507 research outputs found
Theoretical Aspects of the Equivalence Principle
We review several theoretical aspects of the Equivalence Principle (EP). We
emphasize the unsatisfactory fact that the EP maintains the absolute character
of the coupling constants of physics while General Relativity, and its
generalizations (Kaluza-Klein,..., String Theory), suggest that all absolute
structures should be replaced by dynamical entities. We discuss the
EP-violation phenomenology of dilaton-like models, which is likely to be
dominated by the linear superposition of two effects: a signal proportional to
the nuclear Coulomb energy, related to the variation of the fine-structure
constant, and a signal proportional to the surface nuclear binding energy,
related to the variation of the light quark masses. We recall the various
theoretical arguments (including a recently proposed anthropic argument)
suggesting that the EP be violated at a small, but not unmeasurably small
level. This motivates the need for improved tests of the EP. These tests are
probing new territories in physics that are related to deep, and mysterious,
issues in fundamental physics.Comment: 21 pages, no figures; submitted to a "focus issue" of Classical and
Quantum Gravity on Tests of the Weak Equivalence Principle, organized by
Clive Speake and Clifford Wil
Constraining the Equation of State with Moment of Inertia Measurements
We estimate that the moment of inertia of star A in the recently discovered
double pulsar system PSR J0737-3039 may be determined after a few years of
observation to something like 10% accuracy. This would enable accurate
estimates of the radius of the star and the presure of matter in the vicinity
of 1 to 2 times the nuclear saturation density, which would in turn provide
strong constraints on the equation of state of neutron stars and the physics of
their interiors.Comment: Submitted to ApJ, 4 figure
Post-Newtonian Theory for Precision Doppler Measurements of Binary Star Orbits
The determination of velocities of stars from precise Doppler measurements is
described here using relativistic theory of astronomical reference frames so as
to determine the Keplerian and post-Keplerian parameters of binary systems. We
apply successive Lorentz transformations and the relativistic equation of light
propagation to establish the exact treatment of Doppler effect in binary
systems both in special and general relativity theories. As a result, the
Doppler shift is a sum of (1) linear in terms, which include the
ordinary Doppler effect and its variation due to the secular radial
acceleration of the binary with respect to observer; (2) terms proportional to
, which include the contributions from the quadratic Doppler effect
caused by the relative motion of binary star with respect to the Solar system,
motion of the particle emitting light and diurnal rotational motion of
observer, orbital motion of the star around the binary's barycenter, and
orbital motion of the Earth; and (3) terms proportional to , which
include the contributions from redshifts due to gravitational fields of the
star, star's companion, Galaxy, Solar system, and the Earth. After
parameterization of the binary's orbit we find that the presence of
periodically changing terms in the Doppler schift enables us disentangling
different terms and measuring, along with the well known Keplerian parameters
of the binary, four additional post-Keplerian parameters, including the
inclination angle of the binary's orbit, . We briefly discuss feasibility of
practical implementation of these theoretical results, which crucially depends
on further progress in the technique of precision Doppler measurements.Comment: Minor changes, 1 Figure included, submitted to Astrophys.
Development of ex vivo organ culture models to mimic human corneal scarring
PURPOSE: To develop ex vivo organ culture models of human corneal scarring suitable for pharmacological testing and the study of the molecular mechanisms leading to corneal haze after laser surgery or wounding. METHODS: Corneas from human donors were cultured ex vivo for 30 days, either at the air-liquid interface (AL) or immersed (IM) in the culture medium. Histological features and immunofluorescence for fibronectin, tenascin C, thrombospondin-1, and α-smooth muscle actin were graded from 0 to 3 for control corneas and for corneas wounded with an excimer laser. The effects of adding 10 ng/ml transforming growth factor-β1 (TGF-β1) to the culture medium and of prior complete removal of the epithelium and limbus, thus preventing reepithelialization, were also analyzed on wounded corneas. Collagen III expression was detected with real-time PCR. RESULTS: Wounding alone was sufficient to induce keratocyte activation and stromal disorganization, but it was only in the presence of added TGF-β1 that intense staining for fibronectin and tenascin C was found in the AL and IM models (as well as thrombospondin-1 in the AL model) and that α-smooth muscle actin became detectable. The scar-like appearance of the corneas was exacerbated when TGF-β1 was added and reepithelialization was prevented, resulting in the majority of corneas becoming opaque and marked upregulation of collagen III. CONCLUSIONS: THE MAIN FEATURES OF CORNEAL SCARRING WERE REPRODUCED IN THESE TWO COMPLEMENTARY MODELS: the AL model preserved differentiation of the epithelium and permits the topical application of active molecules, while the IM model ensures better perfusion by soluble compounds
Testing the equivalence principle: why and how?
