Browsing by Author "Yagi, Kent"

Now showing 1 - 10 of 10

Approximate universal relations among tidal parameters for neutron star binaries
(2017-01) Yagi, Kent; Yunes, Nicolás
One of the largest uncertainties in nuclear physics is the relation between the pressure and density of supranuclear matter: the equation of state. Some of this uncertainty may be removed through future gravitational wave observations of neutron star binaries by extracting the tidal deformabilities (or Love numbers) of neutron stars, a novel way to probe nuclear physics in the high-density regime. Previous studies have shown that only a certain combination of the individual (quadrupolar) deformabilities of each body (the so-called chirp tidal deformability) can be measured with second-generation, gravitational wave interferometers, such as Adv. LIGO, due to correlations between the individual deformabilities. To overcome this, we search for approximately universal (i.e. approximately equation-of-state independent) relations between two combinations of the individual tidal deformabilities, such that once one of them as been measured, the other can be automatically obtained and the individual ones decoupled through these relations. We find an approximately universal relation between the symmetric and the anti-symmetric combination of the individual tidal deformabilities, that is equation-of-state-insensitive to 20% for binaries with masses less than 1.7M(circle dot). We show that these relations can be used to eliminate a combination of the tidal parameters from the list of model parameters, thus breaking degeneracies and improving the accuracy in parameter estimation. A simple (Fisher) study shows that the universal binary Love relations can improve the accuracy in the extraction of the symmetric combination of tidal parameters by as much as an order of magnitude, making the overall accuracy in the extraction of this parameter slightly better than that of the chirp tidal deformability. These new universal relations and the improved measurement accuracy on tidal parameters not only are important to astrophysics and nuclear physics, but also impact our ability to probe extreme gravity with gravitational waves and cosmology.
Approximate universal relations for neutron stars and quark stars
(2017-04) Yagi, Kent; Yunes, Nicolás
Neutron stars and quark stars are ideal laboratories to study fundamental physics at supra nuclear densities and strong gravitational fields. Astrophysical observables, however, depend strongly on the star’s internal structure, which is currently unknown due to uncertainties in the equation of state. Universal relations, however, exist among certain stellar observables that do not depend sensitively on the star’s internal structure. One such set of relations is between the star’s moment of inertia (I), its tidal Love number (Love) and its quadrupole moment (Q), the so-called I–Love–Q relations. Similar relations hold among the star’s multipole moments, which resemble the well-known black hole no-hair theorems. Universal relations break degeneracies among astrophysical observables, leading to a variety of applications: (i) X-ray measurements of the nuclear matter equation of state, (ii) gravitational wave measurements of the intrinsic spin of inspiraling compact objects, and (iii) gravitational and astrophysical tests of General Relativity that are independent of the equation of state. We here review how the universal relations come about and all the applications that have been devised to date.
Black Hole Spectroscopy with Coherent Mode Stacking
(2017-04) Yang, Huan; Yagi, Kent; Blackman, Jonathan; Lehner, Luis; Paschalidis, Vasileios; Pretorius, Frans; Yunes, Nicolás
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.
Four-hair relations for differentially rotating neutron stars in the weak-field limit
(2015-10) Bretz, Joseph; Yagi, Kent; Yunes, Nicolás
The opportunity to study physics at supra-nuclear densities through x-ray observations of neutron stars has led to in-depth investigations of certain approximately universal relations that can remove degeneracies in pulse profile models. One such set of relations determines all of the multipole moments of a neutron star just from the first three (the mass monopole, the current dipole and the mass quadrupole moment) approximately independently of the equation of state. These three-hair relations were found to hold in neutron stars that rotate rigidly, as is the case in old pulsars, but neutron stars can also rotate differentially, as is the case for proto-neutron stars and hypermassive transient remnants of binary mergers. We here extend the three-hair relations to differentially rotating stars for the first time with a generic rotation law using two approximations: a weak-field scheme (an expansion in powers of the neutron star compactness) and a perturbative differential rotation scheme (an expansion about rigid rotation). These approximations allow us to analytically derive approximately universal relations that allow us to determine all of the multipole moments of a (perturbative) differentially rotating star in terms of only the first four moments. These new four-hair relations for differentially rotating neutron stars are found to be approximately independent of the equation of state to a higher degree than the three-hair relations for uniformly rotating stars. Our results can be instrumental in the development of four-hair relations for rapidly differentially rotating stars in full general relativity using numerical simulations.
