278 research outputs found
Brane Localization and Stabilization via Regional Physics
Extra-dimensional scenarios have become widespread among particle and
gravitational theories of physics to address several outstanding problems,
including cosmic acceleration, the weak hierarchy problem, and the quantization
of gravity. In general, the topology and geometry of the full spacetime
manifold will be non-trivial, even if our ordinary dimensions have the topology
of their covering space. Most compact manifolds are inhomogeneous, even if they
admit a homogeneous geometry, and it will be physically relevant where in the
extra-dimensions one is located. In this letter, we explore the use of both
local and global effects in a braneworld scenario to naturally provide
position-dependent forces that determine and stabilize the location of a single
brane. For illustrative purposes, we consider the 2-dimensional hyperbolic horn
and the Euclidean cone as toy models of the extra-dimensional manifold, and add
a brane wrapped around one of the two spatial dimensions. We calculate the
total energy due to brane tension and bending (extrinsic curvature) as well as
that due to the Casimir energy of a bulk scalar satisfying a Dirchlet boundary
condition on the brane. From the competition of at least two of these effects
there can exist a stable minimum of the effective potential for the brane
location. However, on more generic spaces (on which more symmetries are broken)
any one of these effects may be sufficient to stabilize the brane. We discuss
this as an example of physics that is neither local nor global, but regional.Comment: 4 pages, 2 figures. PRL submitte
Flavordynamics with Conformal Matter and Gauge Theories on Compact Hyperbolic Manifolds in Extra Dimensions
We outline a toy model in which a unique mechanism may trigger a dynamical
chain resulting in key low-energy regularities. The starting points are a
negative cosmological term in the bulk and conformally invariant nongravity
sector. These elements ensure compactification of the extra dimensional space
on a compact hyperbolic manifold (with the negative and constant scalar
curvature). The overall geometry is then M_4 x B_n. The negative curvature on
B_n triggers the formation of the four-dimensional defect which provides in
turn a dynamical localization of ordinary particles. It also leads,
simultaneously, to a spontaneous breaking of gauge symmetry through a Higgs
mechanism. Masses of the fermions, gauge bosons and scalars all derive from the
curvature of the internal manifold such that the Higgs boson is generally
heavier than the gauge bosons. The factorizable geometry M_4 x B_n and flatness
of M_4 require fine-tuning.Comment: 16 pp, added references and a figure with improvements in text;
journal versio
Interaction of a brane with a moving bulk black hole
We study the interaction of an n-dimensional topological defect (n-brane)
described by the Nambu-Goto action with a higher-dimensional Schwarzschild
black hole moving in the bulk spacetime. We derive the general form of the
perturbation equations for an n-brane in the weak field approximation and solve
them analytically in the most interesting cases. We specially analyze
applications to brane world models. We calculate the induced geometry on the
brane generated by a moving black hole. From the point of view of a brane
observer, this geometry can be obtained by solving (n+1)-dimensional Einstein's
equations with a non-vanishing right hand side. We calculate the effective
stress-energy tensor corresponding to this `shadow-matter'. We explicitly show
that there exist regions on the brane where a brane observer sees an apparent
violation of energy conditions. We also study the deflection of light
propagating in the region of influence of this `shadow matter'.Comment: version accepted for publication in Phys. Rev.
Radion Stabilization in Compact Hyperbolic Extra Dimensions
We consider radion stabilization in hyperbolic brane-world scenarios. We
demonstrate that in the context of Einstein gravity, matter fields which
stabilize the extra dimensions must violate the null energy condition. This
result is shown to hold even allowing for FRW-like expansion on the brane. In
particular, we explicitly demonstrate how one putative source of stabilizing
matter fails to work, and how others violate the above condition. We speculate
on a number of ways in which we may bypass this result, including the effect of
Casimir energy in these spaces. A brief discussion of supersymmetry in these
backgrounds is also given.Comment: 16 pages, 1 figur
Statistical isotropy of the Cosmic Microwave Background
The breakdown of statistical homogeneity and isotropy of cosmic perturbations
is a generic feature of ultra large scale structure of the cosmos, in
particular, of non trivial cosmic topology. The statistical isotropy (SI) of
the Cosmic Microwave Background temperature fluctuations (CMB anisotropy) is
sensitive to this breakdown on the largest scales comparable to, and even
beyond the cosmic horizon. We propose a set of measures,
() which for non-zero values indicate and quantify statistical
isotropy violations in a CMB map. We numerically compute the predicted
spectra for CMB anisotropy in flat torus universe models.
