arXiv subjects

Kent Yagi

Publications and source records attributed to Kent Yagi.

At least 109 records · Page 6Linked to original sources

arXiv2018

Multipole Love Relations

Gravitational-wave observations in the near future may allow us to measure tidal deformabilities of neutron stars, which leads us to the understanding of physics at nuclear density. In principle, the gravitational waveform depends on various tidal parameters, which correlate strongly. Therefore, it would be useful if one can express such tidal parameters with a single parameter. Here, we report on universal relations among various $\ell$-th (dimensionless) electric, magnetic and shape tidal deformabilities in neutron stars and quark stars that do not depend sensitively on the equation of state. Such relations allow us to break the degeneracy among the tidal parameters. In this paper, we focus on gravitational waves from non-spinning neutron-star binary inspirals. We first derive the leading contribution of the $\ell$-th electric and $\ell=2$ magnetic tidal deformabilities to the gravitational-wave phase, which enters at $2\ell +1$ and $6$ post-Newtonian orders relative to the leading Newtonian one respectively. We then calculate the useful number of gravitational-wave cycles and show that not only the $\ell=2$ but also $\ell=3$ electric tidal deformabilities are important for parameter estimation with third-generation gravitational-wave detectors such as LIGO III and Einstein Telescope. Although the correlation between the $\ell=2$ and $\ell=3$ electric tidal deformabilities deteriorate the measurement accuracy of the former deformability parameter, one can increase its measurement accuracy significantly by using the universal relation. We provide a fitting formula for the LIGO III noise curve in the Appendix.

gr-qc↗

arXiv2018

Implications from GW170817 and I-Love-Q relations for relativistic hybrid stars

Gravitational wave observations of GW170817 placed bounds on the tidal deformabilities of compact stars allowing one to probe equations of state for matter at supranuclear densities. Here we design new parametrizations for hybrid hadron-quark equations of state, that give rise to low-mass twin stars, and test them against GW170817. We find that GW170817 is consistent with the coalescence of a binary hybrid star--neutron star. We also test and find that the I-Love-Q relations for hybrid stars in the third family agree with those for purely hadronic and quark stars within $\sim 3\%$ for both slowly and rapidly rotating configurations, implying that these relations can be used to perform equation-of-state independent tests of general relativity and to break degeneracies in gravitational waveforms for hybrid stars in the third family as well.

astro-ph.HE↗

arXiv2018

Extreme Gravity Tests with Gravitational Waves from Compact Binary Coalescences: (II) Ringdown

The LIGO/Virgo detections of binary black hole mergers marked a watershed moment in astronomy, ushering in the era of precision tests of Kerr dynamics. We review theoretical and experimental challenges that must be overcome to carry out black hole spectroscopy with present and future gravitational wave detectors. Among other topics, we discuss quasinormal mode excitation in binary mergers, astrophysical event rates, tests of black hole dynamics in modified theories of gravity, parameterized "post-Kerr" ringdown tests, exotic compact objects, and proposed data analysis methods to improve spectroscopic tests of Kerr dynamics by stacking multiple events.

gr-qc↗

arXiv2018

Extreme Gravity Tests with Gravitational Waves from Compact Binary Coalescences: (I) Inspiral-Merger

The observation of the inspiral and merger of compact binaries by the LIGO/Virgo collaboration ushered in a new era in the study of strong-field gravity. We review current and future tests of strong gravity and of the Kerr paradigm with gravitational-wave interferometers, both within a theory-agnostic framework (the parametrized post-Einsteinian formalism) and in the context of specific modified theories of gravity (scalar-tensor, Einstein-dilaton-Gauss-Bonnet, dynamical Chern-Simons, Lorentz-violating, and extra dimensional theories). In this contribution we focus on (i) the information carried by the inspiral radiation, and (ii) recent progress in numerical simulations of compact binary mergers in modified gravity.

gr-qc↗

arXiv2018

Gravitational wave spectroscopy of binary neutron star merger remnants with mode stacking

