987 research outputs found
Neutrino Oscillations Induced by Gravitational Recoil Effects
Quantum gravitational fluctuations of the space-time background, described by
virtual D branes, may induce neutrino oscillations if a tiny violation of the
Lorentz invariance (or a violation of the equivalence principle) is imposed. In
this framework, the oscillation length of massless neutrinos turns out to be
proportional to M/E^2, where E is the neutrino energy and M is the mass scale
characterizing the topological fluctuations in the vacuum. Such a functional
dependence on the energy is the same obtained in the framework of loop quantum
gravity.Comment: 5 pages, LaTex fil
Developmental trends in voice onset time: some evidence for sex differences
This study reports on an investigation into the voice onset time (VOT) patterns of the plosives /p b t d/ in a group of 30 children aged 7 (n = 10), 9 (n = 10) and 11 (n = 10) years. Equal numbers of girls and boys participated in the study. Each child named a series of letter objects to elicit /p b t d/ in a syllable onset position with a fixed vowel context. VOT data were examined for age, sex and plosive differences with the following hypotheses: Firstly, that there would be sex differences in the VOT patterns of preadolescent children. Secondly, that the sex differences in VOT patterns would be linked to age and development, and that these would eventually become marked by the age of 11 years, by which time adult-like VOT values should have been achieved. Finally, that the extent of sex and age differences would be dependent upon the plosive being investigated. Results indicated patterns of decrease with age in the VOT values of /p b/ for the boys, with some evidence of increases in the VOT values of /t/ for the girls. In addition, 'voiced' and 'voiceless' cognates showed a more marked bimodal distribution in the girls' VOT patterns. This bimodal distribution was investigated by examining the degree of difference between the VOT values of voiced and voiceless cognate pairs /p b/ and /t d/, and examining the effects of age, sex and cognate pair. These results indicated that more marked sex differences in the 'voiced'/'voiceless' contrast emerged between the data of the 9- and 11-year-olds, a pattern, which was more marked for the alveolar plosives. These preliminary results confirmed all three hypotheses. The findings are presented and discussed both within a developmental and sociophonetic framework
Dynamics of Inflationary Universes with Positive Spatial Curvature
If the spatial curvature of the universe is positive, then the curvature term
will always dominate at early enough times in a slow-rolling inflationary
epoch. This enhances inflationary effects and hence puts limits on the possible
number of e-foldings that can have occurred, independently of what happened
before inflation began and in particular without regard for what may have
happened in the Planck era. We use a simple multi-stage model to examine this
limit as a function of the present density parameter and the epoch
when inflation ends.Comment: 9 Pages RevTex4. Revised and update
Dynamical Formation of Horizons in Recoiling D Branes
A toy calculation of string/D-particle interactions within a world-sheet
approach indicates that quantum recoil effects - reflecting the gravitational
back-reaction on space-time foam due to the propagation of energetic particles
- induces the appearance of a microscopic event horizon, or `bubble', inside
which stable matter can exist. The scattering event causes this horizon to
expand, but we expect quantum effects to cause it to contract again, in a
`bounce' solution. Within such `bubbles', massless matter propagates with an
effective velocity that is less than the velocity of light in vacuo, which may
lead to observable violations of Lorentz symmetry that may be tested
experimentally. The conformal invariance conditions in the interior geometry of
the bubbles select preferentially three for the number of the spatial
dimensions, corresponding to a consistent formulation of the interaction of D3
branes with recoiling D particles, which are allowed to fluctuate independently
only on the D3-brane hypersurface.Comment: 25 pages LaTeX, 4 eps figures include
Physics in the Real Universe: Time and Spacetime
The Block Universe idea, representing spacetime as a fixed whole, suggests
the flow of time is an illusion: the entire universe just is, with no special
meaning attached to the present time. This view is however based on
time-reversible microphysical laws and does not represent macro-physical
behaviour and the development of emergent complex systems, including life,
which do indeed exist in the real universe. When these are taken into account,
the unchanging block universe view of spacetime is best replaced by an evolving
block universe which extends as time evolves, with the potential of the future
continually becoming the certainty of the past. However this time evolution is
not related to any preferred surfaces in spacetime; rather it is associated
with the evolution of proper time along families of world linesComment: 28 pages, including 9 Figures. Major revision in response to referee
comment
Thermal properties of spacetime foam
Spacetime foam can be modeled in terms of nonlocal effective interactions in
a classical nonfluctuating background. Then, the density matrix for the