Characteristic signature of different models in the spectrum are
noted.Comment: Presented at PASCOS'03, January 3-8, 2003, in TIFR, Mumbai; to be
published in a special issue of 'Pramana' (4 pages, 1 figure, style files
included
Neutrino-Lasing in The Early Universe
Recently, Madsen has argued that relativistic decays of massive neutrinos
into lighter fermions and bosons may lead, via thermalization, to the formation
of a Bose condensate. If correct, this could generate mixed hot and cold dark
matter, with important consequences for structure formation.
From a detailed study of such decays, we arrive at substantially different
conclusions; for a wide range of masses and decay times, we find that
stimulated emission of bosons dominates the decay. This phenomenon can best be
described as a neutrino laser, pumped by the QCD phase transition. We discuss
the implications for structure formation and the dark-matter problem.Comment: 7 pages, 3 figures included as uuencoded file, CITA/93/
A New Technique for Detecting Supersymmetric Dark Matter
We estimate the event rate for excitation of atomic transition by
photino-like dark matter. For excitations of several eV, this event rate can
exceed naive cross-section by many orders of magnitude. Although the event rate
for these atomic excitation is smaller than that of nuclear recoil off of
non-zero spin nuclei, the photons emitted by the deexcitation are easier to
detect than low-energy nuclear recoils. For many elements, there are several
low-lying states with comparable excitation rates, thus, spectral ratios could
be used to distinguish signal from background.Comment: 6 pages plain te
Cushing's syndrome after treatment: Changes in cortisol and ACTH levels, and amelioration of the depressive syndrome
Twenty-three patients with pituitary adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome were studied before and after treatment. The relationship between the amelioration of the depressive syndrome and changes in cortisol and ACTH levels was investigated. There was a significant difference in mean change in 24-hour urinary free cortisol (UFC) excretion for changes in the depressed mood score from first to last visit. There were also significant correlations between decreases in UFC and decreases in both the depressed mood score and the modified Hamilton depression score. These relationships were not found for ACTH. Furthermore, with cortisol decreased to normal levels, continued high ACTH levels did not prevent improvement in depressed mood. The possibility that cortisol may also play a role in the pathogenesis and/or maintenance of the mood disorder in psychiatric patients is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25987/1/0000053.pd
Topology of the Universe: background and recent observational approaches
Is the Universe (a spatial section thereof) finite or infinite? Knowing the
global geometry of a Friedmann-Lema\^{\i}tre (FL) universe requires knowing
both its curvature and its topology. A flat or hyperbolic (``open'') FL
universe is {\em not} necessarily infinite in volume.
Multiply connected flat and hyperbolic models are, in general, as consistent
with present observations on scales of 1-20{\hGpc} as are the corresponding
simply connected flat and hyperbolic models. The methods of detecting multiply
connected models (MCM's) are presently in their pioneering phase of development
and the optimal observationally realistic strategy is probably yet to be
calculated. Constraints against MCM's on ~1-4 h^{-1} Gpc scales have been
claimed, but relate more to inconsistent assumptions on perturbation statistics
rather than just to topology. Candidate 3-manifolds based on hypothesised
multiply imaged objects are being offered for observational refutation.
The theoretical and observational sides of this rapidly developing subject
have yet to make any serious contact, but the prospects of a significant
detection in the coming decade may well propel the two together.Comment: 5 pages, proceedings of the Workshop ``Cosmology: Observations
Confront Theories,'' 11-17 Jan 1999, IIT Kharagpur, West Bengal, to appear in
Pramana - Journal of Physic
Exact Polynomial Eigenmodes for Homogeneous Spherical 3-Manifolds
Observational data hints at a finite universe, with spherical manifolds such
as the Poincare dodecahedral space tentatively providing the best fit.
Simulating the physics of a model universe requires knowing the eigenmodes of
the Laplace operator on the space. The present article provides explicit
polynomial eigenmodes for all globally homogeneous 3-manifolds: the Poincare
dodecahedral space S3/I*, the binary octahedral space S3/O*, the binary
tetrahedral space S3/T*, the prism manifolds S3/D_m* and the lens spaces
L(p,1).Comment: v3. Final published version. 27 pages, 1 figur
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