A binary neutron star (BNS) merger event has recently been observed in gravitational waves (GWs). As in the case of binary black holes, GWs generated by BNS consist of inspiral, merger, and post-merger components. Detecting the latter is important because it encodes information about the nuclear equation of state (EOS) in a regime that cannot be probed prior to merger. The post-merger signal, however, can be observed by current detectors only out to~ 10 Mpc. We carry out Monte-Carlo simulations showing that the dominant post-merger signal (the 22 mode) from individual events will likely not be observable even with the Einstein Telescope and Cosmic Explorer (CE), assuming a full year of operation, the latest merger rates, and a detection threshold with signal-to-noise ratio of 5. For this reason, we propose two methods that stack the post-merger signal from multiple events to boost the detection probability. The first method follows a commonly-used practice of multiplying the Bayes factors of individual events. The second method relies on an assumption that the mode phase can be determined from the inspiral waveform, so that coherent mode stacking of the data from different events becomes possible. Both methods significantly improve the chances of detecting the dominant post-merger signal, making a detection very likely after a year of observation with CE for certain EOS. We also show that in terms of detection, coherent stacking is more efficient in accumulating confidence. Moreover, assuming a 22 mode is detected with CE via stacking, we estimate through a Fisher analysis that the peak frequency can be measured to a statistical error of ~ 4-20 Hz for certain equations of state. Such an error corresponds to a NS radius measurement of ~ 15-56 m, a fractional relative error ~ 4 %, suggesting that systematic errors from theoretical modeling (~ 100 m) may dominate the error budget.

gr-qc↗

arXiv2017

Black hole perturbation under 2 + 2 decomposition in the action

Black hole perturbation theory is useful for studying the stability of black holes and calculating ringdown gravitational waves after the collision of two black holes. Most previous calculations were carried out at the level of the field equations instead of the action. In this work, we compute the Einstein-Hilbert action to quadratic order in linear metric perturbations about a spherically symmetric vacuum background in Regge-Wheeler gauge. Using a 2+2 splitting of spacetime, we expand the metric perturbations into a sum over scalar, vector, and tensor spherical harmonics, and dimensionally reduce the action to two dimensions by integrating over the two sphere. We find that the axial perturbation degree of freedom is described by a two dimensional massive vector action, and that the polar perturbation degree of freedom is described by a two dimensional dilaton massive gravity action. Varying the dimensionally reduced actions, we rederive covariant and gauge-invariant master equations for the axial and polar degrees of freedom. Thus, the two dimensional massive vector and massive gravity actions we derive by dimensionally reducing the perturbed Einstein-Hilbert action describe the dynamics of a well studied physical system: the metric perturbations of a static black hole. The $2+2$ formalism we present can be generalized to $m+n$ dimensional spacetime splittings, which may be useful in more generic situations, such as expanding metric perturbations in higher dimensional gravity. We provide a self-contained presentation of $m+n$ formalism for vacuum spacetime splittings.

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arXiv2017

Probing Gravitational Parity Violation with Gravitational Waves from Stellar-mass Black Hole Binaries

The recent discovery of gravitational wave events has offered us unique testbeds of gravity in the strong and dynamical field regime. One possible modification to General Relativity is the gravitational parity violation that gives rise to the amplitude birefringence in gravitational waves, where one of the circularly-polarized mode is amplified while the other one is suppressed during their propagation. In this paper, we study how well one can measure gravitational parity violation via the amplitude birefringence of gravitational waves from stellar-mass black hole binaries. We choose Chern-Simons gravity as an example and work within an effective field theory formalism. We consider gravitational waves from both individual sources and stochastic gravitational wave backgrounds. Regarding bounds from individual sources, we estimate them using a Fisher analysis and carry out Monte Carlo simulations by randomly distributing sources over their sky location and binary orientation. We find that the bounds on the scalar field evolution in Chern-Simons gravity from GW150914 are too weak to satisfy the weak Chern-Simons approximation, while aLIGO with its design sensitivity can place meaningful bounds. Regarding bounds from stochastic gravitational wave backgrounds, we set the threshold signal-to-noise ratio for detection of the parity-violation mode as 5 and estimate projected bounds with future detectors assuming that signals are consistent with no parity violation. We find that a network of two third-generation detectors is able to place bounds that are slightly stronger than current binary pulsar bounds. Since gravitational wave observations probe either the difference in parity violation between the source and the detector or the line-of-sight integration of the scalar field, such bounds are complementary to local measurements from solar system experiments and binary pulsar observations.

gr-qc↗

arXiv2017

I-Love-Q Relations in Neutron Stars and their Applications to Astrophysics, Gravitational Waves and Fundamental Physics

