arXiv subjects

Enrico Barausse

Publications and source records attributed to Enrico Barausse.

At least 145 records · Page 8Linked to original sources

arXiv2015

The imprint of massive black-hole mergers on the correlation between nuclear clusters and their host galaxies

A literature compilation of nuclear star cluster (NSC) masses is used to study the correlation between global and nuclear properties. A comparison of observational data to the predictions of semi-analytical galaxy formation models places constraints on the co-evolution of NSCs, massive black holes (MBHs) and host galaxies. Both data and theoretical predictions show an increased scatter in the NSC scaling correlations at high galaxy masses, and we show that this is due to the progressively more efficient ejection of stars from NSCs caused by MBH binaries in more massive stellar spheroids. Our results provide a natural explanation of why in nucleated galaxies hosting a MBH, the ratio (M_NSC+M_MBH)/M_bulge (with M_bulge the host spheroid's mass) shows significantly less scatter than M_NSC/M_bulge, and suggest that the formation of MBHs and NSCs are not mutually exclusive, as also supported by observations of co-existing systems. Both MBHs and NSCs represent generic products of galaxy formation, with NSCs being destroyed or modified by the merger evolution of their companion MBHs.

astro-ph.GA↗

arXiv2015

Electromagnetic outflows in a class of scalar-tensor theories: binary neutron star coalescence

As we showed in previous work, the dynamics and gravitational emission of binary neutron star systems in certain scalar-tensor theories can differ significantly from that expected from General Relativity in the coalescing stage. In this work we examine whether the characteristics of the electromagnetic counterparts to these binaries -- driven by magnetosphere interactions prior to the merger event -- can provide an independent way to test gravity in the most strongly dynamical stages of binary mergers. We find that the electromagnetic flux emitted by binaries in these scalar-tensor theories can show deviations from the GR prediction in particular cases. These differences are quite subtle, thus requiring delicate measurements to differentiate between GR and the type of scalar-tensor theories considered in this work using electromagnetic observations alone. However, if coupled with a gravitational-wave detection, electromagnetic measurements might provide a way to increase the confidence with which GR will be confirmed (or ruled out) by gravitational observations.

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arXiv2015

Massive Black Hole Science with eLISA

The evolving Laser Interferometer Space Antenna (eLISA) will revolutionize our understanding of the formation and evolution of massive black holes along cosmic history by probing massive black hole binaries in the $10^3-10^7$ solar mass range out to redshift $z\gtrsim 10$. High signal-to-noise ratio detections of $\sim 10-100$ binary coalescences per year will allow accurate measurements of the parameters of individual binaries (such as their masses, spins and luminosity distance), and a deep understanding of the underlying cosmic massive black hole parent population. This wealth of unprecedented information can lead to breakthroughs in many areas of physics, including astrophysics, cosmology and fundamental physics. We review the current status of the field, recent progress and future challenges.

astro-ph.HE↗

arXiv2015

Environmental Effects for Gravitational-wave Astrophysics

The upcoming detection of gravitational waves by terrestrial interferometers will usher in the era of gravitational-wave astronomy. This will be particularly true when space-based detectors will come of age and measure the mass and spin of massive black holes with exquisite precision and up to very high redshifts, thus allowing for better understanding of the symbiotic evolution of black holes with galaxies, and for high-precision tests of General Relativity in strong-field, highly dynamical regimes. Such ambitious goals require that astrophysical environmental pollution of gravitational-wave signals be constrained to negligible levels, so that neither detection nor estimation of the source parameters are significantly affected. Here, we consider the main sources for space-based detectors -- the inspiral, merger and ringdown of massive black-hole binaries and extreme mass-ratio inspirals -- and account for various effects on their gravitational waveforms, including electromagnetic fields, cosmological evolution, accretion disks, dark matter, "firewalls" and possible deviations from General Relativity. We discover that the black-hole quasinormal modes are sharply different in the presence of matter, but the ringdown signal observed by interferometers is typically unaffected. The effect of accretion disks and dark matter depends critically on their geometry and density profile, but is negligible for most sources, except for few special extreme mass-ratio inspirals. Electromagnetic fields and cosmological effects are always negligible. We finally explore the implications of our findings for proposed tests of General Relativity with gravitational waves, and conclude that environmental effects will not prevent the development of precision gravitational-wave astronomy.

astro-ph.CO↗

arXiv2014

Projected Constraints on Scalarization with Gravitational Waves from Neutron Star Binaries

