Zoltán Keresztes - Academia.edu (original) (raw)

Papers by Zoltán Keresztes

Physical Review D

Based on a recently derived secular spin evolution of black holes, neutron stars, gravastars, or ... more Based on a recently derived secular spin evolution of black holes, neutron stars, gravastars, or boson stars in precessing compact binaries on eccentric orbit, we carry out a linear stability analysis of fix point configurations. We identify the aligned and more generic coplanar configurations of the spins and orbital angular momentum as fix points. Through a dynamical system analysis, we investigate their linear stability as function of the mass quadrupole parameter. Our most important results are as follows. Marginal stability holds for the binary configurations with both spins antialigned to the orbital angular momentum, for both spins aligned to the orbital angular momentum (with the exception of certain quadrupolar parameter ranges of neutron stars and boson stars), and for the extremal mass ratio. For equal masses, the configurations of one of the spins aligned and the other antialigned is stable for gravastar binaries, for neutron star binaries in the high quadrupolar parameter range, and for boson star binaries. For some unequal mass gravastar binaries, black hole binaries or neutron star binaries, a transition from stability to instability can occur during the inspiral, when one of the spins is aligned, while the other is antialigned to the orbital angular momentum. We also discover a transitional instability regime during the inspiral of certain gravastar, neutron star, or boson star binaries with opposing spins. For coplanar configurations we recover the marginally stable configurations leading to the libration phenomenon identified in previous numerical investigations lacking mass quadrupole contributions, and we analyze how it is affected by the quadrupolar structure of the sources. We also investigate the linear stability of black hole, neutron star, and boson star binaries, also of mixed black hole-gravastar, black hole-neutron star and black hole-boson star binaries. We find instabilities only for the gravastar-gravastar, boson star-boson star and black hole-boson star binaries. For a given spin configuration, marginal stability strongly depends on the value of the quadrupolar parameters. The stability region is larger for neutron star binaries than for black hole binaries, while the mixed systems have a restricted stability parameter region. 1 If we were to use an alternative set of angular variables defined with the direction of the total orbital angular momentum L, rather than the Newtonian angular momentum L N , differences would be induced only through the SO contribution to L (as the PN and 2PN contributions lie in the direction of L N). For a discussion in terms of angles related to L of the inspiral waveforms for the coplanar resonant configurations [31] and consequences in data analysis, see Ref. [33].

arXiv (Cornell University), Sep 19, 2016

We derive the conservative secular evolution of precessing compact binaries to second post-Newton... more We derive the conservative secular evolution of precessing compact binaries to second post-Newtonian order accuracy, with leading-order spin-orbit, spin-spin and mass quadrupole-monopole contributions included. The emerging closed system of first-order differential equations evolves the pairs of polar and azimuthal angles of the spin and orbital angular momentum vectors together with the periastron angle. In contrast with the instantaneous dynamics, the secular dynamics is autonomous. This secular dynamics reliably characterizes the system over timescales starting from a few times the radial period to several precessional periods, but less than the radiation reaction timescale. We numerically compare the instantaneous and secular evolutions and estimate the number of periods for which dissipation has no significant effect, e.g. the conservative timescale. We apply the analytic equations to study the spin flip-flop effect, recently found by numerical relativity methods. Our investigations show that the effect does not generalize beyond its original parameter settings, although we reveal distinct configurations exhibiting one half flip-flops. In addition, we find a flip-flopping evolution of the orbital angular momentum vector, which ventures from one pole to another through several precessional periods. This is a new effect, occurring for mass ratios much less than one.

