8,335 research outputs found
Post-Newtonian constraints on f(R) cosmologies in metric formalism
We compute the complete post-Newtonian limit of the metric form of f(R)
gravities using a scalar-tensor representation. By comparing the predictions of
these theories with laboratory and solar system experiments, we find a set of
inequalities that any lagrangian f(R) must satisfy. The constraints imposed by
those inequalities allow us to find explicit bounds to the possible nonlinear
terms of the lagrangian. We conclude that the lagrangian f(R) must be almost
linear in R and that corrections that grow at low curvatures are incompatible
with observations. This result shows that modifications of gravity at very low
cosmic densities cannot be responsible for the observed cosmic speed-up.Comment: 10 pages, no figures, revtex
Stability and Quasinormal Modes of Black holes in Tensor-Vector-Scalar theory: Scalar Field Perturbations
The imminent detection of gravitational waves will trigger precision tests of
gravity through observations of quasinormal ringing of black holes. While
General Relativity predicts just two polarizations of gravitational waves, the
so-called plus and cross polarizations, numerous alternative theories of
gravity predict up to six different polarizations which will potentially be
observed in current and future generations of gravitational wave detectors.
Bekenstein's Tensor-Vector-Scalar (TeVeS) theory and its generalization fall
into one such class of theory that predict the full gamut of six polarizations
of gravitational waves. In this paper we begin the study of quasinormal modes
(QNMs) in TeVeS by studying perturbations of the scalar field in a spherically
symmetric background. We show that, at least in the case where superluminal
propagation of perturbations is not present, black holes are generically stable
to this kind of perturbation. We also make a unique prediction that, as the
limit of the various coupling parameters of the theory tend to zero, the QNM
spectrum tends to times the QNM spectrum induced by scalar
perturbations of a Schwarzschild black hole in General Relativity due to the
intrinsic presence of the background vector field. We further show that the QNM
spectrum does not vary significantly from this value for small values of the
theory's coupling parameters, however can vary by as much as a few percent for
larger, but still physically relevant parameters.Comment: Published in Physical Review
Probing Strong-Field Scalar-Tensor Gravity with Gravitational Wave Asteroseismology
We present an alternative way of tracing the existence of a scalar field
based on the analysis of the gravitational wave spectrum of a vibrating neutron
star. Scalar-tensor theories in strong-field gravity can potentially introduce
much greater differences in the parameters of a neutron star than the
uncertainties introduced by the various equations of state. The detection of
gravitational waves from neutron stars can set constraints on the existence and
the strength of scalar fields. We show that the oscillation spectrum is
dramatically affected by the presence of a scalar field, and can provide unique
confirmation of its existence.Comment: 14 pages, 7 figure
Cerenkov's Effect and Neutrino Oscillations in Loop Quantum Gravity
Bounds on the scale parameter {\cal L} arising in loop quantum gravity theory
are derived in the framework of Cerenkov's effect and neutrino oscillations.
Assuming that {\cal L} is an universal constant, we infer {\cal L}>
10^{-18}eV^{-1}, a bound compatible with ones inferred in different physical
context.Comment: 6 pages, no figures, in print on MPL
A Metric for Rapidly Spinning Black Holes Suitable for Strong-Field Tests of the No-Hair Theorem
According to the no-hair theorem, astrophysical black holes are uniquely
characterized by their masses and spins and are described by the Kerr metric.
