[en] Numerical study of order parameter evolution in the course of symmetry breaking transitions with Landau-Ginzburg endash like dynamics shows that the density of topological defects, kinks which form during the quench, is proportional to the fourth root of its rate. This is a limited (1D) test of the more general theory of domain-size evolution in the course of symmetry breaking transformations proposed by one of us. Using these ideas, it is possible to compute the density of topological defects from the quench time scale and from the equilibrium scaling of the correlation length and relaxation time near the critical point. copyright 1997 The American Physical Society

[en] We consider the contribution of gravitational self-interaction to the mass per unit length μ and the deficit angle Δphi of a long, static, straight cosmic string. The equations describing the structure and gravitational field of the string are expanded in powers of the gravitational strength of the string. To lowest order these equations yield the usual results for the Nielsen-Olesen string including Vilenkin's formula Δphi = 8πμ. In the next order a numerical treatment of the equations gives the gravitational corrections to Δphi and μ

[en] We numerically investigate collisions of cosmic strings carrying different winding numbers. We find that for strings with winding numbers n₁ and n₂, intercommutation occurs by peeling a string of winding number chemical bondn₁-n₂chemical bond from the string with the larger winding number. The resulting string connects the original colliding strings to form a state of three joined strings, but because of the peeling the eventual result is a reduction in the winding numbers of the network. Stable astrophysical strings with large winding number are thus unlikely to persist. All simulations have gauge/scalar-field mass ratio = 2. .AE

[en] To compare the utility of helical computed tomographic (CT) angiography and digital subtraction arteriography in the detection of intracranial aneurysms in the presence of acute subarachnoid hemorrhage (SAH) or suspicion of intracranial aneurysm. Thirty-one patients, 28 with SAH confirmed by CT or lumbar puncture and three with signs of cranial nerve compression, were studied prospectively between March 1997 and March 1998 by two different radiologists blinded to each others findings. One radiologist performed helical CT after intravenous injection of 100 cc of iohexol (240 mg/ml), followed by 2 mm-tick reconstructions of the circle of Willis taken every 0.5 mm. These images were processed in a workstation equipped with a 3 D shaded surface display (3 D-SSD). In five cases, maximum intensity projection (MIP) reconstructions were also carried out. Arteriography was performed subsequently by the other radiologist, who was not informed of the results of CT angiography. He selectively catheterized both carotid and vertebral arteries, obtaining anteroposterior, lateral and oblique images, with matrix of 512 x 512 pixels. CT angiography identified 21 aneurysms in 18 patients. Twelve patients presented no abnormal findings and one was found to have a subdural-subarachnoid hematoma. We had one false negative and one false positive. The minimum size of correctly diagnosed aneurysms was 4 mm and the maximum size was 34 mm. The aneurysm in the case of false positive measured 2,5 mm. The sensitivity of this technique for the detection of aneurysms was 95,2% and the specificity was 92.8%. We consider helical CT angiography with 3 D-SSD reconstruction to be a highly valuable tool in the diagnosis of aneurysms of the circle of Willis. In selected or extremely urgent cases, it may even eliminate the need to perform preoperative convectional angiography. (Author) 16 refs

[en] We investigate numerically the spacetime geometry in the presence of an infinitely long, straight, static, U(1)-gauge cosmic string formed during phase transitions at energy scales larger than the grand-unified-theory scale. As the energy scale of symmetry breaking increases, we find that at radial infinity the geometry around a string changes from Minkowskian minus a wedge to an analog of a Kasner spacetime. The geometry transition occurs at Δφ=2π, where the deficit angle Δφ is defined in the sense of comparison with flat spacetime in the absence of the string. Phase transitions producing such supermassive strings should occur before inflation to avoid contradictions with current observations

[en] We present a spatially three-dimensional study for solving the initial-value problem in general relativity for inhomogeneous cosmologies. We use York's conformal approach to solve the constraint equations of Einstein's field equations for scalar field sources and find the initial data which will be used in the evolution. This work constitutes the first stage in the development of a code to analyze the effects of matter and spacetime inhomogeneities on inflation

