MG11 
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Abstract

Evidence for White Dwarfs with Strange Matter Cores

We summarize masses and radii for a number of white dwarfs as deduced from a combination of proper motion studies, Hipparcos parallax distances, effective temperatures, and binary or spectroscopic masses. We construct a projection of white-dwarf radii for fixed effective mass and conclude that there is at least marginal evidence for bimodality in the radius distribution for white dwarfs. Some stars appear to have radii which are significantly smaller than that expected for a standard electron-degenerate white-dwarf equations of state. We argue that if such compact white dwarfs exist it is unlikely that they contain an iron core. We show that the data exhibit several features consistent with the expected mass-radius relation of strange dwarfs. We identify eight nearby white dwarfs which are possible candidates for strange matter cores and suggest observational tests of this hypothesis.

Dark Energy and Decaying Dark Matter

We discuss a cosmology in which cold dark-matter particles decay into relativistic particles. We show that such decays can lead naturally to a negative pressure term in the cosmic energy-momentum tensor and is a candidate for dark energy. We derive the modified cosmic equations of motion for this fluid and show that if dark matter particles decay at the late times and on a short timescale compared to the present hubble time than an epoch of cosmic acceleration is produced without vacuum enbergy. As a illustration , we investigate the effects of decaying cold dark matter in a Lambda = 0, flat, initially matter dominated cosmology. We show that this model satisfies the cosmological constraint from Type Ia supernovae at high redshift. Possible candidates for drcaying and additional observational tests of this new cosmological paradigm are suggested.

An Axisymmetric Object-Based Search for a Flat Compact Dimension

A method is presented to search for a hypertorus symmetry axis by the alignment of distant objects. This offers greater sensitivity than previously proposed object-based methods that rely on accurate true distances. When applied to the catalog of objects with z > 1, we find no evidence for a compact dimension. We deduce a lower limit to the compact dimension size D > 0.9 of the distance to the cosmic horizon. This is consistent with independent constraints from the recent analysis of the WMAP microwave background data.

Constraints on Accelerating Brane Cosmology with Exchange between the Bulk and Brane

We explore observational constraints on a cosmological brane-world scenario in which the bulk is not empty. Rather, exchange of mass-energy between the bulk and the bane is allowed. The evolution of matter fields to an observer on the brane is modified due to new terms in the energy momentum tensor describing this exchange. We investigate the constraints from various cosmological observations on the flow of matter from the bulk into the brane. Interestingly, we show that it is possible to have a Lambda = 0 cosmology to an observer in the brane which satisfies standard cosmological constraints including the CMB temperature fluctuations, Type Ia supernovae at high redshift, and the matter power spectrum. This model even accounts for the observed suppression of the CMB power spectrum at low multipoles. In this cosmology, the observed cosmic acceleration is attributable to the flow of matter from the bulk to the brane. Possible additional observational tests of this new cosmological paradigm are suggested.

Multiple Orbits for Close Binary Neutron Stars in Strong-Field Relativity

We have numerically integrated the relativistic hydrodynamics of binary neutron stars near the last stable orbit for up to several thousand orbits and for various for various equations of state. We utilize a conformally flat metric and an adaptive mesh to minimize matter flow with respect to the grid. We utilize a multipole expansion to extract the gravity wave template for out to l= 4 including the slow-motion correction. We find that there is an interesting equation of state dependence of these final orbits which may be detectable as a significant deviation from the chirp as expected from post-Newtonian or Newtonian estimates.

Relativistic Hydrodynamic Simulations of Multiple Orbits for Close Neutron Star Binaries

We discuss numerical general relativistic hydrodynamics simulations for close neutron star binary systems. The hydrodynamic and field equations are solved at each time slice with a spatial 3-metric chosen to be conformally flat. By using an adaptive mesh we are able to reliably numerically integrate many orbits of the binary system. The gravitational radiation signal is extracted via a multipole expansion to the hexadecapole (l = 4) order including both mass and current moments and a correction for the slow motion approximation. We compute quasiequilibrium circular orbit conditions for two equal-mass neutron stars as a function of total angular momentum from the post-Newtonian regime to near the last stable circular orbit. We have considered a variety of neutron star equations of state. From the angular momentum and power loss rate we reconstruct the gravitational wave form and find that there is a detectable equation of state dependence of the computed gravity-wave signal.