The pregalactic cosmic gravitational wave background

An outline is given that estimates the expected gravitational wave background, based on plausible pregalactic sources. Some cosmologically significant limits can be put on incoherent gravitational wave background arising from pregalactic cosmic evolution. The spectral region of cosmically generated and cosmically limited radiation is, at long periods, P greater than 1 year, in contrast to more recent cosmological sources, which have P approx. 10 to 10(exp -3)

Kerr-de Sitter Universe

It is now widely accepted that the universe as we understand it is accelerating in expansion and fits the de Sitter model rather well. As such, a realistic assumption of black holes must place them on a de Sitter background and not Minkowski as is typically done in General Relativity. The most astrophysically relevant black hole is the uncharged, rotating Kerr solution, a member of the more general Kerr-Newman metrics. A generalization of the rotating Kerr black hole to a solution of the Einstein's equation with a cosmological constant $\Lambda$ was discovered by Carter \cite{DWDW}. It is typically referred to as the Kerr-de Sitter spacetime. Here, we discuss the horizon structure of this spacetime and its dependence on $\Lambda$. We recall that in a \La>0 universe, the term `extremal black hole' refers to a black hole with angular momentum $J > M^2$. We obtain explicit numerical results for the black hole's maximal spin value and get a distribution of admissible Kerr holes in the ($\Lambda$, spin) parameter space. We look at the conformal structure of the extended spacetime and the embedding of the 3-geometry of the spatial hypersurfaces. In analogy with Reissner-Nordstr\"{o}m -de Sitter spacetime, in particular by considering the Kerr-de Sitter causal structure as a distortion of the Reissner-Nordstr\"{o}m-de Sitter one, we show that spatial sections of the extended spacetime are 3-spheres containing 2-dimensional topologically spherical sections of the horizons of Kerr holes at the poles. Depending on how a $t=$ constant 3-space is defined these holes may be seen as black or white holes (four possible combinations).Comment: 20 pages, 9 figure

Light Propagation in Inhomogeneous Universes. III. Distributions of Image Separations

Using an analytical model, we compute the distribution of image separations resulting from gravitational lensing of distant sources, for 7 COBE-normalized CDM models with various combinations of Omega_0 and lambda_0. Our model assumes that multiple imaging results from strong lensing by individual galaxies. We model galaxies as nonsingular isothermal spheres, and take into account the finite angular size of the sources. Our model neglects the contribution of the background matter distribution, and assumes that lensing is entirely caused by galaxies. To test the validity of this assumption, we performed a series of ray-tracing experiments to study the effect of the background matter on the distribution of image separations. The analytical model predicts that the distributions of image separations are virtually indistinguishable for flat, cosmological constant models with different values of Omega_0. For models with no cosmological constant, the distributions of image separations do depend upon Omega_0, but this dependence is weak. We conclude that while the number of multiple-imaged sources can put strong constraints on the cosmological parameters, the distribution of image separations does not constrain the cosmological models in any significant way, and mostly provides constraints on the structure of the galaxies responsible for lensing.Comment: One Plain TeX file, with 12 postscript figures. Accepted for publication in The Astrophysical Journa

The Expulsion of Stellar Envelopes in Core-Collapse Supernovae

We examine the relation between presupernova stellar structure and the distribution of ejecta in core-collapse supernovae, assuming adiabatic, spherically symmetric flow. We develop a simple yet accurate formula for the blastwave shock velocity, and demonstrate that the entire final density distribution can be approximated with simple models for the final pressure distribution, along with the approximate shock-deposited entropy, in a way that matches the results of simulations. We find that the distribution of density in a star's ejecta depends on whether its outer envelope is radiative or convective, and if convective, on the composition structure of the star; simple approximate forms are presented for red and blue supergiant ejecta. Our models are most accurate for the high-velocity ejecta from the periphery of a star, where the shock dynamics are predictable. We present formulae for the final density distribution of this material, for both radiative and efficiently convective envelopes. These formulae limit to the well-known planar, self-similar solutions for mass shells approaching the stellar surface. But, the assumption of adiabatic flow fails at low optical depth, so this planar limit need not be attained. Formulae are given for the observable properties of the X-ray burst accompanying shock emergence, and their dependence on the parameters of the explosion. Motivated by the relativistic expansion recently inferred by Kulkarni et al. (1998) for the synchrotron shell around SN1998bw, we estimate the criterion for relativistic mass ejection and the rest mass of relativistic ejecta.Comment: 57 pages, 10 eps figures, aaspp4, submitted to Ap