78,875 research outputs found
A primer on the mortgage market and mortgage finance
This article is a primer on mortgage finance. It discusses the basics of the mortgage market and mortgage finance. In so doing, it provides useful information that can aid individuals in making better mortgage finance decisions. The discussion and the tools are presented within the context of mortgage finance; however, these same principles and tools can be applied to a wide range of financial decisions.Mortgages
Gravitational lensing by elliptical galaxies
The fraction of high-redshift sources which are multiply-imaged by
intervening galaxies is strongly dependent on the cosmological constant, and so
can be a useful probe of the cosmological model. However its power is limited
by various systematic (and random) uncertainties in the calculation of lensing
probabilities, one of the most important of which is the dynamical
normalisation of elliptical galaxies. Assuming ellipticals' mass distributions
can be modelled as isothermal spheres, the mass normalisation depends on: the
velocity anisotropy; the luminosity density; the core radius; and the area over
which the velocity dispersion is measured. The differences in the lensing
probability and optical depth produced by using the correct normalisation can
be comparable to the differences between even the most extreme cosmological
models. The existing data is not sufficient to determine the correct
normalisation with enough certainty to allow lensing statistics to be used to
their full potential. However, as the correct lensing probability is almost
certainly higher than is usually assumed, upper bounds on the cosmological
constant are not weakened by these possibilities.Comment: MNRAS, in press; 13 pages, 22 figure
Enhancing Light-Atom Interactions via Atomic Bunching
There is a broad interest in enhancing the strength of light-atom
interactions to the point where injecting a single photon induces a nonlinear
material response. Here, we show theoretically that sub-Doppler-cooled,
two-level atoms that are spatially organized by weak optical fields give rise
to a nonlinear material response that is greatly enhanced beyond that
attainable in a homogeneous gas. Specifically, in the regime where the
intensity of the applied optical fields is much less than the off-resonant
saturation intensity, we show that the third-order nonlinear susceptibility
scales inversely with atomic temperature and, due to this scaling, can be two
orders of magnitude larger than that of a homogeneous gas for typical
experimental parameters. As a result, we predict that spatially bunched
two-level atoms can exhibit single-photon nonlinearities. Our model is valid
for all atomic temperature regimes and simultaneously accounts for the
back-action of the atoms on the optical fields. Our results agree with previous
theoretical and experimental results for light-atom interactions that have
considered only a limited range of temperatures. For lattice beams tuned to the
low-frequency side of the atomic transition, we find that the nonlinearity
transitions from a self-focusing type to a self-defocusing type at a critical
intensity. We also show that higher than third-order nonlinear optical
susceptibilities are significant in the regime where the dipole potential
energy is on the order of the atomic thermal energy. We therefore find that it
is crucial to retain high-order nonlinearities to accurately predict
interactions of laser fields with spatially organized ultracold atoms. The
model presented here is a foundation for modeling low-light-level nonlinear
optical processes for ultracold atoms in optical lattices
Gravitational lensing in galaxy redshift surveys
Gravitationally-lensed quasars should be discovered as a by-product of large
galaxy redshift surveys, being discovered spectroscopically when a low-redshift
galaxy exhibits high-redshift quasar emission lines. The number of lenses
expected is higher than previously estimated, mainly due to the fact that the
presence of the quasar images brings faint deflector galaxies above the survey
limit. Thus the a posteriori likelihood of the discovery of Q 2237+0305 in the
Center for Astrophysics redshift survey is approximately 0.03. In the future,
the 2 degree Field survey should yield at least 10 lensed quasars, and the
Sloan Digitial Sky Survey up to 100.Comment: Gravitational Lensing: Recent Progress and Future Goals, C.S.
