Enrico Barausse

Testing general relativity with present and future astrophysical observations

One century after its formulation, Einsteins general relativity (GR) has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that GR should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of GR. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einsteins theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

CONSTRAINING THE QUADRUPOLE MOMENT OF STELLAR-MASS BLACK HOLE CANDIDATES WITH THE CONTINUUM FITTING METHOD

Black holes in general relativity are known as Kerr black holes and are characterized solely by two parameters, the mass M and the spin J. All the higher multipole moments of the gravitational field are functions of these two parameters. For instance, the quadrupole moment is Q = –J 2/M, which implies that a measurement of M, J, and Q for black hole candidates would allow one to test whether these objects are really black holes as described by general relativity. While future gravitational-wave experiments will be able to test the Kerr nature of these objects with very high accuracy, in this paper we show that it is possible to put constraints on the quadrupole moment of stellar-mass black hole candidates by using presently available X-ray data of the thermal spectrum of their accretion disk.

The evolution of massive black holes and their spins in their galactic hosts

Future space-based gravitational-wave detectors, such as the Laser Interferometer Space Antenna (LISA/SGO) or a similar European mission (eLISA/NGO), will measure the masses and spins of massive black holes up to very high redshift, and in principle discriminate among different models for their evolution. Because the masses and spins change as a result of both accretion from the interstellar medium and the black hole mergers that are expected to naturally occur in the hierarchical formation of galaxies, their evolution is inextricably entangled with that of their galactic hosts. On the one hand, the amount of gas present in galactic nuclei regulates the changes in the black hole masses and spins through accretion, and affects the mutual orientation of the spins before mergers by exerting gravitomagnetic torques on them. On the other hand, massive black holes play a central role in galaxy formation because of the feedback exerted by active galactic nuclei on the growth of structures. In this paper, we study the mass and spin evolution of massive black holes within a semi-analytical galaxy-formation model that follows the evolution of dark-matter haloes along merger trees, as well as that of the baryonic components (hot gas, stellar and gaseous bulges, and stellar and gaseous galactic discs). This allows us to study the mass and spin evolution in a self-consistent way, by taking into account the effect of the gas present in galactic nuclei both during the accretion phases and during mergers. Also, we present predictions, as a function of redshift, for the fraction of gas-rich black hole mergers – in which the spins prior to the merger are aligned due to the gravitomagnetic torques exerted by the circumbinary disc – as opposed to gas-poor mergers, in which the orientation of the spins before the merger is roughly isotropic. These predictions may be tested by LISA or similar spaced-based gravitational-wave detectors such as eLISA/NGO or SGO.

Test bodies and naked singularities: Is the self-force the cosmic censor?

Jacobson and Sotiriou showed that rotating black holes could be spun up past the extremal limit by the capture of nonspinning test bodies, if one neglects radiative and self-force effects. This would represent a violation of the cosmic censorship conjecture in four-dimensional, asymptotically flat spacetimes. We show that for some of the trajectories giving rise to naked singularities, radiative effects can be neglected. However, for these orbits the conservative self-force is important, and seems to have the right sign to prevent the formation of naked singularities.

LINKING THE SPIN EVOLUTION OF MASSIVE BLACK HOLES TO GALAXY KINEMATICS

We present the results of a semianalytical model that evolves the masses and spins of massive black holes together with the properties of their host galaxies across the cosmic history. As a consistency check, our model broadly reproduces a number of observations, e.g., the cosmic star formation history; the black hole mass, luminosity, and galaxy mass functions at low redshift; the black hole-bulge mass relation; and the morphological distribution at low redshift. For the first time in a semianalytical investigation, we relax the simplifying assumptions of perfect coherency or perfect isotropy of the gas fueling the black holes. The dynamics of gas is instead linked to the morphological properties of the host galaxies, resulting in different spin distributions for black holes hosted in different galaxy types. We compare our results with the observed sample of spin measurements obtained through broad Kα iron line fitting. The observational data disfavor both accretion along a fixed direction and isotropic fueling. Conversely, when the properties of the accretion flow are anchored to the kinematics of the host galaxy, we obtain a good match between theoretical expectations and observations. A mixture of coherent accretion and phases of activity in which the gas dynamics is similar to that of the stars in bulges (i.e., with a significant velocity dispersion superimposed to a net rotation) best describes the data, adding further evidence in support of the coevolution of massive black holes and their hosts.

