24,937 research outputs found
New freshwater sponges from Amazonian waters
Four species of freshwater sponges have their first register of occurrence for Amazonian waters upon specimens collected from Culuene and Sete de Setembro Rivers (Xingú basin), Tapirapés River (Araguaia basin) and Cuieiras River. Trochospongilla pennsylvanica (POTTS 1882) and Trochospongilla variabilis BONETTO & EZCURRA DE DRAGO (1973) occurred as minute specimens inside large sponges of the genus Metania GRAY (1867) or of the genus Drulia GRAY (1867). T. pennsylvanica has its first register of occurrence for the Neotropical region. Spongilla spoliata n. sp. resembles Spongilla inarmata ANNADALE (1918) and Spongilla aspinosa POTTS (1880) but is readily separated from these two species on account of the characteristic spines on its microscleres. Radiospongilla amazonensis n. sp. differs from its congeners by the particular characteristics of its megascleres and gemmoscleres
Spatial correlations in parametric down-conversion
The transverse spatial effects observed in photon pairs produced by
parametric down-conversion provide a robust and fertile testing ground for
studies of quantum mechanics, non-classical states of light, correlated imaging
and quantum information. Over the last 20 years there has been much progress in
this area, ranging from technical advances and applications such as quantum
imaging to investigations of fundamental aspects of quantum physics such as
complementarity relations, Bell's inequality violation and entanglement. The
field has grown immensely: a quick search shows that there are hundreds of
papers published in this field. The objective of this article is to review the
building blocks and major theoretical and experimental advances in the field,
along with some possible technical applications and connections to other
research areas.Comment: 116 pages, 35 figures. To appear in Physics Report
Generalized Chaplygin gas with and the cosmological model
The generalized Chaplygin gas model is characterized by the equation of state
. It is generally stated that the case is equivalent to a model with cosmological constant and dust (). In this work we show that, if this is true for the background equations,
this is not true for the perturbation equations. Hence, the mass spectrum
predicted for both models may differ.Comment: Latex file, 4 pages, 2 figures in eps forma
Image and Coherence Transfer in the Stimulated Down-conversion Process
The intensity transverse profile of the light produced in the process of
stimulated down-conversion is derived. A quantum-mechanical treatment is used.
We show that the angular spectrum of the pump laser can be transferred to the
stimulated down-converted beam, so that images can also be transferred from the
pump to the down-converted beam. We also show that the transfer can occur from
the stimulating beam to the down-converted one. Finally, we study the process
of diffraction through an arbitrarily shaped screen. For the special case of a
double-slit, the interference pattern is explicitly obtained. The visibility
for the spontaneous emitted light is in accordance with the van Cittert -
Zernike theorem for incoherent light, while the visibility for the stimulated
emitted light is unity. The overall visibility is in accordance with previous
experimental results
Matching LTB and FRW spacetimes through a null hypersurface
Matching of a LTB metric representing dust matter to a background FRW
universe across a null hypersurface is studied. In general, an unrestricted
matching is possible only if the background FRW is flat or open. There is in
general no gravitational impulsive wave present on the null hypersurface which
is shear-free and expanding. Special cases of the vanishing pressure or energy
density on the hypersurface is discussed. In the case of vanishing energy
momentum tensor of the null hypersurface, i.e. in the case of a null boundary,
it turns out that all possible definitions of the Hubble parameter on the null
hypersurface, being those of LTB or that of FRW, are equivalent, and that a
flat FRW can only be joined smoothly to a flat LTB.Comment: 9 page
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