arXiv subjects

C. O. Lousto

Publications and source records attributed to C. O. Lousto.

At least 37 records · Page 2Linked to original sources

arXiv2000

Pragmatic approach to gravitational radiation reaction in binary black holes

We study the relativistic orbit of binary black holes in systems with small mass ratio. The trajectory of the smaller object (another black hole or a neutron star), represented as a particle, is determined by the geodesic equation on the perturbed massive black hole spacetime. The particle itself generates the gravitational perturbations leading to a problem that needs regularization. Here we study perturbations around a Schwarzschild black hole using Moncrief's gauge invariant formalism. We decompose the perturbations into $\ell-$multipoles to show that all $\ell-$metric coefficients are $C^0$ at the location of the particle. Summing over $\ell$, to reconstruct the full metric, gives a formally divergent result. We succeed in bringing this sum to a generalized Riemann's $ζ-$function regularization scheme and show that this is tantamount to subtract the $\ell\to\infty$ piece to each multipole. We explicitly carry out this regularization and numerically compute the first order geodesics. Application of this method to general orbits around rotating black holes would generate accurate templates for gravitational wave laser interferometric detectors.

gr-qc↗

arXiv2000

Gravitational waves from black hole collisions via an eclectic approach

We present the first results in a new program intended to make the best use of all available technologies to provide an effective understanding of waves from inspiralling black hole binaries in time for imminent observations. In particular, we address the problem of combining the close-limit approximation describing ringing black holes and full numerical relativity, required for essentially nonlinear interactions. We demonstrate the effectiveness of our approach using general methods for a model problem, the head-on collision of black holes. Our method allows a more direct physical understanding of these collisions indicating clearly when non-linear methods are important. The success of this method supports our expectation that this unified approach will be able to provide astrophysically relevant results for black hole binaries in time to assist gravitational wave observations.

gr-qc↗

arXiv1999

Perturbative evolution of nonlinear initial data for binary black holes: Zerilli vs. Teukolsky

We consider the problem of evolving nonlinear initial data in the close limit regime. Metric and curvature perturbations of nonrotating black holes are equivalent to first perturbative order, but Moncrief waveform in the former case and Weyl scalar $ψ_4$ in the later differ when nonlinearities are present. For exact Misner initial data (two equal mass black holes initially at rest), metric perturbations evolved via the Zerilli equation suffer of a premature break down (at proper separation of the holes $L/M\approx2.2$) while the exact Weyl scalar $ψ_4$ evolved via the Teukolsky equation keeps a very good agreement with full numerical results up to $L/M\approx3.5$. We argue that this inequivalent behavior holds for a wider class of conformally flat initial data than those studied here. We then discuss the relevance of these results for second order perturbative computations and for perturbations to take over full numerical evolutions of Einstein equations.

gr-qc↗

arXiv1999

Second order gauge invariant gravitational perturbations of a Kerr black hole

We investigate higher than the first order gravitational perturbations in the Newman-Penrose formalism. Equations for the Weyl scalar $ψ_4,$ representing outgoing gravitational radiation, can be uncoupled into a single wave equation to any perturbative order. For second order perturbations about a Kerr black hole, we prove the existence of a first and second order gauge (coordinates) and tetrad invariant waveform, $ψ_I$, by explicit construction. This waveform is formed by the second order piece of $ψ_4$ plus a term, quadratic in first order perturbations, chosen to make $ψ_I$ totally invariant and to have the appropriate behavior in an asymptotically flat gauge. $ψ_I$ fulfills a single wave equation of the form ${\cal T}ψ_I=S,$ where ${\cal T}$ is the same wave operator as for first order perturbations and $S$ is a source term build up out of (known to this level) first order perturbations. We discuss the issues of imposition of initial data to this equation, computation of the energy and momentum radiated and wave extraction for direct comparison with full numerical approaches to solve Einstein equations.

gr-qc↗

arXiv1996

Are Higher Order Membranes stable in Black Hole Spacetimes?

