35,670 research outputs found
Weak Coupling, Degeneration and Log Calabi-Yau Spaces
We establish a new weak coupling limit in F-theory. The new limit may be
thought of as the process in which a local model bubbles off from the rest of
the Calabi-Yau. The construction comes with a small deformation parameter
such that computations in the local model become exact as . More
generally, we advocate a modular approach where compact Calabi-Yau geometries
are obtained by gluing together local pieces (log Calabi-Yau spaces) into a
normal crossing variety and smoothing, in analogy with a similar cutting and
gluing approach to topological field theories. We further argue for a
holographic relation between F-theory on a degenerate Calabi-Yau and a dual
theory on its boundary, which fits nicely with the gluing construction.Comment: 59 pp, 2 figs, LaTe
Analysis of integration error in the ODP-L program and the effect of selenopotential parameters on the solution vector
Analysis of integration error in lunar orbit trajectory program and effect of selenopotential parameters on solution vecto
Enhanced Gauge Symmetry in Type II String Theory
We show how enhanced gauge symmetry in type II string theory compactified on
a Calabi--Yau threefold arises from singularities in the geometry of the target
space. When the target space of the type IIA string acquires a genus curve
of singularities, we find that an gauge theory with
adjoint hypermultiplets appears at the singularity. The new massless states
correspond to solitons wrapped about the collapsing cycles, and their dynamics
is described by a twisted supersymmetric gauge theory on . We
reproduce this result from an analysis of the -dual -manifold. We check
that the predictions made by this model about the nature of the Higgs branch,
the monodromy of period integrals, and the asymptotics of the one-loop
topological amplitude are in agreement with geometrical computations. In one of
our examples we find that the singularity occurs at strong coupling in the
heterotic dual proposed by Kachru and Vafa.Comment: 43 pages using harvmac's `big' option. (Minor correction concerning
magnetically charged states.
Implications of the Visible and X-Ray Counterparts to GRB970228
The gamma-ray burst source GRB970228 has been observed after a delay of 8--12
hours in X-rays and after one day in visible and near infrared light. This
marks the first detection of emission at lower frequencies following the
gamma-ray observation of a GRB and the first detection of any visible
counterpart to a GRB. We consider possible delayed visible and X-ray emission
mechanisms, and conclude that the intrinsic gamma-ray activity continued at a
much reduced intensity for at least a day. There are hints of such continued
activity in other GRB, and future observations can decide if this is true of
GRB in general. The observed multi-band spectrum of GRB970228 agrees with the
predictions of relativistic shock theory when the flux is integrated over a
time longer than that required for a radiating electron to lose its energy.Comment: 5 pp., tex, 1 figur
New Local Duals in Eternal Inflation
Global-local duality is the equivalence of seemingly different regulators in
eternal inflation. For example, the light-cone time cutoff (a global measure,
which regulates time) makes the same predictions as the causal patch (a local
measure that cuts off space). We show that global-local duality is far more
general. It rests on a redundancy inherent in any global cutoff: at late times,
an attractor regime is reached, characterized by the unlimited exponential
self-reproduction of a certain fundamental region of spacetime. An equivalent
local cutoff can be obtained by restricting to this fundamental region.
We derive local duals to several global cutoffs of interest. The New Scale
Factor Cutoff is dual to the Short Fat Geodesic, a geodesic of fixed
infinitesimal proper width. Vilenkin's CAH Cutoff is equivalent to the
Hubbletube, whose width is proportional to the local Hubble volume. The famous
youngness problem of the Proper Time Cutoff can be readily understood by
considering its local dual, the Incredible Shrinking Geodesic.Comment: 30 pages, 3 figure
Additional extensions to the NASCAP computer code, volume 1
Extensions and revisions to a computer code that comprehensively analyzes problems of spacecraft charging (NASCAP) are documented. Using a fully three dimensional approach, it can accurately predict spacecraft potentials under a variety of conditions. Among the extensions are a multiple electron/ion gun test tank capability, and the ability to model anisotropic and time dependent space environments. Also documented are a greatly extended MATCHG program and the preliminary version of NASCAP/LEO. The interactive MATCHG code was developed into an extremely powerful tool for the study of material-environment interactions. The NASCAP/LEO, a three dimensional code to study current collection under conditions of high voltages and short Debye lengths, was distributed for preliminary testing
Additional extensions to the NASCAP computer code, volume 2
Particular attention is given to comparison of the actural response of the SCATHA (Spacecraft Charging AT High Altitudes) P78-2 satellite with theoretical (NASCAP) predictions. Extensive comparisons for a variety of environmental conditions confirm the validity of the NASCAP model. A summary of the capabilities and range of validity of NASCAP is presented, with extensive reference to previously published applications. It is shown that NASCAP is capable of providing quantitatively accurate results when the object and environment are adequately represented and fall within the range of conditions for which NASCAP was intended. Three dimensional electric field affects play an important role in determining the potential of dielectric surfaces and electrically isolated conducting surfaces, particularly in the presence of artificially imposed high voltages. A theory for such phenomena is presented and applied to the active control experiments carried out in SCATHA, as well as other space and laboratory experiments. Finally, some preliminary work toward modeling large spacecraft in polar Earth orbit is presented. An initial physical model is presented including charge emission. A simple code based upon the model is described along with code test results
Gravitational energy
Observers at rest in a stationary spacetime flat at infinity can measure
small amounts of rest-mass+internal energies+kinetic energies+pressure energy
in a small volume of fluid attached to a local inertial frame. The sum of these
small amounts is the total "matter energy" for those observers. The total
mass-energy minus the matter energy is the binding gravitational energy.
Misner, Thorne and Wheeler evaluated the gravitational energy of a
spherically symmetric static spacetime. Here we show how to calculate
gravitational energy in any static and stationary spacetime for isolated
sources with a set of observers at rest.
The result of MTW is recovered and we find that electromagnetic and
gravitational 3-covariant energy densities in conformastatic spacetimes are of
opposite signs. Various examples suggest that gravitational energy is negative
in spacetimes with special symmetries or when the energy-momentum tensor
satisfies usual energy conditions.Comment: 12 pages. Accepted for publication in Class. Quantum Gra
On the Evolutionary History of Stars and their Fossil Mass and Light
The total extragalactic background radiation can be an important test of the
global star formation history (SFH). Using direct observational estimates of
the SFH, along with standard assumptions about the initial mass function (IMF),
we calculate the total extragalactic background radiation and the observed
stellar density today. We show that plausible SFHs allow a significant range in
each quantity, but that their ratio is very tightly constrained. Current
estimates of the stellar mass and extragalactic background are difficult to
reconcile, as long as the IMF is fixed to the Salpeter slope above 1 Msun. The
joint confidence interval of these two quantities only agrees with that
determined from the allowed range of SFH fits at the 3-sigma level, and for our
best-fit values the discrepancy is about a factor of two. Alternative energy
sources that contribute to the background, such as active galactic nuclei
(AGN), Population III stars, or decaying particles, appear unlikely to resolve
the discrepancy. However, changes to the IMF allow plausible solutions to the
background problem. The simplest is an average IMF with an increased
contribution from stars around 1.5--4 Msun. A ``paunchy'' IMF of this sort
could emerge as a global average if low mass star formation is suppressed in
galaxies experiencing rapid starbursts. Such an IMF is consistent with
observations of star-forming regions, and would help to reconcile the fossil
record of star formation with the directly observed SFH.Comment: 21 pages, 7 figures, 3 tables; submitted to Monthly Notice
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