3,383 research outputs found
Warped compactification on curved manifolds
The characterization of a six- (or seven)-dimensional internal manifold with
metric as having positive, zero or negative curvature is expected to be an
important aspect of warped compactifications in supergravity. In this context,
Douglas and Kallosh recently pointed out that a compact internal space with
negative curvature could help to construct four-dimensional de Sitter solutions
only if the extra dimensions are strongly warped or there are large stringy
corrections. That is, the problem of finding 4-dimensional de Sitter solutions
is well posed, if all extra dimensions are physically compact, which is called
a no-go theorem. Here, we show that the above conclusion does not extend to a
general class of warped compactifications in classical supergravity that allow
a non-compact direction or cosmological solutions for which the internal space
is asymptotic to a cone over a product of compact Einstein spaces or spheres.
For clarity, we present classical solutions that compactify higher-dimensional
spacetime to produce a Robertson--Walker universe with de Sitter-type expansion
plus one extra non-compact direction. Such models are found to admit both an
effective four-dimensional Newton constant that remains finite and a
normalizable zero-mode graviton wavefunction. We also exhibit the possibility
of obtaining 4D de Sitter solutions by including the effect of fluxes (p-form
field strengths).Comment: 24 pages, 1 figure; v5 significant changes in the presentation,
published (journal) versio
Warped Vacuum Statistics
We consider the effect of warping on the distribution of type IIB flux vacua
constructed with Calabi-Yau orientifolds. We derive an analytical form of the
distribution that incorporates warping and find close agreement with the
results of a Monte Carlo enumeration of vacua. Compared with calculations that
neglect warping, we find that for any finite volume compactification, the
density of vacua is highly diluted in close proximity to the conifold point,
with a steep drop-off within a critical distance.Comment: 30 pages, 2 figure
Megamaser Disks in Active Galactic Nuclei
Recent spectroscopic and VLBI-imaging observations of bright extragalactic
water maser sources have revealed that the megamaser emission often originates
in thin circumnuclear disks near the centers of active galactic nuclei (AGNs).
Using general radiative and kinematic considerations and taking account of the
observed flux variability, we argue that the maser emission regions are clumpy,
a conclusion that is independent of the detailed mechanism (X-ray heating,
shocks, etc.) driving the collisionally pumped masers. We examine scenarios in
which the clumps represent discrete gas condensations (i.e., clouds) and do not
merely correspond to velocity irregularities in the disk. We show that even two
clouds that overlap within the velocity coherence length along the line of
sight could account (through self-amplification) for the entire maser flux of a
high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and
we suggest that cloud self-amplification likely contributes also to the flux of
the background-amplifying ``systemic'' features in these objects. Analogous
interpretations have previously been proposed for water maser sources in
Galactic star-forming regions. We argue that this picture provides a natural
explanation of the time-variability characteristics of extragalactic megamaser
sources and of their apparent association with Seyfert 2-like galaxies. We also
show that the requisite cloud space densities and internal densities are
consistent with the typical values of nuclear (broad emission-line region-type)
clouds.Comment: 55 pages, 7 figures, AASTeX4.0, to appear in The Astrophysical
Journal (1999 March 1 issue
On the dynamics of a twisted disc immersed in a radiation field
We study the dynamics of a twisted tilted disc under the influence of an
external radiation field. Assuming the effect of absorption and
reemission/scattering is that a pressure is applied to the disc surface where
the local optical depth is of order unity, we determine the response of the
vertical structure and the influence it has on the possibility of instability
to warping. We derive a pair of equations describing the evolution of a small
tilt as a function of radius in the small amplitude regime that applies to both
the diffusive and bending wave regimes. We also study the non linear vertical
response of the disc numerically using an analogous one dimensional slab model.
For global warps, we find that in order for the disc vertical structure to
respond as a quasi uniform shift or tilt, as has been assumed in previous work,
the product of the ratio of the external radiation momentum flux to the local
disc mid plane pressure, where it is absorbed, with the disc aspect ratio
should be significantly less than unity. Namely, this quantity should be of the
order of or smaller than the ratio of the disc gas density corresponding to the
layer intercepting radiation to the mid plane density, . When
this condition is not satisfied the disc surface tends to adjust so that the
local normal becomes perpendicular to the radiation propagation direction. In
this case dynamical quantities determined by the disc twist and warp tend to
oscillate with a large characteristic period ,
where is some 'typical' orbital period of a gas element in the disc.