Romanian Astronomical Journal, 2020

We investigate the evolution of spinning bodies moving along zoom-whirl orbits in different rotat... more We investigate the evolution of spinning bodies moving along zoom-whirl orbits in different rotating (singular/regular) black hole spacetimes. The spinning body approaches the central black hole along the zoom-whirl orbits so much that it enters the ergosphere periodically. The initial data is chosen such that the body would move in the equatorial plane without spin. We illustrate that the spin precessional angular velocity is highly increased near and within the ergosphere when the spin of the body is not nearly parallel to the rotation axis of the central black hole. We discuss the signs of the different black hole spacetimes occurring in the spin precessional dynamics.

arXiv (Cornell University), Dec 13, 2013

The study of the rotation curves of spiral galaxies reveals a nearly constant cored density distr... more The study of the rotation curves of spiral galaxies reveals a nearly constant cored density distribution of Cold Dark Matter. N-body simulations however lead to a cuspy distribution on the galactic scale, with a central peak. A Bose-Einstein condensate (BEC) of light particles naturally solves this problem by predicting a repulsive force, obstructing the formation of the peak. After succinctly presenting the BEC model, we test it against rotation curve data for a set of 3 High Surface Brightness (HSB), 3 Low Surface Brightness (LSB) and 3 dwarf galaxies. The BEC model gives a similar fit to the Navarro-Frenk-White (NFW) dark matter model for all HSB and LSB galaxies in the sample. For dark matter dominated dwarf galaxies the addition of the BEC component improved more upon the purely baryonic fit than the NFW component. Thus despite the sharp cutoff of the halo density, the BEC dark matter candidate is consistent with the rotation curve data of all types of galaxies.

The radial component of the motion of compact binary systems composed of neutron stars and/or bla... more The radial component of the motion of compact binary systems composed of neutron stars and/or black holes on eccentric orbit is integrated. We consider all type of perturbations that emerge up to second post-Newtonian order. These perturbations are either of relativistic origin or are related to the spin, mass quadrupole and magnetic dipole moments of the binary components. We derive a generalized Kepler equation and investigate its domain of validity, in which it properly describes the radial motion.

arXiv: General Relativity and Quantum Cosmology, 2019

The evolutions of spinning test bodies are investigated in rotating (Kerr, Bardeen-like and Haywa... more The evolutions of spinning test bodies are investigated in rotating (Kerr, Bardeen-like and Hayward-like) black hole spacetimes. Spin vector precessional equations are derived in both comoving and zero 3-momentum frames from the Mathisson-Papapetrou-Dixon (MPD) equations using either the Frenkel-Mathisson-Pirani or the Tulczyjew-Dixon spin supplementary condition. The comoving and the zero 3-momentum frames are set up from either the static or the zero angular momentum observer frame by instantaneous Lorentz-boosts. However when the body passes over the ergosphere only the boosted zero angular momentum frame can be used for description of the spin dynamics during the whole evolution. Far from the black hole the difference between the boosted static and zero angular momentum frames is unsignificant. Numerical applications are presented for spinning bodies moving along spherical-like, zoom-whirl (thus their existence is confirmed based on the MDP equations) and unbound orbits. The spi...

Physical Review D, 2020

We revisit the issue of the fluid description of minimally coupled scalar fields. While in a cosm... more We revisit the issue of the fluid description of minimally coupled scalar fields. While in a cosmological setup the interpretation of a time-evolving scalar field as a perfect fluid is well-understood, the situation is more intricate when the scalar field is static, but has a spatial gradient, a situation motivated by black hole perturbations in scalar-tensor theories. Then the scalar field is interpreted as either a particular imperfect fluid of type I or a superposition of a pair of leftgoing (incoming) and rightgoing (outgoing) null dusts with a perfect fluid. Finally, when the scalar gradient is null, it is equivalent to an imperfect fluid of type II, degenerating into null dust when the energy conditions are imposed. We also propose the suitable action in terms of the fluid pressure components for each case and discuss the variational principle for a generic class of minimally coupled scalar fields.

