621 research outputs found
Induced cosmology on a codimension-2 brane in a conical bulk
We study the cosmology of a 5-dimensional brane, which represents a
regularization of a 4-dimensional codimension-2 brane, embedded in a conical
bulk. The brane is assumed to be tensional, and to contain a curvature term.
Cosmology is obtained by letting the brane move trough the bulk, and
implementing dynamical junction conditions. Our results shows that, with
suitable choices of the parameters, the resulting cosmological dynamics mimics
fairly well standard 4-dimensional cosmology.Comment: 14 pages, 1 figure. Corrections and clarifications troughout the
text. Match the published version on NP
Lorentz-violation and cosmological perturbations: a toy brane-world model
We study possible effects of Lorentz-violation on the generation of
cosmological perturbations at inflation by introducing a simple inflating
five-dimensional brane-world setup with violation of four-dimensional
Lorentz-invariance at an energy scale . We consider massless scalar field,
meant to mimic perturbations of inflaton and/or gravitational field, in this
background. At three-momenta below , there exists a zero mode localized on
the brane, whose behaviour coincides with that in four-dimensional theory. On
the contrary, at three-momenta above , the localized mode is absent and
physics is entirely five-dimensional. As three-momenta get redshifted, more
modes get localized on the brane, the phenomenon analogous to ``mode
generation''. We find that for , where is the inflationary Hubble
scale, the spectrum of perturbations coincides with that in four-dimensional
theory. For and time-dependent bulk parameters, the spectrum deviates,
possibly strongly, from the flat spectrum even for pure de Sitter inflation.Comment: 5 figures, iopart, minor changes, appendix adde
Real-time D-brane condensation
Unstable D-branes or brane-antibrane systems can decay to lower-dimensional
branes. In the effective field theory description, the final state branes are
defects in the tachyon field which describes the initial instability. We study
the dynamical formation of codimension one defects (kinks) using Sen's ansatz
for the tachyon Lagrangian. It is shown that the slope of the kink diverges
within a finite amount of time after the tachyon starts to roll. We discuss the
relevance for reheating after brane-antibrane inflation.Comment: 10 pages, 5 figures; added reference to hep-th/020710
On Bouncing Brane-Worlds, S-branes and Branonium Cosmology
We present several higher-dimensional spacetimes for which observers living
on 3-branes experience an induced metric which bounces. The classes of examples
include boundary branes on generalised S-brane backgrounds and probe branes in
D-brane/anti D-brane systems. The bounces we consider normally would be
expected to require an energy density which violates the weak energy condition,
and for our co-dimension one examples this is attributable to bulk curvature
terms in the effective Friedmann equation. We examine the features of the
acceleration which provides the bounce, including in some cases the existence
of positive acceleration without event horizons, and we give a geometrical
interpretation for it. We discuss the stability of the solutions from the point
of view of both the brane and the bulk. Some of our examples appear to be
stable from the bulk point of view, suggesting the possible existence of stable
bouncing cosmologies within the brane-world framework.Comment: 35 pages, 7 figures, JHEP style. Title changed and references adde
Branonium
We study the bound states of brane/antibrane systems by examining the motion
of a probe antibrane moving in the background fields of N source branes. The
classical system resembles the point-particle central force problem, and the
orbits can be solved by quadrature. Generically the antibrane has orbits which
are not closed on themselves. An important special case occurs for some
Dp-branes moving in three transverse dimensions, in which case the orbits may
be obtained in closed form, giving the standard conic sections but with a
nonstandard time evolution along the orbit. Somewhat surprisingly, in this case
the resulting elliptical orbits are exact solutions, and do not simply apply in
the limit of asymptotically-large separation or non-relativistic velocities.