We continue the study of the existence and stability of static spherical membrane configurations in curved spacetimes. We first consider higher order membranes described by a Lagrangian which, besides the Dirac term, includes a term proportional to the scalar curvature of the world--volume ${}^{(3)}R$. Notably, in this case, the equations of motion can be reduced to second order ones and an effective potential analysis can be made. The conditions for stability are then explicitly derived. We find a self--consistent static spherical membrane, determining the spacetime generated by the membrane itself. In this case we find, however, that the total energy of the membrane has to be negative, and no {\it stable} equilibrium can be achieved. We then generalize the discussion to a membrane described by a Lagrangian including all possible second derivative terms. We conclude the paper with some discussion on the generality of the results obtained.

gr-qc↗

arXiv1996

On the Stability of Spherical Membranes in Curved Spacetimes

We study the existence and stability of spherical membranes in curved spacetimes. For Dirac membranes in the Schwarzschild--de Sitter background we find that there exists an equilibrium solution. By fine--tuning the dimensionless parameter $ΛM^2,$ the static membrane can be at any position outside the black hole event horizon, even at the stretched horizon, but the solution is unstable. We show that modes having $l=0$ (and for $ΛM^2<16/243$ also $l=1$) are responsible for the instability. We also find that spherical higher order membranes (membranes with extrinsic curvature corrections), contrary to what happens in flat Minkowski space, {\it do} have equilibrium solutions in a general curved background and, in particular, also in the ``plain'' Schwarzschild geometry (while Dirac membranes do not have equilibrium solutions there). These solutions, however, are also unstable. We shall discuss a way of by--passing these instability problems, and we also relate our results to the recent ideas of representing the black hole event horizon as a relativistic bosonic membrane.

gr-qc↗

arXiv1996

Some Thermodynamic Aspects of Black Holes and Singularities

We review and correct the classical critical exponents characterizing the transition from negative to positive black hole's heat capacity at high charge--angular momentum. We discuss the stability properties of black holes as a thermodynamic system in equilibrium with a radiation bath (canonical ensamble) by using the Helmholtz free energy potential. We finally analytically extend the analysis to negative mass holes and study its thermodynamical stability behavior.

gr-qc↗

arXiv1995

GUTs in Curved Spacetime: Running Gravitational Constants, Newtonian Potential and the Quantum Corrected Gravitational Equations

The running coupling constants (in particular, the gravitational one) are studied in asymptotically free GUTs and in finite GUTs in curved spacetime, with explicit examples. The running gravitational coupling is used to calculate the leading quantum GUT corrections to the Newtonian potential, which turn out to be of logarithmic form in asymptotically free GUTs. A comparison with the effective theory for the conformal factor ---where leading quantum corrections to the Newtonian potential are again logarithmic--- is made. A totally asymptotically free $O(N)$ GUT with quantum higher derivative gravity is then constructed, using the technique of introducing renormalization group (RG) potentials in the space of couplings. RG equations for the cosmological and gravitational couplings in this theory are derived, and solved numerically, showing the influence of higher-derivative quantum gravity on the Newtonian potential. The RG-improved effective gravitational Lagrangian for asymptotically free massive GUTs is calculated in the strong (almost constant) curvature regime, and the non-singular De Sitter solution to the quantum corrected gravitational equations is subsequently discussed. Finally, possible extensions of the results here obtained are briefly outlined.

hep-th↗

arXiv1994

Perturbative Metric of Charged Black Holes in Quadratic Gravity

We consider perturbative solutions to the classical field equations coming from a quadratic gravitational lagrangian in four dimensions. We study the charged, spherically symmetric black hole and explicitly give corrections up to third order (in the coupling constant $β$ multiplying the $R_{μν}R^{μν}$ term) to the Reissner--Nordström hole metric. We consider the thermodynamics of such black holes, in particular, we compute explicitly its temperature and entropy--area relation which deviates from the usual $S=A/4$ expression.

gr-qc↗

arXiv1994

Short-distance Cut-off from the Quantum Dynamics of Fields in a Black Hole Spacetime