The exterior gravitational field of a slowly-rotating neutron star can be characterized by its multipole moments, the first few being the neutron star mass, moment of inertia, and quadrupole moment to quadratic order in spin. In principle, all of these quantities depend on the neutron star's internal structure, and thus, on unknown nuclear physics at supra-nuclear energy densities. We here find relations between the moment of inertia, the Love numbers and the quadrupole moment (I-Love-Q relations) that do not depend sensitively on the neutron star's internal structure. Three important consequences derive from these I-Love-Q relations. On an observational astrophysics front, the measurement of a single member of the I-Love-Q trio would automatically provide information about the other two, even when the latter may not be observationally accessible. On a gravitational wave front, the I-Love-Q relations break the degeneracy between the quadrupole moment and the neutron-star spins in binary inspiral waveforms, allowing second-generation ground-based detectors to determine the (dimensionless) averaged spin to $\mathcal{O}(10)%$, given a sufficiently large signal-to-noise ratio detection. On a fundamental physics front, the I-Love-Q relations allow for tests of General Relativity in the neutron-star strong-field that are both theory- and internal structure-independent. As an example, by combining gravitational-wave and electromagnetic observations, one may constrain dynamical Chern-Simons gravity in the future by more than 6 orders of magnitude more stringently than Solar System and table-top constraints.

gr-qc↗

arXiv2017

The Gravitational Wave Stress-Energy (pseudo)-Tensor in Modified Gravity

The recent detections of gravitational waves by the advanced LIGO and Virgo detectors open up new tests of modified gravity theories in the strong-field and dynamical, extreme gravity regime. Such tests rely sensitively on the phase evolution of the gravitational waves, which is controlled by the energy-momentum carried by such waves out of the system. We here study four different methods for finding the gravitational wave stress-energy pseudo-tensor in gravity theories with any combination of scalar, vector, or tensor degrees of freedom. These methods rely on the second variation of the action under short-wavelength averaging, the second perturbation of the field equations in the short-wavelength approximation, the construction of an energy complex leading to a Landau-Lifshitz tensor, and the use of Noether's theorem in field theories about a flat background. We apply these methods in General Relativity, scalar-tensor theories and Einstein-Æther theory to find the gravitational wave stress-energy pseudo-tensor and calculate the rate at which energy and linear momentum is carried away from the system. The stress-energy tensor and the rate of linear momentum loss in Einstein-Æther theory are presented here for the first time. We find that all methods yield the same rate of energy loss, although the stress-energy pseudo-tensor can be functionally different. We also find that the Noether method yields a stress-energy tensor that is not symmetric or gauge-invariant, and symmetrization via the Belinfante procedure does not fix these problems because this procedure relies on Lorentz invariance, which is spontaneously broken in Einstein-Æther theory. The methods and results found here will be useful for the calculation of predictions in modified gravity theories that can then be contrasted with observations.

gr-qc↗

arXiv2017

I-Love-Q Relations for Neutron Stars in dynamical Chern Simons Gravity

Neutron stars are ideal to probe, not only nuclear physics, but also strong-field gravity. Approximate universal relations insensitive to the star's internal structure exist among certain observables and are useful in testing General Relativity, as they project out the uncertainties in the equation of state. One such set of universal relations between the moment of inertia $(I)$, the tidal Love number and the quadrupole moment $(Q)$ has been studied both in General Relativity and in modified theories. In this paper, we study the relations in dynamical Chern-Simons gravity, a well-motivated, parity-violating effective field theory, extending previous work in various ways. First, we study how projected constraints on the theory using the I-Love relation depend on the measurement accuracy of $I$ with radio observations and that of the Love number with gravitational-wave observations. Provided these quantities can be measured with future observations, we find that the latter could place bounds on dynamical Chern-Simons gravity that are six orders of magnitude stronger than current bounds. Second, we study the I-Q and Q-Love relations in this theory by constructing slowly-rotating neutron star solutions to quadratic order in spin. We find that the approximate universality continues to hold in dynamical Chern-Simons gravity, and in fact, it becomes stronger than in General Relativity, although its existence depends on the normalization of the dimensional coupling constant of the theory. Finally, we study the variation of the eccentricity of isodensity contours inside a star and its relation to the degree of universality. We find that, in most cases, the eccentricity variation is smaller in dynamical Chern-Simons gravity than in General Relativity, providing further support to the idea that the approximate self-similarity of isodensity contours is responsible for universality.

gr-qc↗

arXiv2017

Constraints on Einstein-Æther theory and Horava gravity from binary pulsar observations

Binary pulsars are ideal to test the foundations of General Relativity, such as Lorentz symmetry, which requires that experiments produce the same results in all free-falling (i.e.inertial) frames. We here break this symmetry in the gravitational sector by specifying a preferred time direction, and thus a preferred frame, at each spacetime point. We then examine the consequences of this gravitational Lorentz symmetry breaking in the orbital evolution of binary pulsars, focusing on the dissipative effects. We find that Lorentz symmetry breaking modifies these effects, and thus the orbital dynamics, in two different ways. First, it generically causes the emission of dipolar radiation, which makes the orbital separation decrease faster than in General Relativity. Second, the quadrupole component of the emission is also modified. The orbital evolution depends critically on the sensitivities of the stars, which measure how their binding energies depend on the motion relative to the preferred frame. We calculate the sensitivities numerically and compute the predicted orbital decay rate of binary pulsars in Lorentz-violating gravity. By testing these predictions against observations, we place very stringent constraints on gravitational Lorentz violation.

