2,431 research outputs found
Extremal Black Holes in Dynamical Chern-Simons Gravity
Rapidly rotating black hole solutions in theories beyond general relativity
play a key role in experimental gravity, as they allow us to compute
observables in extreme spacetimes that deviate from the predictions of general
relativity. Such solutions are often difficult to find in
beyond-general-relativity theories due to the inclusion of additional fields
that couple to the metric non-linearly and non-minimally. In this paper, we
consider rotating black hole solutions in one such theory, dynamical
Chern-Simons gravity, where the Einstein-Hilbert action is modified by the
introduction of a dynamical scalar field that couples to the metric through the
Pontryagin density. We treat dynamical Chern-Simons gravity as an effective
field theory and work in the decoupling limit, where corrections are treated as
small perturbations from general relativity. We perturb about the
maximally-rotating Kerr solution, the so-called extremal limit, and develop
mathematical insight into the analysis techniques needed to construct solutions
for generic spin. First we find closed-form, analytic expressions for the
extremal scalar field, and then determine the trace of the metric perturbation,
giving both in terms of Legendre decompositions. Retaining only the first three
and four modes in the Legendre representation of the scalar field and the
trace, respectively, suffices to ensure a fidelity of over 99% relative to full
numerical solutions. The leading-order mode in the Legendre expansion of the
trace of the metric perturbation contains a logarithmic divergence at the
extremal Kerr horizon, which is likely to be unimportant as it occurs inside
the perturbed dynamical Chern-Simons horizon. The techniques employed here
should enable the construction of analytic, closed-form expressions for the
scalar field and metric perturbations on a background with arbitrary rotation.Comment: 25+9 pages (single column), 10 figures, 1 table; matches published
versio
The Barbero-Immirzi Parameter as a Scalar Field: K-Inflation from Loop Quantum Gravity?
We consider a loop-quantum gravity inspired modification of general
relativity, where the Holst action is generalized by making the Barbero-Immirzi
(BI) parameter a scalar field, whose value could be dynamically determined. The
modified theory leads to a non-zero torsion tensor that corrects the field
equations through quadratic first-derivatives of the BI field. Such a
correction is equivalent to general relativity in the presence of a scalar
field with non-trivial kinetic energy. This stress-energy of this field is
automatically covariantly conserved by its own dynamical equations of motion,
thus satisfying the strong equivalence principle. Every general relativistic
solution remains a solution to the modified theory for any constant value of
the BI field. For arbitrary time-varying BI fields, a study of cosmological
solutions reduces the scalar field stress-energy to that of a pressureless
perfect fluid in a comoving reference frame, forcing the scale factor dynamics
to be equivalent to those of a stiff equation of state. Upon ultraviolet
completion, this model could provide a natural mechanism for k-inflation, where
the role of the inflaton is played by the BI field and inflation is driven by
its non-trivial kinetic energy instead of a potential.Comment: Phys. Rev. D78, 064070 (2008
Computing Jacobi Forms
We describe an implementation for computing holomorphic and skew-holomorphic
Jacobi forms of integral weight and scalar index on the full modular group.
This implementation is based on formulas derived by one of the authors which
express Jacobi forms in terms of modular symbols of elliptic modular forms.
Since this method allows to generate a Jacobi eigenform directly from a given
modular eigensymbol without reference to the whole ambient space of Jacobi
forms it makes it possible to compute Jacobi Hecke eigenforms of large index.
We illustrate our method with several examples.Comment: 14 pages, 5 tables, Cython implementation of algorithm included.
Revised version. To appear in the LMS Journal of Computation and Mathematic
The role of intermediaries in the synchronization of pulse-coupled oscillators
The role of intermediaries in the synchronization of small groups of light
controlled oscillators (LCO) is addressed. A single LCO is a two-time-scale
phase oscillator. When pulse-coupling two LCOs, the synchronization time
decreases monotonously as the coupling strength increases, independent of the
initial conditions and frequency detuning. In this work we study numerically
the effects that a third LCO induces to the collective behavior of the system.
