198 research outputs found
Light deflection by gravitational waves from localized sources
We study the deflection of light (and the redshift, or integrated time delay)
caused by the time-dependent gravitational field generated by a localized
material source lying close to the line of sight. Our calculation explicitly
takes into account the full, near-zone, plus intermediate-zone, plus wave-zone,
retarded gravitational field. Contrary to several recent claims in the
literature, we find that the deflections due to both the wave-zone 1/r
gravitational wave and the intermediate-zone 1/r^2 retarded fields vanish
exactly. The leading total time-dependent deflection caused by a localized
material source, such as a binary system, is proven to be given by the
quasi-static, near-zone quadrupolar piece of the gravitational field, and
therefore to fall off as the inverse cube of the impact parameter.Comment: 12 pages, REVTeX 3.0, no figur
Nonstationary Stochastic Resonance in a Single Neuron-Like System
Stochastic resonance holds much promise for the detection of weak signals in
the presence of relatively loud noise. Following the discovery of nondynamical
and of aperiodic stochastic resonance, it was recently shown that the
phenomenon can manifest itself even in the presence of nonstationary signals.
This was found in a composite system of differentiated trigger mechanisms
mounted in parallel, which suggests that it could be realized in some
elementary neural networks or nonlinear electronic circuits. Here, we find that
even an individual trigger system may be able to detect weak nonstationary
signals using stochastic resonance. The very simple modification to the trigger
mechanism that makes this possible is reminiscent of some aspects of actual
neuron physics. Stochastic resonance may thus become relevant to more types of
biological or electronic systems injected with an ever broader class of
realistic signals.Comment: Plain Latex, 7 figure
The stability of modified gravity models
Conditions for the existence and stability of de Sitter space in modified
gravity are derived by considering inhomogeneous perturbations in a
gauge-invariant formalism. The stability condition coincides with the
corresponding condition for stability with respect to homogeneous
perturbations, while this is not the case in scalar-tensor gravity. The
stability criterion is applied to various modified gravity models of the early
and the present universe.Comment: 22 pages, LaTeX, to appear in Phys. Rev.
Nonstationary Stochastic Resonance
It is by now established that, remarkably, the addition of noise to a
nonlinear system may sometimes facilitate, rather than hamper the detection of
weak signals. This phenomenon, usually referred to as stochastic resonance, was
originally associated with strictly periodic signals, but it was eventually
shown to occur for stationary aperiodic signals as well. However, in several
situations of practical interest, the signal can be markedly nonstationary. We
demonstrate that the phenomenon of stochastic resonance extends to
nonstationary signals as well, and thus could be relevant to a wider class of
biological and electronic applications. Building on both nondynamic and
aperiodic stochastic resonance, our scheme is based on a multilevel trigger
mechanism, which could be realized as a parallel network of differentiated
threshold sensors. We find that optimal detection is reached for a number of
thresholds of order ten, and that little is gained by going much beyond that
number. We raise the question of whether this is related to the fact that
evolution has favored some fixed numbers of precisely this order of magnitude
in certain aspects of sensory perception.Comment: Plain Latex, 6 figure
Non-chaotic dynamics in general-relativistic and scalar-tensor cosmology
In the context of scalar-tensor models of dark energy and inflation, the
dynamics of vacuum scalar-tensor cosmology are analysed without specifying the
coupling function or the scalar field potential. A conformal transformation to
the Einstein frame is used and the dynamics of general relativity with a
minimally coupled scalar field are derived for a generic potential. It is shown
that the dynamics are non-chaotic, thus settling an existing debate.Comment: 20 pages, LaTeX, to appear in Class. Quantum Gra
Multiple imaging by gravitational waves
Gravitational waves act like lenses for the light propagating through them.
This phenomenon is described using the vector formalism employed for ordinary
gravitational lenses, which was proved to be applicable also to a
non-stationary spacetime, with the appropriate modifications. In order to have
multiple imaging, an approximate condition analogous to that for ordinary
gravitational lenses must be satisfied. Certain astrophysical sources of
gravitational waves satisfy this condition, while the gravitational wave
background, on average, does not. Multiple imaging by gravitational waves is,
in principle, possible, but the probability of observing such a phenomenon is
extremely low.Comment: 23 pages, LaTeX, no figures, to appear in Int. J. Mod. Phys.
Complete constraints on a nonminimally coupled chaotic inflationary scenario from the cosmic microwave background
We present complete constraints imposed from observations of the cosmic
microwave background radiation (CMBR) on the chaotic inflationary scenario with
a nonminimally coupled inflaton field proposed by Fakir and Unruh (FU). Our
constraints are complete in the sense that we investigate both the scalar
density perturbation and the tensor gravitational wave in the Jordan frame, as
well as in the Einstein frame. This makes the constraints extremely strong
without any ambiguities due to the choice of frames. We find that the FU
scenario generates tiny tensor contributions to the CMBR relative to chaotic
models in minimal coupling theory, in spite of its spectral index of scalar
perturbation being slightly tilted. This means that the FU scenario will be
excluded if any tensor contributions to CMBR are detected by the forthcoming
satellite missions. Conversely, if no tensor nature is detected despite the
tilted spectrum, a minimal chaotic scenario will be hard to explain and the FU
scenario will be supported.Comment: 7 pages, no figure, RevTeX, to appear in Phys.Rev. D59 (Mar. 15,
1999
Bending of Light by Gravity Waves
We describe the statistical properties of light rays propagating though a
random sea of gravity waves and compare with the case for scalar metric
perturbations from density inhomogeneities. For scalar fluctuations the
deflection angle grows as the square-root of the path length in the manner
of a random walk, and the rms displacement of a ray from the unperturbed
trajectory grows as . For gravity waves the situation is very
different. The mean square deflection angle remains finite and is dominated by
the effect of the metric fluctuations at the ends of the ray, and the mean
square displacement grows only as the logarithm of the path length. In terms of
power spectra, the displacement for scalar perturbations has while for gravity waves the trajectories of photons have which is a scale-invariant or `flicker-noise' process, and departures from
rectilinear motion are suppressed, relative to the scalar case, by a factor
where is the characteristic scale of the
metric fluctuations and is the path length. This result casts doubt on the
viability of some recent proposals for detecting or constraining the gravity
wave background by astronomical measurements.Comment: 14 pages, aastex, submitted to Astrophysical Journa
Inflation and quintessence with nonminimal coupling
The nonminimal coupling (NMC) of the scalar field to the Ricci curvature is
unavoidable in many cosmological scenarios. Inflation and quintessence models
based on nonminimally coupled scalar fields are studied, with particular
attention to the balance between the scalar potential and the NMC term in the
action. NMC makes acceleration of the universe harder to achieve for the usual
potentials, but it is beneficial in obtaining cosmic acceleration with unusual
potentials. The slow-roll approximation with NMC, conformal transformation
techniques, and other aspects of the physics of NMC are clarified.Comment: 36 pages, LaTeX. Typos in Eq. (2.5) correcte
Phase Transition in Conformally Induced Gravity with Torsion
We have considered the quantum behavior of a conformally induced gravity in
the minimal Riemann-Cartan space. The regularized one-loop effective potential
considering the quantum fluctuations of the dilaton and the torsion fields in
the Coleman-Weinberg sector gives a sensible phase transition for an
inflationary phase in De Sitter space. For this effective potential, we have
analyzed the semi-classical equation of motion of the dilaton field in the
slow-rolling regime.Comment: 7pages, no figur
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