gr-qc↗

arXiv2017

Slowly Rotating Black Holes in Dynamical Chern-Simons Gravity: Deformation Quadratic in the Spin

We derive a stationary and axisymmetric black hole solution to quadratic order in the spin angular momentum. The previously found, linear-in-spin terms modify the odd-parity sector of the metric, while the new corrections appear in the even-parity sector. These corrections modify the quadrupole moment, as well as the (coordinate-dependent) location of the event horizon and the ergoregion. Although the linear-in-spin metric is of Petrov type D, the quadratic order terms render it of type I. The metric does not possess a second-order Killing tensor or a Carter-like constant. The new metric does not possess closed timelike curves or spacetime regions that violate causality outside of the event horizon. The new, even-parity modifications to the Kerr metric decay less rapidly at spatial infinity than the leading-order in spin, odd-parity ones, and thus, the former are more important when considering black holes that are rotating moderately fast. We calculate the modifications to the Hamiltonian, binding energy and Kepler's third law. These modifications are crucial for the construction of gravitational wave templates for black hole binaries, which will enter at second post-Newtonian order, just like dissipative modifications found previously.

gr-qc↗

arXiv2017

Detector configuration of DECIGO/BBO and identification of cosmological neutron-star binaries

The primary target for the planned space-borne gravitational wave interferometers DECIGO/BBO is a primordial gravitational wave background (PGWB). However there exist astrophysical foregrounds and among them, gravitational waves from neutron star (NS) binaries are the solid and strong component that must be identified and subtracted. In this paper, we discuss the geometry of detector configurations preferable for identifying the NS/NS binary signals. As a first step, we analytically estimate the minimum signal-to-noise ratios (SNRs) of the binaries for several static detector configurations that are characterized by adjustable geometrical parameters, and determine the optimal values for these parameters. Next we perform numerical simulations to take into account the effect of detector motions, and find reasonable agreements with the analytical results. We show that, with the standard network formed by 4 units of triangle detectors, the proposed BBO sensitivity would be sufficient in receiving gravitational waves from all the NS/NS binaries at $z\le 5$ with SNRs higher than 25. We also discuss the minimum sensitivity of DECIGO required for the foreground identification.

astro-ph.CO↗

arXiv2017

Black hole spectroscopy with coherent mode stacking

The measurement of multiple ringdown modes in gravitational waves from binary black hole mergers will allow for testing fundamental properties of black holes in General Relativity, and to constrain modified theories of gravity. To enhance the ability of Advanced LIGO/Virgo to perform such tasks, we propose a coherent mode stacking method to search for a chosen target mode within a collection of multiple merger events. We first rescale each signal so that the target mode in each of them has the same frequency, and then sum the waveforms constructively. A crucial element to realize this coherent superposition is to make use of a priori information extracted from the inspiral-merger phase of each event. To illustrate the method, we perform a study with simulated events targeting the l=m=3 ringdown mode of the remnant black holes. We show that this method can significantly boost the signal-to-noise ratio of the collective target mode compared to that of the single loudest event. Using current estimates of merger rates we show that it is likely that advanced-era detectors can measure this collective ringdown mode with one year of coincident data gathered at design sensitivity.

gr-qc↗

arXiv2016

Testing the formation scenarios of binary neutron star systems with measurements of the neutron star moment of inertia

Two low mass neutron stars, J0737-3039B and the companion to J1756-2251, show strong evidence of being formed from the collapse of an ONeMg core in an electron capture supernova (ECSN) or in an ultra-stripped iron core collapse supernova (FeCCSN). Using three different systematically generated sets of equations of state we explore the relationship between the moment of inertia of J0737-3039A and the binding energy of the two low mass neutron stars. We find this relationship, a less strict variant of the recently discovered I-Love-Q relations, is nevertheless more robust than a previously explored correlation between the binding energy and the slope of the nuclear symmetry energy L. We find that, if either J0737-3039B or the J1756-2251 companion were formed in an ECSN, no more than 0.06 solar masses could have been lost from the progenitor core, more than four times the mass loss predicted by current supernova modeling. A measurement of the moment of inertia of J0737-3039A to within 10% accuracy from pulsar timing, possible within a decade, can discriminate between formation scenarios such as ECSN or ultra-stripped FeCCSN and, given current constraints on the predicted core mass loss, potentially rule them out. Using the I-Love-Q relations we find that an Advanced LIGO can potentially measure the neutron star tidal polarizability to equivalent accuracy in a neutron star-neutron star merger at a distance of 200 Mpc, thus obtaining similar constraints on the formation scenarios. Such information on the occurrence of ECSNe is important for population synthesis calculations, especially for estimating the rate of binary neutron star mergers and resulting electromagnetic and gravitational wave signals. Further progress needs to be made modeling the core collapse process that leads to low-mass neutron stars, particularly in making robust predictions for the mass loss from the progenitor core.