We analyze the new system by dealing with directed heterogeneous couplings
among the units. We report a novel and robust phenomenon, absent when coupling
two LCOs, which consists of a discontinuous relationship between the
synchronization time and coupling strength or initial conditions. The mechanism
responsible for the appearance of such discontinuities is discussed.Comment: 11 pages, 8 figure
Many-body effects in magnetic inelastic electron tunneling spectroscopy
Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp
increases in conductance when a new conductance channel associated to a change
in magnetic structure is open. Typically, the magnetic moment carried by an
adsorbate can be changed by collision with a tunneling electron; in this
process the spin of the electron can flip or not. A previous one-electron
theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009)] successfully explained both
the conductance thresholds and the magnitude of the conductance variation. The
elastic spin flip of conduction electrons by a magnetic impurity leads to the
well known Kondo effect. In the present work, we compare the theoretical
predictions for inelastic magnetic tunneling obtained with a one-electron
approach and with a many-body theory including Kondo-like phenomena. We apply
our theories to a singlet-triplet transition model system that contains most of
the characteristics revealed in magnetic IETS. We use two self-consistent
treatments (non-crossing approximation and self-consistent ladder
approximation). We show that, although the one-electron limit is properly
recovered, new intrinsic many-body features appear. In particular, sharp peaks
appear close to the inelastic thresholds; these are not localized exactly at
thresholds and could influence the determination of magnetic structures from
IETS experiments.Analysis of the evolution with temperature reveals that these
many-body features involve an energy scale different from that of the usual
Kondo peaks. Indeed, the many-body features perdure at temperatures much larger
than the one given by the Kondo energy scale of the system.Comment: 10 pages and 6 figure
Non-perturbative renormalization group for the Kardar-Parisi-Zhang equation
We present a simple approximation of the non-perturbative renormalization
group designed for the Kardar-Parisi-Zhang equation and show that it yields the
correct phase diagram, including the strong-coupling phase with reasonable
scaling exponent values in physical dimensions. We find indications of a
possible qualitative change of behavior around . We discuss how our
approach can be systematically improved.Comment: 4 pages, 1 figure, references added, minor change
Approximate black hole binary spacetime via asymptotic matching
We construct a fully analytic, general relativistic, nonspinning black hole
binary spacetime that approximately solves the vacuum Einstein equations
everywhere in space and time for black holes sufficiently well separated. The
metric is constructed by asymptotically matching perturbed Schwarzschild
metrics near each black hole to a two-body post-Newtonian metric far from them,
and a two-body post-Minkowskian metric farther still. Asymptotic matching is
done without linearizing about a particular time slice, and thus it is valid
dynamically and for all times, provided the binary is sufficiently well
separated. This approximate global metric can be used for long dynamical
evolutions of relativistic magnetohydrodynamical, circumbinary disks around
inspiraling supermassive black holes to study a variety of phenomena.Comment: 17 pages, 8 figures, 1 table. Appendix added to match published
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Constraining Lorentz-violating, Modified Dispersion Relations with Gravitational Waves
Modified gravity theories generically predict a violation of Lorentz
invariance, which may lead to a modified dispersion relation for propagating
modes of gravitational waves. We construct a parametrized dispersion relation
that can reproduce a range of known Lorentz-violating predictions and
investigate their impact on the propagation of gravitational waves. A modified
dispersion relation forces different wavelengths of the gravitational wave
train to travel at slightly different velocities, leading to a modified phase
evolution observed at a gravitational-wave detector. We show how such
corrections map to the waveform observable and to the parametrized
post-Einsteinian framework, proposed to model a range of deviations from
General Relativity. Given a gravitational-wave detection, the lack of evidence
for such corrections could then be used to place a constraint on Lorentz
violation. The constraints we obtain are tightest for dispersion relations that
scale with small power of the graviton's momentum and deteriorate for a steeper
scaling.Comment: 11 pages, 3 figures, 2 tables: title changed slightly, published
versio
Constraining the evolutionary history of Newton's constant with gravitational wave observations
Space-borne gravitational wave detectors, such as the proposed Laser
Interferometer Space Antenna, are expected to observe black hole coalescences
to high redshift and with large signal-to-noise ratios, rendering their
gravitational waves ideal probes of fundamental physics. The promotion of
Newton's constant to a time-function introduces modifications to the binary's
binding energy and the gravitational wave luminosity, leading to corrections in
the chirping frequency. Such corrections propagate into the response function
and, given a gravitational wave observation, they allow for constraints on the
first time-derivative of Newton's constant at the time of merger. We find that
space-borne detectors could indeed place interesting constraints on this
quantity as a function of sky position and redshift, providing a
{\emph{constraint map}} over the entire range of redshifts where binary black
hole mergers are expected to occur. A LISA observation of an equal-mass
inspiral event with total redshifted mass of 10^5 solar masses for three years
should be able to measure at the time of merger to better than
10^(-11)/yr.Comment: 11 pages, 2 figures, replaced with version accepted for publication
in Phys. Rev. D
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