arXiv subjects

Ulrich Sperhake

Publications and source records attributed to Ulrich Sperhake.

At least 37 records · Page 2Linked to original sources

arXiv2021

Anomalies in the gravitational recoil of eccentric black-hole mergers with unequal mass ratios

The radiation of linear momentum imparts a recoil (or "kick") to the center of mass of a merging black hole binary system. Recent numerical relativity calculations have shown that eccentricity can lead to an approximate 25% increase in recoil velocities for equal-mass, spinning binaries with spins lying in the orbital plane ("superkick" configurations) [U Sperhake et al. Phys. Rev. D 101 (2020) 024044 (arXiv:1910.01598)]. Here we investigate the impact of nonzero eccentricity on the kick magnitude and gravitational-wave emission of nonspinning, unequal-mass black hole binaries. We confirm that nonzero eccentricities at merger can lead to kicks which are larger by up to ~25% relative to the quasicircular case. We also find that the kick velocity $v$ has an oscillatory dependence on eccentricity, that we interpret as a consequence of changes in the angle between the infall direction at merger and the apoapsis (or periapsis) direction.

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arXiv2020

Core collapse in massive scalar-tensor gravity

This paper provides an extended exploration of the inverse-chirp gravitational-wave signals from stellar collapse in massive scalar-tensor gravity reported in [Phys. Rev. Lett. {\bf 119}, 201103]. We systematically explore the parameter space that characterizes the progenitor stars, the equation of state and the scalar-tensor theory of the core collapse events. We identify a remarkably simple and straightforward classification scheme of the resulting collapse events. For any given set of parameters, the collapse leads to one of three end states, a weakly scalarized neutron star, a strongly scalarized neutron star or a black hole, possibly formed in multiple stages. The latter two end states can lead to strong gravitational-wave signals that may be detectable in present continuous-wave searches with ground-based detectors. We identify a very sharp boundary in the parameter space that separates events with strong gravitational-wave emission from those with negligible radiation.

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arXiv2020

Structure of neutron stars in massive scalar-tensor gravity

We compute families of spherically symmetric neutron-star models in two-derivative scalar-tensor theories of gravity with a massive scalar field. The numerical approach we present allows us to compute the resulting spacetimes out to infinite radius using a relaxation algorithm on a compactified grid. We discuss the structure of the weakly and strongly scalarized branches of neutron-star models thus obtained and their dependence on the linear and quadratic coupling parameters $α_0$, $β_0$ between the scalar and tensor sectors of the theory, as well as the scalar mass $μ$. For highly negative values of $β_0$, we encounter configurations resembling a "gravitational atom", consisting of a highly compact baryon star surrounded by a scalar cloud. A stability analysis based on binding-energ calculations suggests that these configurations are unstable and we expect them to migrate to models with radially decreasing baryon density {\it and} scalar field strength.

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arXiv2019

The high-energy collision of black holes in higher dimensions

We compute the gravitational wave energy $E_{\rm rad}$ radiated in head-on collisions of equal-mass, nonspinning black holes in up to $D=8$ dimensional asymptotically flat spacetimes for boost velocities $v$ up to about $90\,\%$ of the speed of light. We identify two main regimes: Weak radiation at velocities up to about $40\,\%$ of the speed of light, and exponential growth of $E_{\rm rad}$ with $v$ at larger velocities. Extrapolation to the speed of light predicts a limit of $12.9\,\%$ $(10.1,~7.7,~5.5,~4.5)\,\%$. of the total mass that is lost in gravitational waves in $D=4$ $(5,\,6,\,7,\,8)$ spacetime dimensions. In agreement with perturbative calculations, we observe that the radiation is minimal for small but finite velocities, rather than for collisions starting from rest. Our computations support the identification of regimes with super Planckian curvature outside the black-hole horizons reported by Okawa, Nakao, and Shibata [Phys.~Rev.~D {\bf 83} 121501(R) (2011)].

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arXiv2019

Wide nutation: binary black-hole spins repeatedly oscillating from full alignment to full anti-alignment

Within the framework of 2PN black-hole binary spin precession, we explore configurations where one of the two spins oscillates from being completely aligned with the orbital angular momentum to being completely anti-aligned with it during a single precession cycle. This "wide nutation" is the extreme limit of the generic phenomenon of spin nutation in black-hole binaries. Crucially, wide nutation happens on the short precession time scale and it is not a secular effect due to gravitational-wave radiation reaction. The spins of these binaries, therefore, flip repeatedly as one of these special configurations is entered. Binaries with total mass $M$, mass ratio $q$, and dimensionless spin $χ_1$ ($χ_2$) of the more (less) massive black hole are allowed to undergo wide nutation at binary separations $r \leq r_{\rm wide} \equiv [(q χ_2 - χ_1)/(1-q)]^2 M$. Sources that are more likely to nutate widely have similar masses and effective spins close to zero.

