628 research outputs found
Testing gravitational parity violation with coincident gravitational waves and short gamma-ray bursts
Gravitational parity violation is a possibility motivated by particle
physics, string theory and loop quantum gravity. One effect of it is amplitude
birefringence of gravitational waves, whereby left and right
circularly-polarized waves propagate at the same speed but with different
amplitude evolution. Here we propose a test of this effect through coincident
observations of gravitational waves and short gamma-ray bursts from binary
mergers involving neutron stars. Such gravitational waves are highly left or
right circularly-polarized due to the geometry of the merger. Using
localization information from the gamma-ray burst, ground-based gravitational
wave detectors can measure the distance to the source with reasonable accuracy.
An electromagnetic determination of the redshift from an afterglow or host
galaxy yields an independent measure of this distance. Gravitational parity
violation would manifest itself as a discrepancy between these two distance
measurements. We exemplify such a test by considering one specific effective
theory that leads to such gravitational parity-violation, Chern-Simons gravity.
We show that the advanced LIGO-Virgo network and all-sky gamma-ray telescopes
can be sensitive to the propagating sector of Chern-Simons gravitational parity
violation to a level roughly two orders of magnitude better than current
stationary constraints from the LAGEOS satellites.Comment: 21 pages, 2 figures, submitted to Phys. Rev.
Frequency-domain waveform approximants capturing Doppler shifts
Gravitational wave astrophysics has only just begun, and as current detectors
are upgraded and new detectors are built, many new, albeit faint, features in
the signals will become accessible. One such feature is the presence of
time-dependent Doppler shifts, generated by the acceleration of the center of
mass of the gravitational-wave emitting system. We here develop a generic
method that takes a frequency-domain, gravitational-wave model devoid of
Doppler shifts and introduces modifications that incorporate them. Building
upon a perturbative expansion that assumes the Doppler-shift velocity is small
relative to the speed of light, the method consists of the inclusion of a
single term in the Fourier phase and two terms in the Fourier amplitude. We
validate the method through matches between waveforms with a Doppler shift in
the time domain and waveforms constructed with our method for two toy problems:
constant accelerations induced by a distant third body and Gaussian
accelerations that resemble a kick profile. We find mismatches below
for all of the astrophysically relevant cases considered, and
improve further at smaller velocities. The work presented here will allow for
the use of future detectors to extract new, faint features in the signal from
the noise.Comment: 11 pages, 5 figures, submitted to Phys. Rev.
Constraining alternative theories of gravity using pulsar timing arrays
The opening of the gravitational wave window by ground-based laser
interferometers has made possible many new tests of gravity, including the
first constraints on polarization. It is hoped that within the next decade
pulsar timing will extend the window by making the first detections in the
nano-Hertz frequency regime. Pulsar timing offers several advantages over
ground-based interferometers for constraining the polarization of gravitational
waves due to the many projections of the polarization pattern provided by the
different lines of sight to the pulsars, and the enhanced response to
longitudinal polarizations. Here we show that existing results from pulsar
timing arrays can be used to place stringent limits on the energy density of
longitudinal stochastic gravitational waves. Paradoxically however, we find
that longitudinal modes will be very difficult to detect due to the large
variance in the pulsar-pulsar correlation patterns for these modes. Existing
upper limits on the power spectrum of pulsar timing residuals imply that the
amplitude of vector longitudinal and scalar longitudinal modes at frequencies
of 1/year are constrained: and , while the bounds on the energy density for a
scale invariant cosmological background are: and .Comment: 5 pages, 4 figure
A new PPN parameter to test Chern-Simons gravity
We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN)
framework through a weak-field solution of the modified field equations. We
find that CS gravity possesses the same PPN parameters as general relativity,
except for the inclusion of a new term, proportional to the CS coupling and the
curl of the PPN vector potential. This new term leads to a modification of
frame dragging and gyroscopic precession and we provide an estimate of its
size. This correction might be used in experiments, such as Gravity Probe B, to
bound CS gravity and test string theory.Comment: 4 pages, replaced with version accepted for publication in Phys. Rev.
Letters (December, 2007
Extreme Mass-Ratio Inspirals in the Effective-One-Body Approach: Quasi-Circular, Equatorial Orbits around a Spinning Black Hole
We construct effective-one-body waveform models suitable for data analysis
with LISA for extreme-mass ratio inspirals in quasi-circular, equatorial orbits
about a spinning supermassive black hole. The accuracy of our model is
established through comparisons against frequency-domain, Teukolsky-based
waveforms in the radiative approximation. The calibration of eight high-order
post-Newtonian parameters in the energy flux suffices to obtain a phase and
fractional amplitude agreement of better than 1 radian and 1 % respectively
over a period between 2 and 6 months depending on the system considered. This
agreement translates into matches higher than 97 % over a period between 4 and
9 months, depending on the system. Better agreements can be obtained if a
larger number of calibration parameters are included. Higher-order mass ratio
terms in the effective-one-body Hamiltonian and radiation-reaction introduce
phase corrections of at most 30 radians in a one year evolution. These
corrections are usually one order of magnitude larger than those introduced by
the spin of the small object in a one year evolution. These results suggest
that the effective-one-body approach for extreme mass ratio inspirals is a good
compromise between accuracy and computational price for LISA data analysis
purposes.Comment: 21 pages, 8 figures, submitted to Phys. Rev.
