Testing the black hole "no-hair" hypothesis

Black holes in General Relativity are very simple objects. This property, that goes under the name of "no-hair," has been refined in the last few decades and admits several versions. The simplicity of black holes makes them ideal testbeds of fundamental physics and of General Relativity itself. Here we discuss the no-hair property of black holes, how it can be measured in the electromagnetic or gravitational window, and what it can possibly tell us about our universe.Comment: Commissioned by Classical and Quantum Gravit

Scalar, Electromagnetic and Gravitational Perturbations of Kerr-Newman Black Holes in the Slow-Rotation Limit

In Einstein-Maxwell theory, according to classic uniqueness theorems, the most general stationary black-hole solution is the axisymmetric Kerr-Newman metric, which is defined by three parameters: mass, spin and electric charge. The radial and angular dependence of gravitational and electromagnetic perturbations in the Kerr-Newman geometry do not seem to be separable. In this paper we circumvent this problem by studying scalar, electromagnetic and gravitational perturbations of Kerr-Newman black holes in the slow-rotation limit. We extend (and provide details of) the analysis presented in a recent Letter [arXiv:1304.1160]. Working at linear order in the spin, we present the first detailed derivation of the axial and polar perturbation equations in the gravito-electromagnetic case, and we compute the corresponding quasinormal modes for any value of the electric charge. Our study is the first self-consistent stability analysis of the Kerr-Newman metric, and in principle it can be extended to any order in the small rotation parameter. We find numerical evidence that the axial and polar sectors are isospectral at first order in the spin, and speculate on the possible implications of this result.Comment: 15 pages, 3 figures. Mathematica notebook with derivation of the axial and polar equations available at http://blackholes.ist.utl.pt/?page=Files and at http://www.phy.olemiss.edu/~berti/qnms.htm

A hybrid approach to black hole perturbations from extended matter sources

We present a new method for the calculation of black hole perturbations induced by extended sources in which the solution of the nonlinear hydrodynamics equations is coupled to a perturbative method based on Regge-Wheeler/Zerilli and Bardeen-Press-Teukolsky equations when these are solved in the frequency domain. In contrast to alternative methods in the time domain which may be unstable for rotating black-hole spacetimes, this approach is expected to be stable as long as an accurate evolution of the matter sources is possible. Hence, it could be used under generic conditions and also with sources coming from three-dimensional numerical relativity codes. As an application of this method we compute the gravitational radiation from an oscillating high-density torus orbiting around a Schwarzschild black hole and show that our method is remarkably accurate, capturing both the basic quadrupolar emission of the torus and the excited emission of the black hole.Comment: 12 pages, 4 figures. Phys. Rev. D, in pres

Tidal Love numbers of a slowly spinning neutron star

By extending our recent framework to describe the tidal deformations of a spinning compact object, we compute for the first time the tidal Love numbers of a spinning neutron star to linear order in the angular momentum. The spin of the object introduces couplings between electric and magnetic distortions and new classes of spin-induced ("rotational") tidal Love numbers emerge. We focus on stationary tidal fields, which induce axisymmetric perturbations. We present the perturbation equations for both electric-led and magnetic-led rotational Love numbers for generic multipoles and explicitly solve them for various tabulated equations of state and for a tidal field with an electric (even parity) and magnetic (odd parity) component with $\ell=2,3,4$. For a binary system close to the merger, various components of the tidal field become relevant. In this case we find that an octupolar magnetic tidal field can significantly modify the mass quadrupole moment of a neutron star. Preliminary estimates, assuming a spin parameter $\chi\approx0.05$, show modifications $\gtrsim10\%$ relative to the static case, at an orbital distance of five stellar radii. Furthermore, the rotational Love numbers as functions of the moment of inertia are much more sensitive to the equation of state than in the static case, where approximate universal relations at the percent level exist. For a neutron-star binary approaching the merger, we estimate that the approximate universality of the induced mass quadrupole moment deteriorates from $1\%$ in the static case to roughly $6\%$ when $\chi\approx0.05$. Our results suggest that spin-tidal couplings can introduce important corrections to the gravitational waveforms of spinning neutron-star binaries approaching the merger.Comment: v1: 16+11 pages, 6 appendices, 11 figures. v2: improved estimates of the tidal-spin corrections to the quadrupole moment of spinning neutron-star binaries approaching the merger. v3: version published in PR

