15583656
Black hole pair creation and the entropy factor.
It is shown that in the instanton approximation the rate of creation of black holes is always enhanced by a factor of the exponential of the black hole entropy relative to the rate of creation of compact matter distributions (stars). This result holds for any generally covariant theory of gravitational and matter fields that can be expressed in Hamiltonian form. It generalizes the result obtained previously for the pair creation of magnetically charged black holes by a magnetic field in Einstein--Maxwell theory. The particular example of pair creation of electrically charged black holes by an electric field in Einstein--Maxwell theory is discussed in detail.
42904289
Numerical simulations with a first order BSSN formulation of Einstein's field equations
We present a new fully first-order strongly hyperbolic representation of the Baumgarte-Shapiro-Shibata-Nakamura formulation of Einstein's equations with optional constraint damping terms. We describe the characteristic fields of the system, discuss its hyperbolicity properties, and present two numerical implementations and simulations: one using finite differences, adaptive mesh refinement, and, in particular, binary black holes, and another one using the discontinuous Galerkin method in spherical symmetry. The results of this paper constitute a first step in an effort to combine the robustness of Baumgarte-Shapiro-Shibata-Nakamura evolutions with very high accuracy numerical techniques, such as spectral collocation multidomain or discontinuous Galerkin methods.
119326995
DYNAMICAL ROTATIONAL INSTABILITY AT LOW T/W
Dynamical instability is shown to occur in differentially rotating polytropes with N = 3.33 and T/|W| 0.14. This instability has a strong m = 1 mode, although the m = 2, 3, and 4 modes also appear. Such instability may allow a centrifugally hung core to begin collapsing to neutron star densities on a dynamical timescale. The gravitational radiation emitted by such unstable cores may be detectable with advanced ground-based detectors, such as LIGO-II. If the instability occurs in a supermassive star, it may produce gravitational radiation detectable by the space-based detector LISA.
6269112
Action functionals for relativistic perfect fluids
Action functionals describing relativistic perfect fluids are presented. Two of these actions apply to fluids whose equations of state are specified by giving the fluid energy density as a function of particle number density and entropy per particle. Other actions apply to fluids whose equations of state are specified in terms of other choices of dependent and independent fluid variables. Particular cases include actions for isentropic fluids and pressureless dust. The canonical Hamiltonian forms of these actions are derived, symmetries and conserved charges are identified, and the boundary value and initial value problems are discussed. As in previous works on perfect fluid actions, the action functionals considered depend on certain Lagrange multipliers and Lagrangian coordinate fields. Particular attention is paid to the interpretations of these variables and to their relationships to the physical properties of the fluid.
16734842
Black Hole Thermodynamics in a Box
Simple calculations indicate that the partition function for a black hole is defined only if the temperature is fixed on a finite boundary. Consequences of this result are discussed. (Contribution to the Proceedings of the Lanczos Centenary Conference.)
119005465
Modifying the Einstein equations off the constraint hypersurface
A new technique is presented for modifying the Einstein evolution equations off the constraint hypersurface. With this approach the evolution equations for the constraints can be specified freely. The equations of motion for the gravitational field variables are modified by the addition of terms that are linear and nonlocal in the constraints. These terms are obtained from solutions of the linearized Einstein constraints.
35977510
Quadrupole moments of sd-shell nuclei.
Shell-model calculations are presented for electric quadrupole moments of A = 17-39 nuclei. The shell-model wave functions, which span the complete 0d/sub 5/2/-ls/sub 1/2/-0d/sub 3/2/ model space in all instances, are obtained by diagonalizing an sd-shell Hamiltonian which consists of a fixed single-particle energy spectrum and a single set of two-body matrix elements scaled as a function of the mass number A. Results obtained with harmonic oscillator and Saxon-Woods single-particle radial wave functions and matched effective-charge models are presented for ground states of odd-even and doubly odd nuclei and for J/sup ..pi../ = 2/sup +/ first-excited states of doubly even sd-shell nuclei.
11983780
Is there large weak mixing in heavy nuclei?