We stress the strongly observer-dependent nature of the fine-grained black hole entropy as computed by usual methods. From the point of view of a fiducial observer, the 1-loop renormalized energy-momentum tensor diverges at the horizon. Considering, more consistently, the computation of the black hole entropy in the back-reaction-corrected curved background an ultraviolet cut-off, needed to regularize the entropy, naturally araises. Examples of the classical dynamics of test particles, strings and relativistic membranes in the corrected metric are also given to complete the picture.

gr-qc↗

arXiv1994

Scattering Processes at the Planck Scale

The ultrarelativistic limit of the Kerr - Newman geometry is studied in detail. We find the gravitational shock wave background associated with this limit. We study the scattering of scalar fields in the gravitational shock wave geometries and compare this with the scattering by ultrarelativistic extended sources and with the scattering of fundamental strings. We also study planckian energy string collisions in flat spacetime as the scattering of a string in the effective curved background produced by the others as the impact parameter $b$ decreases. We find the effective energy density distribution generated by these collisions. The effective metric generated by these collisions is a gravitational shock wave with profile $f(ρ)\sim pρ^{4-D}$, for large impact parameter $b$. For intermediate $b$, $f(ρ)\sim qρ^2$, corresponding to an extended source of momentum $q$. We finally study the emergence of string instabilities in $D$ - dimensional black hole spacetimes and De Sitter space. We solve the first order string fluctuations around the center of mass motion at spatial infinity, near the horizon and at the spacetime singularity. We find that the time components are always well behaved in the three regions and in the three backgrounds. The radial components are unstable: imaginary frequencies develop in the oscillatory modes near the horizon, and the evolution is like $(τ-τ_0)^ {-P}$, $(P>0)$, near the spacetime singularity, $r\to0$, where the world - sheet time $(τ-τ_0)\to0$, and the proper string length grows infinitely.

gr-qc↗

arXiv1994

The Emergence of an Effective two - dimensional Quantum Description from the study of Critical Phenomena in Black Holes

We study the occurrence of critical phenomena in four - dimensional, rotating and charged black holes, derive the critical exponents and show that they fulfill the scaling laws. Correlation functions critical exponents and Renormalization Group considerations assign an effective (spatial) dimension, $d=2$, to the system. The two - dimensional Gaussian approximation to critical systems is shown to reproduce all the black hole's critical exponents. Higher order corrections (which are always relevant) are discussed. Identifying the two - dimensional surface with the event horizon and noting that generalization of scaling leads to conformal invariance and then to string theory, we arrive to 't Hooft's string interpretation of black holes. From this, a model for dealing with a coarse grained black hole quantization is proposed. We also give simple arguments that lead to a rough quantization of the black hole mass in units of the Planck mass, i. e. $M\simeq{1\over\sqrt{2}}M_{pl} \sqrt{l}$ with a $l$ positive integer and then, from this result, to the proportionality between quantum entropy and area.

gr-qc↗

arXiv1994

A Perturbative Method to solve fourth-order Gravity Field Equations

We develop a method for solving the field equations of a quadratic gravitational theory coupled to matter. The quadratic terms are written as a function of the matter stress tensor and its derivatives in such a way to have, order by order, a set of Einstein field equations with an effective $T_{μν}$. We study the cosmological scenario recovering the de Sitter exact solution, and the first order (in the coupling constants $α$ and $β$ appearing in the gravitational Lagrangian) solution to the gauge cosmic string metric and the charged black hole. For this last solution we discuss the consequences on the thermodynamics of black holes, and in particular, the entropy - area relation which gets additional terms to the usual ${1\over4} A$ value.

