732 research outputs found
An exposition on Friedmann Cosmology with Negative Energy Densities
How would negative energy density affect a classic Friedmann cosmology?
Although never measured and possibly unphysical, certain realizations of
quantum field theories leaves the door open for such a possibility. In this
paper we analyze the evolution of a universe comprising varying amounts of
negative energy forms. Negative energy components have negative normalized
energy densities, . They include negative phantom energy with an
equation of state parameter , negative cosmological constant: ,
negative domain walls: , negative cosmic strings: , negative
mass: , negative radiation: and negative ultralight: .
Assuming that such energy forms generate pressure like perfect fluids, the
attractive or repulsive nature of negative energy components are reviewed. The
Friedmann equation is satisfied only when negative energy forms are coupled to
a greater magnitude of positive energy forms or positive curvature. We show
that the solutions exhibit cyclic evolution with bounces and turnovers.The
future and fate of such universes in terms of curvature, temperature,
acceleration, and energy density are reviewed. The end states are dubbed Big
Crunch, Big Void, or Big Rip and further qualified as "Warped", "Curved", or
"Flat", "Hot" versus "Cold", "Accelerating" versus "Decelerating" versus
"Coasting". A universe that ends by contracting to zero energy density is
termed "Big Poof." Which contracting universes "bounce" in expansion and which
expanding universes "turnover" into contraction are also reviewed.Comment: Abridged version with minor correction
Flux Enhancement of Slow-moving Particles by Sun or Jupiter: Can they be Detected on Earth?
Slow-moving particles capable of interacting solely with gravity might be
detected on Earth as a result of the gravitational lensing induced focusing
action of the Sun. The deflection experienced by these particles are inversely
proportional to the square of their velocities and as a result their focal
lengths will be shorter. We investigate the velocity dispersion of these
slow-moving particles, originating from distant point-like sources, for
imposing upper and lower bounds on the velocities of such particles in order
for them to be focused onto Earth. We find that fluxes of such slow-moving and
non-interacting particles must have speeds between ~0.01 and ~0.14 times the
speed of light, . Particles with speeds less than ~0.01 c will undergo way
too much deflection to be focused, although such individual particles could be
detected. At the caustics, the magnification factor could be as high as ~10E+6.
We impose lensing constraints on the mass of these particles in order for
them to be detected with large flux enhancements to be greater than E-9 eV. An
approximate mass density profile for Jupiter is used to constrain particle
velocities for lensing by Jupiter. We show that Jupiter could potentially focus
particles with speeds as low as ~0.001c, which the Sun cannot. As a special
case, the perfect alignment of the planet Jupiter with the Sun is also
considered.Comment: 20 Pages, 5 figure
Femoral Retrotorsion as the Source of Sacroiliac Symptoms – A Case Report
The presence of femoral retrotorsion limits the ability to functionally utilize internal rotation IR of the hip. Postures which require the internal rotation of the hip can place more stress on the sacroiliac SI area as a compensation. This case report highlights a patient s self management of the SI symptoms by avoiding the postural positions requiring internal rotation. Retrospective analysis shows a significantly higher correlation of femoral retroversion (57%) in patients with these pain descriptors versus asymptomatic patients in whom the incidence of retroversion is only 5-15% (Prather 2019). The purpose of this case report is to demonstrate that patient education is an important component of pain management for this population
How do treadmill speed and terrain visibility influence neuromuscular control of guinea fowl locomotion?
Locomotor control mechanisms must flexibly adapt to both anticipated and unexpected terrain changes to maintain movement and avoid a fall. Recent studies revealed that ground birds alter movement in advance of overground obstacles, but not treadmill obstacles, suggesting context-dependent shifts in the use of anticipatory control. We hypothesized that differences between overground and treadmill obstacle negotiation relate to differences in visual sensory information, which influence the ability to execute anticipatory manoeuvres. We explored two possible explanations: (1) previous treadmill obstacles may have been visually imperceptible, as they were low contrast to the tread, and (2) treadmill obstacles are visible for a shorter time compared with runway obstacles, limiting time available for visuomotor adjustments. To investigate these factors, we measured electromyographic activity in eight hindlimb muscles of the guinea fowl (Numida meleagris, N=6) during treadmill locomotion at two speeds (0.7 and 1.3 m s−1) and three terrain conditions at each speed: (i) level, (ii) repeated 5 cm low-contrast obstacles (90% contrast, black/white). We hypothesized that anticipatory changes in muscle activity would be higher for (1) high-contrast obstacles and (2) the slower treadmill speed, when obstacle viewing time is longer. We found that treadmill speed significantly influenced obstacle negotiation strategy, but obstacle contrast did not. At the slower speed, we observed earlier and larger anticipatory increases in muscle activity and shifts in kinematic timing. We discuss possible visuomotor explanations for the observed context-dependent use of anticipatory strategies
Decreased Mobility of Cuboid Rotation as the Source of Achilles Tendinopathy--A Case Report
Joint mobility is necessary for adequate function of musculature. Internal rotation of the cuboid functions to allow midtarsal and hindfoot pronation in weight bearing. Inability of the cuboid to internally rotate limits mid tarsal pronation, which limits contraction of the gastroc soleus on weight bearing, ultimately contributing to muscle/tendon stress. The continued stress on the contractile structure leads to a tendinopathy which results in limited functional use with pain of the gastroc/soleus. The purpose of this case report is to outline the successful treatment of a patient with this clinical presentation
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