Certain scalar-tensor theories have the property of endowing stars with scalar hair, sourced either by the star's own compactness (spontaneous scalarization) or, for binary systems, by the companion's scalar hair (induced scalarization) or by the orbital binding energy (dynamical scalarization). Scalarized stars in binaries present different conservative dynamics than in General Relativity, and can also excite a scalar mode in the metric perturbation that carries away dipolar radiation. As a result, the binary orbit shrinks faster than predicted in General Relativity, modifying the rate of decay of the orbital period. In spite of this, scalar-tensor theories can pass existing binary pulsar tests, because observed pulsars may not be compact enough or sufficiently orbitally bound to activate scalarization. Gravitational waves emitted during the last stages of compact binary inspirals are thus ideal probes of scalarization effects. For the standard projected sensitivity of advanced LIGO, we here show that, if neutron stars are sufficiently compact to enter the detector's sensitivity band already scalarized, then gravitational waves could place constraints at least comparable to binary pulsars. If the stars dynamically scalarize while inspiraling in band, then constraints are still possible provided the scalarization occurs sufficiently early in the inspiral, roughly below an orbital frequency of 50Hz. In performing these studies, we derive an easy-to-calculate data analysis measure, an integrated phase difference between a General Relativistic and a modified signal, that maps directly to the Bayes factor so as to determine whether a modified gravity effect is detectable. Finally, we find that custom-made templates are equally effective as model-independent, parameterized post-Einsteinian waveforms at detecting such modified gravity effects at realistic signal-to-noise ratios.

gr-qc↗

arXiv2014

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 along 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 and luminosity function and the galaxy mass function at low redshift, the black hole to 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 K$α$ 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 to the coevolution of massive black holes and their hosts.

astro-ph.CO↗

arXiv2014

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.

gr-qc↗

arXiv2014

Strong Binary Pulsar Constraints on Lorentz Violation in Gravity

Binary pulsars are excellent laboratories to test the building blocks of Einstein's theory of General Relativity. One of these is Lorentz symmetry which states that physical phenomena appear the same for all inertially moving observers. We study the effect of violations of Lorentz symmetry in the orbital evolution of binary pulsars and find that it induces a much more rapid decay of the binary's orbital period due to the emission of dipolar radiation. The absence of such behavior in recent observations allows us to place the most stringent constraints on Lorentz violation in gravity, thus verifying one of the cornerstones of Einstein's theory much more accurately than any previous gravitational observation.

gr-qc↗

arXiv2014

On the mass radiated by coalescing black-hole binaries

We derive an analytic phenomenological expression that predicts the final mass of the black-hole remnant resulting from the merger of a generic binary system of black holes on quasi-circular orbits. Besides recovering the correct test-particle limit for extreme mass-ratio binaries, our formula reproduces well the results of all the numerical-relativity simulations published so far, both when applied at separations of a few gravitational radii, and when applied at separations of tens of thousands of gravitational radii. These validations make our formula a useful tool in a variety of contexts ranging from gravitational-wave physics to cosmology. As representative examples, we first illustrate how it can be used to decrease the phase error of the effective-one-body waveforms during the ringdown phase. Second, we show that, when combined with the recently computed self-force correction to the binding energy of nonspinning black-hole binaries, it provides an estimate of the energy emitted during the merger and ringdown. Finally, we use it to calculate the energy radiated in gravitational waves by massive black-hole binaries as a function of redshift, using different models for the seeds of the black-hole population.

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arXiv2014

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

[Abridged] [...] We study the mass and spin evolution of massive black holes within a semianalytical galaxy-formation model that follows the evolution of dark-matter halos along merger trees, as well as that of the baryonic components (hot gas, stellar and gaseous bulges, and stellar and gaseous galactic disks). This allows us to study the mass and spin evolution of massive black holes 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 gravito-magnetic torques exerted by the circumbinary disk -- 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.

astro-ph.CO↗

arXiv2014

Dynamical scalarization of neutron stars in scalar-tensor gravity theories

We present a framework to study generic neutron-star binaries in scalar-tensor theories of gravity. Our formalism achieves this goal by suitably interfacing a post-Newtonian orbital evolution (described by a set of ordinary differential equations) with a set of non-linear algebraic equations, which provide a description of the scalar charge of each binary's component along the evolution in terms of isolated-star data. We validate this semi-analytical procedure by comparing its results to those of fully general-relativistic simulations, and use it to investigate the behavior of binary systems in large portions of the parameter space of scalar-tensor theories. This allows us to shed further light on the phenomenon of "dynamical scalarization", which we uncovered in [Barausse, Palenzuela, Ponce and Lehner, Phys. Rev. D 87, 081506(R) (2013)] and which takes place in tight binaries, even for stars that have exactly zero scalar charge in isolation. We also employ our formalism to study representative binary systems, obtain their gravitational-wave signals and discuss the extent to which deviations from General Relativity can be detected. The insights gained by this framework allow us to additionally show that eccentric binaries can undergo scalarization/de-scalarization phenomena.

