Publications (183)293.38 Total impact
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ABSTRACT: The influence of an arbitrary spin orientation on the quadrupolar structure of an extended body moving in a Schwarzschild spacetime is investigated. The body dynamics is described according to the MathissonPapapetrouDixon model without any restriction on the motion or simplifying assumption on the associated spin vector and quadrupole tensor, generalizing previous works. The equations of motion are solved analytically in the limit of small values of the characteristic length scales associated with the spin and quadrupole variables with respect to the characteristic length of the background curvature. The solution provides all corrections to the circular geodesic on the equatorial plane taken as the reference trajectory due to both dipolar and quadrupolar structure of the body as well as the conditions which the nonvanishing components of the quadrupole tensor must fulfill in order that the problem be selfconsistent.12/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the picture of a thin accretion disc around a black hole, radiating mainly in the direction perpendicular to its plane, we study the motion of test particles interacting with a test geodesic radiation flux originating in the equatorial plane of a Schwarzschild spacetime and propagating initially in the perpendicular direction. We assume that the interaction with the test particles is modelled by an effective term corresponding to the Thomsontype interaction which governs the PoyntingRobertson effect. After approximating the individual photon trajectories adequately, we solve the continuity equation approximately in order to find a consistent flux density with a certain plausible prescribed equatorial profile. The combined effects of gravity and radiation are illustrated in several typical figures which confirm that the particles are generically strongly influenced by the flux. In particular, they are both collimated and accelerated in the direction perpendicular to the disc, but this acceleration is not enough to explain highly relativistic outflows emanating from some black holedisc sources. The model can however be improved in a number of ways before posing further questions which are summarized in concluding remarks.Monthly Notices of the Royal Astronomical Society 12/2014; 446(3). · 5.23 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The motion of a test particle in the gravitational field of a nonspherical source endowed with both mass and mass quadrupole moment is investigated when a test radiation field is also present. The background is described by the ErezRosen solution, which is a static spacetime belonging to the Weyl class of solutions to the vacuum Einstein's field equations, and reduces to the familiar Schwarzschild solution when the quadrupole parameter vanishes. The radiation flux has a fixed but arbitrary (nonzero) angular momentum. The interaction with the radiation field is assumed to be Thomsonlike, i.e., the particles absorb and reemit radiation, thus suffering for a frictionlike drag force. Such an additional force is responsible for the PoyntingRobertson effect, which is well established in the framework of Newtonian gravity and has been recently extended to the general theory of relativity. The balance between gravitational attraction, centrifugal force and radiation drag leads to the occurrence of equilibrium circular orbits which are attractors for the surrounding matter for every fixed value of the interaction strength. The presence of the quadrupolar structure of the source introduces a further degree of freedom: there exists a whole family of equilibrium orbits parametrized by the quadrupole parameter, generalizing previous works. This scenario is expected to play a role in the context of accretion matter around compact objects.Monthly Notices of the Royal Astronomical Society 10/2014; 446(1). · 5.23 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Tidal interactions have a significant influence on the late dynamics of compact binary systems, which constitute the prime targets of the upcoming network of gravitationalwave detectors. We refine the theoretical description of tidal interactions (hitherto known only to the second postNewtonian level) by extending our recently developed analytic selfforce formalism, for extreme massratio binary systems, to the computation of several tidal invariants. Specifically, we compute, to linear order in the mass ratio and to the 7.5$^{\rm th}$ postNewtonian order, the following tidal invariants: the square and the cube of the gravitoelectric quadrupolar tidal tensor, the square of the gravitomagnetic quadrupolar tidal tensor, and the square of the gravitoelectric octupolar tidal tensor. Our highaccuracy analytic results are compared to recent numerical selfforce tidal data by Dolan et al. \cite{Dolan:2014pja}, and, notably, provide an analytic understanding of the light ring asymptotic behavior found by them. We transcribe our kinematical tidalinvariant results in the more dynamically significant effective onebody description of the tidal interaction energy. By combining, in a synergetic manner, analytical and numerical results, we provide simple, accurate analytic representations of the global, strongfield behavior of the gravitoelectric quadrupolar tidal factor. A striking finding is that the linearinmassratio piece in the latter tidal factor changes sign in the strongfield domain, to become negative (while its previously known second postNewtonian approximant was always positive). We, however, argue that this will be more than compensated by a probable fast growth, in the strongfield domain, of the nonlinearinmassratio contributions in the tidal factor.Physical Review D 09/2014; 90(12). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the dynamics of test particles undergoing friction forces in a Friedmann–Robertson–Walker (FRW) spacetime. The interaction with the background fluid is modeled by introducing a Poynting–Robertsonlike friction force in the equations of motion, leading to measurable (at least in principle) deviations of the particle trajectories from geodesic motion. The effect on the peculiar velocities of the particles is investigated for various equations of state of the background fluid and different standard cosmological models. The friction force is found to have major effects on particle motion in closed FRW universes, where it turns the timeasymptotic value (approaching the recollapse) of the peculiar particle velocity from ultrarelativistic (close to light speed) to a comoving one, i.e., zero peculiar speed. On the other hand, for open or flat universes the effect of the friction is not so significant, because the timeasymptotic peculiar particle speed is largely nonrelativistic also in the geodesic case.European Physical Journal C 08/2014; 73(2). · 5.44 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The equatorial motion of extended bodies in a Kerr spacetime is investigated in the framework of the MathissonPapapetrouDixon model, including the full set of effective components of the quadrupole tensor. The numerical integration of the associated equations shows the specific role of the mass and current quadrupole moment components. While most of the literature on this topic is limited to spininduced (purely electric) quadrupole tensor, the present analysis highlights the effect of a completely general quadrupole tensor on the dynamics. The contribution of the magnetictype components is indeed related to a number of interesting features, e.g., enhanced inward/outward spiraling behavior of the orbit and spinfliplike effects, which may have observational counterparts. Finally, the validity limit of the MathissonPapapetrouDixon model is also discussed through explicit examples.Classical and Quantum Gravity 08/2014; 31(7). · 3.10 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Some strong field effects on test particle motion associated with the propagation of a plane electromagnetic wave in the exact theory of general relativity are investigated. Two different profiles of the associated radiation flux are considered in comparison, corresponding to either constant or oscillating electric and magnetic fields with respect to a natural family of observers. These are the most common situations to be experimentally explored, and have a well known counterpart in the flat spacetime limit. The resulting line elements are determined by a single metric function, which turns out to be expressed in terms of standard trigonometric functions in the case of a constant radiation flux, and in terms of special functions in the case of oscillating flux, leading to different features of test particle motion. The world line deviation between both uncharged and charged particles on different spacetime trajectories due to the combined effect of gravitational and electromagnetic forces is studied. The interaction of charged particles with the background radiation field is also discussed through a general relativistic description of the inverse Compton effect. Motion as well as deviation effects on particles endowed with spin are studied too. Special situations may occur in which the direction of the spin vector change during the interaction, leading to obsevables effects like spinflip.Physical Review D 08/2014; 89(10). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A simple observation about the action for geodesics in a stationary spacetime with separable geodesic equations leads to a natural class of slicings of that spacetime whose orthogonal geodesic trajectories represent the world lines of freely falling fiducial observers. The time coordinate function can then be taken to be the observer proper time, leading to a unit lapse function, although the time coordinate lines still follow Killing trajectories to retain the explicitly stationary nature of the coordinate grid. This explains some of the properties of the original PainlevéGullstrand coordinates on the Schwarzschild spacetime and their generalization to the KerrNewman family of spacetimes, reproducible also locally for the Gödel spacetime. For the static spherically symmetric case the slicing can be chosen to be intrinsically flat with spherically symmetric geodesic observers, leaving all the gravitational field information in the shift vector field.General Relativity and Gravitation 08/2014; 44(3). · 1.73 Impact Factor 
Article: Particle scattering by a test fluid on a Schwarzschild spacetime: the equation of state matters
