3 eps figure
Deviation From \Lambda CDM With Cosmic Strings Networks
In this work, we consider a network of cosmic strings to explain possible
deviation from \Lambda CDM behaviour. We use different observational data to
constrain the model and show that a small but non zero contribution from the
string network is allowed by the observational data which can result in a
reasonable departure from \Lambda CDM evolution. But by calculating the
Bayesian Evidence, we show that the present data still strongly favour the
concordance \Lambda CDM model irrespective of the choice of the prior.Comment: 15 Pages, Latex Style, 4 eps figures, Revised Version, Accepted for
publication in European Physical Journal
Liouville D-branes in Two-Dimensional Strings and Open String Field Theory
We study open strings in the noncritical bosonic string theory
compactified on a circle at self-dual radius. These strings live on D-branes
that are extended along the Liouville direction ({\it FZZT} branes). We present
explicit expressions for the disc two- and three-point functions of boundary
operators in this theory, as well as the bulk-boundary two-point function. The
expressions obtained are divergent because of resonant behaviour at self-dual
radius. However, these can be regularised and renormalized in a precise way to
get finite results. The boundary correlators are found to depend only on the
differences of boundary cosmological constants, suggesting a fermionic
behaviour. We initiate a study of the open-string field theory localised to the
physical states, which leads to an interesting matrix model.Comment: 29 pages, harvma
Thermodynamics of Modified Chaplygin Gas and Tachyonic Field
Here we generalize the results of the work of ref. [10] in modified Chaplygin
gas model and tachyonic field model. Here we have studied the thermodynamical
behaviour and the equation of state in terms of volume and temperature for both
models. We have used the solution and the corresponding equation of state of
our previous work [12] for tachyonic field model. We have also studied the
thermodynamical stability using thermal equation of state for the tachyonic
field model and have shown that there is no critical points during
thermodynamical expansion. The determination of due to expansion for
the tachyonic field have been discussed by assuming some initial conditions.
Here, the thermal quantities have been investigated using some reduced
parameters.Comment: 10 page
Anisotropy of the Upper Critical Field and Critical Current in Single Crystal MgB
We report on specific heat, high magnetic field transport and
susceptibility measurements on magnesium diboride single crystals. The
upper critical field for magnetic fields perpendicular and parallel to
the Mg and B planes is presented for the first time in the entire temperature
range. A very different temperature dependence has been observed in the two
directions which yields to a temperature dependent anisotropy with 5 at low temperatures and about 2 near . A peak effect is observed
in susceptibility measurements for 2 T parallel to the axis and
the critical current density presnts a sharp maximum for parallel to the
ab-plane.Comment: 6 pages, 5 figure
Statefinder Parameter for Varying G in Three Fluid System
In this work, we have considered variable G in flat FRW universe filled with
the mixture of dark energy, dark matter and radiation. If there is no
interaction between the three fluids, the deceleration parameter and
statefinder parameters have been calculated in terms of dimensionless density
parameters which can be fixed by observational data. Also the interaction
between three fluids has been analyzed due to constant . The statefinder
parameters also calculated in two cases: pressure is constant and pressure is
variable.Comment: 5 pages, Accepted for publication in "Astrophysics and Space Science
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