Recent results in parity non-conserving neutron-nucleus scattering show a tendency for the parity non-conserving effects to predominantly have the same sign. Explanations of this phenomenon all require a weak matrix element between single-particle levels in heavy nuclei to be [similar to]100 eV. This paper contains a discussion of this effect and how the same phenomenon will manifest itself in other systems. The gamma decay of [sup 207]Pb is given as an example of a system where the weak matrix element between single-particle levels in a heavy nucleus can be measured directly.
20082114
Probe of the shell crossing at A=40 via beta decay: Experiment and theory.
The ..beta../sup -/ decays of /sup 35/P, /sup 37/S, /sup 38/S, and /sup 38/Cl were studied both experimentally and via shell-model calculations. The four decaying nuclei were formed by bombardment of 81% enriched /sup 36/S by triton and deuteron beams. Gamma-ray spectroscopy was carried out with Ge detectors, either bare or surrounded by a Compton suppression NaI(Tl) shield. One new Gamow-Teller decay was observed in /sup 35/P decay, and one new first-forbidden decay was observed in /sup 38/S decay. Otherwise ..gamma..-ray measurements of E/sub ..gamma../ and I-/sub ..gamma../ (and limits on I/sub ..gamma../ for unobserved transitions) were improved significantly over previous results. Shell-model studies were undertaken in order to bring these beta decay results to bear on an understanding of the shell structure in the (sd) to (fp) transition region at A = 40. An interaction was constructed in the 1d/sub 5/2/, 1d/sub 3/2/, 2s/sub 1/2/, 1f/sub 7/2/, 1f/sub 5/2/, 1p/sub 3/2/, 1p/sub 1/2/ model space. This interaction started from the Wildenthal ''USD'' (sd) interaction and the van Hees-Glaudemans (fp) interaction, which were connected by the cross-shell Millener-Kurath interaction. Certain important two-body matrix elements and single-particle energies were adjusted to fit experimental binding energies in A = 40, 41, 42.more » This interaction was then used to calculate level spectra for the daughter nuclei /sup 35/S, /sup 37/Cl, /sup 38/Cl, and /sup 38/Ar as well as Gamow-Teller and unique first-forbidden beta decays leading to these nuclei. The techniques used in these relatively large scale calculations are discussed in detail, as are the results.« less
21422029
Shape diffusion in the shell model.
The diffusion coefficient for quadrupolar shape changes is derived in a model based on the mixing of static Hartree-Fock configurations by the residual interaction. The model correctly predicts the width of single-particle configurations. We find a diffusion rate depending on temperature as {ital T}{sup 3}, consistent with at least one other theoretical estimate. However, our diffusion rate is an order of magnitude lower than two values extracted from data.
7024068
Erratum: Radiative decays of the 16.6 and 16.9 MeV states in 8Be and tests of the conservation of the vector current in the A=8 multiplet
We measured the $^{4}\mathrm{He}$(\ensuremath{\alpha},\ensuremath{\gamma}) cross section at ${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$=90\ifmmode^\circ\else\textdegree\fi{} across the 16.6 and 16.9 MeV resonances in $^{8}\mathrm{Be}$ and obtained angular distributions on the peaks of both resonances. The decays of the 16 MeV doublet to the 3.0 MeV ${2}^{+}$ final state have an (isovector E2)/(isovector M1) mixing ratio of (0.01\ifmmode\pm\else\textpm\fi{}0.03), an (isoscalar E2)/(isovector M1) mixing ratio of (0.21\ifmmode\pm\else\textpm\fi{}0.04), and an isovector M1 width of (2.80\ifmmode\pm\else\textpm\fi{}0.18) eV. Our result for the isovector E2 strength disagrees markedly with previous work. We observe ground-state E2 decays of both members of the doublet. We use our results, plus existing $^{8}\mathrm{Li}$ and $^{8}\mathrm{B}$ \ensuremath{\beta}-decay data, to test the conservation of the vector current and to constrain the second-class induced-tensor form factor. Finally, we compare our results to shell-model calculations.