126 research outputs found
Footballs, Conical Singularities and the Liouville Equation
We generalize the football shaped extra dimensions scenario to an arbitrary
number of branes. The problem is related to the solution of the Liouville
equation with singularities and explicit solutions are presented for the case
of three branes. The tensions of the branes do not need to be tuned with each
other but only satisfy mild global constraints.Comment: 15 pages, Refs. added, minor changes. Typo in eq. 4.3 corrected.
Version to be published in PR
A String and M-theory Origin for the Salam-Sezgin Model
An M/string-theory origin for the six-dimensional Salam-Sezgin chiral gauged
supergravity is obtained, by embedding it as a consistent Pauli-type reduction
of type I or heterotic supergravity on the non-compact hyperboloid times . We can also obtain embeddings of larger, non-chiral,
gauged supergravities in six dimensions, whose consistent truncation yields the
Salam-Sezgin theory. The lift of the Salam-Sezgin (Minkowski)
ground state to ten dimensions is asymptotic at large distances to the
near-horizon geometry of the NS5-brane.Comment: Latex, 18 pages; minor correction
Brane Universes and the Cosmological Constant
The cosmological constant problem and brane universes are reviewed briefly.
We discuss how the cosmological constant problem manifests itself in various
scenarios for brane universes. We review attempts - and their difficulties -
that aim at a solution of the cosmological constant problem.Comment: corrected typos, added references, 13 pages, accepted by MPLA as
brief revie
A New Anomaly-Free Gauged Supergravity in Six Dimensions
We present a new anomaly-free gauged N=1 supergravity model in six
dimensions. The gauge group is E_7xG_2xU(1)_R, with all hyperinos transforming
in the product representation {56,14). The theory admits monopole
compactifications to R^4xS^2, leading to D=4 effective theories with broken
supersymmetry and massless fermions.Comment: 9 pages, RevTeX
Spheres, Deficit Angles and the Cosmological Constant
We consider compactifications of six dimensional gravity in four dimensional
Minkowski or de Sitter space times a two dimensional sphere, S^2. As has been
recently pointed out, it is possible to introduce 3-branes in these backgrounds
with arbitrary tension without affecting the effective four dimensional
cosmological constant, since its only effect is to induce a deficit angle in
the sphere. We show that if a monopole like configuration of a 6D U(1) gauge
field is used to produce the spontaneous compactification of the two extra
dimensions in a sphere a fine tuning between brane and bulk parameters is
reintroduced once the quantization condition for the gauge field is taken into
account, so the 4D cosmological constant depends on the brane tension. This
problem is absent if instead of the monopole we consider a four form field
strength in the bulk to obtain the required energy-momentum tensor. Also,
making use of the four form field, we generalize the solution to an arbitrary
number of dimensions (\ge 6), keeping always four noncompact dimensions and
compactifying the rest in a n-dimensional sphere. We show that a (n+1)-brane
with arbitrary tension can be introduced in this background without affecting
the effective 4D cosmological constant.Comment: 14 pages, LaTe
Can codimension-two branes solve the cosmological constant problem?
It has been suggested that codimension-two braneworlds might naturally
explain the vanishing of the 4D effective cosmological constant, due to the
automatic relation between the deficit angle and the brane tension. To
investigate whether this cancellation happens dynamically, and within the
context of a realistic cosmology, we study a codimension-two braneworld with
spherical extra dimensions compactified by magnetic flux. Assuming Einstein
gravity, we show that when the brane contains matter with an arbitrary equation
of state, the 4D metric components are not regular at the brane, unless the
brane has nonzero thickness. We construct explicit 6D solutions with thick
branes, treating the brane matter as a perturbation, and find that the universe
expands consistently with standard Friedmann-Robertson-Walker (FRW) cosmology.
The relation between the brane tension and the bulk deficit angle becomes
for a general equation of state. However, this
relation does not imply a self-tuning of the effective 4D cosmological constant
to zero; perturbations of the brane tension in a static solution lead to
deSitter or anti-deSitter braneworlds. Our results thus confirm other recent
work showing that codimension-two braneworlds in nonsupersymmetric Einstein
gravity do not lead to a dynamical relaxation of the cosmological constant, but
they leave open the possibility that supersymmetric versions can be compatible
with self-tuning.Comment: Revtex4, 17 pages, references added, typos corrected, minor points
clarified. Matches published versio
Series solutions for a static scalar potential in a Salam-Sezgin Supergravitational hybrid braneworld
The static potential for a massless scalar field shares the essential
features of the scalar gravitational mode in a tensorial perturbation analysis
about the background solution. Using the fluxbrane construction of [8] we
calculate the lowest order of the static potential of a massless scalar field
on a thin brane using series solutions to the scalar field's Klein Gordon
equation and we find that it has the same form as Newton's Law of Gravity. We
claim our method will in general provide a quick and useful check that one may
use to see if their model will recover Newton's Law to lowest order on the
brane.Comment: 5 pages, no figure
A systematic search for anomaly-free supergravities in six dimensions
We conduct a systematic search for anomaly-free six-dimensional N=1 chiral
supergravity theories. Under a certain set of restrictions on the allowed gauge
groups and the representations of the hypermultiplets, we enumerate all
possible Poincare and gauged supergravities with one tensor multiplet
satisfying the 6D anomaly cancellation criteria.Comment: 39 pages, JHEP3 class, references added, minor typos correcte
Natural Quintessence in String Theory
We introduce a natural model of quintessence in string theory where the light
rolling scalar is radiatively stable and couples to Standard Model matter with
weaker-than- Planckian strength. The model is embedded in an anisotropic type
IIB compactification with two exponentially large extra dimensions and
TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the
scale of the gravitino mass is of the order of the observed value of the
cosmological constant. The quintessence field is a modulus parameterising the
size of an internal four-cycle which naturally develops a potential of the
order (gravitino mass)^4, leading to a small dark energy scale without tunings.
The mass of the quintessence field is also radiatively stable since it is
protected by supersymmetry in the bulk. Moreover, this light scalar couples to
ordinary matter via its mixing with the volume mode. Due to the fact that the
quintessence field is a flat direction at leading order, this mixing is very
small, resulting in a suppressed coupling to Standard Model particles which
avoids stringent fifth-force constraints. On the other hand, if dark matter is
realised in terms of Kaluza-Klein states, unsuppressed couplings between dark
energy and dark matter can emerge, leading to a scenario of coupled
quintessence within string theory. We study the dynamics of quintessence in our
set-up, showing that its main features make it compatible with observations.Comment: 26 page
Supersymmetric codimension-two branes and U(1)_R mediation in 6D gauged supergravity
We construct a consistent supersymmetric action for brane chiral and vector
multiplets in a six-dimensional chiral gauged supergravity. A nonzero brane
tension can be accommodated by allowing for a brane-localized Fayet-Iliopoulos
term proportional to the brane tension. When the brane chiral multiplet is
charged under the bulk U(1)_R, we obtain a nontrivial coupling to the extra
component of the U(1)_R gauge field strength as well as a singular scalar
self-interaction term. Dimensionally reducing to 4D on a football
supersymmetric solution, we discuss the implication of such interactions for
obtaining the U(1)_R D-term in the 4D effective supergravity. By assuming the
bulk gaugino condensates and nonzero brane F- and/or D-term for the uplifting
potential, we have all the moduli stabilized with a vanishing cosmological
constant. The brane scalar with nonzero R charge then gets a soft mass of order
the gravitino mass. The overall sign of the soft mass squared depends on the
sign of the R charge as well as whether the brane F- or D-term dominates.Comment: 28 pages, no figures, version to appear in JHE
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