115 research outputs found
Fluid-driven deformation of a soft granular material
Compressing a porous, fluid-filled material will drive the interstitial fluid
out of the pore space, as when squeezing water out of a kitchen sponge.
Inversely, injecting fluid into a porous material can deform the solid
structure, as when fracturing a shale for natural gas recovery. These
poromechanical interactions play an important role in geological and biological
systems across a wide range of scales, from the propagation of magma through
the Earth's mantle to the transport of fluid through living cells and tissues.
The theory of poroelasticity has been largely successful in modeling
poromechanical behavior in relatively simple systems, but this continuum theory
is fundamentally limited by our understanding of the pore-scale interactions
between the fluid and the solid, and these problems are notoriously difficult
to study in a laboratory setting. Here, we present a high-resolution
measurement of injection-driven poromechanical deformation in a system with
granular microsctructure: We inject fluid into a dense, confined monolayer of
soft particles and use particle tracking to reveal the dynamics of the
multi-scale deformation field. We find that a continuum model based on
poroelasticity theory captures certain macroscopic features of the deformation,
but the particle-scale deformation field exhibits dramatic departures from
smooth, continuum behavior. We observe particle-scale rearrangement and
hysteresis, as well as petal-like mesoscale structures that are connected to
material failure through spiral shear banding
Spectrum and Duration of Delayed MeV-GeV Emission of Gamma-Ray Bursts in Cosmic Background Radiation Fields
We generally analyze prompt high-energy emission above a few hundreds of GeV
due to synchrotron self-Compton scattering in internal shocks. However, such
photons cannot be detected because they may collide with cosmic infrared
background photons, leading to electron/positron pair production.
Inverse-Compton scattering of the resulting electron/positron pairs off cosmic
microwave background photons will produce delayed MeV-GeV emission, which may
be much stronger than a typical high-energy afterglow in the external shock
model. We expand on the Cheng & Cheng model by deriving the emission spectrum
and duration in the standard fireball shock model. A typical duration of the
emission is ~ 10^3 seconds, and the time-integrated scattered photon spectrum
is nu^{-(p+6)/4}, where p is the index of the electron energy distribution
behind internal shocks. This is slightly harder than the synchrotron photon
spectrum, nu^{-(p+2)/2}. The lower energy property of the scattered photon
spectrum is dependent on the spectral energy distribution of the cosmic
infrared background radiation. Therefore, future observations on such delayed
MeV-GeV emission and the higher-energy spectral cutoff by the Gamma-Ray Large
Area Space Telescope (GLAST) would provide a probe of the cosmic infrared
background radiation.Comment: 5 pages, accepted for publication in Ap
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Longevity Risk and Capital Markets: The 2015-16 Update
International audienc
Probing Galaxy Formation with TeV Gamma Ray Absorption
We present here the extragalactic background light (EBL) predicted by
semi-analytic models of galaxy formation, and show how measurements of the
absorption of gamma rays of TeV energies via pair production on the EBL
can probe cosmology and the formation of galaxies. Semi-analytic models permit
a physical treatment of the key processes of galaxy formation -- including
gravitational collapse and merging of dark matter halos, gas cooling and
dissipation, star formation, supernova feedback and metal production -- and
have been shown to reproduce key observations at low and high redshift. Using
this approach, we investigate the consequences of variations in input
assumptions such as the stellar initial mass function and the underlying
cosmology. We conclude that observational studies of the absorption of TeV gamma rays will help to constrain the star formation
history of the universe, and the nature and extent of the extinction of
starlight due to dust and reradiation of the absorbed energy at infrared
wavelengths.Comment: 17 pages, 8 figures, presented at the VERITAS Workshop on TeV
Astrophysics of Extragalactic Sources, eds. M. Catanese and T. Weekes, to be
published in Astroparticle Physic
Evidence for Intergalactic Absorption in the TeV Gamma-Ray Spectrum of Mkn 501
The recent HEGRA observations of the blazar Mkn 501 show strong curvature in
the very high energy gamma-ray spectrum. Applying the gamma-ray opacity derived
from an empirically based model of the intergalactic infrared background
radiation field (IIRF), to these observations, we find that the intrinsic
spectrum of this source is consistent with a power-law: dN/dE~ E^-alpha with
alpha=2.00 +/- 0.03 over the range 500 GeV - 20 TeV. Within current synchrotron
self-Compton scenarios, the fact that the TeV spectral energy distribution of
Mkn 501 does not vary with luminosity, combined with the correlated, spectrally
variable emission in X-rays, as observed by the BeppoSAX and RXTE instruments,
also independently implies that the intrinsic spectrum must be close to
alpha=2. Thus, the observed curvature in the spectrum is most easily understood
as resulting from intergalactic absorption.Comment: 7 pages, 1 figure, accepted in ApJ Letters 1999 April
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Longevity Risk and Hedging Solutions
Longevity risk—the risk of unanticipated increases in life expectancy—has only recently been recognized as a significant global risk that has materially raised the costs of providing pensions and annuities. We first discuss historical trends in the evolution of life expectancy and then analyze the hedging solutions that have been developed for managing longevity risk. One set of solutions has come directly from the insurance industry: pension buyouts, buy-ins, and bulk annuity transfers. Another complementary set of solutions has come from the capital markets: longevity swaps and q-forwards. This has led to hybrid solutions such as synthetic buy-ins. We then review the evolution of the market for longevity risk transfer, which began in the UK in 2006 and is arguably the most important sector of the broader “life market.” An important theme in the development of the longevity market has been the innovation originating from the combined involvement of insurance, banking, and private equity participants
Multiple mortality modeling in Poisson Lee-Carter framework
The academic literature in longevity field has recently focused on models for detecting multiple population trends (D'Amato et al., 2012b; Njenga and Sherris, 2011; Russolillo et al., 2011, etc.). In particular, increasing interest has been shown about "related" population dynamics or "parent" populations characterized by similar socioeconomic conditions and eventually also by geographical proximity. These studies suggest dependence across multiple populations and common long-run relationships between countries (for instance, see Lazar et al., 2009). In order to investigate cross-country longevity common trends, we adopt a multiple population approach. The algorithm we propose retains the parametric structure of the Lee-Carter model, extending the basic framework to include some cross-dependence in the error term. As far as time dependence is concerned, we allow for all idiosyncratic components (both in the common stochastic trend and in the error term) to follow a linear process, thus considering a highly flexible specification for the serial dependence structure of our data. We also relax the assumption of normality, which is typical of early studies on mortality (Lee and Carter, 1992) and on factor models (see e.g., the textbook by Anderson, 1984). The empirical results show that the multiple Lee-Carter approach works well in the presence of dependence
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