gr-qc↗

arXiv1993

Charged Black Holes In Quadratic Theories

We point out that in general the Reissner-Nordström (RN) charged black holes of general relativity are not solutions of the four dimensional quadratic gravitational theories. They are, e.g., exact solutions of the $R+R^2$ quadratic theory but not of a theory where a $R_{ab}R^{ab}$ term is present in the gravitational Lagrangian. In the case where such a non linear curvature term is present with sufficiently small coupling, we obtain an approximate solution for a charged black hole of charge $Q$ and mass $M$. For $Q\ll M$ the validity of this solution extends down to the horizon. This allows us to explore the thermodynamic properties of the quadratic charged black hole and we find that, to our approximation, its thermodynamics is identical to that of a RN black hole. However our black hole's entropy is not equal to the one fourth of the horizon area. Finally we extend our analysis to the rotating charged black hole and qualitatively similar results are obtained.

gr-qc↗

arXiv1993

The Fourth Law of Black Hole Thermodynamics

We show that black holes fulfill the scaling laws arising in critical transitions. In particular, we find that in the transition from negative to positive values the heat capacities $C_{JQ}$, $C_{ΩQ}$ and $C_{JΦ}$ give rise to critical exponents satisfying the scaling laws. The three transitions have the same critical exponents as predicted by the universality Hypothesis. We also briefly discuss the implications of this result with regards to the connections among gravitation, quantum mechanics and statistical physics.

gr-qc↗

arXiv1993

The Quantization of the Spacetime Geometry Generated by Planckian Energy Particles

We study the quantization of the curved spacetime created by ultrarelativistic particles at Planckian energies. We consider a minisuperspace model based on the classical shock wave metric generated by these particles, and for which the Wheeler - De Witt equation is solved exactly. The wave function of the geometry is a Bessel function whose argument is the classical action. This allows us to describe not only the semiclassical regime $(S\to\infty),$ but also the strong quantum regime $(S\to0).$ We analyze the interaction with a scalar field $ϕ$ and apply the third quantization formalism to it. The quantum gravity effects make the system to evolve from a highly curved semiclassical geometry (a gravitational wave metric) into a strongly quantum state represented by a weakly curved geometry (essentially flat spacetime).

gr-qc↗

arXiv1993

Topological Defects in Gravitational Theories with Non Linear Lagrangians

The gravitational field of monopoles, cosmic strings and domain walls is studied in the quadratic gravitational theory $R+αR^2$ with $α|R|\ll 1$, and is compared with the result in Einstein's theory. The metric aquires modifications which correspond to a short range `Newtonian' potential for gauge cosmic strings, gauge monopoles and domain walls and to a long range one for global monopoles and global cosmic strings. In this theory the corrections turn out to be attractive for all the defects. We explain, however, that the sign of these corrections in general depends on the particular higher order derivative theory and topological defect under consideration. The possible relevance of our results to the study of the evolution of topological defects in the early universe is pointed out.

gr-qc↗

arXiv1993

String Instabilities in Black Hole Spacetimes

We study the emergence of string instabilities in $D$ - dimensional black hole spacetimes (Schwarzschild and Reissner - Nordstrøm), and De Sitter space (in static coordinates to allow a better comparison with the black hole case). We solve the first order string fluctuations around the center of mass motion at spatial infinity, near the horizon and at the spacetime singularity. We find that the time components are always well behaved in the three regions and in the three backgrounds. The radial components are {\it unstable}: imaginary frequencies develop in the oscillatory modes near the horizon, and the evolution is like $(τ-τ_0)^{-P}$, $(P>0)$, near the spacetime singularity, $r\to0$, where the world - sheet time $(τ-τ_0)\to0$, and the proper string length grows infinitely. In the Schwarzschild black hole, the angular components are always well - behaved, while in the Reissner - Nordstrøm case they develop instabilities inside the horizon, near $r\to0$ where the repulsive effects of the charge dominate over those of the mass. In general, whenever large enough repulsive effects in the gravitational background are present, string instabilities develop. In De Sitter space, all the spatial components exhibit instability. The infalling of the string to the black hole singularity is like the motion of a particle in a potential $γ(τ-τ_0)^{-2}$ where $γ$ depends on the $D$ spacetime dimensions and string angular momentum, with $γ>0$ for Schwarzschild and $γ<0$ for Reissner - Nordstrøm black holes. For $(τ-τ_0)\to0$ the string ends trapped by the black hole singularity.

gr-qc↗