[en] Numerical simulations show that bosonic superconducting U(1) gauge cosmic strings interact by reconnecting and chopping off in a fashion similar to nonconducting strings. Cancellation of the electromagnetic current occurs when, in one of the strings, the direction of the U(1) gauge magnetic field is opposite to the electromagnetic current flow. Electric charge accumulates on the segments of the reconnected strings where the current is discontinuous or vanishes. A virtual photon appears after the collision and intercommutation, and a bubble of electromagnetic radiation emerges as the currents in the reconnected strings equalize. These phenomena suggest new possible mechanisms for void production in the large-scale distribution of galaxies

[en] In this study, a framework for the characterization of the dynamic interactions between RR variability (RRV) and systolic arterial pressure variability (SAPV) is proposed. The methodology accounts for the intrinsic non-stationarity of the cardiovascular system and includes the assessment of both the strength and the prevalent direction of local coupling. The smoothed pseudo-Wigner–Ville distribution (SPWVD) is used to estimate the time–frequency (TF) power, coherence, and phase-difference spectra with fine TF resolution. The interactions between the signals are quantified by time-varying indices, including the local coupling, phase differences, time delay, and baroreflex sensitivity (BRS). Every index is extracted from a specific TF region, localized by combining information from the different spectra. In 14 healthy subjects, a head-up tilt provoked an abrupt decrease in the cardiovascular coupling; a rapid change in the phase difference (from 0.37 ± 0.23 to −0.27 ± 0.22 rad) and time delay (from 0.26 ± 0.14 to −0.16 ± 0.16 s) in the high-frequency band; and a decrease in the BRS (from 23.72 ± 7.66 to 6.92 ± 2.51 ms mmHg"−"1). In the low-frequency range, during a head-up tilt, restoration of the baseline level of cardiovascular coupling took about 2 min and SAPV preceded RRV by about 0.85 s during the whole test. The analysis of the Eurobavar data set, which includes subjects with intact as well as impaired baroreflex, showed that the presented methodology represents an improved TF generalization of traditional time-invariant methodologies and can reveal dysfunctions in subjects with baroreflex impairment. Additionally, the results also suggest the use of non-stationary signal-processing techniques to analyze signals recorded under conditions that are usually supposed to be stationary. (paper)

[en] In this paper we assessed the possibility of using the pulse rate variability (PRV) extracted from the photoplethysmography signal as an alternative measurement of the HRV signal in non-stationary conditions. The study is based on analysis of the changes observed during a tilt table test in the heart rate modulation of 17 young subjects. First, the classical indices of HRV analysis were compared to the indices from PRV in intervals where stationarity was assumed. Second, the time-varying spectral properties of both signals were compared by time-frequency (TF) and TF coherence analysis. Third, the effect of replacing PRV with HRV in the assessment of the changes of the autonomic modulation of the heart rate was considered. Time-invariant HRV and PRV indices showed no statistically significant differences (p > 0.05) and high correlation (>0.97). Time-frequency analysis revealed that the TF spectra of both signals were highly correlated (0.99 ± 0.01); the difference between the instantaneous power, in the LF and HF bands, obtained from HRV and PRV was small (<10"−"3 s"−"2) and their temporal patterns were highly correlated (0.98 ± 0.04 and 0.95 ± 0.06 in the LF and HF bands, respectively) and TF coherence in the LF and HF bands was high (0.97 ± 0.04 and 0.89 ± 0.08, respectively). Finally, the instantaneous power in the LF band was observed to significantly increase during head-up tilt by both HRV and PRV analysis. These results suggest that although some differences in the time-varying spectral indices extracted from HRV and PRV exist, mainly in the HF band associated with respiration, PRV could be used as a surrogate of HRV during non-stationary conditions, at least during the tilt table test

[en] Symmetry-breaking phase transitions may leave behind topological defects with a density dependence on the quench rate. We investigate the dynamics of such quenches for the one-dimensional, Landau-Ginzburg case and show that the density of kinks, n, scales differently with the quench time scale, τ_Q, depending on whether the dynamics in the vicinity of the critical point is overdamped (n∝τ_Q^-1/4) or underdamped (n∝τ_Q^-1/3). Either of these cases may be relevant to the early Universe, and we derive bounds on the initial density of topological defects in cosmological phase transitions. copyright 1998 The American Physical Society