666 research outputs found
Is nonrelativistic gravity possible?
We study nonrelativistic gravity using the Hamiltonian formalism. For the
dynamics of general relativity (relativistic gravity) the formalism is well
known and called the Arnowitt-Deser-Misner (ADM) formalism. We show that if the
lapse function is constrained correctly, then nonrelativistic gravity is
described by a consistent Hamiltonian system. Surprisingly, nonrelativistic
gravity can have solutions identical to relativistic gravity ones. In
particular, (anti-)de Sitter black holes of Einstein gravity and IR limit of
Horava gravity are locally identical.Comment: 4 pages, v2, typos corrected, published in Physical Review
Cavity polariton optomechanics: Polariton path to fully resonant dispersive coupling in optomechanical resonators
Resonant photoelastic coupling in semiconductor nanostructures opens new
perspectives for strongly enhanced light-sound interaction in optomechanical
resonators. One potential problem, however, is the reduction of the cavity
Q-factor induced by dissipation when the resonance is approached. We show in
this letter that cavity-polariton mediation in the light-matter process
overcomes this limitation allowing for a strongly enhanced photon-phonon
coupling without significant lifetime reduction in the strongly-coupled regime.
Huge optomechanical coupling factors in the PetaHz/nm range are envisaged,
three orders of magnitude larger than the backaction produced by the mechanical
displacement of the cavity mirrors.Comment: 6 pages, 4 figure
Microscopic nanomechanical dissipation in gallium arsenide resonators
We report on a systematic study of nanomechanical dissipation in
high-frequency (approximatively 300 MHz) gallium arsenide optomechanical disk
resonators, in conditions where clamping and fluidic losses are negligible.
Phonon-phonon interactions are shown to contribute with a loss background
fading away at cryogenic temperatures (3 K). Atomic layer deposition of alumina
at the surface modifies the quality factor of resonators, pointing towards the
importance of surface dissipation. The temperature evolution is accurately
fitted by two-level systems models, showing that nanomechanical dissipation in
gallium arsenide resonators directly connects to their microscopic properties.
Two-level systems, notably at surfaces, appear to rule the damping and
fluctuations of such high-quality crystalline nanomechanical devices, at all
temperatures from 3 to 300K
Hawking radiation in an electro-magnetic wave-guide?
It is demonstrated that the propagation of electro-magnetic waves in an
appropriately designed wave-guide is (for large wave-lengths) analogous to that
within a curved space-time -- such as around a black hole. As electro-magnetic
radiation (e.g., micro-weaves) can be controlled, amplified, and detected (with
present-day technology) much easier than sound, for example, we propose a
set-up for the experimental verification of the Hawking effect. Apart from
experimentally testing this striking prediction, this would facilitate the
investigation of the trans-Planckian problem. PACS: 04.70.Dy, 04.80.-y,
42.50.-p, 84.40.Az.Comment: 4 pages RevTeX, 1 figur
Double Distribution of Dark Matter Halos with respect to Mass and Local Overdensity
We present a double distribution function of dark matter halos, with respect
to both object mass and local over- (or under-) density. This analytical tool
provides a statistical treatment of the properties of matter surrounding
collapsed objects, and can be used to study environmental effects on
hierarchical structure formation. The size of the "local environment" of a
collapsed object is defined to depend on the mass of the object. The
Press-Schechter mass function is recovered by integration of our double
distribution over the density contrast. We also present a detailed treatment of
the evolution of overdensities and underdensities in Einstein-deSitter and flat
LCDM universes, according to the spherical evolution model. We explicitly
distinguish between true and linearly extrapolated overdensities and provide
conversion relations between the two quantities.Comment: 25 pages, 10 figures, comments welcom
A stable static Universe?
Starting from the assumption that general relativity might be an emergent
phenomenon showing up at low-energies from an underlying microscopic structure,
we re-analyze the stability of a static closed Universe filled with radiation.
In this scenario, it is sensible to consider the effective general-relativistic
configuration as in a thermal contact with an "environment" (the role of
environment can be played, for example, by the higher-dimensional bulk or by
the trans-Planckian degrees of freedom). We calculate the free energy at a
fixed temperature of this radiation-filled static configuration. Then, by
looking at the free energy we show that the static Einstein configuration is
stable under the stated condition.Comment: 6 pages, no figures; uses revtex4; version accepted for publication;
some phrasing changed; wrong coefficient correcte
Stable manifolds and homoclinic points near resonances in the restricted three-body problem
The restricted three-body problem describes the motion of a massless particle
under the influence of two primaries of masses and that circle
each other with period equal to . For small , a resonant periodic
motion of the massless particle in the rotating frame can be described by
relatively prime integers and , if its period around the heavier primary
is approximately , and by its approximate eccentricity . We give a
method for the formal development of the stable and unstable manifolds
associated with these resonant motions. We prove the validity of this formal
development and the existence of homoclinic points in the resonant region.
In the study of the Kirkwood gaps in the asteroid belt, the separatrices of
the averaged equations of the restricted three-body problem are commonly used
to derive analytical approximations to the boundaries of the resonances. We use
the unaveraged equations to find values of asteroid eccentricity below which
these approximations will not hold for the Kirkwood gaps with equal to
2/1, 7/3, 5/2, 3/1, and 4/1.
Another application is to the existence of asymmetric librations in the
exterior resonances. We give values of asteroid eccentricity below which
asymmetric librations will not exist for the 1/7, 1/6, 1/5, 1/4, 1/3, and 1/2
resonances for any however small. But if the eccentricity exceeds these
thresholds, asymmetric librations will exist for small enough in the
unaveraged restricted three-body problem
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