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ABSTRACT: The motion of a massive test particle in a Schwarzschild spacetime surrounded by a perfect fluid with equation of state p 0=wρ 0 is investigated. Deviations from geodesic motion are analyzed as a function of the parameter w, ranging from w=1, which corresponds to the case of massive free scalar fields, down into the socalled “phantom” energy, with w<−1. It is found that the interaction with the fluid leads to capture (escape) of the particle trajectory in the case 1+w>0 (<0), respectively. Based on this result, it is argued that inspection of the trajectories of test particles in the vicinity of a Schwarzschild black hole with matter around may offer a new means of gaining insights into the nature of cosmic matter.European Physical Journal C 08/2014; 72(3). · 5.44 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The motion of test particles along circular orbits in the vacuum $C$ metric is studied in the FrenetSerret formalism. Special orbits and corresponding intrinsically defined geometrically relevant properties are selectively studied.08/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Circular orbits are examined in static spacetimes belonging to the Weyl class of vacuum solutions which represent (nonlinear) superposition of the gravitational fields generated by certain collinear distributions of matter. In particular, solutions representing two and three ChazyCurzon particles  all of them endowed with conical singularities  are considered. Conditions for geodesic motion in certain symmetry planes are discussed and results are summarized in a number of graphics too. All the discussion is developed in the framework of observerdependent analysis of motion.International Journal of Modern Physics D 08/2014; 13(06). · 1.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We analytically compute, to linear order in the massratio, the "geodetic" spin precession frequency of a small spinning body orbiting a large (nonspinning) body to the eightandahalf postNewtonian order, thereby extending previous analytical knowledge which was limited to the third postNewtonian level. These results are obtained applying analytical gravitational selfforce theory to the firstderivative level generalization of Detweiler's gaugeinvariant redshift variable. We compare our analytic results with strongfield numerical data recently obtained by S.~R.~Dolan et al. [Phys.\ Rev.\ D {\bf 89}, 064011 (2014)]. Our new, highpostNewtonianorder results capture the strongfield features exhibited by the numerical data. We argue that the spinprecession will diverge as $\approx 0.14/(13y)$ as the lightring is approached. We transcribe our kinematical spinprecession results into a corresponding improved analytic knowledge of one of the two (gaugeinvariant) effective gyrogravitomagnetic ratios characterizing spinorbit couplings within the effectiveonebody formalism. We provide simple, accurate analytic fits both for spinprecession and the effective gyrogravitomagnetic ratio. The latter fit predicts that the linearinmassratio correction to the gyrogravitomagnetic ratio changes sign before reaching the lightring. This strongfield prediction might be important for improving the analytic modeling of coalescing spinning binaries.Physical Review D 04/2014; 90(2). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We {\it analytically} compute, to the eightandahalf postNewtonian order, and to linear order in the mass ratio, the radial potential describing (within the effective onebody formalism) the gravitational interaction of two bodies, thereby extending previous analytic results. These results are obtained by applying analytical gravitational selfforce theory (for a particle in circular orbit around a Schwarzschild black hole) to Detweiler's gaugeinvariant redshift variable. We emphasize the increase in \lq\lq transcendentality" of the numbers entering the postNewtonian expansion coefficients as the order increases, in particular we note the appearance of $\zeta(3)$ (as well as the square of Euler's constant $\gamma$) starting at the seventh postNewtonian order. We study the convergence of the postNewtonian expansion as the expansion parameter $u=GM/(c^2r)$ leaves the weakfield domain $u\ll 1$ to enter the strong field domain $u=O(1)$.Physical Review D 03/2014; 89(10). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The world lines of null particles admit arbitrary parametrizations. In the presence of a family of observers one may introduce along a null world line an extension of the socalled Cattaneo’s relative standard time parameter (valid for massive particles) which plays a special role. Another possibility is to use the coordinate time itself as a parameter. The relation between relative standard time and coordinate time allows for the introduction of an observerdependent optical path and associated refraction index. Both these quantities are studied here working out explicit examples concerning familiar null orbits and observers in black hole spacetimes.International Journal of Geometric Methods in Modern Physics 03/2014; 11(3). · 0.