7,017 research outputs found
The last common bilaterian ancestor
Many regulatory genes appear to be utilized in at least superficially similar ways in the development of particular body parts in Drosophila and in chordates. These similarities have been widely interpreted as functional homologies, producing the conventional view of the last common protostome-deuterostome ancestor (PDA) as a complex organism that possessed some of the same body parts as modern bilaterians. Here we discuss an alternative view, in which the last common PDA had a less complex body plan than is frequently conceived. This reconstruction alters expectations for Neoproterozoic fossil remains that could illustrate the pathways of bilaterian evolution
Post clamp
A pair of spaced collars are mounted at right angles on a clamp body by retaining rings which enable the collars to rotate with respect to the clamp body. Mounting posts extend through aligned holes in the collars and clamp body. Each collar can be clamped onto the inserted post while the clamp body remains free to rotate about the post and collar. The clamp body is selectively clamped onto each post
Inconsistency of an inflationary sector coupled only to Einstein gravity
From a model-building perspective, the inflationary sector might very well
have no direct couplings to other species, apart from inevitable gravitational
interactions. Within the context of General Relativity, a thermal universe can
still emerge after inflation if: some radiation sector is excited towards
the end of inflation, and the post-inflationary equation of state becomes
sufficiently stiff , with a
threshold depending on the inflationary scale and the initial
radiation-to-inflaton energy ratio . Furthermore, a stiff period in
the expansion history enhances significantly the inflationary gravitational
wave (GW) background, making this signal (potentially) observable by aLIGO,
LISA and other experiments. The very same enhancement leads however to an
inconsistency of the scenario: the energy of the GWs becomes too large compared
to the rest of the radiation sector, violating standard BBN and CMB bounds on
GW backgrounds. Except for very special scenarios where the initial radiation
sector comprises hundreds of fields with couplings tuned to specific values,
our result applies independently of , and . This suggests
that in order to reheat the universe, the inflationary sector should be coupled
directly to other particle species. Alternatively the inflationary sector could
be implemented in modified gravity theories.Comment: Comments added to match published version in JCAP, 22 pages (+
appendix + references), 4 figure
Ability of LIGO and LISA to probe the equation of state of the early Universe
The expansion history of the Universe between the end of inflation and the
onset of radiation-domination (RD) is currently unknown. If the equation of
state during this period is stiffer than that of radiation, , the
gravitational wave (GW) background from inflation acquires a blue-tilt
at
frequencies corresponding to modes re-entering the horizon
during the stiff-domination (SD), where is the frequency today of
the horizon scale at the SD-to-RD transition. We characterized in detail the
transfer function of the GW energy density spectrum, considering both 'instant'
and smooth modelings of the SD-to-RD transition. The shape of the spectrum is
controlled by , , and (the Hubble scale of
inflation). We determined the parameter space compatible with a detection of
this signal by LIGO and LISA, including possible changes in the number of
relativistic degrees of freedom, and the presence of a tensor tilt. Consistency
with upper bounds on stochastic GW backgrounds, however, rules out a
significant fraction of the observable parameter space. We find that this
renders the signal unobservable by Advanced LIGO, in all cases. The GW
background remains detectable by LISA, though only in a small island of
parameter space, corresponding to scenarios with an equation of state in the
range and a high inflationary scale , but low reheating temperature (equivalently, ). Implications for early
Universe scenarios resting upon an SD epoch are briefly discussed.Comment: Matching published version in JCAP, 32 pages, 8 figure
Damping of an oscillating scalar field indirectly coupled to a thermal bath
The damping process of a homogeneous oscillating scalar field that indirectly
interacts with a thermal bath through a mediator field is investigated over a
wide range of model parameters. We consider two types of mediator fields, those
that can decay to the thermal bath and those that are individually stable but
pair annihilate. The former case has been extensively studied in the literature
by treating the damping as a local effect after integrating out the assumed
close-to-equilibrium mediator field. The same approach does not apply if the
mediator field is stable and freezes out of equilibrium. To account for the
latter case, we adopt a non-local description of damping that is only
meaningful when we consider full half-oscillations of the field being damped.
The damping rates of the oscillating scalar field and the corresponding heating
rate of the thermal bath in all bulk parameter regions are calculated in both
cases, corroborating previous results in the direct decay case. Using the
obtained results, the time it takes for the amplitude of the scalar field to be
substantially damped is estimated.Comment: 39 pages, 9 figures, 1 table; typos corrected, references adde
Method of stabilizing flueric vortex valves and vortex amplifiers
Inducing losses in the vortex chamber of vortex valves and vortex amplifiers resolves the problem of unstable operation caused by a sufficiently large positive feedback. Induced losses also reduce pressure gain and throttling range of vortex pressure amplifier
RESOURCE OR NUISANCE? MANAGING AFRICAN ELEPHANTS AS A MULTI-USE SPECIES
Increasing human interference with natural systems causes us to re-think our perception of wildlife species and the economic choices society makes with regards to their management. Accordingly, we generalize existing 'bioeconomic' models by proposing an economically-based classification of species. The theoretical model is applied to the case of African elephant management. We demonstrate that the classification of the steady state population of a species depends on both species' density and economic factors. Our main results are threefold. First, we demonstrate the classification-dependent possibility of multiple equilibria and perverse comparative statics for multi-use species. Second, upon comparing the optimal stock of a multi-use species to the stock under an open access regime, we find that the ranking in terms of abundance is ambiguous. Finally, and consistent with existing literature on resource management in a second-best world, our case study supports the idea that trade measures have ambiguous effects on wildlife abundance under open access.Resource /Energy Economics and Policy,
Kings and Vikings: On the Dynamics of Competitive Agglomeration
This paper studies the Viking age – the roughly 300 year period beginning in 800 AD – from the perspective of the economics of conflict. The Viking age is interesting because throughout the time period, the scale of conflict increased – small scale raiding behaviour eventually evolved into large scale clashes between armies. With this observation in mind, we present a theoretical model describing the incentives both the defending population and the invading population had to agglomerate into larger groups to better defend against attacks, and engage in attacks, respectively. The result is what might be called a theory of competitive agglomeration. We also apply our model in assessing the factors behind the onset of Vikings raids at the end of the 8th century.
The Resource Curse Revisited and Revised: A Tale of Paradoxes and Red Herrings
We critically evaluate the empirical basis for the so-called resource curse and find that, despite the topic’s popularity in economics and political science research, this apparent paradox is a red herring. The most commonly used measure of ‘resource abundance’ can be more usefully interpreted as a proxy for ‘resource dependence’—endogenous to underlying institutional factors. In multiple estimations that combine resource abundance and dependence, institutional and constitutional variables, we find that (i) resource abundance, constitutions and institutions determine resource dependence, (ii) resource dependence does not affect growth, and (iii) resource abundance positively affects growth and institutional quality.Natural resource curse, economic growth, growth regressions, political regimes, institutions, constitutions
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