Symmetry, 2018

We confront a non-relativistic Bose–Einstein Condensate (BEC) model of light bosons interacting g... more We confront a non-relativistic Bose–Einstein Condensate (BEC) model of light bosons interacting gravitationally either through a Newtonian or a Yukawa potential with the observed rotational curves of 12 dwarf galaxies. The baryonic component is modeled as an axisymmetric exponential disk and its characteristics are derived from the surface luminosity profile of the galaxies. The purely baryonic fit is unsatisfactory, hence a dark matter component is clearly needed. The rotational curves of five galaxies could be explained with high confidence level by the BEC model. For these galaxies, we derive: (i) upper limits for the allowed graviton mass; and (ii) constraints on a velocity-type and a density-type quantity characterizing the BEC, both being expressed in terms of the BEC particle mass, scattering length and chemical potential. The upper limit for the graviton mass is of the order of 10 - 26 eV/c2, three orders of magnitude stronger than the limit derived from recent gravitational...

Advances in High Energy Physics, 2017

We present a comparative confrontation of both the Bose-Einstein Condensate (BEC) and the Navarro... more We present a comparative confrontation of both the Bose-Einstein Condensate (BEC) and the Navarro-Frenk-White (NFW) dark halo models with galactic rotation curves. We employ 6 High Surface Brightness (HSB), 6 Low Surface Brightness (LSB), and 7 dwarf galaxies with rotation curves falling into two classes. In the first class rotational velocities increase with radius over the observed range. The BEC and NFW models give comparable fits for HSB and LSB galaxies of this type, while for dwarf galaxies the fit is significantly better with the BEC model. In the second class the rotational velocity of HSB and LSB galaxies exhibits long flat plateaus, resulting in better fit with the NFW model for HSB galaxies and comparable fits for LSB galaxies. We conclude that due to its central density cusp avoidance the BEC model fits better dwarf galaxy dark matter distribution. Nevertheless it suffers from sharp cutoff in larger galaxies, where the NFW model performs better. The investigated galaxy s...

AIP Conference Proceedings, 2007

A class of generalized Randall-Sundrum type II (RS) brane-world models with Weyl fluid are confro... more A class of generalized Randall-Sundrum type II (RS) brane-world models with Weyl fluid are confronted with the Gold supernovae data set and BBN constraints. We consider three models with different evolutionary history of the Weyl fluid, characterized by the parameter α. For α = 0 the Weyl curvature of the bulk appears as dark radiation on the brane, while for α = 2 and 3 the brane radiates, leaving a Weyl fluid on the brane with energy density decreasing slower than that of (dark) matter. In each case the contribution Ω d of the Weyl fluid represents but a few percent of the energy content of the Universe. All models fit reasonably well the Gold2006 data. The best fit model for α = 0 is for Ω d = 0.04. In order to obey BBN constraints in this model however, the brane had to radiate at earlier times.

Physical Review D, 2015

We consider cosmological tests of a scalar-vector-tensor gravitational model, in which the dark e... more We consider cosmological tests of a scalar-vector-tensor gravitational model, in which the dark energy is included in the total action through a gauge invariant, electromagnetic type contribution. The ground state of dark energy, corresponding to a constant potential V is a Bose-Einstein type condensate with spontaneously broken U(1) symmetry. In another words dark energy appears as a massive vector field emerging from a superposition of a massless vector and a scalar field, the latter corresponding to the Goldstone boson. Two particular cosmological models, corresponding to pure electric and pure magnetic type potentials, respectively are confronted with Type IA Supernovae and Hubble parameter data. In the electric case good fit is obtained along a narrow inclined stripe in the Ωm − ΩV parameter plane, which includes the ΛCDM limit as the best fit. The other points on this admissible region represent superconducting dark energy as a sum of a cosmological constant and a time-evolving contribution. In the magnetic case the cosmological test selects either i) parameter ranges of the superconducting dark energy allowing for the standard baryonic plus dark matter or ii) a unified superconducting dark matter and dark energy model, additionally including only the baryonic sector.