A no-go theorem for polytropic spheres in Palatini f(R) gravity

Non-vacuum static spherically symmetric solutions in Palatini f(R) gravity are examined. It is shown that for generic choices of f(R), there are commonly used equations of state for which no satisfactory physical solution of the field equations can be found within this framework, apart from in the special case of general relativity, casting doubt on whether Palatini f(R) gravity can be considered as giving viable alternatives to general relativity.

Can environmental effects spoil precision gravitational-wave astrophysics?

[abridged abstract] No, within a broad class of scenarios. With the advent of gravitational-wave (GW) astronomy, environmental effects on the GW signal will eventually have to be quantified. Here we present a wide survey of the corrections due to these effects in two situations of great interest for GW astronomy: the black hole (BH) ringdown emission and the inspiral of two compact objects. We take into account various effects such as: electric charges, magnetic fields, cosmological evolution, possible deviations from General Relativity, firewalls, and various forms of matter such as accretion disks and dark matter halos. Our analysis predicts the existence of resonances dictated by the external mass distribution, which dominate the very late-time behavior of merger/ringdown waveforms. The mode structure can drastically differ from the vacuum case, yet the BH response to external perturbations is unchanged at the time scales relevant for detectors. This is because although the vacuum Schwarzschild resonances are no longer quasinormal modes of the system, they still dominate the response at intermediate times. Our results strongly suggest that both parametrized and ringdown searches should use at least two-mode templates. Our analysis of compact binaries shows that environmental effects are typically negligible for most eLISA sources, with the exception of very few special extreme mass ratio inspirals. We show in particular that accretion and hydrodynamic drag generically dominate over self-force effects for geometrically thin disks, whereas they can be safely neglected for geometrically thick disk environments, which are the most relevant for eLISA. Finally, we discuss how our ignorance of the matter surrounding compact objects implies intrinsic limits on the ability to constrain strong-field deviations from General Relativity.

Prototype effective-one-body model for nonprecessing spinning inspiral-merger-ringdown waveforms

This paper presents a tunable effective-one-body (EOB) model for black-hole (BH) binaries of arbitrary mass ratio and aligned spins. This new EOB model incorporates recent results of small-mass-ratio simulations based on Teukolsky’s perturbative formalism. The free parameters of the model are calibrated to numerical-relativity simulations of nonspinning BH-BH systems of five different mass ratios and to equal-mass nonprecessing BH-BH systems with dimensionless BH spins χ_i≃±0.44. The present analysis focuses on the orbital dynamics of the resulting EOB model, and on the dominant (l,m)=(2,2) gravitational-wave mode. The calibrated EOB model can generate inspiral-merger-ringdown waveforms for nonprecessing, spinning BH binaries with any mass ratio and with individual BH spins -1≤χ_i≲0.7. Extremizing only over time and phase shifts, the calibrated EOB model has overlaps larger than 0.997 with each of the seven numerical-relativity waveforms for total masses between 20M_⊙ and 200M_⊙, using the Advanced LIGO noise curve. We compare the calibrated EOB model with two additional equal-mass highly spinning (χ_i≃-0.95,+0.97) numerical-relativity waveforms, which were not used during calibration. We find that the calibrated model has an overlap larger than 0.995 with the simulation with nearly extremal antialigned spins. Extension of this model to black holes with aligned spins χ_i≳0.7 requires improvements of our modeling of the plunge dynamics and inclusion of higher-order PN spin terms in the gravitational-wave modes and radiation-reaction force.

Curvature singularities, tidal forces and the viability of Palatini f(R) gravity

In a previous paper we showed that static spherically symmetric objects which, in the vicinity of their surface, are well-described by a polytropic equation of state with 3/2<Gamma<2 exhibit a curvature singularity in Palatini f(R) gravity. We argued that this casts serious doubt on the validity of Palatini f(R) gravity as a viable alternative to General Relativity. In the present paper we further investigate this characteristic of Palatini f(R) gravity in order to clarify its physical interpretation and consequences.

Neutron-star mergers in scalar-tensor theories of gravity

Scalar-tensor theories of gravity are natural phenomenological alternatives to General Relativity, where the gravitational interaction is mediated by a scalar degree of freedom, besides the usual tensor gravitons. In regions of the parameter space of these theories where constraints from both solar system experiments and binary-pulsar observations are satisfied, we show that binaries of neutron stars present marked differences from General Relativity in both the late-inspiral and merger phases. In particular, phenomena related to the spontaneous scalarization of isolated neutron stars take place in the late stages of the evolution of binary systems, with important effects in the ensuing dynamics. We comment on the relevance of our results for the upcoming Advanced LIGO/Virgo detectors.