I-Love-Q relations: from compact stars to black holes
(2016-04) Yagi, Kent; Yunes, Nicolás
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 recently discovered 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 (BHs) in General Relativity as shown by the no-hair theorems, mainly because BHs are vacuum solutions with event horizons. 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 BHs. To do so, we consider a toy model for compact stars, i.e. incompressible stars with anisotropic pressure, which allows us to model an equilibrium sequence of stars with ever increasing compactness that approaches the BH limit arbitrarily closely. We numerically construct such a sequence in the slow-rotation and in the small-tide approximations by extending the Hartle-Thorne formalism, and then extract the I-Love-Q trio from the asymptotic behavior of the metric tensor at spatial infinity. We find that the I-Love-Q relations approach the BH 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. Through a generalization of 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 because the star becomes prolate. We also prove analytically that the stellar moment of inertia reaches the BH limit as the compactness reaches a critical BH value in the strongly anisotropic limit. Modeling the BH limit through a sequence of anisotropic stars, however, can fail when considering other theories of gravity. We calculate the scalar dipole charge and the moment of inertia in a particular parity-violating modified theory and find that these quantities do not tend to their BH counterparts as the anisotropic stellar sequence approaches the BH limit.
Probing the internal composition of neutron stars with gravitational waves
(2015-11) Chatziioannou, Katerina; Yagi, Kent; Klein, Antoine; Cornish, Neil J.; Yunes, Nicolás
Gravitational waves from neutron star binary inspirals contain information about the as yet unknown equation of state of supranuclear matter. In the absence of definitive experimental evidence that determines the correct equation of state, a number of diverse models that give the pressure inside a neutron star as function of its density have been constructed by nuclear physicists. These models differ not only in the approximations and techniques they employ to solve the many-body Schrödinger equation, but also in the internal neutron star composition they assume. We study whether gravitational wave observations of neutron star binaries in quasicircular inspirals up to contact will allow us to distinguish between equations of state of differing internal composition, thereby providing important information about the properties and behavior of extremely high density matter. We carry out a Bayesian model selection analysis, and find that second generation gravitational wave detectors can heavily constrain equations of state that contain only quark matter, but hybrid stars containing both normal and quark matter are typically harder to distinguish from normal matter stars. A gravitational wave detection with a signal-to-noise ratio of 20 and masses around 1.4M⊙ would provide indications of the existence or absence of strange quark stars, while a signal-to-noise ratio 30 detection could either detect or rule out strange quark stars with a 20 to 1 confidence. The presence of kaon condensates or hyperons in neutron star inner cores cannot be easily confirmed. For example, for the equations of state studied in this paper, even a gravitational wave signal with a signal-to-noise ratio as high as 60 would not allow us to claim a detection of kaon condensates or hyperons with confidence greater than 5 to 1. On the other hand, if kaon condensates and hyperons do not form in neutron stars, a gravitational wave signal with similar signal-to-noise ratio would be able to constrain their existence with an 11 to 1 confidence for high-mass systems. We, finally, find that combining multiple lower signal-to-noise ratio detections (stacking) must be handled with caution since it could fail in cases where the prior information dominates over new information from the data.
Projected Constraints on Lorentz-Violating Gravity with Gravitational Waves
(2015-04) Hansen, Devin; Yunes, Nicolás; Yagi, Kent
Gravitational waves are excellent tools to probe the foundations of General Relativity in the strongly dynamical and non-linear regime. One such foundation is Lorentz symmetry, which can be broken in the gravitational sector by the existence of a preferred time direction, and thus, a preferred frame at each spacetime point. This leads to a modification in the orbital decay rate of binary systems, and also in the generation and chirping of their associated gravitational waves. We here study whether waves emitted in the late, quasi-circular inspiral of non-spinning, neutron star binaries can place competitive constraints on two proxies of gravitational Lorentz-violation: Einstein-\AE{}ther theory and khronometric gravity. We model the waves in the small-coupling (or decoupling) limit and in the post-Newtonian approximation, by perturbatively solving the field equations in small deformations from General Relativity and in the small-velocity/weak-gravity approximation. We assume a gravitational wave consistent with General Relativity has been detected with second- and third-generation, ground-based detectors, and with the proposed space-based mission, DECIGO, with and without coincident electromagnetic counterparts. Without a counterpart, a detection consistent with General Relativity of neutron star binaries can only place competitive constraints on gravitational Lorentz violation when using future, third-generation or space-based instruments. On the other hand, a single counterpart is enough to place constraints that are 10 orders of magnitude more stringent than current binary pulsar bounds, even when using second-generation detectors. This is because Lorentz violation forces the group velocity of gravitational waves to be different from that of light, and this difference can be very accurately constrained with coincident observations.
