762 research outputs found
Born-Infeld type Gravity
Generalizations of gravitational Born-Infeld type lagrangians are
investigated. Phenomenological constraints (reduction to Einstein-Hilbert
action for small curvature, spin two ghost freedom and absence of Coulomb like
Schwarschild singularity) select one effective lagrangian whose dynamics is
dictated by the tensors g_{\mu\nu} and R_{\mu\nu\rho\sigma}(not R_{\mu\nu} or
the scalar R).Comment: 7 pages, 3 figures, revte
Gravitational dynamics for all tensorial spacetimes carrying predictive, interpretable and quantizable matter
Only a severely restricted class of tensor fields can provide classical
spacetime geometries, namely those that can carry matter field equations that
are predictive, interpretable and quantizable. These three conditions on matter
translate into three corresponding algebraic conditions on the underlying
tensorial geometry, namely to be hyperbolic, time-orientable and
energy-distinguishing. Lorentzian metrics, on which general relativity and the
standard model of particle physics are built, present just the simplest
tensorial spacetime geometry satisfying these conditions. The problem of
finding gravitational dynamics---for the general tensorial spacetime geometries
satisfying the above minimum requirements---is reformulated in this paper as a
system of linear partial differential equations, in the sense that their
solutions yield the actions governing the corresponding spacetime geometry.
Thus the search for modified gravitational dynamics is reduced to a clear
mathematical task.Comment: 47 pages, no figures, minor update
Shortcuts to Spherically Symmetric Solutions: A Cautionary Note
Spherically symmetric solutions of generic gravitational models are
optimally, and legitimately, obtained by expressing the action in terms of the
two surviving metric components. This shortcut is not to be overdone, however:
a one-function ansatz invalidates it, as illustrated by the incorrect solutions
of [1].Comment: 2 pages. Amplified derivation, accepted for publication in Class
Quant Gra
The `s-rule' exclusion principle and vacuum interpolation in worldvolume dynamics
We show how the worldvolume realization of the Hanany-Witten effect for a
supersymmetric D5-brane in a D3 background also provides a classical
realization of the `s-rule' exclusion principle. Despite the supersymmetry, the
force on the D5-brane vanishes only in the D5 `ground state', which is shown to
interpolate between 6-dimensional Minkowski space and an -invariant
geometry. The M-theory analogue of these results is briefly
discussed.Comment: 25 pages, 9 figures, LaTeX JHEP styl
Canonical differential geometry of string backgrounds
String backgrounds and D-branes do not possess the structure of Lorentzian
manifolds, but that of manifolds with area metric. Area metric geometry is a
true generalization of metric geometry, which in particular may accommodate a
B-field. While an area metric does not determine a connection, we identify the
appropriate differential geometric structure which is of relevance for the
minimal surface equation in such a generalized geometry. In particular the
notion of a derivative action of areas on areas emerges naturally. Area metric
geometry provides new tools in differential geometry, which promise to play a
role in the description of gravitational dynamics on D-branes.Comment: 20 pages, no figures, improved journal versio
On Dual Formulation of Gravity
In this paper we consider a possibility to construct dual formulation of
gravity where the main dynamical field is the Lorentz connection
\omega_\mu^{ab} and not that of tetrad e_\mu^a or metric g_\mu\nu. Our approach
is based on the usual dualization procedure which uses first order parent
Lagrangians but in (Anti) de Sitter space and not in the flat Minkowski one. It
turns out that in d=3 dimensions such dual formulation is related with the so
called exotic parity-violating interactions for massless spin-2 particles.Comment: 7 pages, plain LaTe
Thermal Casimir Force between Magnetic Materials
We investigate the Casimir pressure between two parallel plates made of
magnetic materials at nonzero temperature. It is shown that for real
magnetodielectric materials only the magnetic properties of ferromagnets can
influence the Casimir pressure. This influence is accomplished through the
contribution of the zero-frequency term of the Lifshitz formula. The
possibility of the Casimir repulsion through the vacuum gap is analyzed
depending on the model used for the description of the dielectric properties of
the metal plates.Comment: 9 pages, 3 figures. Contribution to the Proceedings of QFEXT09,
Norman, OK, September 21-25, 200
Gravity a la Born-Infeld
A simple technique for the construction of gravity theories in Born-Infeld
style is presented, and the properties of some of these novel theories are
investigated. They regularize the positive energy Schwarzschild singularity,
and a large class of models allows for the cancellation of ghosts. The possible
correspondence to low energy string theory is discussed. By including curvature
corrections to all orders in alpha', the new theories nicely illustrate a
mechanism that string theory might use to regularize gravitational
singularities.Comment: 21 pages, 2 figures, new appendix B with corrigendum: Class. Quantum
Grav. 21 (2004) 529
Spontaneous decompactification
Positive vacuum energy together with extra dimensions of space imply that our
four-dimensional Universe is unstable, generically to decompactification of the
extra dimensions. Either quantum tunneling or thermal fluctuations carry one
past a barrier into the decompactifying regime. We give an overview of this
process, and examine the subsequent expansion into the higher- dimensional
geometry. This is governed by certain fixed-point solutions of the evolution
equations, which are studied for both positive and negative spatial curvature.
In the case where there is a higher-dimensional cosmological constant, we also
outline a possible mechanism for compactification to a four-dimensional de
Sitter cosmology.Comment: 27 pages, 5 figures, harvmac. v2: refs added, minor notation change
The alpha-prime stretched horizon in the Heterotic string
The linear alpha-prime corrections and the field redefinition ambiguities are
studied for half-BPS singular backgrounds representing a wrapped fundamental
string. It is showed that there exist schemes in which the inclusion of all the
linear alpha-prime corrections converts these singular solutions to black holes
with a regular horizon for which the modified Hawking-Bekenstein entropy is in
agreement with the statistical entropy.Comment: 22 pages JHEP; new discussions and more details added to section
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