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recent work in the literature has found a suppression or, instead, an enhancement of the Cosmic Microwave Background power spectrum in quantum gravity, although the effect is too small to be observed, in both cases. The present paper studies in detail the equations recently proposed for a BornOppenheimertype analysis of the problem. By using a perturbative approach to the analysis of the nonlinear ordinary differential equation obeyed by the twopoint function for scalar fluctuations, we find various explicit forms of such a twopoint function, with the associated power spectrum. In particular, a new family of power spectra is obtained and studied. The theoretical prediction of power enhancement at large scales is hence confirmed.Physical Review D 02/2014; 89(8). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A nonideal fluid obeying a ShanChenlike equation of state is used as the background source of a flat FriedmannRobertsonWalker cosmological spacetime to describe the inflationary epoch of our universe. The usual formulation in terms of a selfinteracting scalar field in an external potential is developed, completely determined by the ShanChen energy density and pressure. Within the slowroll approximation, a number of interesting features are presented, including the possibility to fulfill current observational constraints. Furthermore, this model provides a natural exit mechanism from the inflationary epoch.01/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The optical medium analogy of a given spacetime was developed decades ago and has since then been widely applied to different gravitational contexts. Here we consider the case of a colliding gravitational wave spacetime, generalizing previous results concerning single gravitational pulses. Given the complexity of the nonlinear interaction of two gravitational waves in the framework of general relativity, typically leading to the formation of either horizons or singularities, the optical medium analogy proves helpful to simply capture some interesting effects of photon propagation.General Relativity and Gravitation 01/2014; 46(1). · 1.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We extend the analytical determination of the main radial potential describing (within the effective onebody formalism) the gravitational interaction of two bodies beyond the 4th postNewtonian approximation recently obtained by us. This extension is done to linear order in the mass ratio by applying analytical gravitational selfforce theory (for a particle in circular orbit around a Schwarzschild black hole) to Detweiler's gaugeinvariant redshift variable. By using the version of black hole perturbation theory developed by Mano, Suzuki and Takasugi, we have pushed the analytical determination of the (linear in mass ratio) radial potential to the 6th postNewtonian order (passing through 5 and 5.5 postNewtonian terms). In principle, our analytical method can be extended to arbitrarily high postNewtonian orders.Physical Review D 12/2013; 89(6). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The deviation from geodesic motion of the world line of an extended body endowed with multipolar structure up to the mass quadrupole moment is studied in the Kerr background according to the MathissonPapapetrouDixon model. The properties of the quadrupole tensor are clarified by identifying the relevant components which enter the equations of motion, leading to the definition of an effective quadrupole tensor sharing its own algebraic symmetries, but also obeying those implied by the MathissonPapapetrouDixon model itself. The equations of motion are then solved analytically in the limit of small values of the characteristic length scales associated with the spin and quadrupole variables in comparison with the one associated with the background curvature and under special assumptions on body's structure and motion. The resulting quasicircular orbit is parametrized in a Keplerianlike form, so that temporal, radial and azimuthal eccentricities as well as semimajor axis, period and periastron advance are explicitly computed and expressed in terms of gaugeinvariant variables in the weak field and slow motion limit. A companion numerical study of the equations of motion is performed too.Physical Review D 11/2013; 89(4). · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider a FriedmannRobertsonWalker universe with a fluid source obeying a nonideal equation of state with ‘‘asymptotic freedom,” namely ideal gas behavior (pressure changes directly proportional to density changes) both at low and high density regimes, following a fluid dynamical model due to Shan and Chen. It is shown that, starting from an ordinary energy density component, such fluids naturally evolve towards a universe with a substantial ‘‘dark energy” component at the present time, with no need of invoking any cosmological constant. Moreover, we introduce a quantitative indicator of darkness abundance, which provides a consistent picture of the actual matterenergy content of the Universe.Physical Review D 09/2013; 88(6). · 4.86 Impact Factor
Publication Stats
941  Citations  
293.38  Total Impact Points  
Top Journals
Institutions

2014

INFN  Istituto Nazionale di Fisica Nucleare
Frascati, Latium, Italy


1990–2014

Sapienza University of Rome
 Department of Physics
Roma, Latium, Italy


1994–2013

The American University of Rome
Roma, Latium, Italy


1998–2010

International Center for Relativistic Astrophysics
Roma, Latium, Italy


2000–2007

Villanova University
 Department of Mathematics and Statistics
Norristown, Pennsylvania, United States
