17,966 research outputs found
Chameleon fields and solar physics
In this article we discuss some aspects of solar physics from the standpoint
of the so-called chameleon fields (i.e. quantum fields, typically scalar, where
the mass is an increasing function of the matter density of the environment).
Firstly, we analyze the effects of a chameleon-induced deviation from standard
gravity just below the surface of the Sun. In particular, we develop solar
models which take into account the presence of the chameleon and we show that
they are inconsistent with the helioseismic data. This inconsistency presents
itself not only with the typical chameleon set-up discussed in the literature
(where the mass scale of the potential is fine-tuned to the meV), but also if
we remove the fine-tuning on the scale of the potential. Secondly, we point out
that, in a model recently considered in the literature (we call this model
"Modified Fujii's Model"), a conceivable interpretation of the solar
oscillations is given by quantum vacuum fluctuations of a chameleon.Comment: 17 pages including figure
Electrodynamics of a Cosmic Dark Fluid
Cosmic Dark Fluid is considered as a non-stationary medium, in which
electromagnetic waves propagate, and magneto-electric field structures emerge
and evolve. A medium - type representation of the Dark Fluid allows us to
involve into analysis the concepts and mathematical formalism elaborated in the
framework of classical covariant electrodynamics of continua, and to
distinguish dark analogs of well-known medium-effects, such as optical
activity, pyro-electricity, piezo-magnetism, electro- and magneto-striction and
dynamo-optical activity. The Dark Fluid is assumed to be formed by a duet of a
Dark Matter (a pseudoscalar axionic constituent) and Dark Energy (a scalar
element); respectively, we distinguish electrodynamic effects induced by these
two constituents of the Dark Fluid. The review contains discussions of ten
models, which describe electrodynamic effects induced by Dark Matter and/or
Dark Energy. The models are accompanied by examples of exact solutions to the
master equations, correspondingly extended; applications are considered for
cosmology and space-times with spherical and pp-wave symmetries. In these
applications we focused the attention on three main electromagnetic phenomena
induced by the Dark Fluid: first, emergence of Longitudinal Magneto-Electric
Clusters; second, generation of anomalous electromagnetic responses; third,
formation of Dark Epochs in the Universe history.Comment: 39 pages, 0 figures, replaced by the version published in MDPI
Journal "Symmetry" (Special Issue: Symmetry: Feature Papers 2016); typos
correcte
Cosmological Effects of Radion Oscillations
We show that the redshift of pressureless matter density due to the expansion
of the universe generically induces small oscillations in the stabilized radius
of extra dimensions (the radion field). The frequency of these oscillations is
proportional to the mass of the radion and can have interesting cosmological
consequences. For very low radion masses () these low frequency oscillations lead to oscillations in
the expansion rate of the universe. The occurrence of acceleration periods
could naturally lead to a resolution of the coincidence problem, without need
of dark energy. Even though this scenario for low radion mass is consistent
with several observational tests it has difficulty to meet fifth force
constraints. If viewed as an effective Brans-Dicke theory it predicts
( is the number of extra dimensions), while
experiments on scales larger than imply . By deriving the
generalized Newtonian potential corresponding to a massive toroidally compact
radion we demonstrate that Newtonian gravity is modified only on scales smaller
than . Thus, these constraints do not apply for
(high frequency oscillations) corresponding to scales less than the current
experiments (). Even though these high frequency oscillations can not
resolve the coincidence problem they provide a natural mechanism for dark
matter generation. This type of dark matter has many similarities with the
axion.Comment: Accepted in Phys. Rev. D. Clarifying comments added in the text and
some additional references include
Symmetron Cosmology
The symmetron is a scalar field associated with the dark sector whose
coupling to matter depends on the ambient matter density. The symmetron is
decoupled and screened in regions of high density, thereby satisfying local
constraints from tests of gravity, but couples with gravitational strength in
regions of low density, such as the cosmos. In this paper we derive the
cosmological expansion history in the presence of a symmetron field, tracking
the evolution through the inflationary, radiation- and matter-dominated epochs,
using a combination of analytical approximations and numerical integration. For
a broad range of initial conditions at the onset of inflation, the scalar field
reaches its symmetry-breaking vacuum by the present epoch, as assumed in the
local analysis of spherically-symmetric solutions and tests of gravity. For the
simplest form of the potential, the energy scale is too small for the symmetron
to act as dark energy, hence we must add a cosmological constant to drive
late-time cosmic acceleration. We briefly discuss a class of generalized,
non-renormalizable potentials that can have a greater impact on the late-time
cosmology, though cosmic acceleration requires a delicate tuning of parameters
in this case.Comment: 42 page
Beyond-one-loop quantum gravity action yielding both inflation and late-time acceleration
A unified description of early-time inflation with the current cosmic
acceleration is achieved by means of a new theory that uses a quadratic model
of gravity, with the inclusion of an exponential -gravity contribution
for dark energy. High-curvature corrections of the theory come from
higher-derivative quantum gravity and yield an effective action that goes
beyond the one-loop approximation. It is shown that, in this theory, viable
inflation emerges in a natural way, leading to a spectral index and
tensor-to-scalar ratio that are in perfect agreement with the most reliable
Planck results. At low energy, late-time accelerated expansion takes place. As
exponential gravity, for dark energy, must be stabilized during the matter and
radiation eras, we introduce a curing term in order to avoid nonphysical
singularities in the effective equation of state parameter. The results of our
analysis are confirmed by accurate numerical simulations, which show that our
model does fit the most recent cosmological data for dark energy very
precisely.Comment: 20 pages, to appear in NP
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