low-energy fields evolves, in the weak-coupling approximation, according to a
master equation that contains a diffusion term. Furthermore, it is argued that
spacetime foam behaves as a quantum thermal field that, apart from inducing
loss of coherence, gives rise to effects such as gravitational Lamb and Stark
shifts as well as quantum damping in the evolution of the low-energy
observables. These effects can be, at least in principle, experimentally
tested.Comment: RevTeX 3.01, 11 pages, no figure
Loop Quantum Cosmology II: Volume Operators
Volume operators measuring the total volume of space in a loop quantum theory
of cosmological models are constructed. In the case of models with rotational
symmetry an investigation of the Higgs constraint imposed on the reduced
connection variables is necessary, a complete solution of which is given for
isotropic models; in this case the volume spectrum can be calculated
explicitly. It is observed that the stronger the symmetry conditions are the
smaller is the volume spectrum, which can be interpreted as level splitting due
to broken symmetries. Some implications for quantum cosmology are presented.Comment: 21 page
Testing A (Stringy) Model of Quantum Gravity
I discuss a specific model of space-time foam, inspired by the modern
non-perturbative approach to string theory (D-branes). The model views our
world as a three brane, intersecting with D-particles that represent stringy
quantum gravity effects, which can be real or virtual. In this picture, matter
is represented generically by (closed or open) strings on the D3 brane
propagating in such a background. Scattering of the (matter) strings off the
D-particles causes recoil of the latter, which in turn results in a distortion
of the surrounding space-time fluid and the formation of (microscopic, i.e.
Planckian size) horizons around the defects. As a mean-field result, the
dispersion relation of the various particle excitations is modified, leading to
non-trivial optical properties of the space time, for instance a non-trivial
refractive index for the case of photons or other massless probes. Such models
make falsifiable predictions, that may be tested experimentally in the
foreseeable future. I describe a few such tests, ranging from observations of
light from distant gamma-ray-bursters and ultra high energy cosmic rays, to
tests using gravity-wave interferometric devices and terrestrial particle
physics experients involving, for instance, neutral kaons.Comment: 25 pages LATEX, four figures incorporated, uses special proceedings
style. Invited talk at the third international conference on Dark Matter in
Astro and Particle Physics, DARK2000, Heidelberg, Germany, July 10-15 200
Stringy Space-Time Foam and High-Energy Cosmic Photons
In this review, I discuss briefly stringent tests of Lorentz-violating
quantum space-time foam models inspired from String/Brane theories, provided by
studies of high energy Photons from intense celestial sources, such as Active
Galactic Nuclei or Gamma Ray Bursts. The theoretical models predict
modifications to the radiation dispersion relations, which are quadratically
suppressed by the string mass scale, and time delays in the arrival times of
photons (assumed to be emitted more or less simultaneously from the source),
which are proportional to the photon energy, so that the more energetic photons
arrive later. Although the astrophysics at the source of these energetic
photons is still not understood, and such non simultaneous arrivals, that have
been observed recently, might well be due to non simultaneous emission as a
result of conventional physics effects, nevertheless, rather surprisingly, the
observed time delays can also fit excellently the stringy space-time foam
scenarios, provided the space-time defect foam is inhomogeneous. The key
features of the model, that allow it to evade a plethora of astrophysical
constraints on Lorentz violation, in sharp contrast to other field-theoretic
Lorentz-violating models of quantum gravity, are: (i) transparency of the foam
to electrons and in general charged matter, (ii) absence of birefringence
effects and (iii) a breakdown of the local effective lagrangian formalism.Comment: 26 pages Latex, 4 figures, uses special macros. Keynote Lecture in
the International Conference "Recent Developments in Gravity" (NEB14),
Ioannina (Greece) June 8-11 201
Loop Quantum Cosmology I: Kinematics
The framework of quantum symmetry reduction is applied to loop quantum
gravity with respect to transitively acting symmetry groups. This allows to
test loop quantum gravity in a large class of minisuperspaces and to
investigate its features - e.g. the discrete volume spectrum - in certain
cosmological regimes. Contrary to previous studies of quantum cosmology
(minisuperspace quantizations) the symmetry reduction is carried out not at the
classical level but on an auxiliary Hilbert space of the quantum theory before
solving the constraints. Therefore, kinematical properties like volume
quantization survive the symmetry reduction. In this first part the kinematical
framework, i.e. implementation of the quantum symmetry reduction and
quantization of Gauss and diffeomorphism constraints, is presented for Bianchi
class A models as well as locally rotationally symmetric and spatially
isotropic closed and flat models.Comment: 24 page
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