Hawking emission of gravitons in higher dimensions: non-rotating black holes

We compute the absorption cross section and the total power carried by gravitons in the evaporation process of a higher-dimensional non-rotating black hole. These results are applied to a model of extra dimensions with standard model fields propagating on a brane. The emission of gravitons in the bulk is highly enhanced as the spacetime dimensionality increases. The implications for the detection of black holes in particle colliders and ultrahigh-energy cosmic ray air showers are briefly discussed.Comment: 16 pages, no figures, revtex4. v3: Misprints in Tables and four-dimensional power for fermions correcte

Constraining the equation of state of nuclear matter with gravitational wave observations: Tidal deformability and tidal disruption

We study how to extract information on the neutron star equation of state from the gravitational wave signal emitted during the coalescence of a binary system composed of two neutron stars or a neutron star and a black hole. We use post-Newtonian templates which include the tidal deformability parameter and, when tidal disruption occurs before merger, a frequency cut-off. Assuming that this signal is detected by Advanced LIGO/Virgo or ET, we evaluate the uncertainties on these parameters using different data analysis strategies based on the Fisher matrix approach, and on recently obtained analytical fits of the relevant quantities. We find that the tidal deformability is more effective than the stellar compactness to discriminate among different possible equations of state.Comment: 13 pages, 4 figures, 4 tables. Minor changes to match the version appearing on Phys. Rev.

Threshold anomalies in Horava-Lifshitz-type theories

Recently the study of threshold kinematic requirements for particle-production processes has played a very significant role in the phenomenology of theories with departures from Poincare' symmetry. We here specialize these threshold studies to the case of a class of violations of Poincare' symmetry which has been much discussed in the literature on Horava-Lifshitz scenarios. These involve modifications of the energy-momentum ("dispersion") relation that may be different for different types of particles, but always involve even powers of energy-momentum in the correction terms. We establish the requirements for compatibility with the observed cosmic-ray spectrum, which is sensitive to the photopion-production threshold. We find that the implications for the electron-positron pair-production threshold are rather intriguing, in light of some recent studies of TeV emissions by Blazars. Our findings should also provide motivation for examining the fate of the law of energy-momentum conservation in Horava-Lifshitz-type theories.Comment: LaTex, 6 page

Non-linear relativistic perturbation theory with two parameters

An underlying fundamental assumption in relativistic perturbation theory is the existence of a parametric family of spacetimes that can be Taylor expanded around a background. Since the choice of the latter is crucial, sometimes it is convenient to have a perturbative formalism based on two (or more) parameters. A good example is the study of rotating stars, where generic perturbations are constructed on top of an axisymmetric configuration built by using the slow rotation approximation. Here, we discuss the gauge dependence of non-linear perturbations depending on two parameters and how to derive explicit higher order gauge transformation rules.Comment: 5 pages, LaTeX2e. Contribution to the Spanish Relativity Meeting (ERE 2002), Mao, Menorca, Spain, 22-24.September.200

The return of the membrane paradigm? Black holes and strings in the water tap

Several general arguments indicate that the event horizon behaves as a stretched membrane. We propose using this relation to understand gravity and dynamics of black objects in higher dimensions. We provide evidence that (i) the gravitational Gregory-Laflamme instability has a classical counterpart in the Rayleigh-Plateau instability of fluids. Each known feature of the gravitational instability can be accounted for in the fluid model. These features include threshold mode, dispersion relation, time evolution and critical dimension of certain phase transitions. Thus, we argue that black strings break in much the same way as water from a faucet breaks up into small droplets. (ii) General rotating black holes can also be understood with this analogy. In particular, instability and bifurcation diagrams for black objects can easily be inferred. This correspondence can and should be used as a guiding tool to understand and explore physics of gravity in higher dimensions.Comment: This essay received an honorable mention in the Gravity Research Foundation Essay Competition, 2007. v2: Published versio