3,980 research outputs found
Can rigidly rotating polytropes be sources of the Kerr metric?
We use a recent result by Cabezas et al. to build up an approximate solution
to the gravitational field created by a rigidly rotating polytrope. We solve
the linearized Einstein equations inside and outside the surface of zero
pressure including second-order corrections due to rotational motion to get an
asymptotically flat metric in a global harmonic coordinate system. We prove
that if the metric and their first derivatives are continuous on the matching
surface up to this order of approximation, the multipole moments of this metric
cannot be fitted to those of the Kerr metric.Comment: LaTeX, 17 pages, submitted to CQ
Photoproduction of the Eta-Prime Mesons as a New Tool to Probe Baryon Resonances
We examine eta prime photoproduction, as a novel tool to study baryon
resonances around 2 GeV, of particular interest to the quark shell model, which
predicts a number of them. We find important roles of the form factors at the
strong vertices, and show that the N^*(2080) can be probed efficiently by this
reaction.Comment: Will be published in Phys. Rev.
Instantons and unitarity in quantum cosmology with fixed four-volume
We find a number of complex solutions of the Einstein equations in the
so-called unimodular version of general relativity, and we interpret them as
saddle points yielding estimates of a gravitational path integral over a space
of almost everywhere Lorentzian metrics on a spacetime manifold with topology
of the "no-boundary" type. In this setting, the compatibility of the
no-boundary initial condition with the definability of the quantum measure
reduces reduces to the normalizability and unitary evolution of the no-boundary
wave function \psi. We consider the spacetime topologies R^4 and RP^4 # R^4
within a Taub minisuperspace model with spatial topology S^3, and the spacetime
topology R^2 x T^2 within a Bianchi type I minisuperspace model with spatial
topology T^3. In each case there exists exactly one complex saddle point (or
combination of saddle points) that yields a wave function compatible with
normalizability and unitary evolution. The existence of such saddle points
tends to bear out the suggestion that the unimodular theory is less divergent
than traditional Einstein gravity. In the Bianchi type I case, the
distinguished complex solution is approximately real and Lorentzian at late
times, and appears to describe an explosive expansion from zero size at T=0.
(In the Taub cases, in contrast, the only complex solution with nearly
Lorentzian late-time behavior yields a wave function that is normalizable but
evolves nonunitarily, with the total probability increasing exponentially in
the unimodular "time" in a manner that suggests a continuous creation of new
universes at zero volume.) The issue of the stability of these results upon the
inclusion of more degrees of freedom is raised.Comment: 32 pages, REVTeX v3.1 with amsfonts. (v2: minor typos etc corrected.
Double Photoproduction off the Proton at Threshold
The reaction has been measured using the TAPS
BaF calorimeter at the tagged photon facility of the Mainz Microtron
accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold
this channel is significantly enhanced compared to double pion final states
with charged pions. In contrast to other reaction channels, the lower order
tree terms are strongly suppressed in 2 photoproduction. The consequence
is the dominance of pion loops in the 2 channel close to threshold - a
result that opens new prospects for the test of ChPT and in particular its
inherent loop terms. The present measurement is the first which is sensitive
enough for a conclusive comparison with the ChPT calculation and is in
agreement with its prediction. The data also show good agreement with a
calculation in the unitary chiral approach.Comment: Submitted to PL
Indication of Anisotropy in Electromagnetic Propagation over Cosmological Distances
We report a systematic rotation of the plane of polarization of
electromagnetic radiation propagating over cosmological distances. The effect
is extracted independently from Faraday rotation, and found to be correlated
with the angular positions and distances to the sources. Monte Carlo analysis
yields probabilistic P-values of order 10^(-3) for this to occur as a
fluctuation. A fit yields a birefringence scale of order 10^(25) meters.
Dependence on redshift z rules out a local effect. Barring hidden systematic
bias in the data, the correlation indicates a new cosmological effect.Comment: 5 pages, 1 figure, ReVTeX. For more information, see
http://www.cc.rochester.edu/college/rtc/Borge/aniso.htm
A first-principles study of the vibrational properties of crystalline tetracene under pressure
Gravity and the Quantum
The goal of this article is to present a broad perspective on quantum gravity
for \emph{non-experts}. After a historical introduction, key physical problems
of quantum gravity are illustrated. While there are a number of interesting and
insightful approaches to address these issues, over the past two decades
sustained progress has primarily occurred in two programs: string theory and
loop quantum gravity. The first program is described in Horowitz's contribution
while my article will focus on the second. The emphasis is on underlying ideas,
conceptual issues and overall status of the program rather than mathematical
details and associated technical subtleties.Comment: A general review of quantum gravity addresed non-experts. To appear
in the special issue `Space-time Hundred Years Later' of NJP; J.Pullin and R.
Price (editors). Typos and an attribution corrected; a clarification added in
section 2.
Photon Rates for Heavy-Ion Collisions from Hidden Local Symmetry
We study photon production from the hidden local symmetry approach that
includes pions, rho and a1 mesons and compute the corresponding photon emission
rates from a hadronic gas in thermal equilibrium. Together with experimental
radiative decay widths of the background, these rates are used in a
relativistic transport model to calculate single photon spectra in heavy-ion
collisions at SPS energies. We then employ this effective theory to test three
scenarios for the chiral phase transition in high-temperature nuclear matter
including decreasing vector meson masses. Although all calculations respect the
upper bound set by the WA80 Collaboration, we find the scenarios could be
distinguished with more detailed data.Comment: 12 pages, 12 Postscript figures; discussion of thermal equilibrium
rates expanded, minor corrections to text and graph
Resonant Structure of and Decays
The resonant structure of the four pion final state in the decay is analyzed using 4.27 million pairs
collected by the CLEO II experiment. We search for second class currents in the
decay using spin-parity analysis and establish an
upper limit on the non-vector current contribution. The mass and width of the
resonance are extracted from a fit to the
spectral function. A partial wave analysis of the resonant structure of the
decay is performed; the spectral decomposition of
the four pion system is dominated by the and final
states.Comment: 34 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLN
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