9,244 research outputs found
Imide and isatin derivatives as β-lactam mimics of β-lactam antibiotics
Activated γ-lactams, which are derivatives of succinimide, phthalimide and isatin with suitable
elements of molecular recognition, have been synthesised as mimics of the ß-lactam antibiotics
and their chemical and biological reactivity determined
Proof of the Generalized Second Law for Quasistationary Semiclassical Black Holes
A simple direct explicit proof of the generalized second law of black hole
thermodynamics is given for a quasistationary semiclassical black hole.Comment: 12 pages, LaTeX, report Alberta-Thy-10-93 (revision of paper in
response to Phys. Rev. Lett. referees' comments, which suffered a series of
long delays
Timing evolution of accreting strange stars
It has been suggested that the QPO phenomenon in LMXB's could be explained
when the central compact object is a strange star. In this work we investigate
within a standard model for disk accretion whether the observed clustering of
spin frequencies in a narrow band is in accordance with this hypothesis. We
show that frequency clustering occurs for accreting strange stars when typical
values of the parameters of magnetic field initial strength and decay time,
accretion rate are chosen. In contrast to hybrid star accretion no mass
clustering effect is found.Comment: 10 pages, 3 figures, version accepted for publication in New
Astronom
The Evolution in the Faint-End Slope of the Quasar Luminosity Function
(Abridged) Based on numerical simulations of galaxy mergers that incorporate
black hole (BH) growth, we predict the faint end slope of the quasar luminosity
function (QLF) and its evolution with redshift. Our simulations have yielded a
new model for quasar lifetimes where the lifetime depends on both the
instantaneous and peak quasar luminosities. This motivates a new interpretation
of the QLF in which the bright end consists of quasars radiating at nearly
their peak luminosities, but the faint end is mostly made up of quasars in less
luminous phases of evolution. The faint-end QLF slope is then determined by the
faint-end slope of the quasar lifetime for quasars with peak luminosities near
the observed break. We determine this slope from the quasar lifetime as a
function of peak luminosity, based on a large set of simulations spanning a
wide variety of host galaxy, merger, BH, and ISM gas properties. Brighter peak
luminosity (higher BH mass) systems undergo more violent evolution, and expel
and heat gas more rapidly in the final stages of quasar evolution, resulting in
a flatter faint-end slope (as these objects fall below the observed break in
the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher
luminosities with increasing redshift, implying a larger typical quasar peak
luminosity, the faint-end QLF slope flattens. From the quasar lifetime as a
function of peak luminosity and this interpretation of the QLF, we predict the
faint-end QLF slope and its evolution with redshift in good agreement with
observations. Although BHs grow anti-hierarchically (with lower-mass BHs formed
primarily at lower redshifts), the observed change in slope and differential or
luminosity dependent density evolution in the QLF is completely determined by
the luminosity-dependent quasar lifetime and physics of quasar feedback.Comment: 13 pages, 4 figures, submitted to ApJ (Replacement with minor
revisions and changed sign convention
Mass and angular momenta of Kerr anti-de Sitter spacetimes in Einstein-Gauss-Bonnet theory
We compute the mass and angular momenta of rotating anti-de Sitter spacetimes
in Einstein-Gauss-Bonnet theory of gravity using a superpotential derived from
standard Noether identities. The calculation relies on the fact that the
Einstein and Einstein-Gauss-Bonnet vacuum equations are the same when
linearized on maximally symmetric backgrounds and uses the recently discovered
D-dimensional Kerr-anti-de Sitter solutions to Einstein's equations
Probability distributions for quantum stress tensors in four dimensions
We treat the probability distributions for quadratic quantum fields, averaged
with a Lorentzian test function, in four-dimensional Minkowski vacuum. These
distributions share some properties with previous results in two-dimensional
spacetime. Specifically, there is a lower bound at a finite negative value, but
no upper bound. Thus arbitrarily large positive energy density fluctuations are
possible. We are not able to give closed form expressions for the probability
distribution, but rather use calculations of a finite number of moments to
estimate the lower bounds, the asymptotic forms for large positive argument,
and possible fits to the intermediate region. The first 65 moments are used for
these purposes. All of our results are subject to the caveat that these
distributions are not uniquely determined by the moments. However, we also give
bounds on the cumulative distribution function that are valid for any
distribution fitting these moments.We apply the asymptotic form of the
electromagnetic energy density distribution to estimate the nucleation rates of
black holes and of Boltzmann brains.Comment: 26 pages, 2 figure
THERMAL RADIATION FROM MAGNETIZED NEUTRON STARS: A look at the Surface of a Neutron Star.
Surface thermal emission has been detected by ROSAT from four nearby young
neutron stars. Assuming black body emission, the significant pulsations of the
observed light curves can be interpreted as due to large surface temperature
differences produced by the effect of the crustal magnetic field on the flow of
heat from the hot interior toward the cooler surface. However, the energy
dependence of the modulation observed in Geminga is incompatible with blackbody
emission: this effect will give us a strong constraint on models of the neutron
star surface.Comment: 10 pages. tar-compressed and uuencoded postcript file. talk given at
the `Jubilee Gamow Seminar', St. Petersburg, Sept. 1994
Mesoscopic phase statistics of diffuse ultrasound in dynamic matter
Temporal fluctuations in the phase of waves transmitted through a dynamic,
strongly scattering, mesoscopic sample are investigated using ultrasonic waves,
and compared with theoretical predictions based on circular Gaussian
statistics. The fundamental role of phase in Diffusing Acoustic Wave
Spectroscopy is revealed, and phase statistics are also shown to provide a
sensitive and accurate way to probe scatterer motions at both short and long
time scales.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
High Temperature Matter and Gamma Ray Spectra from Microscopic Black Holes
The relativistic viscous fluid equations describing the outflow of high
temperature matter created via Hawking radiation from microscopic black holes
are solved numerically for a realistic equation of state. We focus on black
holes with initial temperatures greater than 100 GeV and lifetimes less than 6
days. The spectra of direct photons and photons from decay are
calculated for energies greater than 1 GeV. We calculate the diffuse gamma ray
spectrum from black holes distributed in our galactic halo. However, the most
promising route for their observation is to search for point sources emitting
gamma rays of ever-increasing energy.Comment: 33 pages, 13 figures, to be submitted to PR
Cosmological Measures without Volume Weighting
Many cosmologists (myself included) have advocated volume weighting for the
cosmological measure problem, weighting spatial hypersurfaces by their volume.
However, this often leads to the Boltzmann brain problem, that almost all
observations would be by momentary Boltzmann brains that arise very briefly as
quantum fluctuations in the late universe when it has expanded to a huge size,
so that our observations (too ordered for Boltzmann brains) would be highly
atypical and unlikely. Here it is suggested that volume weighting may be a
mistake. Volume averaging is advocated as an alternative. One consequence may
be a loss of the argument that eternal inflation gives a nonzero probability
that our universe now has infinite volume.Comment: 15 pages, LaTeX, added references for constant-H hypersurfaces and
also an idea for minimal-flux hypersurface
- …