1,419 research outputs found
Resolution of a low-lying placenta and placenta previa diagnosed at the midtrimester anatomy scan
Konjunkturprognosen und wissenschaftlich-technischer Fortschritt
Wissenschaftliche Prognosen unterscheiden sich von sonstigen Vorhersagen nicht zuletzt dadurch, daß mit ihnen der Anspruch auf systematische Verbesserbarkeit verknüpft wird. Hat sich die Treffsicherheit von Konjunkturprognosen als Folge des wissenschaftlich-technischen Fortschritts verbessert? --
Counting statistics of collective photon transmissions
We theoretically study cooperative effects in the steady-state transmission
of photons through a medium of radiators. Using methods from quantum
transport, we find a cross-over in scaling from to in the current and
even higher powers of in the higher cumulants of the photon counting
statistics as a function of the tunable source occupation. The effect should be
observable for atoms confined within a nano-cell with a pumped optical cavity
as photon source.Comment: extended results, 9 pages, 2 figures, to appear in Annals of Physic
The flatness problem and
By way of a complete integration of the Friedmann equations, in terms of
observables, it is shown that for the cosmological constant there
exist non-flat FLRW models for which the total density parameter
remains throughout the entire history of the universe. Further, it is
shown that in a precise quantitative sense these models are not finely tuned.
When observations are brought to bear on the theory, and in particular the WMAP
observations, they confirm that we live in just such a universe. The conclusion
holds when the classical notion of is extended to dark energy.Comment: Final form to appear in Physical Review Letters. Further information
at http://grtensor.org/Robertson
The Polarization of the Cosmic Microwave Background Due to Primordial Gravitational Waves
We review current observational constraints on the polarization of the Cosmic
Microwave Background (CMB), with a particular emphasis on detecting the
signature of primordial gravitational waves. We present an analytic solution to
the Polanarev approximation for CMB polarization produced by primordial
gravitational waves. This simplifies the calculation of the curl, or B-mode
power spectrum associated with gravitational waves during the epoch of
cosmological inflation. We compare our analytic method to existing numerical
methods and also make predictions for the sensitivity of upcoming CMB
polarization observations to the inflationary gravitational wave background. We
show that upcoming experiments should be able either detect the relic
gravitational wave background or completely rule out whole classes of
inflationary models.Comment: 25 pages, 4 figures, review published in IJMP
Theory of superradiant scattering of laser light from Bose-Einstein condensates
In a recent MIT experiment, a new form of superradiant Rayleigh scattering
was observed in Bose-Einstein condensates. We present a detailed theory of this
phenomena in which the directional dependence of the scattering rate and
condensate depletion lead to mode competition which is ultimately responsible
for superradiance. The nonlinear response of the system is highly sensitive to
initial quantum fluctuations which cause large run to run variations in the
observed superradiant pulses.Comment: Updated version with new figures,a numerical simulation with
realistic experimental parameters is now included. Featured in September 1999
Physics Today, in Search and Discovery sectio
Optically controlled spin-glasses in multi-qubit cavity systems
Recent advances in nanostructure fabrication and optical control, suggest
that it will soon be possible to prepare collections of interacting two-level
systems (i.e. qubits) within an optical cavity. Here we show theoretically that
such systems could exhibit novel phase transition phenomena involving
spin-glass phases. By contrast with traditional realizations using magnetic
solids, these phase transition phenomena are associated with both matter and
radiation subsystems. Moreover the various phase transitions should be tunable
simply by varying the matter-radiation coupling strength.Comment: 4 pages, 3 figure
Generation of scalar-tensor gravity effects in equilibrium state boson stars
Boson stars in zero-, one-, and two-node equilibrium states are modeled
numerically within the framework of Scalar-Tensor Gravity. The complex scalar
field is taken to be both massive and self-interacting. Configurations are
formed in the case of a linear gravitational scalar coupling (the Brans-Dicke
case) and a quadratic coupling which has been used previously in a cosmological
context. The coupling parameters and asymptotic value for the gravitational
scalar field are chosen so that the known observational constraints on
Scalar-Tensor Gravity are satisfied. It is found that the constraints are so
restrictive that the field equations of General Relativity and Scalar-Tensor
gravity yield virtually identical solutions. We then use catastrophe theory to
determine the dynamically stable configurations. It is found that the maximum
mass allowed for a stable state in Scalar-Tensor gravity in the present
cosmological era is essentially unchanged from that of General Relativity. We
also construct boson star configurations appropriate to earlier cosmological
eras and find that the maximum mass for stable states is smaller than that
predicted by General Relativity, and the more so for earlier eras. However, our
results also show that if the cosmological era is early enough then only states
with positive binding energy can be constructed.Comment: 20 pages, RevTeX, 11 figures, to appear in Class. Quantum Grav.,
comments added, refs update
Coherent states and global entanglement in an N qubit system
We consider an qubit system and show that in the symmetric subspace,
a state is not globally entangled, iff it is a coherent state. It
is also proven that in the orthogonal complement all states
are globally entangled
Singularity Free (Homogeneous Isotropic) Universe in Graviton-Dilaton Models
We present a class of graviton-dilaton models in which a homogeneous
isotropic universe, such as our observed one, evolves with no singularity at
any time. Such models may stand on their own as interesting models for
singularity free cosmology, and may be studied further accordingly. They may
also arise from string theory. We discuss critically a few such possibilities.Comment: 11 pages. Latex file. Revised in response to referees' Comments.
Results remain same. To appear in Phys. Rev. Let
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