730 research outputs found
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
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
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
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
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.
Power laws, scale invariance, and generalized Frobenius series: Applications to Newtonian and TOV stars near criticality
We present a self-contained formalism for analyzing scale invariant
differential equations. We first cast the scale invariant model into its
equidimensional and autonomous forms, find its fixed points, and then obtain
power-law background solutions. After linearizing about these fixed points, we
find a second linearized solution, which provides a distinct collection of
power laws characterizing the deviations from the fixed point. We prove that
generically there will be a region surrounding the fixed point in which the
complete general solution can be represented as a generalized Frobenius-like
power series with exponents that are integer multiples of the exponents arising
in the linearized problem. This Frobenius-like series can be viewed as a
variant of Liapunov's expansion theorem. As specific examples we apply these
ideas to Newtonian and relativistic isothermal stars and demonstrate (both
numerically and analytically) that the solution exhibits oscillatory power-law
behaviour as the star approaches the point of collapse. These series solutions
extend classical results. (Lane, Emden, and Chandrasekhar in the Newtonian
case; Harrison, Thorne, Wakano, and Wheeler in the relativistic case.) We also
indicate how to extend these ideas to situations where fixed points may not
exist -- either due to ``monotone'' flow or due to the presence of limit
cycles. Monotone flow generically leads to logarithmic deviations from scaling,
while limit cycles generally lead to discrete self-similar solutions.Comment: 35 pages; IJMPA style fil
Improved initial data for black hole binaries by asymptotic matching of post-Newtonian and perturbed black hole solutions
We construct approximate initial data for non-spinning black hole binary
systems by asymptotically matching the 4-metrics of two tidally perturbed
Schwarzschild solutions in isotropic coordinates to a resummed post-Newtonian
4-metric in ADMTT coordinates. The specific matching procedure used here
closely follows the calculation in gr-qc/0503011, and is performed in the so
called buffer zone where both the post-Newtonian and the perturbed
Schwarzschild approximations hold. The result is that both metrics agree in the
buffer zone, up to the errors in the approximations. However, since isotropic
coordinates are very similar to ADMTT coordinates, matching yields better
results than in the previous calculation, where harmonic coordinates were used
for the post-Newtonian 4-metric. In particular, not only does matching improve
in the buffer zone, but due to the similarity between ADMTT and isotropic
coordinates the two metrics are also close to each other near the black hole
horizons. With the help of a transition function we also obtain a global smooth
4-metric which has errors on the order of the error introduced by the more
accurate of the two approximations we match. This global smoothed out 4-metric
is obtained in ADMTT coordinates which are not horizon penetrating. In
addition, we construct a further coordinate transformation that takes the
4-metric from global ADMTT coordinates to new coordinates which are similar to
Kerr-Schild coordinates near each black hole, but which remain ADMTT further
away from the black holes. These new coordinates are horizon penetrating and
lead, for example, to a lapse which is everywhere positive on the t=0 slice.
Such coordinates may be more useful in numerical simulations.Comment: 25 pages, 21 figures. Replaced with accepted versio
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
Model-Independent Test of General Relativity: An Extended post-Einsteinian Framework with Complete Polarization Content
We develop a model-independent test of General Relativity that allows for the
constraint of the gravitational wave (GW) polarization content with GW
detections of binary compact object inspirals. We first consider three modified
gravity theories (Brans-Dicke theory, Rosen's theory and Lightman-Lee theory)
and calculate the response function of ground-based detectors to gravitational
waves in the inspiral phase. This allows us to see how additional polarizations
predicted in these theories modify the General Relativistic prediction of the
response function. We then consider general power-law modifications to the
Hamiltonian and radiation-reaction force and study how these modify the
time-domain and Fourier response function when all polarizations are present.
From these general arguments and specific modified gravity examples, we infer
an improved parameterized post-Einsteinian template family with complete
polarization content. This family enhances General Relativity templates through
the inclusion of new theory parameters, reducing to the former when these
parameters acquire certain values, and recovering modified gravity predictions
for other values, including all polarizations. We conclude by discussing
detection strategies to constrain these new, polarization theory parameters by
constructing certain null channels through the combination of output from
multiple detectors.Comment: 20 pages, 1 figure, added erratum correcting some intermediate
equation
Theory-Agnostic Constraints on Black-Hole Dipole Radiation with Multiband Gravitational-Wave Astrophysics
The aLIGO detection of the black-hole binary GW150914 opens a new era for probing extreme gravity. Many gravity theories predict the emission of dipole gravitational radiation by binaries. This is excluded to high accuracy in binary pulsars, but entire classes of theories predict this effect predominantly ( or only) in binaries involving black holes. Joint observations of GW150914-like systems by aLIGO and eLISA will improve bounds on dipole emission from black-hole binaries by 6 orders of magnitude relative to current constraints, provided that eLISA is not dramatically descoped
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