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arXiv2019

Inverse-chirp signals and spontaneous scalarisation with self-interacting potentials in stellar collapse

We study how the gravitational wave signal from stellar collapse in scalar-tensor gravity varies under the influence of scalar self-interaction. To this end, we extract the gravitational radiation from numerical simulations of stellar collapse for a range of potentials with higher-order terms in addition to the quadratic mass term. Our study includes collapse to neutron stars and black holes and we find the strong inverse-chirp signals obtained for the purely quadratic potential to be exceptionally robust under changes in the potential at higher orders; quartic and sextic terms in the potential lead to noticeable differences in the wave signal only if their contribution is amplified, implying a relative fine-tuning to within 5 or more orders of magnitude between the mass and self-interaction parameters.

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arXiv2019

Black holes, gravitational waves and fundamental physics: a roadmap

The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.

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arXiv2018

Orbiting black-hole binaries and apparent horizons in higher dimensions

We study gravitational wave emission and the structure and formation of apparent horizons in orbiting black-hole binary systems in higher-dimensional general relativity. For this purpose we present an apparent horizon finder for use in higher dimensional numerical simulations and test the finder's accuracy and consistency in single and binary black-hole spacetimes. The black-hole binaries we model in $D=6$ dimensions complete up to about one orbit before merging or scatter off each other without formation of a common horizon. In agreement with the absence of stable circular geodesic orbits around higher-dimensional black holes, we do not find binaries completing multiple orbits without finetuning of the initial data. All binaries radiate about $0.13\,\%$ to $0.2\,\%$ of the total mass-energy in gravitational waves, over an order of magnitude below the radiated energy measured for four-dimensional binaries. The low radiative efficiency is accompanied by relatively slow dynamics of the binaries as expected from the more rapid falloff of the binding gravitational force in higher dimensions.

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arXiv2017

Black-hole head-on collisions in higher dimensions

The collision of black holes and the emission of gravitational radiation in higher-dimensional spacetimes are of interest in various research areas, including the gauge-gravity duality, the TeV gravity scenarios evoked for the explanation of the hierarchy problem, and the large-dimensionality limit of general relativity. We present numerical simulations of head-on collisions of nonspinning, unequal-mass black holes starting from rest in general relativity with $4 \leq D\leq 10$ spacetime dimensions. We compare the energy and linear momentum radiated in gravitational waves with perturbative predictions in the extreme mass ratio limit, demonstrating the strength and limitations of black-hole perturbation theory in this context.

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arXiv2017

Long-lived inverse chirp signals from core collapse in massive scalar-tensor gravity

This letter considers stellar core collapse in massive scalar--tensor theories of gravity. The presence of a mass term for the scalar field allows for dramatic increases in the radiated gravitational wave signal. There are several potential smoking gun signatures of a departure from general relativity associated with this process. These signatures could show up within existing LIGO--Virgo searches.

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arXiv2017

On the equal-mass limit of precessing black-hole binaries

We analyze the inspiral dynamics of equal-mass precessing black-hole binaries using multi-timescale techniques. The orbit-averaged post-Newtonian evolutionary equations admit two constants of motion in the equal-mass limit, namely the magnitude of the total spin $S$ and the effective spin $ξ$. This feature makes the entire dynamics qualitatively different compared to the generic unequal-mass case, where only $ξ$ is constant while the variable $S$ parametrizes the precession dynamics. For fixed individual masses and spin magnitudes, an equal-mass black-hole inspiral is uniquely characterized by the two parameters $(S,ξ)$: these two numbers completely determine the entire evolution under the effect of radiation reaction. In particular, for equal-mass binaries we find that (i) the black-hole binary spin morphology is constant throughout the inspiral, and that (ii) the precessional motion of the two black-hole spins about the total spin takes place on a longer timescale than the precession of the total spin and the orbital plane about the total angular momentum.

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arXiv2016

Gravitational wave extraction in higher dimensional numerical relativity using the Weyl tensor

Gravitational waves are one of the most important diagnostic tools in the analysis of strong-gravity dynamics and have been turned into an observational channel with LIGO's detection of GW150914. Aside from their importance in astrophysics, black holes and compact matter distributions have also assumed a central role in many other branches of physics. These applications often involve spacetimes with $D>4$ dimensions where the calculation of gravitational waves is more involved than in the four dimensional case, but has now become possible thanks to substantial progress in the theoretical study of general relativity in $D>4$. Here, we develop a numerical implementation of the formalism by Godazgar and Reall (Ref.[1]) -- based on projections of the Weyl tensor analogous to the Newman-Penrose scalars -- that allows for the calculation of gravitational waves in higher dimensional spacetimes with rotational symmetry. We apply and test this method in black-hole head-on collisions from rest in $D=6$ spacetime dimensions and find that a fraction $(8.19\pm 0.05)\times 10^{-4}$ of the Arnowitt-Deser-Misner mass is radiated away from the system, in excellent agreement with literature results based on the Kodama-Ishibashi perturbation technique. The method presented here complements the perturbative approach by automatically including contributions from all multipoles rather than computing the energy content of individual multipoles.

