39 research outputs found
Evolution of the Order Parameter after Bubble Collisions
If a first-order phase transition is terminated by collisions of new-phase
bubbles, there will exist a period of nonequilibrium between the time bubbles
collide and the time thermal equilibrium is established. We study the behavior
of the order parameter during this phase. We find that large nonthermal
fluctuations at this stage tend to restore symmetry, i.e., the order parameter
is smaller than its eventual thermal equilibrium value. We comment on possible
consequences for electroweak baryogenesis.Comment: 11 page LaTeX file with two figures, fig1.ps and fig2.p
Dynamics of Symmetry Breaking and Tachyonic Preheating
We reconsider the old problem of the dynamics of spontaneous symmetry
breaking using 3d lattice simulations, and develop a theory of tachyonic
preheating, which occurs due to the spinodal instability of the scalar field.
Tachyonic preheating is so efficient that symmetry breaking typically completes
within a single oscillation of the field distribution as it rolls towards the
minimum of its effective potential. As an application of this theory we
consider preheating in the hybrid inflation scenario, including SUSY-motivated
F-term and D-term inflationary models. We show that preheating in hybrid
inflation is typically tachyonic and the stage of oscillations of a homogeneous
component of the scalar fields driving inflation ends after a single
oscillation. Our results may also be relevant for the theory of the formation
of disoriented chiral condensates in heavy ion collisions.Comment: 7 pages, 6 figures. Higher quality figures and computer generated
movies in gif format illustrating our results can be found at
http://physics.stanford.edu/gfelder/hybri
Postural Responses Following Space Flight and Ground Based Analogs
With the transition from the Shuttle program to the International Space Station (ISS), the opportunity to fly sensorimotor experiments in a weightless environment has become increasingly more difficult to obtain. As a result, more investigations have turned to ground-based analogs as a way of evaluating an experiment's viability. The two primary analogs available to most investigators are 6deg head down bed rest (HDBR) and dry immersion (DI). For the time being, HDBR investigations have been associated with studies conducted in the United States while the Russians and several other European Union states have concentrated their efforts on using DI as the space flight analog of choice. While either model may be viable for cardiovascular, bone and other system changes, vestibular and sensorimotor investigators have retained serious reservations of either analog's potential to serve as a replacement for a true weightless environment. These reservations have merit, but it is worthwhile to consider that not all changes associated with sensorimotor function during space flight are the result of top-down modifications, but may also be due to the lack, or change, of appropriate support surfaces applying force to the bottom of the feet. To this end we have compared quiet stance postural responses between short duration Space Shuttle flights, long duration ISS flights and HDBR of varying duration. Using these three platforms, representing different modifications of support we investigated postural ataxia using a quiet stance model. Quiet stance was obtained by asking the subjects to stand upright on a force plate, eyes open, arms at the side of the body for three min. From the force plate we obtained average sway velocity in two axes as well as length of line (stabilogram). These parameters were then related to EMG activity recorded from the medial gastrocnemius and lateral tibialis. It is significant to note that postural ataxia measured as quiet stance shows analogous changes between HDBR and space flight. Primary differences across short duration, long duration space flight and HDBR are related to the length of exposure associated with both space flight and HDBR
Using low levels of stochastic vestibular stimulation to improve locomotor stability
Low levels of bipolar binaural white noise based imperceptible stochastic electrical stimulation to the vestibular system (stochastic vestibular stimulation, SVS) have been shown to improve stability during balance tasks in normal, healthy subjects by facilitating enhanced information transfer using stochastic resonance (SR) principles. We hypothesize that detection of time-critical sub-threshold sensory signals using low levels of bipolar binaural SVS based on SR principles will help improve stability of walking during support surface perturbations. In the current study 13 healthy subjects were exposed to short continuous support surface perturbations for 60 s while walking on a treadmill and simultaneously viewing perceptually matched linear optic flow. Low levels of bipolar binaural white noise based SVS were applied to the vestibular organs. Multiple trials of the treadmill locomotion test were performed with stimulation current levels varying in the range of 0–1500 μA, randomized across trials. The results show that subjects significantly improved their walking stability during support surface perturbations at stimulation levels with peak amplitude predominantly in the range of 100–500 μA consistent with the SR phenomenon. Additionally, objective perceptual motion thresholds were measured separately as estimates of internal noise while subjects sat on a chair with their eyes closed and received 1 Hz bipolar binaural sinusoidal electrical stimuli. The optimal improvement in walking stability was achieved on average with peak stimulation amplitudes of approximately 35% of perceptual motion threshold. This study shows the effectiveness of using low imperceptible levels of SVS to improve dynamic stability during walking on a laterally oscillating treadmill via the SR phenomenon
Neural Working Memory Changes During a Spaceflight Analog With Elevated Carbon Dioxide: A Pilot Study
Spaceflight missions to the International Space Station (ISS) expose astronauts to
microgravity, radiation, isolation, and elevated carbon dioxide (COâ‚‚), among other
factors. Head down tilt bed rest (HDBR) is an Earth-based analog for spaceflight used to
study body unloading, fluid shifts, and other factors unrelated to gravitational changes.
