9,041 research outputs found
Cosmological string models from Milne spaces and SL(2,Z) orbifold
The -dimensional Milne Universe with extra free directions is used to
construct simple FRW cosmological string models in four dimensions, describing
expansion in the presence of matter with , . We then
consider the n=2 case and make SL(2,Z) orbifold identifications. The model is
surprisingly related to the null orbifold with an extra reflection generator.
The study of the string spectrum involves the theory of harmonic functions in
the fundamental domain of SL(2,Z). In particular, from this theory one can
deduce a bound for the energy gap and the fact that there are an infinite
number of excitations with a finite degeneracy. We discuss the structure of
wave functions and give examples of physical winding states becoming light near
the singularity.Comment: 14 pages, harvma
Comments on Black Holes in String Theory
A very brief review is given of some of the developments leading to our
current understanding of black holes in string theory. This is followed by a
discussion of two possible misconceptions in this subject - one involving the
stability of small black holes and the other involving scale radius duality.
Finally, I describe some recent results concerning quasinormal modes of black
holes in anti de Sitter spacetime, and their implications for strongly coupled
conformal field theories (in various dimensions).Comment: 13 pages. Talk given at Strings '99, Potsdam, German
Crystallization of Carbon Oxygen Mixtures in White Dwarf Stars
We determine the phase diagram for dense carbon/ oxygen mixtures in White
Dwarf (WD) star interiors using molecular dynamics simulations involving liquid
and solid phases. Our phase diagram agrees well with predictions from Ogata et
al. and Medin and Cumming and gives lower melting temperatures than Segretain
et al. Observations of WD crystallization in the globular cluster NGC 6397 by
Winget et al. suggest that the melting temperature of WD cores is close to that
for pure carbon. If this is true, our phase diagram implies that the central
oxygen abundance in these stars is less than about 60%. This constraint, along
with assumptions about convection in stellar evolution models, limits the
effective S factor for the C()O reaction to
S_{300} <= 170 keV barns.Comment: 4 pages, 2 figures, Phys. Rev. Lett. in pres
Neutrino Trapping in a Supernova and Ion Screening
Neutrino-nucleus elastic scattering is reduced in dense matter because of
correlations between ions. The static structure factor for a plasma of
electrons and ions is calculated from Monte Carlo simulations and parameterized
with a least squares fit. Our results imply a large increase in the neutrino
mean free path. This strongly limits the trapping of neutrinos in a supernova
by coherent neutral current interactions.Comment: 9 pages, 1 postscript figure using epsf.st
Diffusion of Neon in White Dwarf Stars
Sedimentation of the neutron rich isotope Ne may be an important
source of gravitational energy during the cooling of white dwarf stars. This
depends on the diffusion constant for Ne in strongly coupled plasma
mixtures. We calculate self-diffusion constants from molecular dynamics
simulations of carbon, oxygen, and neon mixtures. We find that in a
mixture does not differ greatly from earlier one component plasma results. For
strong coupling (coulomb parameter few), has a modest
dependence on the charge of the ion species, .
However depends more strongly on for weak coupling (smaller
). We conclude that the self-diffusion constant for
Ne in carbon, oxygen, and neon plasma mixtures is accurately known so
that uncertainties in should be unimportant for simulations of
white dwarf cooling.Comment: 6 pages, 5 figures, minor changes, Phys. Rev. E in pres
Unitarity, quasi-normal modes and the AdS_3/CFT_2 correspondence
In general, black-hole perturbations are governed by a discrete spectrum of
complex eigen-frequencies (quasi-normal modes). This signals the breakdown of
unitarity. In asymptotically AdS spaces, this is puzzling because the
corresponding CFT is unitary. To address this issue in three dimensions, we
replace the BTZ black hole by a wormhole, following a suggestion by Solodukhin
[hep-th/0406130]. We solve the wave equation for a massive scalar field and
find an equation for the poles of the propagator. This equation yields a rich
spectrum of {\em real} eigen-frequencies. We show that the throat of the
wormhole is , where is Newton's constant. Thus, the quantum
effects which might produce the wormhole are non-perturbative.Comment: 9 page
Strong Correlations in Actinide Redox Reactions
Reduction-oxidation (redox) reactions of the redox couples An(VI)/An(V),
An(V)/An(IV), and An(IV)/An(III), where An is an element in the family of early
actinides (U, Np, and Pu), as well as Am(VI)/Am(V) and Am(V)/Am(III), are
modeled by combining density functional theory with a generalized Anderson
impurity model that accounts for the strong correlations between the 5f
electrons. Diagonalization of the Anderson impurity model yields improved
estimates for the redox potentials and the propensity of the actinide complexes
to disproportionate.Comment: 17 pages, 10 figure, 3 tables. Corrections and clarifications; this
version has been accepted for publication in The Journal of Chemical Physic
Realistic Neutrino Opacities for Supernova Simulations With Correlations and Weak Magnetism
Advances in neutrino transport allow realistic neutrino interactions to be
incorporated into supernova simulations. We add tensor couplings to
relativistic RPA calculations of neutrino opacities. Our results reproduce
free-space neutrino-nucleon cross sections at low density, including weak
magnetism and recoil corrections. In addition, our opacities are
thermodynamically consistent with relativistic mean field equations of state.
We find antineutrino mean free paths that are considerably larger then those
for neutrinos. This difference depends little on density. In a supernova, this
difference could lead to an average energy of that is larger than
that for by an amount that is comparable to the energy difference
between and Comment: 15 pages, 10 figures, submitted to PRC, minor changes to figs. (9,10
Delta Excitations in Neutrino-Nucleus Scattering
We derive the contribution of -h excitations to quasielastic
charged-current neutrino-nucleus scattering in the framework of relativistic
mean-field theory. We discuss the effect of production on the
determination of the axial mass in neutrino scattering experiments.Comment: 14 pages, revtex, 3 postscript figures (available upon request
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