Programme

At Amaldi7,which was held in Sydney in 2007, the Gravitational Wave International Committee (GWIC), which oversees the Amaldi meetings, decided to hold the 8th Edoardo Amaldi Conference on Gravitational Waves at Columbia University in the City of New York. With this decision, Amaldi returned to North America after a decade.The previous two years have seen many advances in the field of gravitational-wave detection. By the summer of 2009 the km-scale ground based interferometric detectors in the USA and Europe were preparing for a second long-term scientific run as a worldwide detector network. The advanced or second-generation detectors had well-developed plans and were ready for the production phase or had started construction. The European–American space mission, LISA Pathfinder, is progressing towards deployment in the foreseeable future and it is expected to pave the way towards gravitational-wave detection in the millihertz regime with LISA. Plans were developed for an additional gravitational-wave detector in Australia and in Japan (in this case underground) to extend the worldwide network of detectors for the advanced detector era. Japanese colleagues also presented plans for a space mission, DECIGO, that would bridge the gap between the LISA and ground-based interferometer frequency range.Compared to previous Amaldi meetings, Amaldi8 had new elements representing emerging trends in the field. For example, with the inclusion of pulsar timing collaborations to the GWIC, gravitational-wave detection using pulsar timing arrays was recognized as one of the prominent directions in the field and was represented at Amaldi8 as a separate session. By 2009, searches for gravitational waves based on external triggers received from electromagnetic observations were already producing significant scientific results and plans existed for pointing telescopes by utilizing gravitational-wave trigger events. Such multimessenger approaches to gravitational-wave detection also received special attention at the meeting.For the first time in the history of Amaldi conferences, plenary and contributed sessions were held to transfer ideas and experience gained with gravitational-wave science inspired education and outreach projects. Additionally, Columbia University faculties working in frontier fields, which currently may not have a direct connection to the field of gravitational-wave science, gave enthralling presentations in the form of a `wake-up' lecture series.The meeting also facilitated the exchange of scientific results and new ideas among all members of gravitational-wave experiment collaborations and the gravitational-wave theory community. Additionally, future directions in gravitational-wave detection were discussed in a special session dedicated to the Gravitational Wave International Committee Roadmap.A highly entertaining and inspiring public talk entitled `Songs from space: black holes and the Big Bang in audio' was given by Janna Levin and it attracted over 300 young and old science enthusiasts from and around New York City.This special issue of Classical and Quantum Gravity (Volume 27, Number 8, 2010) is published as the proceedings of Amaldi8. It contains the overview articles by invited plenary speakers, and some of the highlights of the conference as selected by session chairs and organizers. Other Amaldi8 talks and posters appear in the refereed issue of the electronic Journal of Physics: Conference Series. This issue of Classical and Quantum Gravity and the Journal of Physics: Conference Series issue will be electronically linked.The conference organizers gratefully acknowledge the financial support of Columbia University in the City of New York, the International Union of Pure and Applied Physics and the United States National Science Foundation.We would like to thank the Gravitational Wave International Committee for their scientific oversight, the Local Organizing Committee for program selection, and the session chairs for thoughtful delivery of conference goals and enabling many lively discussions that are much needed for the development of our frontier field. We would also like to thank the hard work of Columbia University Physics Department administrators for handling some of the practical aspects of the organization. Finally, a team of students (Jonathan Berliner, Rutu Das, David Fierroz, Alyssa Miller, David Murphy and Keith Redwine) provided assistance behind the scenes. Their hard work, practical ideas, and dedication are greatly appreciated. We are also grateful for Keith Redwine for his help in organizing the Journal of Physics: Conference Series proceedings articles.Finally, we would like to thank over 310 participants for their interesting and lively contributions that ultimately made the conference a success.