gr-qc↗

arXiv2013

Black holes in Lorentz-violating gravity theories

Lorentz-symmetry and the notion of light cones play a central role in the definition of horizons and the existence of black holes. Current observations provide strong indications that astrophysical black holes do exist in Nature. Here we explore what happens to the notion of a black hole in gravity theories where local Lorentz symmetry is violated, and discuss the relevant astrophysical implications. Einstein-aether theory and Horava gravity are used as the theoretical background for addressing this question. We review earlier results about static, spherically symmetric black holes, which demonstrate that in Lorentz-violating theories there can be a new type of horizon and, hence, a new notion of black hole. We also present both known and new results on slowly rotating black holes in these theories, which provide insights on how generic these new horizons are. Finally, we discuss the differences between black holes in Lorentz-violating theories and in General Relativity, and assess to what extent they can be probed with present and future observations.

gr-qc↗

arXiv2013

A Post-Newtonian approach to black hole-fluid systems

This work devises a formalism to obtain the equations of motion for a black hole-fluid configuration. Our approach is based on a Post-Newtonian expansion and adapted to scenarios where obtaining the relevant dynamics requires long time-scale evolutions. These systems are typically studied with Newtonian approaches, which have the advantage that larger time-steps can be employed than in full general-relativistic simulations, but have the downside that important physical effects are not accounted for. The formalism presented here provides a relatively straightforward way to incorporate those effects in existing implementations, up to 2.5PN order, with lower computational costs than fully relativistic simulations.

gr-qc↗

arXiv2013

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.

gr-qc↗

arXiv2013

Slowly rotating black holes in Horava-Lifshitz gravity

In a recent paper we claimed that there there are no slowly rotating, stationary, axisymmetric black holes in the infrared limit of Horava-Lifshitz gravity, provided that they are regular everywhere apart from the central singularity. Here we point out a subtlety in the equivalence between Einstein-aether theory and the infrared limit of Horava-Lifshitz gravity which was missed in our earlier derivation and drastically modifies our conclusion: our earlier calculations (which are otherwise technically correct) do not really imply that there are no slowly rotating black holes in Horava-Lifshitz gravity, but that there are no slowly rotating black holes in the latter that are also solutions of Einstein-aether theory and vice versa. That is, even though the two theories share the static, spherically symmetric solutions, there are no slowly rotating black holes that are solutions to both theories. We proceed to generate slowly rotating black hole solutions in the infrared limit of Horava-Lifshitz gravity, and we show that the configuration of the foliation-defining scalar remains the same as in spherical symmetry, thus these black holes are expected to posses a universal horizon.

gr-qc↗

arXiv2012

A no-go theorem for slowly rotating black holes in Horava-Lifshitz gravity

We consider slowly rotating, stationary, axisymmetric black holes in the infrared limit of Horava-Lifshitz gravity. We show that such solutions do not exist, provided that they are regular everywhere apart from the central singularity. This has profound implications for the viability of the theory, considering the astrophysical evidence for the existence of black holes with non-zero spin. NOTE ADDED: A subtlety in the dynamical equivalence between Horava-Lifshitz gravity and Einstein-aether theory has been missed and this has seriously affected the conclusions of this paper. Please see arXiv:1212.1334 for a full discussion

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arXiv2012

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

We first use five non-spinning and two mildly spinning (chi_i \simeq -0.44, +0.44) numerical-relativity waveforms of black-hole binaries and calibrate an effective-one-body (EOB) model for non-precessing spinning binaries, notably its dynamics and the dominant (2,2) gravitational-wave mode. Then, we combine the above results with recent outcomes of small-mass-ratio simulations produced by the Teukolsky equation and build a prototype EOB model for detection purposes, which is capable of generating inspiral-merger-ringdown waveforms for non-precessing spinning black-hole binaries with any mass ratio and individual black-hole spins -1 \leq chi_i \lesssim 0.7. We compare the prototype EOB model to two equal-mass highly spinning numerical-relativity waveforms of black holes with spins chi_i = -0.95, +0.97, which were not available at the time the EOB model was calibrated. In the case of Advanced LIGO we find that the mismatch between prototype-EOB and numerical-relativity waveforms is always smaller than 0.003 for total mass 20-200 M_\odot, the mismatch being computed by maximizing only over the initial phase and time. To successfully generate merger waveforms for individual black-hole spins chi_i \gtrsim 0.7, the prototype-EOB model needs to be improved by (i) better modeling the plunge dynamics and (ii) including higher-order PN spin terms in the gravitational-wave modes and radiation-reaction force.

gr-qc↗

arXiv2012

Gravitational Self-Force Correction to the Binding Energy of Compact Binary Systems

Using the first law of binary black-hole mechanics, we compute the binding energy E and total angular momentum J of two non-spinning compact objects moving on circular orbits with frequency Omega, at leading order beyond the test-particle approximation. By minimizing E(Omega) we recover the exact frequency shift of the Schwarzschild innermost stable circular orbit induced by the conservative piece of the gravitational self-force. Comparing our results for the coordinate invariant relation E(J) to those recently obtained from numerical simulations of comparable-mass non-spinning black-hole binaries, we find a remarkably good agreement, even in the strong-field regime. Our findings confirm that the domain of validity of perturbative calculations may extend well beyond the extreme mass-ratio limit.

gr-qc↗