33,166 research outputs found
Quantum Decoherence in a D-Foam Background
Within the general framework of Liouville string theory, we construct a model
for quantum D-brane fluctuations in the space-time background through which
light closed-string states propagate. The model is based on monopole and vortex
defects on the world sheet, which have been discussed previously in a treatment
of 1+1-dimensional black-hole fluctuations in the space-time background, and
makes use of a T-duality transformation to relate formulations with Neumann and
Dirichlet boundary conditions. In accordance with previous general arguments,
we derive an open quantum-mechanical description of this D-brane foam which
embodies momentum and energy conservation and small mean energy fluctuations.
Quantum decoherence effects appear at a rate consistent with previous
estimates.Comment: 16 pages, Latex, two eps figures include
Time-Dependent Vacuum Energy Induced by D-Particle Recoil
We consider cosmology in the framework of a `material reference system' of D
particles, including the effects of quantum recoil induced by closed-string
probe particles. We find a time-dependent contribution to the cosmological
vacuum energy, which relaxes to zero as for large times . If
this energy density is dominant, the Universe expands with a scale factor . We show that this possibility is compatible with recent
observational constraints from high-redshift supernovae, and may also respect
other phenomenological bounds on time variation in the vacuum energy imposed by
early cosmology.Comment: 14 pages LATEX, no figure
M Theory from World-Sheet Defects in Liouville String
We have argued previously that black holes may be represented in a D-brane
approach by monopole and vortex defects in a sine-Gordon field theory model of
Liouville dynamics on the world sheet. Supersymmetrizing this sine-Gordon
system, we find critical behaviour in 11 dimensions, due to defect condensation
that is the world-sheet analogue of D-brane condensation around an extra
space-time dimension in M theory. This supersymmetric description of Liouville
dynamics has a natural embedding within a 12-dimensional framework suggestive
of F theory.Comment: 17 pages LATEX, 1 epsf figure include
The Children of Spring Street: Rickets in an Early Nineteenth-Century Congregation
This paper examines the prevalence of rickets, or vitamin D deficiency, in the subadult skeletal remains from the burial vaults of the Spring Street Presbyterian Church of New York City. The burial vaults of the church were active from approximately 1820â1846 and contain the remains of at least 86 subadults (minimum number of individuals count [MNI] of left tibiae). Over 34% of the subadult tibiae in this collection display pathology consistent with vitamin D deficiency. Since vitamin D is acquired through access to sunlight and specific foods, a high rate of rickets can give clues about living conditions, parenting strategies, and childrenâs behavior in this population. The urbanizing landscape of early 19th-century New York City and the associated cultural changes make this an interesting case study for exploring the relationship between biology and the environment in childrenâs lives
Matter sources for a Null Big Bang
We consider the properties of stress-energy tensors compatible with a Null
Big Bang, i.e., cosmological evolution starting from a Killing horizon rather
than a singularity. For Kantowski-Sachs cosmologies, it is shown that if matter
satisfies the Null Energy Condition (NEC), then (i) regular cosmological
evolution can only start from a Killing horizon, (ii) matter is absent at the
horizon, and (iii) matter can only appear in the cosmological region due to
interaction with vacuum. The latter is understood phenomenologically as a fluid
whose stress tensor is insensitive to boosts in a particular direction. We also
argue that matter is absent in a static region beyond the horizon. All this
generalizes the observations recently obtained for a mixture of dust and a
vacuum fluid. If, however, we admit the existence of phantom matter, its
certain special kinds (with the parameter ) are consistent with a
Null Big Bang without interaction with vacuum (or without vacuum fluid at all).
Then in the static region there is matter with . Alternatively, the
evolution can begin from a horizon in an infinitely remote past, leading to a
scenario combining the features of a Null Big Bang and an emergent universe.Comment: 5 two-column pages, revtex4, no figures. One reference corrected.
Final version accepted for publication in Class. Quantum Gra
Electoral system design: The new international IDEA handbook
The Choice of Electoral System is one of the most important institutional decisions for any democracy. In almost all cases the choice of a particular electoral system has a profound effect on the future political life of the country concerned, and electoral systems, once chosen, often remain fairly constant as political interests solidify around and respond to the incentives presented by them. However, while conscious design has become far more prevalent recently, traditionally it has been rare for electoral systems to be consciously and deliberately selected. Often the choice was essentially accidental, the result of an unusual combination of circumstances, of a passing trend, or of a quirk of history, with the impact of colonialism and the effects of influential neighbours often being especially strong
Dark Matter in SuperGUT Unification Models
After a brief update on the prospects for dark matter in the constrained
version of the MSSM (CMSSM) and its differences with models based on minimal
supergravity (mSUGRA), I will consider the effects of unifying the
supersymmetry-breaking parameters at a scale above M_{GUT}. One of the
consequences of superGUT unification, is the ability to take vanishing scalar
masses at the unification scale with a neutralino LSP dark matter candidate.
This allows one to resurrect no-scale supergravity as a viable phenomenological
model.Comment: 12 pages, 16 figures, To be published in the Proceedings of the 6th
DSU Conference, Leon, Mexico, ed. D. Delepin
Curvature blow up in Bianchi VIII and IX vacuum spacetimes
The maximal globally hyperbolic development of non-Taub-NUT Bianchi IX vacuum
initial data and of non-NUT Bianchi VIII vacuum initial data is C2
inextendible. Furthermore, a curvature invariant is unbounded in the incomplete
directions of inextendible causal geodesics.Comment: 20 pages, no figures. Submitted to Classical and Quantum Gravit
Direct Detection of Dark Matter in the MSSM with Non-Universal Higgs Masses
We calculate dark matter scattering rates in the minimal supersymmetric
extension of the Standard Model (MSSM), allowing the soft
supersymmetry-breaking masses of the Higgs multiplets, m_{1,2}, to be
non-universal (NUHM). Compared with the constrained MSSM (CMSSM) in which
m_{1,2} are required to be equal to the soft supersymmetry-breaking masses m_0
of the squark and slepton masses, we find that the elastic scattering cross
sections may be up to two orders of magnitude larger than values in the CMSSM
for similar LSP masses. We find the following preferred ranges for the
spin-independent cross section: 10^{-6} pb \ga \sigma_{SI} \ga 10^{-10} pb, and
for the spin-dependent cross section: 10^{-3} pb \ga \sigma_{SD}, with the
lower bound on \sigma_{SI} dependent on using the putative constraint from the
muon anomalous magnetic moment. We stress the importance of incorporating
accelerator and dark matter constraints in restricting the NUHM parameter
space, and also of requiring that no undesirable vacuum appear below the GUT
scale. In particular, values of the spin-independent cross section another
order of magnitude larger would appear to be allowed, for small \tan \beta, if
the GUT vacuum stability requirement were relaxed, and much lower cross-section
values would be permitted if the muon anomalous magnetic moment constraint were
dropped.Comment: 30 pages LaTeX, 40 eps figure
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