The Eleventh Marcel Grossmann Meeting - On Recent Developments in Theoretical and Experimental General Relativity, Gravitation and Relativistic Field Theories - Proceedings of the MG11 Meeting on General Relativity, 2008

Journal of Cosmology and Astroparticle Physics, 2014

A recently investigated tachyonic scalar field dark energy dominated universe exhibits a bivalent... more A recently investigated tachyonic scalar field dark energy dominated universe exhibits a bivalent future: depending on initial parameters can run either into a de Sitter exponential expansion or into a traversable future soft singularity followed by a contraction phase. We also include in the model (i) a tiny amount of radiation, (ii) baryonic matter (Ω b h 2 = 0.022161, where the Hubble constant is fixed as h = 0.706) and (iii) cold dark matter (CDM). Out of a variety of six types of evolutions arising in a more subtle classification, we identify two in which in the past the scalar field effectively degenerates into a dust (its pressure drops to an insignificantly low negative value). These are the evolutions of type IIb converging to de Sitter and type III hitting the future soft singularity. We confront these background evolutions with various cosmological tests, including the supernova type Ia Union 2.1 data, baryon acoustic oscillation distance ratios, Hubble parameter-redshift relation and the cosmic microwave background (CMB) acoustic scale. We determine a subset of the evolutions of both types which at 1σ confidence level are consistent with all of these cosmological tests. At perturbative level we derive the CMB temperature power spectrum to find the best agreement with the Planck data for Ω CDM = 0.22. The fit is as good as for the ΛCDM model at high multipoles, but the power remains slightly overestimated at low multipoles, for both types of evolutions. The rest of the CDM is effectively generated by the tachyonic field, which in this sense acts as a combined dark energy and dark matter model.

Astronomische Nachrichten, 2013

We present a new type of observation relating the gravitational wave emission of supermassive bla... more We present a new type of observation relating the gravitational wave emission of supermassive black hole mergers to their electromagnetic counterparts. This dual emission involves variability of a relativistic jet arising from the spin‐orbit precession of the supermassive black hole binary at its base. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Physical Review D, 2012

A cosmological model of a flat Friedmann universe filled with a mixture of anti-Chaplygin gas and... more A cosmological model of a flat Friedmann universe filled with a mixture of anti-Chaplygin gas and dustlike matter exhibits a future soft singularity, where the pressure of the anti-Chaplygin gas diverges (while its energy density is finite). Despite infinite tidal forces the geodesics pass through the singularity. Because of the dust component, the Hubble parameter has a nonzero value at the encounter with the singularity, therefore the dust implies further expansion. With continued expansion however, the energy density and the pressure of the anti-Chaplygin gas would become ill-defined hence from the point of view of the anti-Chaplygin gas only a contraction is allowed. Paradoxically, the universe in this cosmological model would have to expand and contract simultaneously. This obviously could not happen. We solve the paradox by redefining the anti-Chaplygin gas in a distributional sense. Then a contraction could follow the expansion phase at the singularity at the price of a jump in the Hubble parameter. Although such an abrupt change is not common in any cosmological evolution, we explicitly show that the set of Friedmann, Raychaudhuri and continuity equations are all obeyed both at the singularity and in its vicinity. We also prove that the Israel junction conditions are obeyed through the singular spatial hypersurface. In particular we enounce and prove a more general form of the Lanczos equation.