Part of the theoretical motivation for improving the present level of testing
of the equivalence principle is reviewed. The general rationale for optimizing
the choice of pairs of materials to be tested is presented. One introduces a
simplified rationale based on a trichotomy of competing classes of theoretical
models.Comment: 11 pages, Latex, uses ioplppt.sty, submitted to Class. Quantum Gra
Spontaneous Scalarization and Boson Stars
We study spontaneous scalarization in Scalar-Tensor boson stars. We find that
scalarization does not occur in stars whose bosons have no self-interaction. We
introduce a quartic self-interaction term into the boson Lagrangian and show
that when this term is large, scalarization does occur. Strong self-interaction
leads to a large value of the compactness (or sensitivity) of the boson star, a
necessary condition for scalarization to occur, and we derive an analytical
expression for computing the sensitivity of a boson star in Brans-Dicke theory
from its mass and particle number. Next we comment on how one can use the
sensitivity of a star in any Scalar-Tensor theory to determine how its mass
changes when it undergoes gravitational evolution. Finally, in the Appendix, we
derive the most general form of the boson wavefunction that minimises the
energy of the star when the bosons carry a U(1) charge.Comment: 23 pages, 5 postscript figures. Typing errors corrected. Includes
some new text that relates the paper to several previous results. Accepted
for publication in PR
Self-accelerated Universe
It is widely believed that the large redshifts for distant supernovae are
explained by the vacuum energy dominance, or, in other words, by the
cosmological constant in Einstein's equations, which is responsible for the
anti-gravitation effect. A tacit assumption is that particles move along a
geodesic for the background metric. This is in the same spirit as the consensus
regarding the uniform Galilean motion of a free electron. However, there is a
runaway solution to the Lorentz--Dirac equation governing the behavior of a
radiating electron, in addition to the Galilean solution. Likewise, a runaway
solution to the entire system of equations, both gravitation and matter
equations of motion including, may provide an alternative explanation for the
accelerated expansion of the Universe, without recourse to the hypothetic
cosmological constant.Comment: 11 pages; Talk at the 9th Adriatic Meeting, Dubrovnic, Croatia, 4-14
September, 2003, Minor improvement, references added; to appear in ``Progress
in General Relativity and Quantum Cosmology Research'', Nova Science
Publisher
Pulse Arrival-Times from Binary Pulsars with Rotating Black Hole Companions
We present a study of the gravitational time delay of arrival of signals from
binary pulsar systems with rotating black hole companions. In particular, we
investigate the strength of this effect (Shapiro delay) as a function of the
inclination, eccentricity and period of the orbit, as well as the mass and
angular momentum of the black hole. This study is based on direct numerical
integration of null geodesics in a Kerr background geometry. We find that, for
binaries with sufficiently high orbital inclinations () and compact
companion masses , the effect arising from the rotation of the
black hole in the system amounts to a microsecond-level variation of the
arrival times of the pulsar pulses. If measurable, this variation could provide
a unique signature for the presence of a rotating black hole in a binary pulsar
system.Comment: 8 pages, 1 figur
Time-symmetric initial data for binary black holes in numerical relativity
We look for physically realistic initial data in numerical relativity which
are in agreement with post-Newtonian approximations. We propose a particular
solution of the time-symmetric constraint equation, appropriate to two
momentarily static black holes, in the form of a conformal decomposition of the
spatial metric. This solution is isometric to the post-Newtonian metric up to
the 2PN order. It represents a non-linear deformation of the solution of Brill
and Lindquist, i.e. an asymptotically flat region is connected to two
asymptotically flat (in a certain weak sense) sheets, that are the images of
the two singularities through appropriate inversion transformations. The total
ADM mass M as well as the individual masses m_1 and m_2 (when they exist) are
computed by surface integrals performed at infinity. Using second order
perturbation theory on the Brill-Lindquist background, we prove that the
binary's interacting mass-energy M-m_1-m_2 is well-defined at the 2PN order and
in agreement with the known post-Newtonian result.Comment: 27 pages, to appear in Phys. Rev.
Einstein billiards and spatially homogeneous cosmological models
In this paper, we analyse the Einstein and Einstein-Maxwell billiards for all
spatially homogeneous cosmological models corresponding to 3 and 4 dimensional
real unimodular Lie algebras and provide the list of those models which are
chaotic in the Belinskii, Khalatnikov and Lifschitz (BKL) limit. Through the
billiard picture, we confirm that, in D=5 spacetime dimensions, chaos is
present if off-diagonal metric elements are kept: the finite volume billiards
can be identified with the fundamental Weyl chambers of hyperbolic Kac-Moody
algebras. The most generic cases bring in the same algebras as in the
inhomogeneous case, but other algebras appear through special initial
conditions.Comment: 27 pages, 10 figures, additional possibility analysed in section 4.3,
references added, typos correcte
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