The possibility of warping instability then becomes significantly reduced. In
addition, when the vertical response is non uniform, the possible production of
shocks may lead to an important dissipation mechanism.Comment: submitted to MNRA
Reducing “Structure from Motion”: a general framework for dynamic vision. 1. Modeling
The literature on recursive estimation of structure and motion from monocular image sequences comprises a large number of apparently unrelated models and estimation techniques. We propose a framework that allows us to derive and compare all models by following the idea of dynamical system reduction. The “natural” dynamic model, derived from the rigidity constraint and the projection model, is first reduced by explicitly decoupling structure (depth) from motion. Then, implicit decoupling techniques are explored, which consist of imposing that some function of the unknown parameters is held constant. By appropriately choosing such a function, not only can we account for models seen so far in the literature, but we can also derive novel ones
Algebraic constructive quantum field theory: Integrable models and deformation techniques
Several related operator-algebraic constructions for quantum field theory
models on Minkowski spacetime are reviewed. The common theme of these
constructions is that of a Borchers triple, capturing the structure of
observables localized in a Rindler wedge. After reviewing the abstract setting,
we discuss in this framework i) the construction of free field theories from
standard pairs, ii) the inverse scattering construction of integrable QFT
models on two-dimensional Minkowski space, and iii) the warped convolution
deformation of QFT models in arbitrary dimension, inspired from non-commutative
Minkowski space.Comment: Review article, 57 pages, 3 figure
Comparing Brane Inflation to WMAP
We compare the simplest realistic brane inflationary model to recent
cosmological data, including WMAP 3-year cosmic microwave background (CMB)
results, Sloan Digital Sky Survey luminous red galaxies (SDSS LRG) power
spectrum data and Supernovae Legacy Survey (SNLS) Type 1a supernovae distance
measures. Here, the inflaton is simply the position of a -brane which is
moving towards a -brane sitting at the bottom of a throat (a warped,
deformed conifold) in the flux compactified bulk in Type IIB string theory. The
analysis includes both the usual slow-roll scenario and the Dirac-Born-Infeld
scenario of slow but relativistic rolling. Requiring that the throat is inside
the bulk greatly restricts the allowed parameter space. We discuss possible
scenarios in which large tensor mode and/or non-Gaussianity may emerge. Here,
the properties of a large tensor mode deviate from that in the usual slow-roll
scenario, providing a possible stringy signature. Overall, within the brane
inflationary scenario, the cosmological data is providing information about the
properties of the compactification of the extra dimensions.Comment: 45 pages 11 figure
String Necklaces and Primordial Black Holes from Type IIB Strings
We consider a model of static cosmic string loops in type IIB string theory,
where the strings wrap cycles within the internal space. The strings are not
topologically stabilised, however the presence of a lifting potential traps the
windings giving rise to kinky cycloops. We find that PBH formation occurs at
early times in a small window, whilst at late times we observe the formation of
dark matter relics in the scaling regime. This is in stark contrast to previous
predictions based on field theoretic models. We also consider the PBH
contribution to the mass density of the universe, and use the experimental data
to impose bounds on the string theory parameters.Comment: 45 pages, 9 figures, LaTeX; published versio
Bouncing Brane Cosmologies from Warped String Compactifications
We study the cosmology induced on a brane probing a warped throat region in a
Calabi-Yau compactification of type IIB string theory. For the case of a BPS
D3-brane probing the Klebanov-Strassler warped deformed conifold, the cosmology
described by a suitable brane observer is a bouncing, spatially flat
Friedmann-Robertson-Walker universe with time-varying Newton's constant, which
passes smoothly from a contracting to an expanding phase. In the
Klebanov-Tseytlin approximation to the Klebanov-Strassler solution the
cosmology would end with a big crunch singularity. In this sense, the warped
deformed conifold provides a string theory resolution of a spacelike
singularity in the brane cosmology. The four-dimensional effective action
appropriate for a brane observer is a simple scalar-tensor theory of gravity.
In this description of the physics, a bounce is possible because the relevant
energy-momentum tensor can classically violate the null energy condition.Comment: 20 pages, 2 figures; v2, references added and minor correction
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