425 research outputs found
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.
Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks
We examine the electromagnetic (EM) and gravitational wave (GW) signatures of
stellar-mass compact objects (COs) spiraling into a supermassive black hole
(extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure
dominated, accretion disk. At large separations, the tidal effect of the
secondary CO clears a gap. We show that the gap refills during the late
GW-driven phase of the inspiral, leading to a sudden EM brightening of the
source. The accretion disk leaves an imprint on the GW through its angular
momentum exchange with the binary, the mass increase of the binary members due
to accretion, and its gravity. We compute the disk-modified GWs both in an
analytical Newtonian approximation and in a numerical effective-one-body
approach. We find that disk-induced migration provides the dominant
perturbation to the inspiral, with weaker effects from the mass accretion onto
the CO and hydrodynamic drag. Depending on whether a gap is present, the
perturbation of the GW phase is between 10 and 1000 radians per year,
detectable with the future Laser Interferometer Space Antenna (LISA) at high
significance. The Fourier transform of the disk-modified GW in the stationary
phase approximation is sensitive to disk parameters with a frequency trend
different from post-Newtonian vacuum corrections. Our results suggest that
observations of EMRIs may place new sensitive constraints on the physics of
accretion disks.Comment: 42 pages, 8 figures, 3 tables, submitted to Phys. Rev.
Generic bounds on dipolar gravitational radiation from inspiralling compact binaries
Various alternative theories of gravity predict dipolar gravitational
radiation in addition to quadrupolar radiation. We show that gravitational wave
(GW) observations of inspiralling compact binaries can put interesting
constraints on the strengths of the dipole modes of GW polarizations. We put
forward a physically motivated gravitational waveform for dipole modes, in the
Fourier domain, in terms of two parameters: one which captures the relative
amplitude of the dipole mode with respect to the quadrupole mode () and
the other a dipole term in the phase (). We then use this two parameter
representation to discuss typical bounds on their values using GW measurements.
We obtain the expected bounds on the amplitude parameter and the phase
parameter for Advanced LIGO (AdvLIGO) and Einstein Telescope (ET) noise
power spectral densities using Fisher information matrix. AdvLIGO and ET may at
best bound to an accuracy of and and
to an accuracy of and respectively.Comment: Matches with the published versio
Semianalytical estimates of scattering thresholds and gravitational radiation in ultrarelativistic black hole encounters
Ultrarelativistic collisions of black holes are ideal gedanken experiments to
study the nonlinearities of general relativity. In this paper we use
semianalytical tools to better understand the nature of these collisions and
the emitted gravitational radiation. We explain many features of the energy
spectra extracted from numerical relativity simulations using two complementary
semianalytical calculations. In the first calculation we estimate the radiation
by a "zero-frequency limit" analysis of the collision of two point particles
with finite impact parameter. In the second calculation we replace one of the
black holes by a point particle plunging with arbitrary energy and impact
parameter into a Schwarzschild black hole, and we explore the multipolar
structure of the radiation paying particular attention to the near-critical
regime. We also use a geodesic analogy to provide qualitative estimates of the
dependence of the scattering threshold on the black hole spin and on the
dimensionality of the spacetime.Comment: 29 pages, 19 figure, 6 tables, minor changes to match version in
press in Phys.Rev.
Attitudes of Germans towards distributive issues in the German health system
Social health care systems are inevitably confronted with the scarcity of resources and the resulting distributional challenges. Since prioritization implies distributional effects, decisions on respective rules should take citizens’ preferences into account. Thus, knowledge about citizens’ attitudes and preferences regarding different distributional issues implied by the type of financing health care is necessary to judge the public acceptance of a health system. In this study we concentrate on two distributive issues in the German health system: First, we analyse the acceptance of prioritizing decisions concerning the treatment of certain patient groups, in this case patients who all need a heart operation. Here we focus on the fact that a patient is strong smoker or a non-smoker, the criteria of age or the fact that a patient has or does not have young children. Second, we investigate Germans’ opinions towards income dependent health services. The results reveal strong effects of individuals’ attitudes regarding general aspects of the health system on priorities, e.g. that individuals behaving health demanding should not be preferred. In addition, experiences of limited access to health services are found to have a strong influence on citizens’ attitudes, too. Finally, decisions about different prioritization criteria are found to be not independent.
