55 research outputs found
Developmental simulation of the adult cranial morphology of australopithecus sediba.
The type specimen of Australopithecus sediba (MH1) is a late juvenile, prompting some commentators to suggest that had it lived to adulthood its morphology would have changed sufficiently so as to render hypotheses regarding its phylogenetic relations suspect. Considering the potentially critical position of this species with regard to the origins of the genus Homo, a deeper understanding of this change is especially vital. As an empirical response to this critique, a developmental simulation of the MH1 cranium was carried out using geometric morphometric techniques to extrapolate adult morphology using extant male and female chimpanzees, gorillas and humans by modelling remaining development. Multivariate comparisons of the simulated adult A. sediba crania with other early hominin taxa indicate that subsequent cranial development primarily reflects development of secondary sexual characteristics and would not likely be substantial enough to alter suggested morphological affinities of A. sediba. This study also illustrates the importance of separating developmental vectors by sex when estimating ontogenetic change. Results of the ontogenetic projections concur with those from mandible morphology, and jointly affirm the taxonomic validity of A. sediba.Andrew W. Mellon Foundation and National Geographic Society.NCS201
Parton content of the real photon: astrophysical implications
We possess convincing experimental evidence for the fact that the real photon
has non-trivial parton structure. On the other hand, interactions of the cosmic
microwave background photons with high energy particles propagating through the
Universe play an important role in astrophysics. In this context, to invoke the
parton content could be convenient for calculations of the probabilities of
different processes involving these photons. As an example, the cross section
of inclusive resonant boson production in the reaction is calculated by using the parton language. Neutrino--photon deep
inelastic scattering is considered.Comment: 4 pages, 2 figures. The spin states of the initial particles in the
reaction are correctly treated. As a result, the
corresponding cross section becomes two times greater than the one from the
previous version. Some changes in the tex
Different representations for the action principle in 4D N = 2 supergravity
Within the superspace formulation for four-dimensional N = 2 matter-coupled
supergravity developed in arXiv:0805.4683, we elaborate two approaches to
reduce the superfield action to components. One of them is based on the
principle of projective invariance which is a purely N = 2 concept having no
analogue in simple supergravity. In this approach, the component reduction of
the action is performed without imposing any Wess-Zumino gauge condition, that
is by keeping intact all the gauge symmetries of the superfield action,
including the super-Weyl invariance. As a simple application, the c-map is
derived for the first time from superfield supergravity. Our second approach to
component reduction is based on the method of normal coordinates around a
submanifold in a curved superspace, which we develop in detail. We derive
differential equations which are obeyed by the vielbein and the connection in
normal coordinates, and which can be used to reconstruct these objects, in
principle in closed form. A separate equation is found for the
super-determinant of the vielbein, which allows one to reconstruct it without a
detailed knowledge of the vielbein. This approach is applicable to any
supergravity theory in any number of space-time dimensions. As a simple
application of this construction, we reduce an integral over the curved N = 2
superspace to that over the chiral subspace of the full superspace. We also
give a new representation for the curved projective-superspace action principle
as a chiral integral.Comment: 44 pages; V2: typos corrected, a comment added; V3: eq. (3.16)
corrected, version published in JHEP; V4: more typos correcte
Gravity wave analogs of black holes
It is demonstrated that gravity waves of a flowing fluid in a shallow basin
can be used to simulate phenomena around black holes in the laboratory. Since
the speed of the gravity waves as well as their high-wavenumber dispersion
(subluminal vs. superluminal) can be adjusted easily by varying the height of
the fluid (and its surface tension) this scenario has certain advantages over
the sonic and dielectric black hole analogs, for example, although its use in
testing quantum effects is dubious. It can be used to investigate the various
classical instabilities associated with black (and white) holes experimentally,
including positive and negative norm mode mixing at horizons. PACS: 04.70.-s,
47.90.+a, 92.60.Dj, 04.80.-y.Comment: 14 pages RevTeX, 5 figures, section VI modifie
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
Adaptive acceleration in growth and development of salamander hatchlings in cannibalistic situations
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