Publications (202)414.83 Total impact
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ABSTRACT: The features of equatorial motion of an extended body in Kerr spacetime are investigated in the framework of the MathissonPapapetrouDixon model. The body is assumed to stay at quasiequilibrium and respond instantly to external perturbations. Besides the mass, it is completely determined by its spin, the multipolar expansion being truncated at the quadrupole order, with a spininduced quadrupole tensor. The study of the radial effective potential allows to analytically determine the ISCO shift due to spin and the associated frequency of the last circular orbit.  [Show abstract] [Hide abstract]
ABSTRACT: An assessment is made of recent attempts to evaluate how quantum gravity may affect the anisotropy spectrum of the cosmic microwave background. A perturbative scheme for the solution of the WheelerDeWitt equation has been found to allow for enhancement of power at large scales, whereas the alternative predicts a suppression of power at large scales. Both effects are corrections which, although conceptually interesting, turn out to be too small to be detected. Another scheme relies upon a BornOppenheimer analysis: by using a perturbative approach to the nonlinear ordinary differential equation obeyed by the twopoint function for scalar fluctuations, a new family of power spectra have been obtained and studied by the authors.  [Show abstract] [Hide abstract]
ABSTRACT: An extended body orbiting a compact object undergoes tidal deformations by the background gravitational field. Tidal invariants built up with the Riemann tensor and their derivatives evaluated along the worldline of the body are essential tools to investigate both geometrical and physical properties of the tidal interaction. For example, one can determine the tidal potential in the neighborhood of the body by constructing a bodyfixed frame, which requires Fermitype coordinates attached to the body itself, the latter being in turn related to the spacetime metric and curvature along the considered worldline. Similarly, in an effective field theory description of extended bodies, finite size effects are taken into account by adding to the point mass action certain nonminimal couplings which involve integrals of tidal invariants along the orbit of the body. In both cases such a computation of tidal tensors is required. Here we consider the case of a spinning body also endowed with a nonvanishing quadrupole moment in a Kerr spacetime. The structure of the body is modeled by a multipolar expansion around the ``centerofmass line'' according to the MathissonPapapetrouDixon model truncated at the quadrupolar order. The quadrupole tensor is assumed to be quadratic in spin, accounting for rotational deformations. The behavior of tidal invariants of both electric and magnetic type is discussed in terms of gaugeinvariant quantities when the body is moving along a circular orbit as well as in the case of an arbitrary (equatorial) motion. The analysis is completed by examining the associated eigenvalues and eigenvectors of the tidal tensors. The limiting situation of the Schwarzschild solution is also explored both in the strong field regime and in the weak field limit.Physical Review D 04/2015; 91(8). DOI:10.1103/PhysRevD.91.084012 · 4.86 Impact Factor 
Article: Slicing black hole spacetimes
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ABSTRACT: A general framework is developed to investigate the properties of useful choices of stationary spacelike slicings of stationary spacetimes whose congruences of timelike orthogonal trajectories are interpreted as the world lines of an associated family of observers, the kinematical properties of which in turn may be used to geometrically characterize the original slicings. On the other hand, properties of the slicings themselves can directly characterize their utility motivated instead by other considerations like the initial value and evolution problems in the 3plus1 approach to general relativity. An attempt is made to categorize the various slicing conditions or "time gauges" used in the literature for the most familiar stationary spacetimes: black holes and their flat spacetime limit.International Journal of Geometric Methods in Modern Physics 03/2015; DOI:10.1142/S021988781550070X · 0.