Gravitational inertial sensors will be placed on board the Laser Interferometer Space Antenna (LISA) and aboard its precursor mission LISA Pathfinder (LISA-PF) in order to detect low frequency gravitational waves in space. Free-floating test-masses (Au7Pt3 cubes) will be housed in inertial sensors for detecting possible laser signal variations induced by gravitational waves. Charging of the LISA test-masses due to exposure of the spacecraft to cosmic radiation and energetic solar particles will affect operation of gravitational inertial sensors. In this paper we report on the role of diffuse γ-rays in charging the LISA and LISA-PF test-masses with respect to protons and helium nuclei. The diffuse γ-ray flux in the Galaxy has been interpolated taking into account the outcomes of recent calculations. A comparison with γ-ray observations gathered by different experiments (COMPTEL and EGRET, Milagro, Whipple, HEGRA, TIBET) has been carried out. Simulations of the test-mass charging process have been performed by means of the FLUKA2006.3b package. Monte Carlo simulations of the interaction of cosmic particles with the LISA spacecraft indicate that the diffuse γ-ray contribution to the average steady-state test-mass charging rate and to the single-sided power spectrum of the charge rate noise is marginal with respect to that due to galactic cosmic-rays.

The field of observational gravitational wave astronomy has begun in earnest, starting with the detection of the strain signal from the binary black hole merger GW150914 by the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2015. The current incarnation of the LIGO observatories, known as Advanced LIGO, has achieved strain sensitivities on the order of 10−23/√Hz in the hundreds of Hz region, which has enabled unambiguous detection of astrophysical gravitational wave signals. Nevertheless, the scientific output from the LIGO observatories is constrained by the instrumental performance and sensitivity, as there remain many more distant and exotic sources to be observed. This thesis describes a few topics in experimental gravitational physics, broadly unified by the desire to improve the performance and sensitivity of gravitational wave interferometers. First, it describes an experimental effort to search for a novel form of nonlinear mechanical noise that may be relevant for the ultimate performance of the mirror sus- pension systems used throughout the instrument. Next, it summarizes work done at the CalTech 40m LIGO controls prototype to realize its fully operational state, and a novel automated controls algorithm developed and tested there that may be useful in simplifying the control of current and future interferometers. Finally, it describes work done on a system to identify and subtract unwanted noise couplings out of recorded aLIGO strain data in an automated fashion. The noise subtraction system applied to GW150914 is demonstrated to reduce the uncertainties of the black hole mass parameters by about 10%.

The LIGO and Virgo gravitational-wave observatories are complex and extremely sensitive strain detectors that can be used to search for a wide variety of gravitational waves from astrophysical and cosmological sources. In this thesis, I motivate the search for the gravitational wave signals from coalescing black hole binary systems with total mass between 25 and 100 solar masses. The mechanisms for formation of such systems are not well-understood, and we do not have many observational constraints on the parameters that guide the formation scenarios. Detection of gravitational waves from such systems — or, in the absence of detection, the tightening of upper limits on the rate of such coalescences — will provide valuable information that can inform the astrophysics of the formation of these systems. I review the search for these systems and place upper limits on the rate of black hole binary coalescences with total mass between 25 and 100 solar masses. I then show how the sensitivity of this search can be improved by up to 40% by the the application of the multivariate statistical classifier known as a random forest of bagged decision trees to more effectively discriminate between signal and non-Gaussian instrumental noise. I also discuss the use of this classifier in the search for the ringdown signal from the merger of two black holes with total mass between 50 and 450 solar masses and present upper limits. I also apply multivariate statistical classifiers to the problem of quantifying the non-Gaussianity of LIGO data. Despite these improvements, no gravitational-wave signals have been detected in LIGO data so far. However, the use of multivariate statistical classification can significantly improve the sensitivity of the Advanced LIGO detectors to such signals.

