16,439 research outputs found
Modified Dynamical Supergravity Breaking and Off-Diagonal Super-Higgs Effects
We argue that generic off-diagonal vacuum and nonvacuum solutions for
Einstein manifolds mimic physical effects in modified gravity theories (MGTs)
and encode certain models of the , Ho\v{r}ava type with dynamical
Lorentz symmetry breaking, induced effective mass for the graviton etc. Our
main goal is to investigate the dynamical breaking of local supersymmetry
determined by off-diagonal solutions in MGTs and encoded as effective Einstein
spaces. This includes the Deser-Zumino super-Higgs effect, for instance, for a
one-loop potential in a (simple but representative) model of supergravity. We develop and apply new geometrical techniques which allows
us to decouple the gravitational field equations and integrate them in a very
general form with the metric and vielbein fields depending on all the spacetime
coordinates via means of various generating and integration functions and
parameters. We study how solutions in MGTs may be related to the dynamical
generation of a gravitino mass and supersymmetry breaking.Comment: latex2e, 33 pages, with table of content; v3 accepted to Class.
Quant. Gravit
On the Existence of Optimal Exact-Repair MDS Codes for Distributed Storage
The high repair cost of (n,k) Maximum Distance Separable (MDS) erasure codes
has recently motivated a new class of codes, called Regenerating Codes, that
optimally trade off storage cost for repair bandwidth. In this paper, we
address bandwidth-optimal (n,k,d) Exact-Repair MDS codes, which allow for any
failed node to be repaired exactly with access to arbitrary d survivor nodes,
where k<=d<=n-1. We show the existence of Exact-Repair MDS codes that achieve
minimum repair bandwidth (matching the cutset lower bound) for arbitrary
admissible (n,k,d), i.e., k<n and k<=d<=n-1. Our approach is based on
interference alignment techniques and uses vector linear codes which allow to
split symbols into arbitrarily small subsymbols.Comment: 20 pages, 6 figure
Off-Diagonal Deformations of Kerr Metrics and Black Ellipsoids in Heterotic Supergravity
Geometric methods for constructing exact solutions of motion equations with
first order corrections to the heterotic supergravity action
implying a non-trivial Yang-Mills sector and six dimensional, 6-d,
almost-K\"ahler internal spaces are studied. In 10-d spacetimes, general
parametrizations for generic off-diagonal metrics, nonlinear and linear
connections and matter sources, when the equations of motion decouple in very
general forms are considered. This allows us to construct a variety of exact
solutions when the coefficients of fundamental geometric/physical objects
depend on all higher dimensional spacetime coordinates via corresponding
classes of generating and integration functions, generalized effective sources
and integration constants. Such generalized solutions are determined by generic
off-diagonal metrics and nonlinear and/or linear connections. In particular, as
configurations which are warped/compactified to lower dimensions and for
Levi-Civita connections. The corresponding metrics can have (non) Killing
and/or Lie algebra symmetries and/or describe (1+2)-d and/or (1+3)-d domain
wall configurations, with possible warping nearly almost-K\"ahler manifolds,
with gravitational and gauge instantons for nonlinear vacuum configurations and
effective polarizations of cosmological and interaction constants encoding
string gravity effects. A series of examples of exact solutions describing
generic off-diagonal supergravity modifications to black hole/ ellipsoid and
solitonic configurations are provided and analyzed. We prove that it is
possible to reproduce the Kerr and other type black solutions in general
relativity (with certain types of string corrections) in 4-d and to generalize
the solutions to non-vacuum configurations in (super) gravity/ string theories.Comment: latex2e, 44 pages with table of content, v2 accepted to EJPC with
minor typos modifications requested by editor and referee and up-dated
reference
Off-Diagonal Deformations of Kerr Black Holes in Einstein and Modified Massive Gravity and Higher Dimensions
We find general parameterizations for generic off-diagonal spacetime metrics
and matter sources in general relativity (GR) and modified gravity theories
when the field equations decouple with respect to certain types of nonholonomic
frames of reference. This allows us to construct various classes of exact
solutions when the coefficients of the fundamental geometric/ physical objects
depend on all spacetime coordinates via corresponding classes of generating and
integration functions and/or constants. Such (modified) spacetimes display
Killing and non-Killing symmetries, describe nonlinear vacuum configurations
and effective polarizations of cosmological and interaction constants. Our
method can be extended to higher dimensions which simplifies some proofs for
embedded and nonholonomically constrained four-dimensional configurations. We
reproduce the Kerr solution and show how to deform it nonholonomically into new
classes of generic off-diagonal solutions depending on 3-8 spacetime
coordinates. Certain examples of exact solutions are analyzed and that are
determined by contributions of new type of interactions with sources in massive
gravity and/or modified f(R,T) gravity. We conclude that by considering generic
off-diagonal nonlinear parametric interactions in GR it is possible to mimic
various effects in massive and/or modified gravity, or to distinguish certain
classes of "generic" modified gravity solutions which cannot be encoded in GR.Comment: latex 2e, 11pt, 35 pages with table of content; version 2 modified
following Editor's requests and accepted to EPJ
Exact Solutions in Modified Massive Gravity and Off-Diagonal Wormhole Deformations
There are explored off-diagonal deformations of "prime" metrics in Einstein
gravity (for instance, for wormhole configurations) into "target" exact
solutions in f(R,T)-modified and massive/ bi-metric gravity theories. The new
classes of solutions may posses, or not, Killing symmetries and can be
characterized by effective induced masses, anisotropic polarized interactions
and cosmological constants. For nonholonomic deformations with (conformal)
ellipsoid/ toroid and/or solitonic symmetries and, in particular, for small
eccentricity rotoid configurations, we can generate wormholes like objects
matching external black ellipsoid - de Sitter geometries. We conclude that
there are nonholonomic transforms and/or non-trivial limits to exact solutions
in general relativity when modified/ massive gravity effects are modeled by
off-diagonal and/or nonholonomic parametric interactions.Comment: 30 pages, accepted to EJPC; moderators permitted on March 3, 2014, to
resubmit this work from April 1, 201
The Spacetime of Double Field Theory: Review, Remarks, and Outlook
We review double field theory (DFT) with emphasis on the doubled spacetime
and its generalized coordinate transformations, which unify diffeomorphisms and
b-field gauge transformations. We illustrate how the composition of generalized
coordinate transformations fails to associate. Moreover, in dimensional
reduction, the O(d,d) T-duality transformations of fields can be obtained as
generalized diffeomorphisms. Restricted to a half-dimensional subspace, DFT
includes `generalized geometry', but is more general in that local patches of
the doubled space may be glued together with generalized coordinate
transformations. Indeed, we show that for certain T-fold backgrounds with
non-geometric fluxes, there are generalized coordinate transformations that
induce, as gauge symmetries of DFT, the requisite O(d,d;Z) monodromy
transformations. Finally we review recent results on the \alpha' extension of
DFT which, reduced to the half-dimensional subspace, yields intriguing
modifications of the basic structures of generalized geometry.Comment: 50 pages, v2: minor corrections, version to be published in
Fortschritte der Physik, v3: refs. added, discussion of non-geometric
backgrounds extende
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