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Saving on a Rainy Day, Borrowing for a Rainy Day
The aim of this paper is to understand what a recession means for individual consumers, and to model in a life-cycle framework how individuals respond to recessions. Our focus is on the sharp increase in savings rates that have been observed in the current and recent recessions. We show empirically that these saving spikes were short-lived and common to all working age groups. We then study life-cycle models in which recessions involve one or more of: (i) an aggregate permanent negative shock to individual income; (ii) an increase in the variance of idiosyncratic permanent shocks; (iii) a tightening of credit constraints; (iv) asset market crashes. In simulations and in the data we aggregate explicitly from individual behavior. We model credit tightening as a constraint on new borrowing and this generates an option value of borrowing in good times. We show that the rise in the aggregate savings ratio is driven by increases in uncertainty, rather than tightening of credit; temporary shocks to the supply of credit generate increases in saving only among younger agents
Age Dating of a High-Redshift QSO B1422+231 at Z=3.62 and its Cosmological Implications
The observed Fe II(UV+optical)/Mg II lambda lambda 2796,2804 flux ratio from
a gravitationally lensed quasar B1422+231 at z=3.62 is interpreted in terms of
detailed modeling of photoionization and chemical enrichment in the broad-line
region (BLR) of the host galaxy. The delayed iron enrichment by Type Ia
supernovae is used as a cosmic clock. Our standard model, which matches the Fe
II/Mg II ratio, requires the age of 1.5 Gyr for B1422+231 with a lower bound of
1.3 Gyr, which exceeds the expansion age of the Einstein-de Sitter Omega_0=1
universe at a redshift of 3.62 for any value of the Hubble constant in the
currently accepted range, H_0=60-80 km,s^{-1},Mpc^{-1}. This problem of an age
discrepancy at z=3.62 can be unraveled in a low-density Omega_0<0.2 universe,
either with or without a cosmological constant, depending on the allowable
redshift range of galaxy formation. However, whether the cosmological constant
is a required option in modern cosmology awaits a thorough understanding of
line transfer processes in the BLRs.Comment: 7 pages including 3 figures, to appear in ApJ Letter
Light Sheets and the Covariant Entropy Conjecture
We examine the holography bound suggested by Bousso in his covariant entropy
conjecture, and argue that it is violated because his notion of light sheet is
too generous. We suggest its replacement by a weaker bound.Comment: 5 pages, to appear in Classical and Quantum Gravit
Multifluid magnetohydrodynamic turbulent decay
It is generally believed that turbulence has a significant impact on the
dynamics and evolution of molecular clouds and the star formation which occurs
within them. Non-ideal magnetohydrodynamic effects are known to influence the
nature of this turbulence. We present the results of a suite of 512-cubed
resolution simulations of the decay of initially super-Alfvenic and supersonic
fully multifluid MHD turbulence. We find that ambipolar diffusion increases the
rate of decay of the turbulence while the Hall effect has virtually no impact.
The decay of the kinetic energy can be fitted as a power-law in time and the
exponent is found to be -1.34 for fully multifluid MHD turbulence. The power
spectra of density, velocity and magnetic field are all steepened significantly
by the inclusion of non-ideal terms. The dominant reason for this steepening is
ambipolar diffusion with the Hall effect again playing a minimal role except at
short length scales where it creates extra structure in the magnetic field.