Extending the electromagnetic and gravitational radiations from binary black holes with electric and magnetic charges in circular orbits in Liu et al. (Phys. Rev. D 102:103520, 2020), we calculate the total emission rates of energy and angular momentum due to gravitational and electromagnetic radiations from dyonic binary black holes in precessing elliptical orbits. It is shown that the emission rates of energy and angular momentum due to gravitational and electromagnetic radiations have the same dependence on the conic angle for different orbits. Moreover, we obtain the evolutions of orbits and find that a circular orbit remains circular while an elliptic orbit becomes quasi-circular due to electromagnetic and gravitational radiations. Using the evolution of orbits, we derive the waveform models for dyonic binary black hole inspirals and show the amplitudes of the gravitational waves for dyonic binary black hole inspirals differ from those for Schwarzschild binary black hole inspirals, which can be used to test electric and magnetic charges of black holes.

Gravitational waves are basically ripples in spacetime predicted by Einstein's General Theory of Relativity (1915), have revolutionized our understanding of the universe since their first detection in 2015 by the LIGO. These waves are typically generated by extreme cosmological events such as the mergers of two or more black holes, neutron stars, and other compact objects. Black holes, regions of spacetime where gravity is so intense that not even light can escape, are both significant sources and subjects of gravitational wave studies. The observation of gravitational waves from binary black hole mergers has provided unprecedented insights into the nature of black holes, their mass distribution, and the environments in which they form. This paper reviews the current state of research on gravitational waves and black holes, focusing on their astrophysical origins, the mechanisms of wave generation, and the information encoded in the wave signals. I have discussed how gravitational wave detections have confirmed the existence of stellar-mass black hole binaries, revealed new populations of black holes, and provided tests of general relativity in the strong-field regime. Additionally, this study highlights the potential of gravitational wave astronomy to address unresolved questions, such as the formation channels of black hole binaries, the detection of primordial black holes, and the role of black holes in galaxy evolution. By bridging observational data with theoretical models, gravitational wave observations are opening new frontiers in our understanding of both black holes and the fundamental nature of gravity.

As gravitational wave astronomy prepares for the first detections of gravitational waves from compact-object binary inspirals, theoretical work is required on the study of (i) gravitational-wave sources, (ii) the signals emitted by those sources, and (iii) the searches for those signals in detector data. This thesis describes work on all three fronts. (i) We discuss intermediate-mass-ratio inspirals (IMRIs) of black holes or neutron stars into intermediate-mass black holes (IMBHs) that could be detected with Advanced LIGO. We analyze different mechanisms of IMRI formation and compute IMRI event rates of up to tens of events per year for Advanced LIGO. We study the spin evolution of IMBHs that grow through a series of minor mergers. We explore how a deviation of an IMRI's central body from a Kerr black hole influences geodesics, including the possibility of chaotic orbital dynamics. We also address the scientific consequences of extreme-mass-ratio inspiral (EMRI) detections by LISA for astrophysics and general relativity, and the difficulties associated with detecting and analyzing EMRI signals. (ii) We study the periodic standing-wave approximation (PSWA), which can potentially provide accurate waveforms in the last inspiral cycles of a comparable-mass black-hole binary. Using a simple model, we find that the solution to Einstein's equations for inspiraling black holes can be recovered to a high accuracy by the addition a perturbative radiation-reaction field to the standing-wave, noninspiraling solution. (iii) We demonstrate the utility of searching for and analyzing tracks in time-frequency spectrograms of a gravitational-wave signal as a means of estimating the parameters of a massive black-hole binary inspiral, as observed by LISA.

