Verifying the no-hair property of massive compact objects with intermediate-mass-ratio inspirals in advanced gravitational-wave detectors

The detection of gravitational waves from the inspiral of a neutron star or stellar-mass black hole into an intermediate-mass black hole (IMBH) promises an entirely new look at strong-field gravitational physics. Gravitational waves from these intermediate-mass-ratio inspirals (IMRIs), systems with mass ratios from ~10:1 to ~100:1, may be detectable at rates of up to a few tens per year by Advanced LIGO/Virgo and will encode a signature of the central body's spacetime. Direct observation of the spacetime will allow us to use the "no-hair" theorem of general relativity to determine if the IMBH is a Kerr black hole (or some more exotic object, e.g. a boson star). Using modified post-Newtonian (pN) waveforms, we explore the prospects for constraining the central body's mass-quadrupole moment in the advanced-detector era. We use the Fisher information matrix to estimate the accuracy with which the parameters of the central body can be measured. We find that for favorable mass and spin combinations, the quadrupole moment of a non-Kerr central body can be measured to within a ~15% fractional error or better using 3.5 pN order waveforms; on the other hand, we find the accuracy decreases to ~100% fractional error using 2 pN waveforms, except for a narrow band of values of the best-fit non-Kerr quadrupole moment.Comment: Second version, 12 pages, 5 figures, accepted by PR

Binary Black Holes in Dense Star Clusters: Exploring the Theoretical Uncertainties

Recent N-body simulations predict that large numbers of stellar black holes (BHs) could remain bound to globular clusters (GCs) at present, and merging BH--BH binaries are produced dynamically in significant numbers. We systematically vary "standard" assumptions made by numerical simulations related to, e.g., BH formation, stellar winds, binary properties of high-mass stars, and IMF within existing uncertainties, and study the effects on the evolution of the structural properties of GCs, and the BHs in GCs. We find that variations in initial assumptions can set otherwise identical initial clusters on completely different evolutionary paths, significantly affecting their present observable properties, or even affecting the cluster's very survival to the present. However, these changes usually do not affect the numbers or properties of local BH--BH mergers. The only exception is that variations in the assumed winds and IMF can change the masses and numbers of local BH--BH mergers, respectively. All other variations (e.g., in initial binary properties and binary fraction) leave the masses and numbers of locally merging BH--BH binaries largely unchanged. This is in contrast to binary population synthesis models for the field, where results are very sensitive to many uncertain parameters in the initial binary properties and binary stellar-evolution physics. We find that weak winds are required for producing GW150914-like mergers from GCs at low redshifts. LVT151012 can be produced in GCs modeled both with strong and weak winds. GW151226 is lower-mass than typical mergers from GCs modeled with weak winds, but is similar to mergers from GCs modeled with strong winds.Comment: 25 pages, 20 figures, 4 tables, ApJ in pres

Measuring the star formation rate with gravitational waves from binary black holes

A measurement of the history of cosmic star formation is central to understand the origin and evolution of galaxies. The measurement is extremely challenging using electromagnetic radiation: significant modeling is required to convert luminosity to mass, and to properly account for dust attenuation, for example. Here we show how detections of gravitational waves from inspiraling binary black holes made by proposed third-generation detectors can be used to measure the star formation rate of massive stars with high precision up to redshifts of ~10. Depending on the time-delay model, the predicted detection rates ranges from ~1400 to ~16000 per month with the current measurement of local merger rate density. With three months of observations, parameters describing the volumetric star formation rate can be constrained at the few percent level, and the volumetric merger rate can be directly measured to 3% at z~2. Given a parameterized star formation rate, the characteristic delay time between binary formation and merger can be measured to ~60%.Comment: 7 pages, 1 table, 4 fig

Post-Newtonian Dynamics in Dense Star Clusters: Highly-Eccentric, Highly-Spinning, and Repeated Binary Black Hole Mergers

We present models of realistic globular clusters with post-Newtonian dynamics for black holes. By modeling the relativistic accelerations and gravitational-wave emission in isolated binaries and during three- and four-body encounters, we find that nearly half of all binary black hole mergers occur inside the cluster, with about 10% of those mergers entering the LIGO/Virgo band with eccentricities greater than 0.1. In-cluster mergers lead to the birth of a second generation of black holes with larger masses and high spins, which, depending on the black hole natal spins, can sometimes be retained in the cluster and merge again. As a result, globular clusters can produce merging binaries with detectable spins regardless of the birth spins of black holes formed from massive stars. These second-generation black holes would also populate any upper mass gap created by pair-instability supernovae.Comment: 9 pages, 3 figures, 2 appendices. To appear in Physical Review Letter