18,305 research outputs found
The Economic Consequences of Widowhood
We analyzed the economic consequences of a husband’s death using events that occurred between the first two waves of the HRS and AHEAD studies. We compared poverty transitions against published results from Social Security’s Retirement History Survey of the 1970s. Widowhood remains an important risk factor for transition into poverty, although somewhat less so than twenty years ago. Women over age 65 (AHEAD) are less likely to experience severe economic changes than women under age 61 (HRS). Several factors account for the age differences: the declining importance of husband’s earnings with age, the rising importance of Social Security benefits, and the occasionally large out-of-pocket medical expenses associated with husband’s death before Medicare eligibility. The greater economic impact of widowhood at younger ages is consistent with our cross-section evidence that poverty rates rise with duration of widowhood but are only weakly associated with age.
Towards the graviton from spinfoams: higher order corrections in the 3d toy model
We consider the recent calculation gr-qc/0508124 of the graviton propagator
in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102,
we test how the spinfoam formalism can be used to construct the perturbative
expansion of graviton amplitudes. Although the 3d graviton is a pure gauge, one
can choose to work in a gauge where it is not zero and thus reproduce the
structure of the 4d perturbative calculations. We compute explicitly the next
to leading and next to next to leading orders, corresponding to one-loop and
two-loop corrections. We show that while the first arises entirely from the
expansion of the Regge action around the flat background, the latter receives
contributions from the microscopic, non Regge-like, quantum geometry.
Surprisingly, this new contribution reduces the magnitude of the next to next
to leading order. It thus appears that the spinfoam formalism is likely to
substantially modify the conventional perturbative expansion at higher orders.
This result supports the interest in this approach. We then address a number
of open issues in the rest of the paper. First, we discuss the boundary state
ansatz, which is a key ingredient in the whole construction. We propose a way
to enhance the ansatz in order to make the edge lengths and dihedral angles
conjugate variables in a mathematically well-defined way. Second, we show that
the leading order is stable against different choices of the face weights of
the spinfoam model; the next to leading order, on the other hand, is changed in
a simple way, and we show that the topological face weight minimizes it.
Finally, we extend the leading order result to the case of a regular, but not
equilateral, tetrahedron.Comment: 24 pages, many figure
Complex networks created by aggregation
We study aggregation as a mechanism for the creation of complex networks. In
this evolution process vertices merge together, which increases the number of
highly connected hubs. We study a range of complex network architectures
produced by the aggregation. Fat-tailed (in particular, scale-free)
distributions of connections are obtained both for networks with a finite
number of vertices and growing networks. We observe a strong variation of a
network structure with growing density of connections and find the phase
transition of the condensation of edges. Finally, we demonstrate the importance
of structural correlations in these networks.Comment: 12 pages, 13 figure
Crack Front Waves and the dynamics of a rapidly moving crack
Crack front waves are localized waves that propagate along the leading edge
of a crack. They are generated by the interaction of a crack with a localized
material inhomogeneity. We show that front waves are nonlinear entities that
transport energy, generate surface structure and lead to localized velocity
fluctuations. Their existence locally imparts inertia, which is not
incorporated in current theories of fracture, to initially "massless" cracks.
This, coupled to crack instabilities, yields both inhomogeneity and scaling
behavior within fracture surface structure.Comment: Embedded Latex file including 4 figure
Non-degenerate four-wave mixing in rubidium vapor: transient regime
We investigate the transient response of the generated light from Four-Wave
Mixing (FWM) in the diamond configuration using a step-down field excitation.
The transients show fast decay times and oscillations that depend on the
detunings and intensities of the fields. A simplified model taking into account
the thermal motion of the atoms, propagation, absorption and dispersion effects
shows qualitative agreement with the experimental observations with the energy
levels in rubidium (5S1/2, 5P1/2, 5P3/2 and 6S1/2). The atomic polarization
comes from all the contributions of different velocity classes of atoms in the
ensemble modifying dramatically the total transient behavior of the light from
FWM.Comment: 11 pages, 11 figures, to be published in Physical Review
Gas Purity effect on GEM Performance in He and Ne at Low Temperatures
The performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne, He+H2
and Ne+H2 was studied at temperatures in the range of 3-293 K. This paper
reports on previously published measurements and additional studies on the
effects of the purity of the gases in which the GEM performance is evaluated.
In He, at temperatures between 77 and 293 K, triple-GEM structures operate at
rather high gains, exceeding 1000. There is an indication that this high gain
is achieved through the Penning effect as a result of impurities in the gas. At
lower temperatures the gain-voltage characteristics are significantly modified
probably due to the freeze-out of these impurities. Double-GEM and single-GEM
structures can operate down to 3 K at gains reaching only several tens at a gas
density of about 0.5 g/l; at higher densities the maximum gain drops further.
In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in
Ne at low temperatures can be re-established in Penning mixtures of Ne+H2: very
high gains, exceeding 104, have been obtained in these mixtures at 30-77 K, at
a density of 9.2 g/l which corresponds to saturated Ne vapor density at 27 K.
The addition of small amounts of H2 in He also re-establishes large GEM gains
above 30 K but no gain was observed in He+H2 at 4 K and a density of 1.7 g/l
(corresponding to roughly one-tenth of the saturated vapor density). These
studies are, in part, being pursued in the development of two-phase He and Ne
detectors for solar neutrino detection.Comment: 4 pages, 7 figure
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