Surface of rapidly-rotating neutron stars: Implications to neutron star parameter estimation
(American Physical Society, 2021-03) Silva, Hector O.; Pappas, George; Yunes, Nicolás; Yagi, Kent
The Neutron star Interior Composition Explorer (NICER) is currently observing the x-ray pulse profiles emitted by hot spots on the surface of rotating neutron stars allowing for an inference of their radii with unprecedented precision. A critical ingredient in the pulse profile model is an analytical formula for the oblate shape of the star. These formulas require a fitting over a large ensemble of neutron star solutions, which cover a wide set of equations of state, stellar compactnesses and rotational frequencies. However, this procedure introduces a source of systematic error, as (i) the fits do not describe perfectly the surface of all stars in the ensemble and (ii) neutron stars are described by a single equation of state, whose influence on the surface shape is averaged out during the fitting procedure. Here we perform a first study of this systematic error, finding evidence that it is subdominant relative to the statistical error in the radius inference by NICER. We also find evidence that the formula currently used by NICER can be used in the inference of the radii of rapidly rotating stars, outside of the formula’s domain of validity. Moreover, we employ an accurate enthalpy-based method to locate the surface of numerical solutions of rapidly rotating neutron stars and a new highly accurate formula to describe their surfaces. These results can be used in applications that require an accurate description of oblate surfaces of rapidly rotating neutron stars.
Theoretical physics implications of the binary black-hole mergers GW150914 and GW151226
(2016-10) Yunes, Nicolás; Yagi, Kent; Pretorius, Frans
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 Einstein\'s theory of general relativity, constraining the presence of certain parametric anomalies in the signal. This paper expands their analysis to a larger class of anomalies, highlighting the inferences that can be drawn on nonstandard theoretical physics mechanisms that could otherwise have affected the observed signals. We find that these gravitational wave 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, the speed of gravity, 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 coalescence 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 reaching the minimal level of 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, or such a coincidence is observed with future events; this would provide dramatic constraints on the speed of gravity and gravitational Lorentz violation.
Why I-Love-Q: Explaining why universality emerges in compact objects
(American Physical Society, 2014) Yagi, Kent; Stein, Leo C.; Pappas, George; Yunes, Nicolás; Apostolatos, Theocharis A.
Black holes are said to have no hair because all of their multipole moments can be expressed in terms of just their mass, charge and spin angular momentum. The recent discovery of approximately equation-of-state-independent relations among certain multipole moments in neutron stars suggests that they are also approximately bald. We here explore the yet unknown origin for this universality. First, we investigate which region of the neutron star’s interior and of the equation of state is most responsible for the universality. We find that the universal relation between the moment of inertia and the quadrupole moment is dominated by the star’s outer core, a shell of width 50%–95% of the total radius, which corresponds to the density range 1014–1015 g/cm3. In this range, realistic neutron star equations of state are not sufficiently similar to each other to explain the universality observed. Second, we study the impact on the universality of approximating stellar isodensity contours as self-similar ellipsoids. An analytical calculation in the nonrelativistic limit reveals that the shape of the ellipsoids per se does not affect the universal relations much, but relaxing the self-similarity assumption can completely destroy it. Third, we investigate the eccentricity profiles of rotating relativistic stars and find that the stellar eccentricity is roughly constant, with variations of roughly 20%–30% in the region that matters to the universal relations. Fourth, we repeat the above analysis for differentially rotating, noncompact, regular stars and find that this time the eccentricity is not constant, with variations that easily exceed 100%, and moreover universality is lost. These findings suggest that universality arises as an emergent approximate symmetry: as one flows in the stellar-structure phase space from noncompact star region to the relativistic star region, the eccentricity variation inside stars decreases, leading to approximate self-similarity in their isodensity contours, which then leads to the universal behavior observed in their exterior multipole moments.