Kochanek & T.G. Brainerd, eds., in press; 2 pages, 1 figur
Gravitational lensing in modified Newtonian dynamics
Modified Newtonian dynamics (MOND) is an alternative theory of gravity that
aims to explain large-scale dynamics without recourse to any form of dark
matter. However the theory is incomplete, lacking a relativistic counterpart,
and so makes no definite predictions about gravitational lensing. The most
obvious form that MONDian lensing might take is that photons experience twice
the deflection of massive particles moving at the speed of light, as in general
relativity (GR). In such a theory there is no general thin-lens approximation
(although one can be made for spherically-symmetric deflectors), but the
three-dimensional acceleration of photons is in the same direction as the
relativistic acceleration would be. In regimes where the deflector can
reasonably be approximated as a single point-mass (specifically low-optical
depth microlensing and weak galaxy-galaxy lensing), this naive formulation is
consistent with observations. Forthcoming galaxy-galaxy lensing data and the
possibility of cosmological microlensing have the potential to distinguish
unambiguously between GR and MOND. Some tests can also be performed with
extended deflectors, for example by using surface brightness measurements of
lens galaxies to model quasar lenses, although the breakdown of the thin-lens
approximation allows an extra degree of freedom. Nonetheless, it seems unlikely
that simple ellipsoidal galaxies can explain both constraints. Further, the
low-density universe implied by MOND must be completely dominated by the
cosmological constant (to fit microwave background observations), and such
models are at odds with the low frequency of quasar lenses. These conflicts
might be resolved by a fully consistent relativistic extension to MOND; the
alternative is that MOND is not an accurate description of the universe.Comment: MNRAS, in press; 11 pages, 10 figure
Approaches to Faith, Guest Editorial Preface
Springer. We find in contemporary culture starkly contrasting estimates of the value of faith. On the one hand, for many people, faith is a virtue or positive human value, something associated with understanding, hope, and love, something to be inculcated, maintained, and cherished. On the other hand, for many people, faith is a vice, something associated with dogmatism, arrogance, and close-mindedness, something to be avoided at all costs. The papers included in this special (double) issue on approaches to faith explore questions about faith in a variety of settings through a diverse range of examples, both secular and religious. The attempt to deepen our understanding of faith in the context of ordinary human relationships (e.g., between parents and children, friends, generals and their armies, business partners, citizens and the state), a commitment to ideals, or the pursuit of significant goals is clearly of general philosophical interest, as is the examination of potential connections between faith and topics such as trust or reliance
Using the 2dF galaxy redshift survey to detect gravitationally-lensed quasars
Galaxy redshift surveys can be used to detect gravitationally-lensed quasars
if the spectra obtained are searched for the quasars' emission lines. Previous
investigations of this possibility have used simple models to show that the 2
degree Field (2dF) redshift survey could yield several tens of new lenses, and
that the larger Sloan Digital Sky Survey should contain an order of magnitude
more. However the particular selection effects of the samples were not included
in these calculations, limiting the robustness of the predictions; thus a more
detailed simulation of the 2dF survey was undertaken here. The use of an
isophotal magnitude limit reduces both the depth of the sample and the expected
number of lenses, but more important is the Automatic Plate Measuring survey's
star-galaxy separation algorithm, used to generate the 2dF input catalogue. It
is found that most quasar lenses are classed as merged stars, with only the few
lenses with low-redshift deflectors likely to be classified as galaxies.
Explicit inclusion of these selection effects implies that the 2dF survey
should contain 10 lenses on average. The largest remaining uncertainty is the
lack of knowledge of the ease with which any underlying quasars can be
extracted from the survey spectra.Comment: MNRAS, in press; 14 pages, 19 figure
Closing the Loop: Creating Deliverables That Add Value
As special collections librarians and liaison librarians work together to create inÂnovative experiences working with primary source material, it is important to remember students have much to offer in the collaborative design process. In this case study, Prudence Doherty, a special collections librarian, and Daniel DeSanto, an instruction librarian, describe a project they initiated and implemented with upper-level education majors at the University of Vermont (UVM). The students were pre-service teachers (student teachers working toward degree and licensure) enrolled in Social Education and Social Studies, a course that focuses on teaching methods, assessment alternatives, and resources used in the elementary (K–4) classroom. The project gave the pre-service teachers an opportunity to work with three digital collections in order to design lesson plans for elementary-aged stuÂdents. The project closed the loop of learn, create, and teach by requiring students to learn evaluative approaches to working with historical material and then create lesson plans based on those approaches. By creating professional resources for other teachers, the students added value to the digital collections
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