Both dark energy models and modified gravity theories could lead to cosmological evolutions diffe... more Both dark energy models and modified gravity theories could lead to cosmological evolutions different from either the recollapse into a Big Crunch or exponential de Sitter expansion. The newly arising singularities may represent true endpoints of the evolution or alternatively they can allow for the extension of geodesics through them. In the latter case only the components of the Riemann tensor representing tidal forces diverge. A subclass of these soft singularities, the Sudden Future Singularity (SFS) occurs at finite time, finite scale factor and finite Hubble parameter, only the deceleration parameter being divergent. In a Friedmann universe evolving in the framework of general relativity they are realized by perfect fluids with regular energy density and diverging pressure at the SFS. A particular SFS, the Big Brake occurs when the energy density vanishes and the expansion arrives at a full stop at the singularity. Such scenarios are generated by either a particular scalar field (the tachyon field) or the anti-Chaplygin gas. By adding any matter (in particular the simplest, the dust) to these models, an unwanted feature appears: at the finite scale factor of the SFS the matter energy density remains finite, implying (for a spatially flat universe) a finite Hubble parameter, hence finite expansion rate, rather then full stop. The universe would then further expand through the singularity, this nevertheless seems forbidden as the energy density of the tachyonic field / anti-Chaplygin gas would become ill-defined. This paradox is relieved in the case of the anti-Chaplygin gas by redefining its energy density and pressure in terms of distributions peaked on the singularity. The regular cosmological quantities which are continuous across the SFS are then the energy density and the square of the Hubble parameter; those allowing for a jump at the SFS are the Hubble parameter and expansion rate (both being mirror-symmetric). The pressure and the decelaration parameter will contain Dirac delta-function contributions peaked on the SFS, however this is no disadvantage as they anyhow diverge at the singularity.

Physical Review D, 2011

We discuss gravitational lensing in the Kehagias-Sfetsos space-time emerging in the framework of ... more We discuss gravitational lensing in the Kehagias-Sfetsos space-time emerging in the framework of Hořava-Lifshitz gravity. In weak lensing we show that there are three regimes, depending on the value ofλ = 1/ωd 2 , where ω is the Hořava-Lifshitz parameter and d characterizes the lensing geometry. Whenλ is close to zero, light deflection typically produces two images, as in Schwarzschild lensing. For very largeλ the space-time approaches flatness, therefore there is only one undeflected image. In the intermediate range ofλ only the upper focused image is produced due to the existence of a maximal deflection angle δmax, a feature inexistent in the Schwarzschild weak lensing. We also discuss the location of Einstein rings, and determine the range of the Hořava-Lifshitz parameter compatible with present day lensing observations. Finally, we analyze in the strong lensing regime the first two relativistic Einstein rings and determine the constraints on the parameter range to be imposed by forthcoming experiments.

Physical Review D, 2009

We report on a search for gravitational waves from coalescing compact binaries, of total mass bet... more We report on a search for gravitational waves from coalescing compact binaries, of total mass between 2 and 35 M⊙, using LIGO observations between November 14, 2006 and May 18, 2007. No gravitational-wave signals were detected. We report upper limits on the rate of compact binary coalescence as a function of total mass. The LIGO cumulative 90%-confidence rate upper limits of the binary coalescence of neutron stars, black holes and black hole-neutron star systems are 1.4 × 10 −2 , 7.3 × 10 −4 and 3.6 × 10 −3 yr −1 L −1 10 respectively, where L10 is 10 10 times the blue solar luminosity.

Physical Review D, 2013

This paper presents results of an all-sky search for periodic gravitational waves in the frequenc... more This paper presents results of an all-sky search for periodic gravitational waves in the frequency range ½50; 1 190 Hz and with frequency derivative range of $½À20; 1:1 Â 10 À10 Hz s À1 for the fifth LIGO science run (S5). The search uses a noncoherent Hough-transform method to combine the information from coherent searches on time scales of about one day. Because these searches are very computationally intensive, they have been carried out with the Einstein@Home volunteer distributed computing project. Postprocessing identifies eight candidate signals; deeper follow-up studies rule them out. Hence, since no gravitational wave signals have been found, we report upper limits on the intrinsic gravitational wave strain amplitude h 0. For example, in the 0.5 Hz-wide band at 152.5 Hz, we can exclude the presence of signals with h 0 greater than 7:6 Â 10 À25 at a 90% confidence level. This search is about a factor 3 more sensitive than the previous Einstein@Home search of early S5 LIGO data.