400 research outputs found
Adiabatic instability in coupled dark energy-dark matter models
We consider theories in which there exists a nontrivial coupling between the
dark matter sector and the sector responsible for the acceleration of the
universe. Such theories can possess an adiabatic regime in which the
quintessence field always sits at the minimum of its effective potential, which
is set by the local dark matter density. We show that if the coupling strength
is much larger than gravitational, then the adiabatic regime is always subject
to an instability. The instability, which can also be thought of as a type of
Jeans instability, is characterized by a negative sound speed squared of an
effective coupled dark matter/dark energy fluid, and results in the exponential
growth of small scale modes. We discuss the role of the instability in specific
coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and
clarify for these theories the regimes in which the instability can be evaded
due to non-adiabaticity or weak coupling.Comment: 20 pages, 2 figures; final published versio
Rotating saddle trap as Foucault's pendulum
One of the many surprising results found in the mechanics of rotating systems
is the stabilization of a particle in a rapidly rotating planar saddle
potential. Besides the counterintuitive stabilization, an unexpected
precessional motion is observed. In this note we show that this precession is
due to a Coriolis-like force caused by the rotation of the potential. To our
knowledge this is the first example where such force arises in an inertial
reference frame. We also propose an idea of a simple mechanical demonstration
of this effect.Comment: 13 pages, 9 figure
Impact Of Death-Related Television Programming On Advertising Evaluation
Programming that depicts or implies death constitutes a vital component of daily television broadcasts, yet the impact of such programming on the evaluation of embedded advertising remains unexplored. Using terror management theory, we propose that exposure to routine and commonplace death-related television programming will lead to the differential evaluation of ensuing advertisements depending on whether they are perceived to be of domestic or foreign origin, and that this effect is contingent on the nature of the death-related cognitions at play at the time of ad exposure. Further, the death-related program context effects on embedded advertising are unique—ads at the end, not the beginning, of the embedded pod are affected by programming content. Based on the empirical findings from four studies we identify the cognitive processes underlying consumers’ ad evaluation, isolate the pod positions when context effects are observed, and suggest a framework for television commercial scheduling
Smectic-C tilt under shear in Smectic-A elastomers
Stenull and Lubensky [Phys. Rev. E {\bf 76}, 011706 (2007)] have argued that
shear strain and tilt of the director relative to the layer normal are coupled
in smectic elastomers and that the imposition of one necessarily leads to the
development of the other. This means, in particular, that a Smectic-A elastomer
subjected to a simple shear will develop Smectic-C-like tilt of the director.
Recently, Kramer and Finkelmann [arXiv:0708.2024, Phys. Rev. E {\bf 78}, 021704
(2008)] performed shear experiments on Smectic-A elastomers using two different
shear geometries. One of the experiments, which implements simple shear,
produces clear evidence for the development of Smectic-C-like tilt. Here, we
generalize a model for smectic elastomers introduced by Adams and Warner [Phys.
Rev. E {\bf 71}, 021708 (2005)] and use it to study the magnitude of
Smectic-C-like tilt under shear for the two geometries investigated by Kramer
and Finkelmann. Using reasonable estimates of model parameters, we estimate the
tilt angle for both geometries, and we compare our estimates to the
experimental results. The other shear geometry is problematic since it
introduces additional in-plane compressions in a sheet-like sample, thus
inducing instabilities that we discuss.Comment: 8 pages, 5 figure
Harmonic moment dynamics in Laplacian growth
Harmonic moments are integrals of integer powers of z = x+iy over a domain.
