899 research outputs found
A model for time-dependent cosmological constant and its consistency with the present Friedmann universe
We use a model where the cosmological term can be related to the chiral gauge
anomaly of a possible quantum scenario of the initial evolution of the
universe. We show that this term is compatible with the Friedmann behavior of
the present universe.Comment: 5 pages, Revtex 4, twocolumn (minor corrections and improved
reference list. To appear in Classical and Quantum Gravity
Cosmology in GSG
We describe what cosmology looks like in the context of the geometric theory
of gravity (GSG) based on a single scalar field. There are two distinct classes
of cosmological solutions. An interesting feature is the possibility of having
a bounce without invoking exotic equations of state for the cosmic fluid. We
also discuss cosmological perturbation and present the basis of structure
formation by gravitational instability in the framework of the geometric scalar
gravity.Comment: 12 pages, 5 figures, accepted for publication in Phys. Rev.
More about scalar gravity
We discuss a class of models for gravity based on a scalar field. The models
include and generalize the old approach by Nordstr\"om which predated and in
some way inspired General Relativity. The class include also a model that we
have recently introduced and discussed in its cosmological aspects (GSG). We
present here a complete characterisation of the Schwarschild geometry as a
vacuum solution of GSG and sketch a discussion of the first Post-Newtonian
approximation.Comment: 11 pages, 1 figure, accepted for publication in PR
Photosynthetic activity in grafted and ownrooted Erbaluce grapevines trained to four trellis systems
Photosynthetic activity of grafted (Kober 5BB) and ownrooted grapevines (cv. Erbaluce) trained to four trellis systems (alternate curtain, central curtain with one or two canes and pergola) was assessed for three years. The overall photosynthetic response to training, the seasonal and diurnal photosynthetic trends and the response of the training systems to different photosynthetic photon flux densities were evaluated. The training systems had a significant influence on the rate of photosynthetic assimilation: the two central curtains generally had higher rates of CO2 uptake compared to the vines which were trained to pergola and to alternate curtain. With one exception Kober 5BB improved net assimilation rates
The sonic analogue of black hole radiation
A microscopic description of Hawking radiation in sonic black holes has been
recently presented (Giovanazzi S 2005 Phys. Rev. Lett. 94 061302). This exactly
solvable model is formulated in terms of one-dimensional scattering of a Fermi
gas. In this paper, the model is extended to account possible finite size
effects of a realistic geometry. The flow of particles is maintained by a
piston (i.e. an impenetrable barrier) moving slowly towards the sonic horizon.
Using existing technologies the Hawking temperature can be of order of a few
microkelvin in a realistic experiment.Comment: 14 pages, 7 figures, submitted to Journal of Physics B: Atomic,
Molecular & Optical Physic
Vacuum polarization in the spacetime of charged nonlinear black hole
Building on general formulas obtained from the approximate renormalized
effective action, the approximate stress-energy tensor of the quantized massive
scalar field with arbitrary curvature coupling in the spacetime of charged
black hole being a solution of coupled equations of nonlinear electrodynamics
and general relativity is constructed and analysed. It is shown that in a few
limiting cases, the analytical expressions relating obtained tensor to the
general renormalized stress-energy tensor evaluated in the geometry of the
Reissner-Nordstr\"{o}m black hole could be derived. A detailed numerical
analysis with special emphasis put on the minimal coupling is presented and the
results are compared with those obtained earlier for the conformally coupled
field. Some novel features of the renormalized stress-energy tensor are
discussed
Simple quantum cosmology: Vacuum energy and initial state
A static non-singular 10-dimensional closed Friedmann universe of Planck
size, filled with a perfect fluid with an equation of state with w = -2/3, can
arise spontaneously by a quantum fluctuation from nothing in 11-dimensional
spacetime. A quantum transition from this state can initiate the inflationary
quantum cosmology outlined in Ref. 2 [General Relativity and Gravitation 33,
1415, 2001 - gr-qc/0103021]. With no fine-tuning, that cosmology predicts about
60 e-folds of inflation and a vacuum energy density depending only on the
number of extra space dimensions (seven), G, h, c and the ratio between the
strength of gravity and the strength of the strong force. The fraction of the
total energy in the universe represented by this vacuum energy depends on the
Hubble constant. Hubble constant estimates from WMAP, SDSS, the Hubble Key
Project and Sunyaev-Zeldovich and X-ray flux measurements range from 60 to 72
km/(Mpc sec). With a mid-range Hubble constant of 65 km/(Mpc sec), the model in
Ref. 2 predicts Omega-sub-Lambda = 0.7Comment: To be published in General Relativity and Gravitation, Vol. 37, May
2005. 5 pages, no figure
Cosmological Effects of Nonlinear Electrodynamics
It will be shown that a given realization of nonlinear electrodynamics, used
as source of Einstein's equations, generates a cosmological model with
interesting features, namely a phase of current cosmic acceleration, and the
absence of an initial singularity, thus pointing to a way to solve two
important problems in cosmology
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