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Continuing our analytic computation of the firstorder selfforce contribution to the "geodetic" spin precession frequency of a small spinning body orbiting a large (nonspinning) body we provide the exact expressions of the tenth and tenthandahalf postNewtonian terms. We also introduce a new approach to the analytic computation of selfforce regularization parameters based on a WKB analysis of the radial and angular equations satisfied by the metric perturbations.Physical Review D 03/2015; 91(6). DOI:10.1103/PhysRevD.91.064064 · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study Weitzenb\"ock's torsion and discuss its properties. Specifically, we calculate the measured components of Weitzenb\"ock's torsion tensor for a frame field adapted to static observers in a Fermi normal coordinate system that we establish along the world line of an arbitrary accelerated observer in general relativity. A similar calculation is carried out in the standard Schwarzschildlike coordinates for static observers in the exterior Kerr spacetime; we then compare our results with the corresponding curvature components. Our work supports the contention that in the extended general relativistic framework involving both the LeviCivita and Weitzenb\"ock connections, curvature and torsion provide complementary representations of the gravitational field.Physical Review D 02/2015; 91(8). DOI:10.1103/PhysRevD.91.084026 · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Continuing our analytic computation of the firstorder selfforce contribution to Detweiler's redshift variable we provide the exact expressions of the ninth and ninthandahalf postNewtonian terms.Physical Review D 02/2015; 91(6). DOI:10.1103/PhysRevD.91.064050 · 4.86 Impact Factor  [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 space–time 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 Poynting–Robertson 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 hole–disc 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 01/2015; 446(3):2317. DOI:10.1093/mnras/stu2242 · 5.23 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In light of the relativistic precession model, we present here detailed analyses, extending the ones performed in the Schwarzschild and Kerr spacetimes. We consider the kilohertz quasiperiodic oscillations in the HartleThorne spacetime, which describes the gravitational field of a rotating and deformed object. We derive the analytic formulas for the epicyclic frequencies in the HartelThorne spacetime and by means of these frequencies we interpret the kilohertz quasiperiodic oscillations of lowmass Xray binaries of the atoll and Z  sources, on the basis of the relativistic precession model. Particularly we perform analyzes for Z source: GX 51. We show that the quasiperiodic oscillations data can provide information on the parameters, namely, the mass, angular momentum and quadrupole moment of the compact objects in the lowmass Xray binaries.Gravitation and Cosmology 12/2014; 20(4). DOI:10.1134/S0202289314040033 · 0.49 Impact Factor  [Show abstract] [Hide abstract]
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.Physical Review D 12/2014; 91(10). DOI:10.1103/PhysRevD.91.104036 · 4.86 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). DOI:10.1093/mnras/stu2082 · 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). DOI:10.1103/PhysRevD.90.124037 · 4.86 Impact Factor  [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 08/2014; 46(1). DOI:10.1007/s1071401316444 · 1.73 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). DOI:10.1088/02649381/31/7/075024 · 3.10 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). DOI:10.1140/epjc/s1005201323349 · 5.44 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). DOI:10.1103/PhysRevD.89.104049 · 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 freely falling observers. The time coordinate function can then be taken to be the observer proper time, leading to a unit lapse function. This explains some of the properties of the original Painlev\'eGullstrand coordinates on the Schwarzschild spacetime and their generalization to the KerrNewman family of spacetimes, reproducible also locally for the G\"odel 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). DOI:10.1007/s1071401112952 · 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). DOI:10.1140/epjc/s1005201219135 · 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.  [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). DOI:10.1142/S0218271804005031 · 1.42 Impact Factor
Publication Stats
1k  Citations  
414.83  Total Impact Points  
Top Journals
Institutions

2007–2015

INO  Istituto Nazionale di Ottica
Florens, Tuscany, Italy


1990–2015

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


2002–2014

National Research Council
 Institute for Applied Mathematics "Mauro Picone" IAC
Roma, Latium, Italy


1994–2013

The American University of Rome
Roma, Latium, Italy


2011

INFN  Istituto Nazionale di Fisica Nucleare
Frascati, Latium, Italy


1998–2011

International Center for Relativistic Astrophysics
Roma, Latium, Italy


1998–2007

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


2004

Università degli Studi di Salerno
 Department of Physics "E. R. Caianiello" DF
Fisciano, Campania, Italy
