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 $W^+$ boson production in the reaction $\nu \gamma\to W^+X$ 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 $\nu\gamma\to W^+X$ 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