667 research outputs found
A trapped surface in the higher-dimensional self-similar Vaidya spacetime
We investigate a trapped surface and naked singularity in a -dimensional
Vaidya spacetime with a self-similar mass function. A trapped surface is
defined as a closed spacelike -surface which has negative both null
expansions. There is no trapped surface in the Minkowski spacetime. However, in
a four-dimensional self-similar Vaidya spacetime, Bengtsson and Senovilla
considered non-spherical trapped surfaces and showed that a trapped surface can
penetrate into a flat region, if and only if the mass function rises fast
enough [I. Bengtsson and J. M. M. Senovilla, Phys. Rev. D \textbf{79}, 024027
(2009).]. We apply this result to a -dimensional spacetime motivated by the
context of large extra dimensions or TeV-scale gravity. In this paper,
similarly to Bengtsson and Senovilla's study, we match four types of
-surfaces and show that a trapped surface extended into the flat region
can be constructed in the -dimensional Vaidya spacetime, if the increasing
rate of the mass function is greater than 0.4628. Moreover, we show that the
maximum radius of the trapped surface constructed here approaches the
Schwarzschild-Tangherlini radius in the large limit. Also, we show that
there is no naked singularity, if the spacetime has the trapped surface
constructed here.Comment: 13 pages, 5 figure
Naked Singularity Explosion in Higher Dimensions
Motivated by the recent argument that in the TeV-scale gravity
trans-Planckian domains of spacetime as effective naked singularities would be
generated by high-energy particle (and black-hole) collisions, we investigate
the quantum particle creation by naked-singularity formation in general
dimensions. Background spacetime is simply modeled by the self-similar Vaidya
solution, describing the spherical collapse of a null dust fluid. In a generic
case the emission power is found to be proportional to the quadratic inverse of
the remaining time to a Cauchy horizon, as known in four dimensions. On the
other hand, the power is proportional to the quartic inverse for a critical
case in which the Cauchy horizon is `degenerate'. According to these results,
we argue that the backreaction of the particle creation to gravity will be
important in particle collisions, in contrast to the gravitational collapse of
massive stellar objects, since the bulk of energy is carried away by the
quantum radiation even if a quantum gravitational effect cutoff the radiation
just before the appearance of naked singularity.Comment: 19 pages, 2 figures; v2: typos fixe
Electromagnons in the multiferroic state of perovskite manganites with symmetric-exchange striction
We have investigated electrically-active magnetic excitations
(electromagnons) in perovskite manganites with the -type (up-up-down-down)
spin structure by terahertz spectroscopy. EuYMnO (0.11) and YLuMnO (01) without magnetic -moments,
which host collinear sinusoidal, -type, cycloidal, and -type spin orders,
are used to examine the systematics of possible electromagnons. Three-peak
structures (23, 35, 45 cm) of magnetic origin show up in the -type
phase with little composition () dependence of frequencies, making a
contrast with the electromagnons observed in the cycloidal-spin ()
phases. One of these electromagnon is ascribed to the zone-edge magnon mode
based on the calculated magnon dispersions.Comment: 5 pages, 4 figure
Electromagnons in the multiferroic state of perovskite manganites with symmetric-exchange striction
We have investigated electrically-active magnetic excitations
(electromagnons) in perovskite manganites with the -type (up-up-down-down)
spin structure by terahertz spectroscopy. EuYMnO (0.11) and YLuMnO (01) without magnetic -moments,
which host collinear sinusoidal, -type, cycloidal, and -type spin orders,
are used to examine the systematics of possible electromagnons. Three-peak
structures (23, 35, 45 cm) of magnetic origin show up in the -type
phase with little composition () dependence of frequencies, making a
contrast with the electromagnons observed in the cycloidal-spin ()
phases. One of these electromagnon is ascribed to the zone-edge magnon mode
based on the calculated magnon dispersions.Comment: 5 pages, 4 figure
Effects of high-impact exercise on the physical properties of bones of ovariectomized rats fed to a high-protein diet
The aim of this study was to evaluate the effects of high-impact physical exercise as a prophylactic and therapeutic means in osteopenic bones of rats submitted to ovariectomy and protein diet intake. A total of 64 Wistar rats were divided into eight groups (n=8 each), being: OVX, ovx, standard diet and sedentary; OVXE, ovx, standard diet and jump; OVXP, ovx, high-protein diet and sedentary; and OVXEP, ovx, high-protein diet and jump; SH, sham, standard diet and sedentary; SHE, sham, standard diet and jump; SHP, sham, high-protein diet and sedentary; and SHEP, sham, high-protein diet and jump. OVX surgery consists of ovariectomy, and sham was the control surgery. The jumping protocol consisted of 20 jumps/day, 5days/week. The bone structure was evaluated by densitometry, mechanical tests, histomorphometric, and immunohistochemical analyses. A high-protein diet resulted in increased bone mineral density (P=.049), but decreased maximal load (P=.026) and bone volume fraction (P=.023). The benefits of physical exercise were demonstrated by higher values of the maximal load in the trained groups compared to the sedentary groups (P<.001). The sham groups had decreased immunostaining of osteocalcin (P=.004) and osteopontin (P=.010) compared to ovx groups. However, the high-protein diet (P=.005) and jump exercise (P=.017) resulted in lower immunostaining of osteopontin compared to the standard diet and sedentary groups, respectively. In this experimental model, it was concluded that ovariectomy and a high-fat diet can negatively affect bone tissue and the high-impact exercise was not enough to suppress the deleterious effects caused by the protein diet and ovariectomy
Association of falls, fear of falling, handgrip strength and gait speed with frailty levels in the community elderly
Introduction: The Frailty Syndrome is not synonymous with disability, but may be a precursor of etiological and physiological dysfunction, which affects mobility before causing reduced functional capacity, explaining conditions of weakness, weight loss, and abnormal gait. Study design: This is an analytical, observational, cross-sectional study. Objective: To verify the association of handgrip strength, gait speed, fear of falling, and falls with the level of frailty. Methods: Study consisting of 54 participants, aged 65 and older, enrolled in a health care and monitoring government program in the municipality of Uberaba, Minas Gerais, Brazil. The volunteers were assessed for frailty, – non-frail group (NG), pre-frail group (PG), and frail group (FG) – according to the Fried criteria, and based on the outcomes of handgrip strength, gait speed, fear of falling, and falls. An inferential descriptive statistical analysis followed, with Chi-square and KruskallWallis tests performed by the Stata11.0 software. Results: In the comparative analysis between the groups studied there was statistical significance relative to handgrip strength (FG and NG), gait speed (NG and FG / NG and PG), and fear of falling (NG and FG). There was no significant difference between the levels of frailty and falls. Conclusions: Frailty is associated with reduced muscle strength, decreased gait speed, and greater fear of falling in elderly people of the communit
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