2,532 research outputs found
Cylindrical thin-shell wormholes and energy conditions
We prove the impossibility of cylindrical thin-shell wormholes supported by
matter satisfying the energy conditions everywhere, under reasonable
assumptions about the asymptotic behaviour of the - in general different -
metrics at each side of the throat. In particular, we reproduce for singular
sources previous results corresponding to flat and conical asymptotics, and
extend them to a more general asymptotic behaviour. Besides, we establish
necessary conditions for the possibility of non exotic cylindrical thin-shell
wormholes.Comment: 9 pages; slightly improved version of the article accepted in Int. J.
Mod. Phys.
Traversable wormholes in a string cloud
We study spherically symmetric thin-shell wormholes in a string cloud
background in (3+1)-dimensional spacetime. The amount of exotic matter required
for the construction, the traversability and the stability under radial
perturbations, are analyzed as functions of the parameters of the model.
Besides, in the Appendices a non perturbative approach to the dynamics and a
possible extension of the analysis to a related model are briefly discussed.Comment: 21 pages, 10 figures; accepted for publication in Int. J. Mod. Phys.
A farinha do mesocarpo dp babaçu (Orbignya phalerata) como componente de uma mistura protéica em dieta semi-purificada e seus reflexos sobre a colesterolemia e a trigliceridemia em ratos.
Suplemento 1. Edição dos resumos do XV Congresso Latinoamericano de Nutricion e XVI Jornadas de la Sociedad Chilena de Nutrición, Santiago do Chile, 2009
Global phase time and path integral for string cosmological models
A global phase time is identified for homogeneous and isotropic cosmological
models yielding from the low energy effective action of closed bosonic string
theory. When the Hamiltonian constraint allows for the existence of an
intrinsic time, the quantum transition amplitude is obtained by means of the
usual path integral procedure for gauge systems.Comment: 12 pages, added reference
Seasonal variations in the behavioural thermoregulation of roosting Humboldt Penguins (Spheniscus humboldti) in north-central Chile.
We examined the thermoregulatory behaviour (TRB) of roosting Humboldt penguins (Spheniscus humboldti) in north central Chile during summer and winter, when ambient temperatures (Ta) are most extreme. Each body posture was considered to represent a particular TRB, which was ranked in a sequence that reflected different degrees of thermal load and was assigned an arbitrary thermoregulatory score. During summer, birds exhibited eight different TRBs, mainly oriented to heat dissipation, and experienced a wide range of Ta (from 14 to 31°C), occasionally above their thermoneutral zone (TNZ, from 2 to 30°C), this being evident by observations of extreme thermoregulatory responses such as panting. In winter, birds exhibited only three TRBs, mainly oriented to heat retention, and experienced a smaller range of Ta (from 11 to 18°C), always within the TNZ, even at night. The components of behavioural responses increased directly with the heat load which explains the broader behavioural repertoire observed in summer. Since penguins are primarily adapted in morphology and physiology to cope with low water temperatures, our results suggest that behavioural thermoregulation may be important in the maintenance of the thermal balance in Humboldt penguins while on land
Brans-Dicke cylindrical wormholes
Static axisymmetric thin-shell wormholes are constructed within the framework
of the Brans-Dicke scalar-tensor theory of gravity. Examples of wormholes
associated with vacuum and electromagnetic fields are studied. All
constructions must be threaded by exotic matter, except in the case of
geometries with a singularity of finite radius, associated with an electric
field, which can have a throat supported by ordinary matter. These results are
achieved with any of the two definitions of the flare-out condition considered.Comment: 11 pages, 3 figures; v3: corrected version, conclusions unchange
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