2,699 research outputs found
Nonsingular charged black holes \`{a} la Palatini
We argue that the quantum nature of matter and gravity should lead to a
discretization of the allowed states of the matter confined in the interior of
black holes. To support and illustrate this idea, we consider a quadratic
extension of General Relativity formulated \`{a} la Palatini and show that
nonrotating, electrically charged black holes develop a compact core at the
Planck density which is nonsingular if the mass spectrum satisfies a certain
discreteness condition. We also find that the area of the core is proportional
to the number of charges times the Planck area.Comment: 10 single column page
Acceleration radiation, transition probabilities, and trans-Planckian physics
An important question in the derivation of the acceleration radiation, which
also arises in Hawking's derivation of black hole radiance, is the need to
invoke trans-Planckian physics for the quantum field that originates the
created quanta. We point out that this issue can be further clarified by
reconsidering the analysis in terms of particle detectors, transition
probabilities, and local two-point functions. By writing down separate
expressions for the spontaneous- and induced-transition probabilities of a
uniformly accelerated detector, we show that the bulk of the effect comes from
the natural (non trans-Planckian) scale of the problem, which largely
diminishes the importance of the trans-Planckian sector. This is so, at least,
when trans-Planckian physics is defined in a Lorentz invariant way. This
analysis also suggests how to define and estimate the role of trans-Planckian
physics in the Hawking effect itself.Comment: 19 page
Non-singular Universes a la Palatini
It has recently been shown that f(R) theories formulated in the Palatini
variational formalism are able to avoid the big bang singularity yielding
instead a bouncing solution. The mechanism responsible for this behavior is
similar to that observed in the effective dynamics of loop quantum cosmology
and an f(R) theory exactly reproducing that dynamics has been found. I will
show here that considering more general actions, with quadratic contributions
of the Ricci tensor, results in a much richer phenomenology that yields
bouncing solutions even in anisotropic (Bianchi I) scenarios. Some implications
of these results are discussed.Comment: 4 pages, no figures. Contribution to the Spanish Relativity Meeting
(ERE2010), 6-10 Sept. Granada, Spai
New observations of the colpodid ciliate Ottowphrya dragescoi (Ciliophora, Colpodea, Platyophryida, Ottowphryidae) and its confusing taxonomic history
This paper describes a new population of Ottowphrya dragescoi isolated from moss samples in the UK. Its size in vivo is 70-110 x 35-60 μm. Its oral and somatic infraciliature were revealed with silver-carbonate impregnation, which showed 4-7 adoral organelles, and 27-34 somatic kineties. Thus far, three populations have been described in the scientific literature: one German, one from Austria and a Finnish population in addition to specimens from France, Australia and Kenya. The English population, described here, is quite similar to the one from Finland. Our data confirm the observations made by
Foissner (2002) and highlight the taxonomic difficulties in establishing new genera and specie
Extended Loop Quantum Gravity
We discuss constraint structure of extended theories of gravitation (also
known as f(R) theories) in the vacuum selfdual formulation introduced in ref.
[1].Comment: 7 pages, few typos correcte
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