Here the domain is an exterior of a bubble of air growing in an oil layer
between two horizontal closely spaced plates. Harmonic moments are a natural
basis for such Laplacian growth phenomena because, unlike other
representations, these moments linearize the zero surface tension problem
(Richardson, 1972), so that all moments except the lowest one are conserved in
time. For non-zero surface tension, we show that the the harmonic moments decay
in time rather than exhibiting the divergences of other representations. Our
laboratory observations confirm the theoretical predictions and demonstrate
that an interface dynamics description in terms of harmonic moments is
physically realizable and robust. In addition, by extending the theory to
include surface tension, we obtain from measurements of the time evolution of
the harmonic moments a value for the surface tension that is within 20% of the
accepted value.Comment: 10 pages, 7 figure
Shape of the acoustic gravitational wave power spectrum from a first order phase transition
We present results from large-scale numerical simulations of a first order thermal phase transition in the early Universe, in order to explore the shape of the acoustic gravitational wave and the velocity power spectra. We compare the results with the predictions of the recently proposed sound shell model. For the gravitational wave power spectrum, we find that the predicted k(-3) behavior, where k is the wave number, emerges clearly for detonations. The power spectra from deflagrations show similar features, but exhibit a steeper high-k decay and an extra feature not accounted for in the model. There are two independent length scales: the mean bubble separation and the thickness of the sound shell around the expanding bubble of the low temperature phase. It is the sound shell thickness which sets the position of the peak of the power spectrum. The low wave number behavior of the velocity power spectrum is consistent with a causal k(3), except for the thinnest sound shell, where it is steeper. We present parameters for a simple broken power law fit to the gravitational wave power spectrum for wall speeds well away from the speed of sound where this form can be usefully applied. We examine the prospects for the detection, showing that a LISA-like mission has the sensitivity to detect a gravitational wave signal from sound waves with an RMS fluid velocity of about 0.05c, produced from bubbles with a mean separation of about 10(-2) of the Hubble radius. The shape of the gravitational wave power spectrum depends on the bubble wall speed, and it may be possible to estimate the wall speed, and constrain other phase transition parameters, with an accurate measurement of a stochastic gravitational wave background.Peer reviewe
Spectroscopy of Ultracold, Trapped Cesium Feshbach Molecules
We explore the rich internal structure of Cs_2 Feshbach molecules. Pure
ultracold molecular samples are prepared in a CO_2-laser trap, and a multitude
of weakly bound states is populated by elaborate magnetic-field ramping
techniques. Our methods use different Feshbach resonances as input ports and
various internal level crossings for controlled state transfer. We populate
higher partial-wave states of up to eight units of rotational angular momentum
(l-wave states). We investigate the molecular structure by measurements of the
magnetic moments for various states. Avoided level crossings between different
molecular states are characterized through the changes in magnetic moment and
by a Landau-Zener tunneling method. Based on microwave spectroscopy, we present
a precise measurement of the magnetic-field dependent binding energy of the
weakly bound s-wave state that is responsible for the large background
scattering length of Cs. This state is of particular interest because of its
quantum-halo character.Comment: 15 pages, 12 figures, 4 table
Managing terror when self-worth and worldviews collide: Evidence that mortality salience increases reluctance to self-enhance beyond authorities.
a b s t r a c t Terror management theory posits that one's self-esteem and worldview operate jointly to manage mortality concerns. Accordingly, past research shows that mortality salience (MS) increases selfenhancement and worldview defense. The current research is the first to examine MS effects when self-enhancement threatens to undermine aspects of the worldview, in this case the credibility and status of worldview-representative authorities. MS led to reluctance to self-enhance following positive personality test feedback when the test was judged negatively by institutional authorities (Study 1a), as well as unwillingness to contradict self-esteem threatening feedback sanctioned by authorities (Study 1b). Mortality salient participants also rated themselves higher on valued dimensions unless it meant viewing themselves more positively than their parents (Study 2) and admired political icons (Study 3). Taken together, these results show that MS increases self-enhancement unless doing so challenges important representatives of the worldview. Theoretical and practical implications are discussed
Extracting Atoms on Demand with Lasers
We propose a scheme that allows to coherently extract cold atoms from a
reservoir in a deterministic way. The transfer is achieved by means of
radiation pulses coupling two atomic states which are object to different
trapping conditions. A particular realization is proposed, where one state has
zero magnetic moment and is confined by a dipole trap, whereas the other state
with non-vanishing magnetic moment is confined by a steep microtrap potential.
We show that in this setup a predetermined number of atoms can be transferred
from a reservoir, a Bose-Einstein condensate, into the collective quantum state
of the steep trap with high efficiency in the parameter regime of present
experiments.Comment: 11 pages, 8 figure
Can Cosmic Parallax Distinguish Between Anisotropic Cosmologies?
In an anisotropic universe, observers not positioned at a point of special
symmetry should observe cosmic parallax - the relative angular motion of test
galaxies over cosmic time. It was recently argued that the non-observance of
this effect in upcoming precision astrometry missions such as Gaia may be used
to place strong bounds on the position of off-center observers in a void-model
universe described by the Lemaitre-Tolman-Bondi metric. We consider the
analogous effect in anisotropic cosmological models described by an
axisymmetric homogeneous Bianchi type I metric and discuss whether any
observation of cosmic parallax would distinguish between different anisotropic
evolutions.Comment: 24 pages, 6 figure
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