18,721 research outputs found
Stationary Points of Scalar Fields Coupled to Gravity
We investigate the dynamics of gravity coupled to a scalar field using a
non-canonical form of the kinetic term. It is shown that its singular point
represents an attractor for classical solutions and the stationary value of the
field may occur distant from the minimum of the potential. In this paper
properties of universes with such stationary states are considered. We reveal
that such state can be responsible for modern dark energy density.Comment: H. Kroger, invited talk, FFP6, Udine (2004), revised version with
corrected author lis
Shock-Wave Heating Model for Chondrule Formation: Prevention of Isotopic Fractionation
Chondrules are considered to have much information on dust particles and
processes in the solar nebula. It is naturally expected that protoplanetary
disks observed in present star forming regions have similar dust particles and
processes, so study of chondrule formation may provide us great information on
the formation of the planetary systems.
Evaporation during chondrule melting may have resulted in depletion of
volatile elements in chondrules. However, no evidence for a large degree of
heavy-isotope enrichment has been reported in chondrules. In order to meet this
observed constraint, the rapid heating rate at temperatures below the silicate
solidus is required to suppress the isotopic fractionation.
We have developed a new shock-wave heating model taking into account the
radiative transfer of the dust thermal continuum emission and the line emission
of gas molecules and calculated the thermal history of chondrules. We have
found that optically-thin shock waves for the thermal continuum emission from
dust particles can meet the rapid heating constraint, because the dust thermal
emission does not keep the dust particles high temperature for a long time in
the pre-shock region and dust particles are abruptly heated by the gas drag
heating in the post-shock region. We have also derived the upper limit of
optical depth of the pre-shock region using the radiative diffusion
approximation, above which the rapid heating constraint is not satisfied. It is
about 1 - 10.Comment: 58 pages, including 5 tables and 15 figures, accepted for publication
in The Astrophysical Journa
Ghost Interference with Optical Parametric Amplifier
The 'Ghost' interference experiment is analyzed when the source of entangled
photons is a multimode Optical Parametric Amplifier(OPA) whose weak limit is
the two-photon Spontaneous Parametric Downconversion(SPDC) beam. The visibility
of the double-slit pattern is calculated, taking the finite coincidence time
window of the photon counting detectors into account. It is found that the
coincidence window and the bandwidth of light reaching the detectors play a
crucial role in the loss of visibility on coincidence detection, not only in
the 'Ghost' interference experiment but in all experiments involving
coincidence detection. The differences between the loss of visibility with
two-mode and multimode OPA sources is also discussed.
PACS: 42.65.Yj, 42.50.Dv, 42.65.L
Open Questions in Classical Gravity
We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.Comment: UCONN-93-1, plain TeX format, 22 pages (plus 7 figures - send
requests to [email protected]). To appear in a special issue of
Foundations of Physics honoring Professor Fritz Rohrlich on the occasion of
his retirement, L. P. Horwitz and A. van der Merwe Editors, Plenum Publishing
Company, N.Y., Fall 199
Two-Photon Beatings Using Biphotons Generated from a Two-Level System
We propose a two-photon beating experiment based upon biphotons generated
from a resonant pumping two-level system operating in a backward geometry. On
the one hand, the linear optical-response leads biphotons produced from two
sidebands in the Mollow triplet to propagate with tunable refractive indices,
while the central-component propagates with unity refractive index. The
relative phase difference due to different refractive indices is analogous to
the pathway-length difference between long-long and short-short in the original
Franson interferometer. By subtracting the linear Rayleigh scattering of the
pump, the visibility in the center part of the two-photon beating interference
can be ideally manipulated among [0, 100%] by varying the pump power, the
material length, and the atomic density, which indicates a Bell-type inequality
violation. On the other hand, the proposed experiment may be an interesting way
of probing the quantum nature of the detection process. The interference will
disappear when the separation of the Mollow peaks approaches the fundamental
timescales for photon absorption in the detector.Comment: to appear in Phys. Rev. A (2008
Coaching Teachers of English Language Learners
The following qualitative study examined how Reading First Literacy Coaches refined their literacy coaching to meet the cultural and linguistic needs of Hispanic English language learners (ELLs) in 30 elementary schools located along the US Mexico Border. Data were gathered from the coaches through written surveys and a focus group. Findings from the coaches’ practices identified three themes: 1) Coaches understood bilingual programs and the theory underlying such instruction; 2) Coaches supported teachers of ELLs by sharing their knowledge and experiences about ELLs; and 3) Coaches faced challenges in meeting the needs of teachers of Hispanic ELLs. This study is an addition to the literature that describes and contextualizes the work of instructional coaches. It has practical implications for schools seeking to build the capacity of teachers of ELLs. Guidance is suggested related to hiring coaches with special dispositions and the professional development of existing coaches
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