While in space, astronauts need to use mental rotation strategies to facilitate their
adaptation to the ISS environment. Therefore, spatial working memory is essential for
crewmember performance. Although the effects of HDBR on spatial working memory
have recently been studied, the results are still inconclusive. Here, we expand upon
past work and examine the effects of HDBR with elevated COâ‚‚ (HDBR + COâ‚‚) on
brain activation patterns during spatial working memory performance. In addition,
we compare brain activation between 30 days of HDBR + COâ‚‚ and 70 days of
HDBR to test the isolated effect of COâ‚‚. Eleven subjects (6 males, 5 females; mean
age = 34 ± 8 years) underwent six functional magnetic resonance imaging (fMRI)
sessions pre-, during, and post-HDBR + COâ‚‚. During the HDBR + COâ‚‚ intervention,
we observed decreasing activation in the right middle frontal gyrus and left regions of
the cerebellum, followed by post-intervention recovery. We detected several correlations
between brain and behavioral slopes of change with the HDBR + COâ‚‚ intervention.
For example, greater increases in activation in frontal, temporal and parietal regions
were associated with larger spatial working memory improvements. Comparing the
HDBR + COâ‚‚ group to data from our previous 70-day HDBR study, we found greater
decreases in activation in the right hippocampus and left inferior temporal gyrus for
the HDBR + COâ‚‚ group over the course of the intervention. Together, these findings
increase our understanding of the neural mechanisms of HDBR, elevated levels of COâ‚‚
and spaceflight-related changes in spatial working memory performance
Turbulent Thermalization
We study, analytically and with lattice simulations, the decay of coherent
field oscillations and the subsequent thermalization of the resulting
stochastic classical wave-field. The problem of reheating of the Universe after
inflation constitutes our prime motivation and application of the results. We
identify three different stages of these processes. During the initial stage of
``parametric resonance'', only a small fraction of the initial inflaton energy
is transferred to fluctuations in the physically relevant case of sufficiently
large couplings. A major fraction is transfered in the prompt regime of driven
turbulence. The subsequent long stage of thermalization classifies as free
turbulence. During the turbulent stages, the evolution of particle distribution
functions is self-similar. We show that wave kinetic theory successfully
describes the late stages of our lattice calculation. Our analytical results
are general and give estimates of reheating time and temperature in terms of
coupling constants and initial inflaton amplitude.Comment: 27 pages, 13 figure
Towards an Explicit Model of D-brane Inflation
We present a detailed analysis of an explicit model of warped D-brane
inflation, incorporating the effects of moduli stabilization. We consider the
potential for D3-brane motion in a warped conifold background that includes
fluxes and holomorphically-embedded D7-branes involved in moduli stabilization.
Although the D7-branes significantly modify the inflaton potential, they do not
correct the quadratic term in the potential, and hence do not cause a uniform
change in the slow-roll parameter eta. Nevertheless, we present a simple
example based on the Kuperstein embedding of D7-branes, z_1=constant, in which
the potential can be fine-tuned to be sufficiently flat for inflation. To
derive this result, it is essential to incorporate the fact that the
compactification volume changes slightly as the D3-brane moves. We stress that
the compactification geometry dictates certain relationships among the
parameters in the inflaton Lagrangian, and these microscopic constraints impose
severe restrictions on the space of possible models. We note that the shape of
the final inflaton potential differs from projections given in earlier studies:
in configurations where inflation occurs, it does so near an inflection point.