The orbits eventually decay through the radiation of massless modes into the
bulk and onto the branes, and we estimate this decay time. Applications of
these orbits to cosmology are discussed in a companion paper.Comment: 34 pages, LaTeX, 4 figures, uses JHEP
D-Brane Chemistry
We study several different kinds of bound states built from D-branes and
orientifolds. These states are to atoms what branonium - the bound state of a
brane and its anti-brane - is to positronium, inasmuch as they typically
involve a light brane bound to a much heavier object with conserved charges
which forbid the system's decay. We find the fully relativistic motion of a
probe Dp'-brane in the presence of source Dp-branes is integrable by
quadratures. Keplerian conic sections are obtained for special choices for p
and p' and the systems are shown to be equivalent to nonrelativistic systems.
Their quantum behaviour is also equivalent to the corresponding
non-relativistic limit. In particular the p=6, p'=0 case is equivalent to a
non-relativistic dyon in a magnetic monopole background, with the trajectories
in the surface of a cone. We also show that the motion of the probe branes
about D6-branes in IIA theory is equivalent to the motion of the corresponding
probes in the uplift to M-theory in 11 dimensions, for which there are no
D6-branes but their fields are replaced by a particular Taub-NUT geometry. We
further discuss the interactions of D-branes and orientifold planes having the
same dimension. this system behaves at large distances as a brane-brane system
but at shorter distances it does not have the tachyon instability.Comment: ref. added and typos correcte
On modified tachyon DBI action
Recently a modification of the tachyon DBI action has been proposed in which
the tachyon carries the internal CP matrix \sigma_1 and \sigma_2. In this
paper, we find the momentum expansion of the disk level S-matrix element of
four tachyons and one gauge field in superstring theory and show that the first
and second order terms of the expansion are in perfect agreement with the above
tachyon DBI action.Comment: 15 pages, no figure; V2: typos corrected, a reference added; V3: the
final version to appear in Nucl. Phys.
Spontaneous Lorentz Violation and the Long-Range Gravitational Preferred-Frame Effect
Lorentz-violating operators involving Standard Model fields are tightly
constrained by experimental data. However, bounds are more model-independent
for Lorentz violation appearing in purely gravitational couplings. The
spontaneous breaking of Lorentz invariance by the vacuum expectation value of a
vector field selects a universal rest frame. This affects the propagation of
the graviton, leading to a modification of Newton's law of gravity. We compute
the size of the long-range preferred-frame effect in terms of the coefficients
of the two-derivative operators in the low-energy effective theory that
involves only the graviton and the Goldstone bosons.Comment: 11 pages, no figures, revtex4. v4: Replaced to match version to
appear in Phys. Lett. B (minor corrections of form
Generic Modal Cut Elimination Applied to Conditional Logics
We develop a general criterion for cut elimination in sequent calculi for
propositional modal logics, which rests on absorption of cut, contraction,
weakening and inversion by the purely modal part of the rule system. Our
criterion applies also to a wide variety of logics outside the realm of normal
modal logic. We give extensive example instantiations of our framework to
various conditional logics. For these, we obtain fully internalised calculi
which are substantially simpler than those known in the literature, along with
leaner proofs of cut elimination and complexity. In one case, conditional logic
with modus ponens and conditional excluded middle, cut elimination and
complexity were explicitly stated as open in the literature
Brane cosmological solutions in six-dimensional warped flux compactifications
We study cosmology on a conical brane in the six-dimensional
Einstein-Maxwell-dilaton system, where the extra dimensions are compactified by
a magnetic flux. We systematically construct exact cosmological solutions using
the fact that the system is equivalently described by (6+n)-dimensional pure
Einstein-Maxwell theory via dimensional reduction. In particular, we find a
power-law inflationary solution for a general dilatonic coupling. When the
dilatonic coupling is given by that of Nishino-Sezgin chiral supergravity, this
reduces to the known solution which is not inflating. The power-law solution is
shown to be the late-time attractor. We also investigate cosmological tensor
perturbations in this model using the (6+n)-dimensional description. We obtain
the separable equation of motion and find that there always exist a zero mode,
while tachyonic modes are absent in the spectrum. The mass spectrum of
Kaluza-Klein modes is obtained numerically.Comment: 12 pages, 2 figures; v2: references added; v3: version published in
JCA
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