Several parametric deviations from the Kerr metric have been suggested to study
observational signatures in both the electromagnetic and gravitational-wave
spectra that differ from the expected Kerr signals. Due to the no-hair theorem,
however, such spacetimes cannot be regular everywhere outside the event
horizons, if they are solutions to the Einstein field equations; they are often
characterized by naked singularities or closed time-like loops in the regions
of the spacetime that are accessible to an external observer. For observational
tests of the no-hair theorem that involve phenomena in the vicinity of the
circular photon orbit or the innermost stable circular orbit around a black
hole, these pathologies limit the applicability of the metrics only to compact
objects that do not spin rapidly. In this paper, we construct a Kerr-like
metric which depends on a set of free parameters in addition to its mass and
spin and which is regular everywhere outside of the event horizon. We derive
expressions for the energy and angular momentum of a particle on a circular
equatorial orbit around the black hole and compute the locations of the
innermost stable circular orbit and the circular photon orbit. We demonstrate
that these orbits change significantly for even moderate deviations from the
Kerr metric. The properties of our metric make it an ideally suited spacetime
to carry out strong-field tests of the no-hair theorem in the electromagnetic
spectrum using the properties of accretion flows around astrophysical black
holes of arbitrary spin.Comment: 11 pages, 7 figures, accepted for publication in PR
Strong field effects on binary systems in Einstein-aether theory
"Einstein-aether" theory is a generally covariant theory of gravity
containing a dynamical preferred frame. This article continues an examination
of effects on the motion of binary pulsar systems in this theory, by
incorporating effects due to strong fields in the vicinity of neutron star
pulsars. These effects are included through an effective approach, by treating
the compact bodies as point particles with nonstandard, velocity dependent
interactions parametrized by dimensionless "sensitivities". Effective
post-Newtonian equations of motion for the bodies and the radiation damping
rate are determined. More work is needed to calculate values of the
sensitivities for a given fluid source, so precise constraints on the theory's
coupling constants cannot yet be stated. It is shown, however, that strong
field effects will be negligible given current observational uncertainties if
the dimensionless couplings are less than roughly 0.01 and two conditions that
match the PPN parameters to those of pure general relativity are imposed. In
this case, weak field results suffice and imply one further condition on the
couplings. Thus, there exists a one-parameter family of Einstein-aether
theories with "small-enough" couplings that passes all current observational
tests. No conclusion can yet be reached for large couplings.Comment: 23 pages, 1 figure; v2: fixed error in Eqn. (70) and resulting bounds
on c'
Testing Scalar-Tensor Gravity Using Space Gravitational-Wave Interferometers
We calculate the bounds which could be placed on scalar-tensor theories of
gravity of the Jordan, Fierz, Brans and Dicke type by measurements of
gravitational waveforms from neutron stars (NS) spiralling into massive black
holes (MBH) using LISA, the proposed space laser interferometric observatory.
Such observations may yield significantly more stringent bounds on the
Brans-Dicke coupling parameter \omega than are achievable from solar system or
binary pulsar measurements. For NS-MBH inspirals, dipole gravitational
radiation modifies the inspiral and generates an additional contribution to the
phase evolution of the emitted gravitational waveform. Bounds on \omega can
therefore be found by using the technique of matched filtering. We compute the
Fisher information matrix for a waveform accurate to second post-Newtonian
order, including the effect of dipole radiation, filtered using a currently
modeled noise curve for LISA, and determine the bounds on \omega for several
different NS-MBH canonical systems. For example, observations of a 1.4 solar
mass NS inspiralling to a 1000 solar mass MBH with a signal-to-noise ratio of
10 could yield a bound of \omega > 240,000, substantially greater than the
current experimental bound of \omega > 3000.Comment: 18 pages, 4 figures, 1 table; to be submitted to Phys. Rev.
Pharmaceutical M&A Activity: Effects on Prices, Innovation, and Competition
The rise of blockbuster pharmaceutical acquisitions has prompted fears that unprecedented market concentration will weaken competition. Two of the most prominent concerns focus on the upstream and downstream ends of the pharmaceutical industry: (1) the concern that these mergers will concentrate the market for discovery and will therefore lead to fewer discoveries; and (2) the concern that merging large marketing, sales, and distribution forces will strengthen the hands of select pharmaceutical manufacturers and weaken downstream competition. Having considered potential dynamic effects in the industry and conducted a series of preliminary interviews with knowledgeable observers, though, this Article argues that neither of these common fears is systematically warranted. There are, however, potential dangers in market concentration at an intermediate stage during the discovery-to-development path: the stage for regulatory approval. These preliminary findings are a product of dramatic changes that are currently reshaping the structure of the pharmaceutical industry. This Article discusses how these structural changes contribute to the current merger wave, how dynamic responses by industry players in response to the merger wave mitigate the potential harm from competition, and how the political arena might still offer threats to market concentration
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