astro-ph.SR↗

arXiv2016

Theoretical Physics Implications of the Binary Black-Hole Mergers GW150914 and GW151226

The gravitational wave observations GW150914 and GW151226 by Advanced LIGO provide the first opportunity to learn about physics in the extreme gravity environment of coalescing binary black holes. The LIGO Scientific Collaboration and the Virgo Collaboration have verified that this observation is consistent with General Relativity. This paper expands their analysis to a larger class of anomalies, highlighting the inferences that can be drawn on non-standard theoretical physics mechanisms. We find that these events constrain a plethora of mechanisms associated with the generation and propagation of gravitational waves, including the activation of scalar fields, gravitational leakage into large extra dimensions, the variability of Newton's constant, a modified dispersion relation, gravitational Lorentz violation and the strong equivalence principle. Though other observations limit many of these mechanisms already, GW150914 and GW151226 are unique in that they are direct probes of dynamical strong-field gravity and of gravitational wave propagation. We also show that GW150914 constrains inferred properties of exotic compact object alternatives to Kerr black holes. We argue, however, that the true potential for GW150914 to both rule out exotic objects and constrain physics beyond General Relativity is severely limited by the lack of understanding of the merger regime in almost all relevant modified gravity theories. This event thus significantly raises the bar that these theories have to pass, both in terms of having a sound theoretical underpinning, and being able to solve the equations of motion for binary merger events. We conclude with a discussion of the additional inferences that can be drawn if the lower-confidence observation of an electromagnetic counterpart to GW150914 holds true; this would provide dramatic constraints on the speed of gravity and gravitational Lorentz violation.

gr-qc↗

arXiv2016

Binary Love Relations

When in a tight binary, the mutual tidal deformations of neutron stars imprint onto observables, encoding information about their internal structure at supranuclear densities and gravity in the extreme-gravity regime. Gravitational wave observations of their late binary inspiral may serve as a tool to extract the individual tidal deformabilities, but this is made difficult by degeneracies between them in the gravitational wave model. We here resolve this problem by discovering approximately universal relations between dimensionless combinations of the individual tidal deformabilities. We show that these relations break degeneracies in the gravitational wave model, allowing for the accurate extraction of both deformabilities. Such measurements can be used to better differentiate between equation-of-state models, and improve tests of General Relativity and cosmology.

gr-qc↗

arXiv2016

I-Love-Q Relations: From Compact Stars to Black Holes

The relations between most observables associated with a compact star, such as the mass and radius of a neutron star or a quark star, typically depend strongly on their unknown internal structure. The I-Love-Q relations (between the moment of inertia, the tidal deformability and the quadrupole moment) are however approximately insensitive to this structure. These relations become exact for stationary black holes in General Relativity as shown by the no-hair theorems. In this paper, we take the first steps toward studying how the approximate I-Love-Q relations become exact in the limit as compact stars become black holes. To do so, we consider a toy model, i.e. incompressible stars with anisotropic pressure, which allows us to model an equilibrium sequence of stars with their compactness approaching the black hole limit arbitrarily closely. We extract the I-Love-Q trio by numerically constructing such a sequence in the slow-rotation and small-tide approximations. We find that the I-Love-Q relations approach the black hole limit in a nontrivial way, with the quadrupole moment and the tidal deformability changing sign as the compactness and the amount of anisotropy are increased. Generalizing Maclaurin spheroids to anisotropic stars, we show that the multipole moments also change sign in the Newtonian limit as the amount of anisotropy is increased. We also prove analytically that the stellar moment of inertia reaches the black hole limit as the compactness reaches the black hole value in the strongly anisotropic limit. Modeling the black hole limit through a sequence of anisotropic stars, however, fails when considering other theories of gravity. We calculate the scalar dipole charge and the moment of inertia in a parity-violating modified theory and find that these quantities do not tend to their black hole counterparts as the anisotropic stellar sequence approaches the black hole limit.

gr-qc↗