682 research outputs found
Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach
Gravitational wave production from bubble collisions was calculated in the
early nineties using numerical simulations. In this paper, we present an
alternative analytic estimate, relying on a different treatment of
stochasticity. In our approach, we provide a model for the bubble velocity
power spectrum, suitable for both detonations and deflagrations. From this, we
derive the anisotropic stress and analytically solve the gravitational wave
equation. We provide analytical formulae for the peak frequency and the shape
of the spectrum which we compare with numerical estimates. In contrast to the
previous analysis, we do not work in the envelope approximation. This paper
focuses on a particular source of gravitational waves from phase transitions.
In a companion article, we will add together the different sources of
gravitational wave signals from phase transitions: bubble collisions,
turbulence and magnetic fields and discuss the prospects for probing the
electroweak phase transition at LISA.Comment: 48 pages, 14 figures. v2 (PRD version): calculation refined; plots
redone starting from Fig. 4. Factor 2 in GW energy spectrum corrected. Main
conclusions unchanged. v3: Note added at the end of paper to comment on the
new results of 0901.166
En 2010, les PME bénéficient de la reprise mais diffèrent leurs investissements.
En 2010, la reprise de l’activité permet aux PME d’améliorer leur rentabilité et de consolider leurs capitaux propres, même si de fortes disparités demeurent. Le renforcement de la trésorerie et la faiblesse de l’investissement demeurent caractéristiques d’un comportement attentiste.PME, activité, rentabilité, endettement, investissement.
The Value of Spirituality within the Workplace: A Discussion and Proposal for Research
This paper considers the definitions of workplace spirituality, its value, and how the defining foundational concepts are manifested within organizations today. Also, if there is value, what is the process of educating future leaders so that they are prepared to create and sustain workplace spirituality as part of the cultural fabric of their organizations? Last, there is a call for dialogue between management practitioners and theorists as to whether the phrase, Workplace Spirituality, can thrive within the management practitioner’s vernacular. Perhaps it is more effective to refer to the workplace spirituality concepts under their covert labels and avoid using the controversial “S” word
Distributional reaction time properties in the Eriksen task: marked differences or hidden similarities with the Simon task?
In conflict tasks, the irrelevant stimulus attribute needs to be suppressed for the correct response to be produced. In the Simon task, earlier researchers have proposed that this suppression is the reason that, after an initial increase, the interference effect decreases for longer RTs, as reflected by late, negative-going delta plots. This view has been challenged by observations of positive-going delta plots, even for long RTs, in other conflict tasks, despite a similar necessity for suppression. For late negative-going delta plots to be interpreted as reflecting suppression, a necessary, although maybe not sufficient, condition is that similar patterns should be observed for other conflict tasks. We reasoned that a similar suppression could be present, but hidden, in the Eriksen flanker task. By recording and analyzing electromyograms of the muscles involved in response execution, we could compute delta plots separately for trials that elicited a subthreshold incorrect response activation (partial error). Late negative-going delta plots were observable on partial-error trials, although they were weaker than for the Simon task, reducing the impact of this inversion on the overall distribution. We further showed that this pattern is modulated by time pressure. Those results indicate that mechanisms leading to negative-going delta plots, similar to those observed in the Simon task, are also at play in the Eriksen task. The link between negative-going delta plots and executive online control is discussed
Behind the times: a brief history of motivation discourse in problem-based learning
That idea that problem-based learning (PBL) is more motivating that traditional education has been prevalent since the inception of PBL at McMaster University in the late 1960s. Evidencing this through empirical research, however, has proven to be a lot more problematic. This paper retraces how the discourse on motivation started from a laymen's conception in the early days of PBL, and slowly evolved into a field of scientific inquiry in the 1980s and 1990s. However, looking at the evolution of motivation theory over the same period, we show that motivation discourse in the burgeoning literature on motivation and PBL remained largely wedded to the laymen's approach, and failed to catch up with the new achievement-goal theory and self-determination theory approaches. This paper proceeds to analyse the explosion of studies on PBL and motivation after 2000, acknowledging efforts to move away from anecdotal accounts and provide theoretical grounding to the research. However, once again, we show that the majority of the research employed outdated motivational measures that do not fully grasp the complexity of contemporary motivation theory. The paper concludes on the observation that single-course and curriculum-wide research interventions have yielded no conclusive results on the effect of PBL on intrinsic motivation, and that future research should therefore seek to use up-to-date motivational constructs in more targeted interventions
The Indirect Search for Dark Matter with IceCube
We revisit the prospects for IceCube and similar kilometer-scale telescopes
to detect neutrinos produced by the annihilation of weakly interacting massive
dark matter particles (WIMPs) in the Sun. We emphasize that the astrophysics of
the problem is understood; models can be observed or, alternatively, ruled out.