Interestingly we find that, at least at these resolutions, the majority of the
physics of multifluid turbulence can be captured by simply introducing fixed
(in time and space) resistive terms into the induction equation without the
need for a full multifluid MHD treatment. The velocity dispersion is also
examined and, in common with previously published results, it is found not to
be power-law in nature.Comment: 16 pages, 15 figures, Accepted for publication in Ap
Error Voltage Components in Quantitative Voltage Contrast Measurement Systems
This paper presents the results of computer simulation studies into the respective contributions of the potential barrier, the off-normal incidence injection of secondary electrons (SEs) into the retarding field and analyser geometry on Types I and II local field error voltages for a practical 20 mm wide planar retarding field energy analyser. Results show that the error voltage component due to the off-normal incidence injection effect of SEs into the retarding field dominates the Type I local field error. For type II LFE, the error voltage component due to analyser geometry effect is the higher contributing factor. The presence of a neighbouring electrode voltage tends to draw SEs away from the central axis of the energy analyser, thus causing the electron trajectories to be more sensitive to the influence of the analyser geometry
Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence
Hydrodynamic supersonic turbulence can only prevent local gravitational
collapse if the turbulence is driven on scales smaller than the local Jeans
lengths in the densest regions, a very severe requirement (Paper I). Magnetic
fields have been suggested to support molecular clouds either magnetostatically
or via magnetohydrodynamic (MHD) waves. Whereas the first mechanism would form
sheet-like clouds, the second mechanism not only could exert a pressure onto
the gas counteracting the gravitational forces, but could lead to a transfer of
turbulent kinetic energy down to smaller spatial scales via MHD wave
interactions. This turbulent magnetic cascade might provide sufficient energy
at small scales to halt local collapse.
We test this hypothesis with MHD simulations at resolutions up to 256^3
zones, done with ZEUS-3D. We first derive a resolution criterion for
self-gravitating, magnetized gas: in order to prevent collapse of
magnetostatically supported regions due to numerical diffusion, the minimum
Jeans length must be resolved by four zones. Resolution of MHD waves increases
this requirement to roughly six zones. We then find that magnetic fields cannot
prevent local collapse unless they provide magnetostatic support. Weaker
magnetic fields do somewhat delay collapse and cause it to occur more uniformly
across the supported region in comparison to the hydrodynamical case. However,
they still cannot prevent local collapse for much longer than a global
free-fall time.Comment: 32 pages, 14 figures, accepted by Ap
Everything you ever wanted to know about sweetpotato: Reaching agents of change ToT manual. 6: Gender and diversity aspects. Monitoring of OFSP dissemination and uptake.
Linking and causality in globally hyperbolic spacetimes
The linking number is defined if link components are zero homologous.
Our affine linking invariant generalizes to the case of linked
submanifolds with arbitrary homology classes. We apply to the study of
causality in Lorentz manifolds. Let be a spacelike Cauchy surface in a
globally hyperbolic spacetime . The spherical cotangent bundle
is identified with the space of all null geodesics in
Hence the set of null geodesics passing through a point gives an
embedded -sphere in called the sky of Low observed
that if the link is nontrivial, then are causally
related. This motivated the problem (communicated by Penrose) on the Arnold's
1998 problem list to apply link theory to the study of causality. The spheres
are isotopic to fibers of They are nonzero
homologous and is undefined when is closed, while is well defined. Moreover, if is not an
odd-dimensional rational homology sphere. We give a formula for the increment
of \alk under passages through Arnold dangerous tangencies. If is
such that takes values in and is conformal to having all
the timelike sectional curvatures nonnegative, then are causally
related if and only if . We show that in
nonrefocussing are causally unrelated iff can be deformed
to a pair of -fibers of by an isotopy through skies. Low
showed that if (\ss, g) is refocussing, then is compact. We show that the
universal cover of is also compact.Comment: We added: Theorem 11.5 saying that a Cauchy surface in a refocussing
space time has finite pi_1; changed Theorem 7.5 to be in terms of conformal
classes of Lorentz metrics and did a few more changes. 45 pages, 3 figures. A
part of the paper (several results of sections 4,5,6,9,10) is an extension
and development of our work math.GT/0207219 in the context of Lorentzian
geometry. The results of sections 7,8,11,12 and Appendix B are ne
Low-energy interaction of composite spin-half systems with scalar and vector fields
We consider a composite spin-half particle moving in spatially-varying scalar
and vector fields. The vector field is assumed to couple to a conserved charge,
but no assumption is made about either the structure of the composite or its
coupling to the scalar field. A general form for the piece of the spin-orbit
interaction of the composite with the scalar and vector fields which is
first-order in momentum transfer and second-order in the fields is
derived.Comment: 10 pages, RevTe
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