8,548 research outputs found
Sculpting the Extra Dimensions: Inflation from Codimension-2 Brane Back-reaction
We construct an inflationary model in 6D supergravity that is based on
explicit time-dependent solutions to the full higher-dimensional field
equations, back-reacting to the presence of a 4D inflaton rolling on a
space-filling codimension-2 source brane. Fluxes in the bulk stabilize all
moduli except the `breathing' modulus (that is generically present in
higher-dimensional supergravities). Back-reaction to the inflaton roll causes
the 4D Einstein-frame on-brane geometry to expand, a(t) ~ t^p, as well as
exciting the breathing mode and causing the two off-brane dimensions to expand,
r(t) ~ t^q. The model evades the general no-go theorems precluding 4D de Sitter
solutions, since adjustments to the brane-localized inflaton potential allow
the power p to be dialed to be arbitrarily large, with the 4D geometry becoming
de Sitter in the limit p -> infinity (in which case q = 0). Slow-roll solutions
give accelerated expansion with p large but finite, and q = 1/2. Because the
extra dimensions expand during inflation, the present-day 6D gravity scale can
be much smaller than it was when primordial fluctuations were generated -
potentially allowing TeV gravity now to be consistent with the much higher
gravity scale required at horizon-exit for observable primordial gravity waves.
Because p >> q, the 4 on-brane dimensions expand more quickly than the 2
off-brane ones, providing a framework for understanding why the observed four
dimensions are presently so much larger than the internal two. If uplifted to a
10D framework with 4 dimensions stabilized, the 6D evolution described here
could describe how two of the six extra dimensions evolve to become much larger
than the others, as a consequence of the enormous expansion of the 4 large
dimensions we can see.Comment: 27 pages + appendices, 2 figure
Bulk Axions, Brane Back-reaction and Fluxes
Extra-dimensional models can involve bulk pseudo-Goldstone bosons (pGBs)
whose shift symmetry is explicitly broken only by physics localized on branes.
Reliable calculation of their low-energy potential is often difficult because
it requires details of the stabilization of the extra dimensions. In rugby ball
solutions, for which two compact extra dimensions are stabilized in the
presence of only positive-tension brane sources, the effects of brane
back-reaction can be computed explicitly. This allows the calculation of the
shape of the low-energy pGB potential and response of the extra dimensional
geometry as a function of the perturbing brane properties. If the
pGB-dependence is a small part of the total brane tension a very general
analysis is possible, permitting an exploration of how the system responds to
frustration when the two branes disagree on what the proper scalar vacuum
should be. We show how the low-energy potential is given by the sum of brane
tensions (in agreement with common lore) when only the brane tensions couple to
the pGB. We also show how a direct brane coupling to the flux stabilizing the
extra dimensions corrects this result in a way that does not simply amount to
the contribution of the flux to the brane tensions. We calculate the mass of
the would-be zero mode, and briefly describe several potential applications,
including a brane realization of `natural inflation,' and a dynamical mechanism
for suppressing the couplings of the pGB to matter localized on the branes.
Since the scalar can be light enough to be relevant to precision tests of
gravity (in a technically natural way) this mechanism can be relevant to
evading phenomenological bounds.Comment: 36 pages, JHEP styl
Comment on Higgs Inflation and Naturalness
We rebut the recent claim (arXiv:0912.5463) that Einstein-frame scattering in
the Higgs inflation model is unitary above the cut-off energy Lambda ~ Mp/xi.
We show explicitly how unitarity problems arise in both the Einstein and Jordan
frames of the theory. In a covariant gauge they arise from non-minimal Higgs
self-couplings, which cannot be removed by field redefinitions because the
target space is not flat. In unitary gauge, where there is only a single scalar
which can be redefined to achieve canonical kinetic terms, the unitarity
problems arise through non-minimal Higgs-gauge couplings.Comment: 5 pages, 1 figure V3: Journal Versio
Quantum fields in disequilibrium: neutral scalar bosons with long-range, inhomogeneous perturbations
Using Schwinger's quantum action principle, dispersion relations are obtained
for neutral scalar mesons interacting with bi-local sources. These relations
are used as the basis of a method for representing the effect of interactions
in the Gaussian approximation to field theory, and it is argued that a marked
inhomogeneity, in space-time dependence of the sources, forces a discrete
spectrum on the field. The development of such a system is characterized by
features commonly associated with chaos and self-organization (localization by
domain or cell formation). The Green functions play the role of an iterative
map in phase space. Stable systems reside at the fixed points of the map. The
present work can be applied to self-interacting theories by choosing suitable
properties for the sources. Rapid transport leads to a second order phase
transition and anomalous dispersion. Finally, it is shown that there is a
compact representation of the non-equilibrium dynamics in terms of generalized
chemical potentials, or equivalently as a pseudo-gauge theory, with an
imaginary charge. This analogy shows, more clearly, how dissipation and entropy
production are related to the source picture and transform a flip-flop like
behaviour between two reservoirs into the Landau problem in a constant
`magnetic field'. A summary of conventions and formalism is provided as a basis
for future work.Comment: 23 pages revte
LP (a) levels and apo (a) phenotypes in urban black South African men
CITATION: Carstens, M. E., Burgess, L. J. & Taljaard, J. J. F. 1998. LP (a) levels and apo (a) phenotypes in urban black South African men. South African Medical Journal, 88:139-142.The original publication is available at http://www.samj.org.zaObjective. To investigate the lipoprotein (a) (Lp (a)) levels and apolipoprotein (a) (apo (a)) phenotypes in a group of urban black South African men. Design. Cross-sectional design. Setting. Lead acid battery plant, East London, Eastern Cape. Participants. Blood samples from a study on the association between lead and renal failure were kindly donated for the present study and 111 of the donors participated (K Steyn - personal communication). Outcome measures. Lp (a) levels and apo (a) phenotypes. Results. Three groups were identified: those with normal ( 700 U/l) plasma Lp (a) concentrations. Nine apo (a) phenotypes and 26 combinations thereof could be discerned. Apart from the single- and double-band phenotypes described before, triple-band phenotypes were also present. As the Lp (a) values increased, the relative frequency of the single-band phenotype decreased, whereas the relative frequency of the double-band phenotype increased. The relative frequency of the triple-band phenotype was highest in the group with high Lp (a) concentrations. No correlation was evident between the size of the apo (a) isoforms and the Lp (a) concentrations. Conclusions. Raised plasma Lp (a) levels have been associated with coronary heart disease (CHD). In addition, it has been proposed that the apo (a) gene determined plasma Lp (a) concentrations. These studies were performed using plasma from white subjects. CHD is uncommon in black South Africans. The reason may be that, given the lack of relationship between the size of the apo (a) isoforms and the Lp (a) concentrations observed in the present study, factors other than the isoform size may determine the Lp (a) levels in this particular ethnic group.Publisher’s versio
Cosmology and two-body problem of D-branes
In this paper, we investigate the dynamics and the evolution of the scale
factor of a probe Dp-brane which move in the background of source Dp-branes.