In searching for a WIMP with spin-independent interactions with ordinary
matter, IceCube is only competitive with direct detection experiments if the
WIMP mass is sufficiently large. For spin-dependent interactions IceCube
already has improved the best limits on spin-dependent WIMP cross sections by
two orders of magnitude. This is largely due to the fact that models with
significant spin-dependent couplings to protons are the least constrained and,
at the same time, the most promising because of the efficient capture of WIMPs
in the Sun. We identify models where dark matter particles are beyond the reach
of any planned direct detection experiments while being within reach of
neutrino telescopes. In summary, we find that, even when contemplating recent
direct detection results, neutrino telescopes have the opportunity to play an
important as well as complementary role in the search for particle dark matter.Comment: 17 pages, 10 figures, published in the New Journal of Physics 11
105019 http://www.iop.org/EJ/abstract/1367-2630/11/10/105019, new version
submitted to correct Abstract in origina
Cosmological Consequences of Nearly Conformal Dynamics at the TeV scale
Nearly conformal dynamics at the TeV scale as motivated by the hierarchy
problem can be characterized by a stage of significant supercooling at the
electroweak epoch. This has important cosmological consequences. In particular,
a common assumption about the history of the universe is that the reheating
temperature is high, at least high enough to assume that TeV-mass particles
were once in thermal equilibrium. However, as we discuss in this paper, this
assumption is not well justified in some models of strong dynamics at the TeV
scale. We then need to reexamine how to achieve baryogenesis in these theories
as well as reconsider how the dark matter abundance is inherited. We argue that
baryonic and dark matter abundances can be explained naturally in these setups
where reheating takes place by bubble collisions at the end of the strongly
first-order phase transition characterizing conformal symmetry breaking, even
if the reheating temperature is below the electroweak scale GeV. We
also discuss inflation as well as gravity wave smoking gun signatures of this
class of models.Comment: 22 pages, 7 figure
Cosmological Expansion in the Presence of an Extra Dimension
It has recently been pointed out that global solutions of Einstein's
equations for a 3-brane universe embedded in 4 spatial dimensions give rise to
a Friedmann equation of the form H ~ rho on the brane, instead of the usual H ~
(rho)^{1/2}, which is inconsistent with cosmological observations. We remedy
this problem by adding cosmological constants to the brane and the bulk, as in
the recent scenario of Randall and Sundrum. Our observation allows for normal
expansion during nucleosynthesis, but faster than normal expansion in the very
early universe, which could be helpful for electroweak baryogenesis, for
example.Comment: 4pp, latex, 1 figure; added and corrected references; revised
incorrect argument about sign of action on brane; final version to be
published in PR
Energy Budget of Cosmological First-order Phase Transitions
The study of the hydrodynamics of bubble growth in first-order phase
transitions is very relevant for electroweak baryogenesis, as the baryon
asymmetry depends sensitively on the bubble wall velocity, and also for
predicting the size of the gravity wave signal resulting from bubble
collisions, which depends on both the bubble wall velocity and the plasma fluid
velocity. We perform such study in different bubble expansion regimes, namely
deflagrations, detonations, hybrids (steady states) and runaway solutions
(accelerating wall), without relying on a specific particle physics model. We
compute the efficiency of the transfer of vacuum energy to the bubble wall and
the plasma in all regimes. We clarify the condition determining the runaway
regime and stress that in most models of strong first-order phase transitions
this will modify expectations for the gravity wave signal. Indeed, in this
case, most of the kinetic energy is concentrated in the wall and almost no
turbulent fluid motions are expected since the surrounding fluid is kept mostly
at rest.Comment: 36 pages, 14 figure
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