Publications (214)435.27 Total impact

Article: Latetime evolution of cosmological models with fluids obeying a ShanChenlike equation of state
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ABSTRACT: Classical as well as quantum features of the latetime evolution of cosmological models with fluids obeying a ShanChenlike equation of state are studied. The latter is of the type $p=w_{\rm eff}(\rho)\,\rho$, and has been used in previous works to describe, e.g., a possible scenario for the growth of the darkenergy content of the present Universe. At the classical level the fluid dynamics in a spatially flat FriedmannRobertsonWalker background implies the existence of two possible equilibrium solutions depending on the model parameters, associated with (asymptotic) finite pressure and energy density. We show that no future cosmological singularity is developed during the evolution for this specific model. The corresponding quantum effects in the latetime behavior of the system are also investigated within the framework of quantum geometrodynamics, i.e., by solving the (minisuperspace) WheelerDeWitt equation in the BornOppenheimer approximation, constructing wavepackets and analyzing their behavior.  [Show abstract] [Hide abstract]
ABSTRACT: We raise the analytical knowledge of the eccentricityexpansion of the DetweilerBarackSago redshift invariant in a Schwarzschild spacetime up to the 9.5th postNewtonian order (included) for the $e^2$ and $e^4$ contributions, and up to the 4th postNewtonian order for the higher eccentricity contributions through $e^{20}$. We convert this information into an analytical knowledge of the effectiveonebody radial potentials $\bar d(u)$, $\rho(u)$ and $q(u)$ through the 9.5th postNewtonian order. We find that our analytical results are compatible with current corresponding numerical selfforce data.  [Show abstract] [Hide abstract]
ABSTRACT: We analytically compute, through the sixandahalf postNewtonian order, the secondorderineccentricity piece of the DetweilerBarackSago gaugeinvariant redshift function for a small mass in eccentric orbit around a Schwarzschild black hole. Using the first law of mechanics for eccentric orbits [A. Le Tiec, Phys. Rev. D {\bf 92}, 084021 (2015)] we transcribe our result into a correspondingly accurate knowledge of the second radial potential of the effectiveonebody formalism [A. Buonanno and T. Damour, Phys. Rev. D {\bf 59}, 084006 (1999)]. We compare our newly acquired analytical information to several different numerical selfforce data and find good agreement, within estimated error bars. We also obtain, for the first time, independent analytical checks of the recently derived, comparablemass fourthpostNewtonian order dynamics [T. Damour, P. Jaranowski and G. Shaefer, Phys. Rev. D {\bf 89}, 064058 (2014)].  [Show abstract] [Hide abstract]
ABSTRACT: We show that causality violation in a Kerr naked singularity spacetime is constrained by the existence of (radial) potential barriers. We extend to the class of vortical nonequatorial null geodesics confined to $$\theta $$θ $$=$$= constant hyperboloids (boreal orbits) previous results concerning timelike ones (Calvani et al. in Gen Rel Gravit 9:155, 1978), showing that within this class of orbits, the causality principle is rigorously satisfied.  [Show abstract] [Hide abstract]
ABSTRACT: We analytically compute, through the eightandahalf postNewtonian order and the fourthorder in spin, the gravitational selfforce correction to Detweiler's gauge invariant redshift function for a small mass in circular orbit around a Kerr black hole. Using the first law of mechanics for black hole binaries with spin [L.~Blanchet, A.~Buonanno and A.~Le Tiec, Phys.\ Rev.\ D {\bf 87}, 024030 (2013)] we transcribe our results into a knowledge of various spindependent couplings, as encoded within the spinning effectiveonebody model of T.~Damour and A.~Nagar [Phys.\ Rev.\ D {\bf 90}, 044018 (2014)]. We also compare our analytical results to the (corrected) numerical selfforce results of A.~G.~Shah, J.~L.~Friedman and T.~S.~Keidl [Phys.\ Rev.\ D {\bf 86}, 084059 (2012)], from which we show how to directly extract physically relevant spindependent couplings.  [Show abstract] [Hide abstract]
ABSTRACT: We study the motion of test particles in the metric of a localized and slowly rotating astronomical source, within the framework of linear gravitoelectromagnetism, grounded on a PostMinkowskian approximation of general relativity. Special attention is paid to gravitational inductive effects due to timevarying gravitomagnetic fields. We show that, within the limits of the approximation mentioned above, there are cumulative effects on the orbit of the particles either for planetary sources or for binary systems. They turn out to be negligible.  [Show abstract] [Hide abstract]
ABSTRACT: We study the behavior of massless Dirac particles in the vacuum Cmetric spacetime, representing the nonlinear superposition of the Schwarzschild black hole solution and the Rindler flat spacetime associated with uniformly accelerated observers. Under certain conditions, the Cmetric can be considered as a unique laboratory to test the coupling between intrinsic properties of particles and fields with the background acceleration in the full (exact) strongfield regime. The Dirac equation is separable by using, e.g., a sphericallike coordinate system, reducing the problem to onedimensional radial and angular parts. Both radial and angular equations can be solved exactly in terms of general Heun functions. We also provide perturbative solutions to firstorder in a suitably defined acceleration parameter, and compute the accelerationinduced corrections to the particle absorption rate as well as to the angleaveraged cross section of the associated scattering problem in the lowfrequency limit. Furthermore, we show that the angular eigenvalue problem can be put in onetoone correspondence with the analogous problem for a Kerr spacetime, by identifying a map between these "acceleration" harmonics and Kerr spheroidal harmonics. Finally, in this respect we discuss the nature of the coupling between intrinsic spin and spacetime acceleration in comparison with the well known Kerr spinrotation coupling.  [Show abstract] [Hide abstract]
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. 
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.  [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.  [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.  [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.  [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.  [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.  [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.  [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.  [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.  [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.
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2k  Citations  
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Institutions

20072015

INO  Istituto Nazionale di Ottica
Florens, Tuscany, Italy


19902015

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


20022013

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


2012

INFN  Istituto Nazionale di Fisica Nucleare
Frascati, Latium, Italy


19982011

International Center for Relativistic Astrophysics
Roma, Latium, Italy 
University of Rome Tor Vergata
Roma, Latium, Italy


19982007

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


19942006

The American University of Rome
Roma, Latium, Italy


2003

National Institute of Geophysics and Volcanology
Roma, Latium, Italy