Action of the probe brane is described by the Born-Infeld action and the
interaction with the background R-R field. When the probe brane moves away from
the source branes, it expands by power law, whose index depends on the
dimension of the brane. If the energy density of the gauge field on the brane
is subdominant, the expansion is decelerating irrespective of the dimension of
the brane. On the other hand, when the probe brane is a Nambu-Goto brane, the
energy density of the gauge field can be dominant, in which case accelerating
expansion occurs for . The accelerating expansion stops when the
brane has expanded sufficiently so that the energy density of the gauge field
become subdominant.Comment: 6 pages, 7 figures, reference added, accepted for publication in PR
LP (a) levels and apo (a) phenotypes in urban black South African men
Objective. To investigate the lipoprotein (a) (Lp (a)) levels and apolipoprotein (a) (apo (a)) phenotypes in a group of urban black South African men.Design. Cross-sectional design.Setting. Lead acid battery plant, East London, Eastern Cape.Participants. Blood samples from a study on the association between lead and renal failure were kindly donated for the present study and 111 of the donors participated (K Steyn - personal communication).Outcome measures. Lp (a) levels and apo (a) phenotypes.Results. Three groups were identified: those with normal (<300 U/I), intermediate (300 - 700 U/I) and high (> 700 U/I) plasma Lp (a) concentrations. Nine apo (a) phenotypes and 26 combinations thereof could be discerned. Apart from the single- and double-band phenotypes described before, triple-band phenotypes were also present. As the Lp (a) values increased, the relative frequency of the single-band phenotype decreased, whereas the relative frequency of the double-band phenotype increased. The relative frequency of the triple-band phenotype was highest in the group with high Lp (a) concentrations. No correlation was evident between the size of the apo (a) isoforms and the Lp (a) concentrations.Conclusions. Raised plasma Lp (a) levels have been associated with coronary heart disease (CHD). In addition, it has been proposed that the apo (a) gene determined plasma Lp (a) concentrations. These studies were performed using plasma from white subjects. CHD is uncommon in black South Africans. The reason may be that, given the lack of relationship between the size of the apo (a) isoforms and the Lp (a) concentrations observed in the present study, factors other than the isoform size may determine the Lp (a) levels in this particular ethnic group
The hierarchy problem, radion mass, localization of gravity and 4D effective Newtonian potential in string theory on
We present a systematical study of brane worlds in string theory on
. Starting with the toroidal compactification of the NS/NS sector
in (D+d) dimensions, we first obtain an effective -dimensional action, and
then compactify one of the spatial dimensions by introducing two
orbifold branes as its boundaries. By combining the Gauss-Codacci and Lanczos
equations, we write down explicitly the general gravitational field equations
on each of the two branes, while using distribution theory we express the
matter field equations on the branes in terms of the discontinuities of the
first derivatives of the matter fields. Afterwards, we address three important
issues: (i) the hierarchy problem; (ii) the radion mass; and (iii) the
localization of gravity, the 4-dimensional Newtonian effective potential and
the Yukawa corrections due to the gravitational high-order Kaluza-Klein (KK)
modes. With a very conservative estimation, we find that the radion mass is of
the order of . The gravity is localized on the visible brane, and
the spectrum of the gravitational KK modes is discrete and can be of the order
of TeV. The corrections to the 4-dimensional Newtonian potential from the
higher order of gravitational KK modes are exponentially suppressed and can be
safely neglected in current experiments. In an appendix, we also present a
systematical and pedagogical study of the Gauss-Codacci equations and Israel's
junction conditions across a (D-1)-dimensional hypersurface, which can be
either spacelike or timelike.Comment: Considerably extended, Revtex4, 19 pages, 5 figures, published in
IJMPA, 25, 1661-1698 (2010
Zeta Determinant for Laplace Operators on Riemann Caps
The goal of this paper is to compute the zeta function determinant for the
massive Laplacian on Riemann caps (or spherical suspensions). These manifolds
are defined as compact and boundaryless dimensional manifolds deformed by a
singular Riemannian structure. The deformed spheres, considered previously in
the literature, belong to this class. After presenting the geometry and
discussing the spectrum of the Laplacian, we illustrate a method to compute its
zeta regularized determinant. The special case of the deformed sphere is
recovered as a limit of our general formulas.Comment: 19 pages, 1 figur
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