32,335 research outputs found
Chance in the Modern Synthesis
The modern synthesis in evolutionary biology is taken to be that period in
which a consensus developed among biologists about the major causes of
evolution, a consensus that informed research in evolutionary biology for
at least a half century. As such, it is a particularly fruitful period to consider
when reflecting on the meaning and role of chance in evolutionary explanation.
Biologists of this period make reference to “chance” and loose cognates
of “chance,” such as: “random,” “contingent,” “accidental,” “haphazard,” or
“stochastic.” Of course, what an author might mean by “chance” in any specific context varies.
In the following, we first offer a historiographical note on the synthesis.
Second, we introduce five ways in which synthesis authors spoke about
chance. We do not take these to be an exhaustive taxonomy of all possible
ways in which chance meaningfully figures in explanations in evolutionary
biology. These are simply five common uses of the term by biologists at this
period. They will serve to organize our summary of the collected references to
chance and the analysis and discussion of the following questions:
• What did synthesis authors understand by chance?
• How did these authors see chance operating in evolution?
• Did their appeals to chance increase or decrease over time during the synthesis?
That is, was there a “hardening” of the synthesis, as Gould claimed
(1983)
Modelling Private Wealth Accumulation and Spend-down in the Italian Microsimulation Model CAPP_DYN: A Life-Cycle Approach
In microsimulation literature a limited number of
models include a module aimed
at analyzing and projecting the evolution of privat
e wealth over time. However, this issue appears
crucial in order to comprehensively evaluate the li
kely distributional effects of institutional
reforms adopted to cope with population ageing. In
this work we describe the implementation in
the Italian dynamic micro simulation model CAPP_DYN
of a new module in which households\u2019
savings and asset allocation are modelled. In parti
cular, we aim to account for possible
behavioural responses to pension reforms in househo
ld savings. To this end, we rely on an
approximate life cycle structural framework for est
imating saving behaviour, while adopting a
traditional stochastic micro simulation approach fo
r asset allocation. In line with Ando and
Nicoletti Altimari (2004), we emphasize the role of
lifetime economic resources in households\u2019
consumption decisions, yet we further account for i
nternal habit formation and subjective
expectations on pension outcomes in the econometric
stage. In addition, we model
intergenerational transfers of private wealth in a
probabilistic fashio
Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants
High resolution optical spectra of 57 Galactic B-type supergiant stars have
been analyzed to determine their rotational and macroturbulent velocities. In
addition, their atmospheric parameters (effective temperature, surface gravity
and microturbulent velocity) and surface nitrogen abundances have been
estimated using a non-LTE grid of model atmospheres. Comparisons of the
projected rotational velocities have been made with the predictions of stellar
evolutionary models and in general good agreement was found. However for a
small number of targets, their observed rotational velocities were
significantly larger than predicted, although their nitrogen abundances were
consistent with the rest of the sample. We conclude that binarity may have
played a role in generating their large rotational velocities. No correlation
was found between nitrogen abundances and the current projected rotational
velocities. However a correlation was found with the inferred projected
rotational velocities of the main sequence precursors of our supergiant sample.
This correlation is again in agreement with the predictions of single star
evolutionary models that incorporate rotational mixing. The origin of the
macroturbulent and microturbulent velocity fields is discussed and our results
support previous theoretical studies that link the former to sub-photospheric
convection and the latter to non-radial gravity mode oscillations. In addition,
we have attempted to identify differential rotation in our most rapidly
rotating targets.Comment: Submitted to MNRAS, 16 page
The spin temperature of high-redshift damped Lyman- systems
We report results from a programme aimed at investigating the temperature of
neutral gas in high-redshift damped Lyman- absorbers (DLAs). This
involved (1) HI 21cm absorption studies of a large DLA sample, (2) VLBI studies
to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet
spectroscopy to determine DLA metallicities and velocity widths.
Including literature data, our sample consists of 37 DLAs with estimates of
the spin temperature and the covering factor. We find a strong )
difference between the distributions in high-z (z>2.4) and low-z (z<2.4)
DLA samples. The high-z sample contains more systems with high values,
K. The distributions in DLAs and the Galaxy are also
clearly (~) different, with more high- sightlines in DLAs than in
the Milky Way. The high values in the high-z DLAs of our sample arise due
to low fractions of the cold neutral medium.