Finally, we comment on the difficulty of making precise cosmological
predictions in this scenario. This is the companion paper to arXiv:0705.3837.Comment: 68 pages, 6 figures; v2: fixed typos, added refs and clarifications;
v3: expanded discussion of inflection point inflatio
Neural Correlates of Vestibular Processing During a Spaceflight Analog With Elevated Carbon Dioxide (COâ‚‚): A Pilot Study
Astronauts return to Earth from spaceflight missions with impaired mobility and balance;
recovery can last weeks postflight. This is due in large part to the altered vestibular
signaling and sensory reweighting that occurs in microgravity. The neural mechanisms
of spaceflight-induced vestibular changes are not well understood. Head-down-tilt bed
rest (HDBR) is a common spaceflight analog environment that allows for study of
body unloading, fluid shifts, and other consequences of spaceflight. Subjects in this
context still show vestibular changes despite being in Earth’s gravitational environment,
potentially due to sensory reweighting. Previously, we found evidence of sensory
reweighting and reduced neural efficiency for vestibular processing in subjects who
underwent a 70-day HDBR intervention. Here we extend this work by evaluating the
impact of HDBR paired with elevated carbon dioxide (COâ‚‚) to mimic International
Space Station conditions on vestibular neural processing. Eleven participants (6 males,
34 ± 8 years) completed 30 days of HDBR combined with 0.5% atmospheric CO₂
(HDBR + COâ‚‚). Participants underwent six functional magnetic resonance imaging
(fMRI) sessions pre-, during, and post- HDBR + COâ‚‚ while we measured brain activity
in response to pneumatic skull taps (a validated method of vestibular stimulation). We
also measured mobility and balance performance several times before and after the
intervention. We found support for adaptive neural changes within the vestibular system
during bed rest that subsequently recovered in several cortical and cerebellar regions.
Further, there were multiple brain regions where greater pre- to post- deactivation was
associated with reduced pre- to post- balance declines. That is, increased deactivation
of certain brain regions associated with better balance post-HDBR + COâ‚‚. We also
found that, compared to HDBR alone (n = 13 males; 29 ± 3 years) HDBR + CO₂ is
associated with greater increases in activation of multiple frontal, parietal, and temporal
regions during vestibular stimulation. This suggests interactive or additive effects of bed
rest and elevated COâ‚‚. Finally, we found stronger correlations between pre- to postHDBR + COâ‚‚ brain changes and dependence on the visual system during balance
for subjects who developed signs of Spaceflight-Associated Neuro-ocular Syndrome
Frontiers in Systems Neuroscience | www.frontiersin.org 1 January 2020 | Volume 13 | Article 80Hupfeld et al. Neural Vestibular Processing With HDBR + COâ‚‚
(SANS). Together, these findings have clear implications for understanding the neural
mechanisms of bed rest and spaceflight-related changes in vestibular processing, as
well as adaptation to altered sensory inputs
Probing Planckian physics: resonant production of particles during inflation and features in the primordial power spectrum
The phenomenon of resonant production of particles {\it after} inflation has
received much attention in the past few years. In a new application of resonant
production of particles, we consider the effect of a resonance {\em during}
inflation. We show that if the inflaton is coupled to a massive particle,
resonant production of the particle during inflation modifies the evolution of
the inflaton, and may leave an imprint in the form of sharp features in the
primordial power spectrum. Precision measurements of microwave background
anisotropies and large-scale structure surveys could be sensitive to the
features, and probe the spectrum of particles as massive as the Planck scale.Comment: 19 pages, 11 eps figure
Matter Creation via Vacuum Fluctuations in the Early Universe and Observed Ultra-High Energy Cosmic Ray Events
Cosmic rays of the highest energy, above the Greisen-Zatsepin-Kuzmin cut-off
of the spectrum, may originate in decays of superheavy long-living X-particles.
These particles may be produced in the early Universe from vacuum fluctuations
during or after inflation and may constitute a considerable fraction of Cold
Dark Matter. We calculate numerically their abundance for a wide range of
models. X-particles are considered to be either bosons or fermions. Particles
that are several times heavier than inflaton, m_inflaton \approx 10^{13} GeV,
and were produced by this mechanism, can account for the critical mass in the
Universe naturally. In some cases induced isocurvature density fluctuations can
leave an imprint in anisotropy of cosmic microwave background radiation.Comment: LaTeX, 9 page