993 research outputs found
EIT2015: promoting electrical impedance tomography as a non-invasive monitoring technology
Editorial. No abstract
Thermodynamical aspects of running vacuum models
The thermal history of a large class of running vacuum models in which the
effective cosmological term is described by a truncated power series of the
Hubble rate, whose dominant term is , is discussed
in detail. Specifically, by assuming that the ultra-relativistic particles
produced by the vacuum decay emerge into space-time in such a way that its
energy density , the temperature evolution law and the
increasing entropy function are analytically calculated. For the whole class of
vacuum models explored here we findthat the primeval value of the comoving
radiation entropy density (associated to effectively massless particles) starts
from zero and evolves extremely fast until reaching a maximum near the end of
the vacuum decay phase, where it saturates. The late time conservation of the
radiation entropy during the adiabatic FRW phase also guarantees that the whole
class of running vacuum models predicts thesame correct value of the present
day entropy, (in natural units), independently of the
initial conditions. In addition, by assuming Gibbons-Hawking temperature as an
initial condition, we find that the ratio between the late time and primordial
vacuum energy densities is in agreement with naive estimates from quantum field
theory, namely, . Such results
are independent on the power and suggests that the observed Universe may
evolve smoothly between two extreme, unstable, nonsingular de Sitter phases.Comment: 15 pages in free style, 2 figures, to appear in European Phys.
Journal C.,(this work generalizes that of arXiv:1412.5196
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
What is there in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?
The coincidence problems and other dynamical features of dark energy are
studied in cosmological models with variable cosmological parameters and in
models with the composite dark energy. It is found that many of the problems
usually considered to be cosmological coincidences can be explained or
significantly alleviated in the aforementioned models.Comment: 6 pages, 1 figure, talk given at IRGAC2006 (Barcelona, July 11-15,
2006), to appear in J. Phys.
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