For 29 DLAs with metallicity [Z/H] estimates, we confirm the presence of an
anti-correlation between and [Z/H], at significance via a
non-parametric Kendall-tau test. This result was obtained with the assumption
that the DLA covering factor is equal to the core fraction. Monte Carlo
simulations show that the significance of the result is only marginally
decreased if the covering factor and the core fraction are uncorrelated, or if
there is a random error in the inferred covering factor.
We also find evidence for redshift evolution in DLA values even for the
z>1 sub-sample. Since z>1 DLAs have angular diameter distances comparable to or
larger than those of the background quasars, they have similar efficiency in
covering the quasars. Low covering factors in high-z DLAs thus cannot account
for the observed redshift evolution in spin temperatures. (Abstract abridged.)Comment: 37 pages, 22 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Dynamics of a Quantum Phase Transition and Relaxation to a Steady State
We review recent theoretical work on two closely related issues: excitation
of an isolated quantum condensed matter system driven adiabatically across a
continuous quantum phase transition or a gapless phase, and apparent relaxation
of an excited system after a sudden quench of a parameter in its Hamiltonian.
Accordingly the review is divided into two parts. The first part revolves
around a quantum version of the Kibble-Zurek mechanism including also phenomena
that go beyond this simple paradigm. What they have in common is that
excitation of a gapless many-body system scales with a power of the driving
rate. The second part attempts a systematic presentation of recent results and
conjectures on apparent relaxation of a pure state of an isolated quantum
many-body system after its excitation by a sudden quench. This research is
motivated in part by recent experimental developments in the physics of
ultracold atoms with potential applications in the adiabatic quantum state
preparation and quantum computation.Comment: 117 pages; review accepted in Advances in Physic
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Dynamic Wetting and Drag Reduction on Superhydrophobic and Liquid-Infused Surfaces
Reducing drag in fluid flow has been one of the most widely studied topics in fluid dynamics due to the significant impact on improving operational efficiencies and cutting cost in applications from the aerospace, automotive and naval industries. Over the past two decades, superhydrophobic surfaces have been in the spotlight due to their ability to reduce frictional drag on the wall surface in both laminar and turbulent flows. Despite the extensive work on superhydrophobic surfaces, there are still a number of open questions remaining. In this dissertation, we investigate how a moving contact line interacts with a superhydrophobic surface by performing the first dynamic contact angle measurements to better understand the dynamics of droplets and streams on the surfaces. Our measurements found that the dynamic advancing contact angles on a superhydrophobic surface remains constant independent on capillary number while the dynamic receding contact angles decreases with capillary number but at a rate much slower than on a smooth surface. Furthermore, we investigated the role of the air-water interface shapes on the drag reduction. A novel microfluidic device was designed to incorporate superhydrophobic pillars. The shape of the air-water interface was changed with change to the static pressure in the channel. Slip along interface trapped within the superhydrophobic surface was found to result in significant drag reduction. However, the changes in flow geometry due to changes in bubble shape dominated effects due to slip. Reducing the bubble size amplified drag reduction, while increasing bubble size reduced drag reduction and even resulted in drag enhancement. In this dissertation, we also studies liquid-infused superhydrophobic surfaces as an alternative to the air-infused superhydrophobic surfaces. In the studies presented here, various immiscible oils were infused into the structures of precisely patterned and randomly rough superhydrophobic surfaces. A series of experiments were performed to investigate how liquid-infused surface affect drag reduction and droplet impact dynamics. The pressure drop reduction and slip length on the liquid-infused surfaces in microchannels were found to increase as the ratio between viscosity of water and the infused oil was increased. The longevity of these surfaces was also studied with the most effective surface found to be randomly rough. The effect of the viscosity ratio was also investigated on the droplet impact dynamics onto liquid-infused superhydrophobic surfaces. The increase in the viscosity ratio was found to increase a maximum diameter and a spreading/retraction rates of droplets. Taken together, the experimental research presented in this dissertation have allowed us to better understand and optimize the design of air-infused and liquid-infused superhydrophobic surfaces for drag reduction, droplet spreading and liquid mobility. With this new-found knowledge, a sense of new innovative ideas and applications has been or soon will be realized
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