696 research outputs found
Particle spectra of gravity based on internal symmetry of quantum fields
We examine the weak-field, zero-coupling limit of Yang--Mills gravity as
recently formulated by Partanen and Tulkki, viewed as a free quantum field
theory. In this approximation the theory has a ghostly teleparallel vacuum. We
suggest that bimetric, vacuum expectation value, or finite-coupling extensions
should be investigated.Comment: 3 pages, 1 figur
A Purely Gravitational Origin for Einstein-Proca Theory
We construct a theory of gravity in which a propagating massive vector field
arises from a quadratic curvature invariant. The Einstein-Cartan formulation
and a partial suppression of torsion ensure the absence of ghost and
strong-coupling problems, as we prove with nonlinear Lagrangian and Hamiltonian
analysis. Augmenting General Relativity with a propagating torsion vector, our
theory provides a purely gravitational origin of Einstein-Proca models and
constrains their parameter space. As an outlook to phenomenology, we discuss
the gravitational production of fermionic dark matter.Comment: 13 pages, 5 figures, 3 appendice
Application of energy and angular momentum balance to gravitational radiation reaction for binary systems with spin-orbit coupling
We study gravitational radiation reaction in the equations of motion for
binary systems with spin-orbit coupling, at order (v/c)^7 beyond Newtonian
gravity, or O(v/c)^2 beyond the leading radiation reaction effects for
non-spinning bodies. We use expressions for the energy and angular momentum
flux at infinity that include spin-orbit corrections, together with an
assumption of energy and angular momentum balance, to derive equations of
motion that are valid for general orbits and for a class of coordinate gauges.
We show that the equations of motion are compatible with those derived earlier
by a direct calculation.Comment: 12 pages, submitted to General Relativity and Gravitatio
Manifestly covariant variational principle for gauge theories of gravity
A variational principle for gauge theories of gravity is presented, which
maintains manifest covariance under the symmetries to which the action is
invariant, throughout the calculation of the equations of motion and
conservation laws. This is performed by deriving explicit manifestly covariant
expressions for the Euler--Lagrange variational derivatives and Noether's
theorems for a generic action of the form typically assumed in gauge theories
of gravity. The approach is illustrated by application to two scale-invariant
gravitational gauge theories, namely Weyl gauge theory (WGT) and the recently
proposed `extended' Weyl gauge theory (eWGT), where the latter may be
considered as a novel gauging of the conformal group, but the method can be
straightforwardly applied to other theories with smaller or larger symmetry
groups. The approach also enables one easily to establish the relationship
between manifestly covariant forms of variational derivatives obtained when one
or more of the gauge field strengths is set to zero either before or after the
variation is performed. This is illustrated explicitly for both WGT and eWGT in
the case where the translational gauge field strength (or torsion) is set to
zero before and after performing the variation, respectively.Comment: 27 pages, no figures, submitted to PR
New Villagers
It was a gorgeous day in Dubrovnik, Croatia. Brooke Barker, senior in management and international business, kayaked to the Island of Lokrum, the wind blew through her hair as she paddled to keep up with the group. Upon reaching the island, she began the trek to the top, where she ran into some local daredevils preparing to jump into the deep blue water of the Adriatic Sea 35 feet below. Intrigued, Brooke decided to join them
Covariant Calculation of General Relativistic Effects in an Orbiting Gyroscope Experiment
We carry out a covariant calculation of the measurable relativistic effects
in an orbiting gyroscope experiment. The experiment, currently known as Gravity
Probe B, compares the spin directions of an array of spinning gyroscopes with
the optical axis of a telescope, all housed in a spacecraft that rolls about
the optical axis. The spacecraft is steered so that the telescope always points
toward a known guide star. We calculate the variation in the spin directions
relative to readout loops rigidly fixed in the spacecraft, and express the
variations in terms of quantities that can be measured, to sufficient accuracy,
using an Earth-centered coordinate system. The measurable effects include the
aberration of starlight, the geodetic precession caused by space curvature, the
frame-dragging effect caused by the rotation of the Earth and the deflection of
light by the Sun.Comment: 7 pages, 1 figure, to be submitted to Phys. Rev.
Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. III. Radiation reaction for binary systems with spinning bodies
Using post-Newtonian equations of motion for fluid bodies that include
radiation-reaction terms at 2.5 and 3.5 post-Newtonian (PN) order (O[(v/c)^5]
and O[(v/c)^7] beyond Newtonian order), we derive the equations of motion for
binary systems with spinning bodies. In particular we determine the effects of
radiation-reaction coupled to spin-orbit effects on the two-body equations of
motion, and on the evolution of the spins. For a suitable definition of spin,
we reproduce the standard equations of motion and spin-precession at the first
post-Newtonian order. At 3.5PN order, we determine the spin-orbit induced
reaction effects on the orbital motion, but we find that radiation damping has
no effect on either the magnitude or the direction of the spins. Using the
equations of motion, we find that the loss of total energy and total angular
momentum induced by spin-orbit effects precisely balances the radiative flux of
those quantities calculated by Kidder et al. The equations of motion may be
useful for evolving inspiraling orbits of compact spinning binaries.Comment: 19 pages, small corrections, equivalent to published versio
Representing and extending ensembles of parsimonious evolutionary histories with a directed acyclic graph
In many situations, it would be useful to know not just the best phylogenetic
tree for a given data set, but the collection of high-quality trees. This goal
is typically addressed using Bayesian techniques, however, current Bayesian
methods do not scale to large data sets. Furthermore, for large data sets with
relatively low signal one cannot even store every good tree individually,
especially when the trees are required to be bifurcating. In this paper, we
develop a novel object called the "history subpartition directed acyclic graph"
(or "history sDAG" for short) that compactly represents an ensemble of trees
with labels (e.g. ancestral sequences) mapped onto the internal nodes. The
history sDAG can be built efficiently and can also be efficiently trimmed to
only represent maximally parsimonious trees. We show that the history sDAG
allows us to find many additional equally parsimonious trees, extending
combinatorially beyond the ensemble used to construct it. We argue that this
object could be useful as the "skeleton" of a more complete uncertainty
quantification.Comment: To appear in JM
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