Constraining Parity Violation in Gravity with Measurements of Neutron-Star Moments of Inertia
Neutron stars are sensitive laboratories for testing general relativity,
especially when considering deviations where velocities are relativistic and
gravitational fields are strong. One such deviation is described by dynamical,
Chern-Simons modified gravity, where the Einstein-Hilbert action is modified
through the addition of the gravitational parity-violating Pontryagin density
coupled to a field. This four-dimensional effective theory arises naturally
both in perturbative and non-perturbative string theory, loop quantum gravity,
and generic effective field theory expansions. We calculate here Chern-Simons
modifications to the properties and gravitational fields of slowly spinning
neutron stars. We find that the Chern-Simons correction affects only the
gravitomagnetic sector of the metric to leading order, thus introducing
modifications to the moment of inertia but not to the mass-radius relation. We
show that an observational determination of the moment of inertia to an
accuracy of 10%, as is expected from near-future observations of the double
pulsar, will place a constraint on the Chern-Simons coupling constant of
\xi^{1/4} < 5 km, which is at least three-orders of magnitude stronger than the
previous strongest bound.Comment: 14 pages, 6 figures, replaced with version accepted for publication
in Phys. Rev.
Chern-Simons Modified Gravity as a Torsion Theory and its Interaction with Fermions
We study the tetrad formulation of Chern-Simons (CS) modified gravity, which
adds a Pontryagin term to the Einstein-Hilbert action with a
spacetime-dependent coupling field. We first verify that CS modified gravity
leads to a theory with torsion, where this tensor is given by an antisymmetric
product of the Riemann tensor and derivatives of the CS coupling. We then
calculate the torsion in the far field of a weakly gravitating source within
the parameterized post-Newtonian formalism, and specialize the result to Earth.
We find that CS torsion vanishes only if the coupling vanishes, thus
generically leading to a modification of gyroscopic precession, irrespective of
the coupling choice. Perhaps most interestingly, we couple fermions to CS
modified gravity via the standard Dirac action and find that these further
correct the torsion tensor. Such a correction leads to two new results: (i) a
generic enhancement of CS modified gravity by the Dirac equation and axial
fermion currents; (ii) a new two-fermion interactions, mediated by an axial
current and the CS correction. We conclude with a discussion of the
consequences of these results in particle detectors and realistic astrophysical
systems.Comment: 11 pages, submitted to Phys. Rev.
Testing Alternative Theories of Gravity using LISA
We investigate the possible bounds which could be placed on alternative
theories of gravity using gravitational wave detection from inspiralling
compact binaries with the proposed LISA space interferometer. Specifically, we
estimate lower bounds on the coupling parameter \omega of scalar-tensor
theories of the Brans-Dicke type and on the Compton wavelength of the graviton
\lambda_g in hypothetical massive graviton theories. In these theories,
modifications of the gravitational radiation damping formulae or of the
propagation of the waves translate into a change in the phase evolution of the
observed gravitational waveform. We obtain the bounds through the technique of
matched filtering, employing the LISA Sensitivity Curve Generator (SCG),
available online. For a neutron star inspiralling into a 10^3 M_sun black hole
in the Virgo Cluster, in a two-year integration, we find a lower bound \omega >
3 * 10^5. For lower-mass black holes, the bound could be as large as 2 * 10^6.
The bound is independent of LISA arm length, but is inversely proportional to
the LISA position noise error. Lower bounds on the graviton Compton wavelength
ranging from 10^15 km to 5 * 10^16 km can be obtained from one-year
observations of massive binary black hole inspirals at cosmological distances
(3 Gpc), for masses ranging from 10^4 to 10^7 M_sun. For the highest-mass
systems (10^7 M_sun), the bound is proportional to (LISA arm length)^{1/2} and
to (LISA acceleration noise)^{-1/2}. For the others, the bound is independent
of these parameters because of the dominance of white-dwarf confusion noise in
the relevant part of the frequency spectrum. These bounds improve and extend
earlier work which used analytic formulae for the noise curves.Comment: 16 pages, 9 figures, submitted to Classical & Quantum Gravit
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