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arXiv2016

Numerical simulations of stellar collapse in scalar-tensor theories of gravity

We present numerical-relativity simulations of spherically symmetric core collapse and compact-object formation in scalar-tensor theories of gravity. The additional scalar degree of freedom introduces a propagating monopole gravitational-wave mode. Detection of monopole scalar waves with current and future gravitational-wave experiments may constitute smoking gun evidence for strong-field modifications of General Relativity. We collapse both polytropic and more realistic pre-supernova profiles using a high-resolution shock-capturing scheme and an approximate prescription for the nuclear equation of state. The most promising sources of scalar radiation are protoneutron stars collapsing to black holes. In case of a Galactic core collapse event forming a black hole, Advanced LIGO may be able to place independent constraints on the parameters of the theory at a level comparable to current Solar-System and binary-pulsar measurements. In the region of the parameter space admitting spontaneously scalarised stars, transition to configurations with prominent scalar hair before black-hole formation further enhances the emitted signal. Although a more realistic treatment of the microphysics is necessary to fully investigate the occurrence of spontaneous scalarisation of neutron star remnants, we speculate that formation of such objects could constrain the parameters of the theory beyond the current bounds obtained with Solar-System and binary-pulsar experiments.

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arXiv2016

Numerical Relativity and High Energy Physics: Recent Developments

We review recent progress in the application of numerical relativity techniques to astrophysics and high-energy physics. We focus on some developments that took place within the "Numerical Relativity and High Energy Physics" network, a Marie Curie IRSES action that we coordinated, namely: spin evolution in black hole binaries, high-energy black hole collisions, compact object solutions in scalar-tensor gravity, superradiant instabilities and hairy black hole solutions in Einstein's gravity coupled to fundamental fields, and the possibility to gain insight into these phenomena using analog gravity models.

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arXiv2016

Dimensional reduction in numerical relativity: Modified cartoon formalism and regularization

We present in detail the Einstein equations in the Baumgarte-Shapiro-Shibata-Nakamura formulation for the case of $D$ dimensional spacetimes with $SO(D-d)$ isometry based on a method originally introduced in Ref.1. Regularized expressions are given for a numerical implementation of this method on a vertex centered grid including the origin of the quasi-radial coordinate that covers the extra dimensions with rotational symmetry. Axisymmetry, corresponding to the value $d=D-2$, represents a special case with fewer constraints on the vanishing of tensor components and is conveniently implemented in a variation of the general method. The robustness of the scheme is demonstrated for the case of a black-hole head-on collision in $D=7$ spacetime dimensions with $SO(4)$ symmetry.

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arXiv2016

Distinguishing black-hole spin-orbit resonances by their gravitational wave signatures. II: Full parameter estimation

Gravitational waves from coalescing binary black holes encode the evolution of their spins prior to merger. In the post-Newtonian regime and on the precession timescale, this evolution has one of three morphologies, with the spins either librating around one of two fixed points ("resonances") or circulating freely. In this work we perform full parameter estimation on resonant binaries with fixed masses and spin magnitudes, changing three parameters: a conserved "projected effective spin" $ξ$ and resonant family $ΔΦ=0,π$ (which uniquely label the source), the inclination $θ_{JN}$ of the binary's total angular momentum with respect to the line of sight (which determines the strength of precessional effects in the waveform), and the signal amplitude. We demonstrate that resonances can be distinguished for a wide range of binaries, except for highly symmetric configurations where precessional effects are suppressed. Motivated by new insight into double-spin evolution, we introduce new variables to characterize precessing black hole binaries which naturally reflects the timescale separation of the system and therefore better encode the dynamical information carried by gravitational waves.

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arXiv2015

Testing General Relativity with Present and Future Astrophysical Observations

One century after its formulation, Einstein's general relativity 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 general relativity 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 general relativity. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einstein's 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.

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arXiv2015

Gravity-dominated unequal-mass black hole collisions

We continue our series of studies of high-energy collisions of black holes investigating unequal-mass, boosted head-on collisions in four dimensions. We show that the fraction of the center-of-mass energy radiated as gravitational waves becomes independent of mass ratio and approximately equal to $13\%$ at large energies. We support this conclusion with calculations using black hole perturbation theory and Smarr's zero-frequency limit approximation. These results lend strong support to the conjecture that the detailed structure of the colliding objects is irrelevant at high energies.

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