Relativistic Effects in Extreme Mass Ratio Gravitational Wave Bursts
Extreme mass ratio bursts (EMRBs) have been proposed as a possible source for
future space-borne gravitational wave detectors, such as the Laser
Interferometer Space Antenna (LISA). These events are characterized by
long-period, nearly-radial orbits of compact objects around a central massive
black hole. The gravitational radiation emitted during such events consists of
a short burst, corresponding to periapse passage, followed by a longer, silent
interval. In this paper we investigate the impact of including relativistic
corrections to the description of the compact object's trajectory via a
geodesic treatment, as well as including higher-order multipole corrections in
the waveform calculation. The degree to which the relativistic corrections are
important depends on the EMRB's orbital parameters. We find that relativistic
EMRBs (v_{max}}/c > 0.25) are not rare and actually account for approximately
half of the events in our astrophysical model. The relativistic corrections
tend to significantly change the waveform amplitude and phase relative to a
Newtonian description, although some of this dephasing could be mimicked by
parameter errors. The dephasing over several bursts could be of particular
importance not only to gravitational wave detection, but also to parameter
estimation, since it is highly correlated to the spin of the massive black
hole. Consequently, we postulate that if a relativistic EMRB is detected, such
dephasing might be used to probe the relativistic character of the massive
black hole and obtain information about its spin.Comment: 13 pages, 8 figures, 2 tables. Replaced with version accepted for
publication in the Ap.
Seeking the Loop Quantum Gravity Barbero-Immirzi Parameter and Field in 4D, = 1 Supergravity
We embed the Loop Quantum Gravity Barbero-Immirzi parameter and field within
an action describing 4D, = 1 supergravity and thus within a Low Energy
Effective Action of Superstring/M-Theory. We use the fully gauge-covariant
description of supergravity in (curved) superspace. The gravitational constant
is replaced with the vacuum expectation value of a scalar field, which in local
supersymmetry is promoted to a complex, covariantly chiral scalar superfield.
The imaginary part of this superfield couples to a supersymmetric Holst term.
The Holst term also serves as a starting point in the Loop Quantum Gravity
action. This suggest the possibility of a relation between Loop Quantum Gravity
and supersymmetric string theory, where the Barbero-Immirzi parameter and field
of the former play the role of the supersymmetric axion in the latter. Adding
matter fermions in Loop Quantum Gravity may require the extension of the Holst
action through the Nieh-Yan topological invariant, while in pure, matter-free
supergravity their supersymmetric extensions are the same. We show that, when
the Barbero-Immirzi parameter is promoted to a field in the context of 4D
supergravity, it is equivalent to adding a dynamical complex chiral
(dilaton-axion) superfield with a non-trivial kinetic term (or K\"ahler
potential), coupled to supergravity.Comment: 20 pages, 1 figure. Replaced with accepted version in Phys. Rev.
Cross section, final spin and zoom-whirl behavior in high-energy black hole collisions
We study the collision of two highly boosted equal mass, nonrotating black
holes with generic impact parameter. We find such systems to exhibit zoom-whirl
behavior when fine tuning the impact parameter. Near the threshold of immediate
merger the remnant black hole Kerr parameter can be near maximal (a/M about
0.95) and the radiated energy can be as large as 35% of the center-of-mass
energy.Comment: Rearranged results section; accepted for publication in Phys. Rev.
Let
Superkicks in ultrarelativistic encounters of spinning black holes
We study ultrarelativistic encounters of two spinning, equal-mass black holes
through simulations in full numerical relativity. Two initial data sequences
are studied in detail: one that leads to scattering and one that leads to a
grazing collision and merger. In all cases, the initial black hole spins lie in
the orbital plane, a configuration that leads to the so-called "superkicks". In
astrophysical, quasicircular inspirals, such kicks can be as large as ~3,000
km/s; here, we find configurations that exceed ~15,000 km/s. We find that the
maximum recoil is to a good approximation proportional to the total amount of
energy radiated in gravitational waves, but largely independent of whether a
merger occurs or not. This shows that the mechanism predominantly responsible
for the superkick is not related to merger dynamics. Rather, a consistent
explanation is that the "bobbing" motion of the orbit causes an asymmetric
beaming of the radiation produced by the in-plane orbital motion of the binary,
and the net asymmetry is balanced by a recoil. We use our results to formulate
some conjectures on the ultimate kick achievable in any black hole encounter.Comment: 10 pages, 6 figures, 2 table
Gravitational Waves from Quasi-Circular Black Hole Binaries in Dynamical Chern-Simons Gravity
Dynamical Chern-Simons gravity cannot be strongly constrained with current
experiments because it reduces to General Relativity in the weak-field limit.
This theory, however, introduces modifications in the non-linear, dynamical
regime, and thus, it could be greatly constrained with gravitational waves from
the late inspiral of black hole binaries. We complete the first self-consistent
calculation of such gravitational waves in this theory. For favorable
spin-orientations, advanced ground-based detectors may improve existing
solar-system constraints by 6 orders of magnitude.Comment: 6 pages, 1 figure; errors corrected in Eqs. (8) and (9
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