The Edoardo Amaldi Conference on Gravitational Waves is the premier forum for the fields of gravitational wave science and gravitational wave detection. Held biannually, the Amaldi meetings are organized under the auspices of the Gravitational Wave International Committee (GWIC), which oversees the scientific program for the meeting.Amaldi 12 was held at the Hilton Hotel in Pasadena, CA from July 9-14, 2017. This was the first Amaldi conference since LIGO and Virgo announced the detection of gravitational waves from the merger of two black holes into a single black hole. Unlike previous Amaldi Conferences that focused on anticipated gravitational wave science and detector development, this was the first Amaldi conference to examine the science arising from the LIGO-Virgo detections and the successful LISA Pathfinder mission, the precursor to LISA.During the conference, the full range of gravitational wave science was represented, including plans for the further development of terrestrial and space based detectors, as well as progress in pulsar timing arrays. Emphasis was placed on the connection between gravitational wave science and the fields of observational astrophysics and multi-messenger astronomy, as well as in progress in modeling the waveforms for gravitational waves arising from various astrophysical sources, highlighting the stronger connection between experimental advances, numerical relativity simulations, and astrophysics theory.Guest editorDr. Sydney MeshkovCaltech, LIGO 100 – 36, Pasadena, CA 91125 (syd@ligo.caltech.edu)

We search for gravitational waves from the coalescence (inspiral, merger and ringdown) of binary black holes with data from the Laser Interferometer Gravitational-Wave Observatory (LIGO). Provided with well-described waveform models from General Relativity, matched filtering is employed in the GSTLAL analysis pipeline as the optimal detection technique for weak signals in Gaussian noise. The GSTLAL analysis pipeline filters data with waveform template banks, identifies triggers with SNR greater than 4, forms coincident triggers between multiple detectors in the LSC-Virgo Collaboration, and attempts to optimally separate signal from detector background noise fluctuations using a Chisquared test. We analyze high-statistics simulations of binary merger waveforms injected into LIGO recolored S6 data to evaluate the pipeline search sensitivity and to test the readiness of the pipeline for Advanced LIGO. With Advanced LIGO fully in operation by 2015 and the upgraded analysis pipelines, the expected detection rate is increased to as much as 100 events/year or more as compared to 0.01–1 events/year in Initial LIGO. Our work will make it possible to detect gravitational waves from binary black hole coalescence in Advanced LIGO data with high confidence. KEYWORDS: LIGO, Gravitational Waves, General Relativity, Coalescence, Black Hole Binaries, Noise Fluctuations, Matched Filtering, Chi-squared Test, Simulations, GSTLAL Analysis Pipeline

Stellar black hole (BH) binaries are one of the most promising gravitational wave (GW) sources for GW detection by the ground-based detectors. Nuclear star clusters (NCs) located at the centre of galaxies are known to harbour massive black holes (MBHs) and to be bounded by a gravitational potential by other galactic components such as the galactic bulge. Such an environment of NCs provides a favourable conditions for the BH–BH binary formation by the gravitational radiation capture due to the high BH number density and velocity dispersion. We carried out detailed numerical study of the formation of BH binaries in the NCs using a series of N-body simulations for equal-mass cases. There is no mass segregation introduced. We have derived scaling relations of the binary formation rate with the velocity dispersion of the stellar system beyond the radius of influence and made estimates of the rate of formation of BH binaries per unit comoving volume and thus expected detection rate by integrating the binary formation rate over galaxy population within the detection distance of the advanced detectors. We find that the overall formation rates for BH–BH binaries per NC is ∼10−10 yr−1 for the Milky Way-like galaxies and weakly dependent on the mass of MBH as Γ ∝ |$M_{\rm MBH}^{3/28}$|⁠. We estimate the detection rate of 0.02–14 yr−1 for advanced LIGO/Virgo considering several factors such as the dynamical evolution of NCs, the variance of the number density of stars and the mass range of MBH giving uncertainties.

Aims. It is speculated that a merger of two massive stellar-origin black holes in a dense stellar environment may lead to the formation of a massive black hole in the pair-instability mass gap (∼50−135 M⊙). Such a merger-formed black hole is expected to typically have a high spin (a ∼ 0.7). If such a massive black hole acquires another black hole it may lead to another merger detectable by LIGO/Virgo in gravitational waves. Acquiring a companion may be hindered by gravitational-wave kick/recoil, which accompanies the first merger and may quickly remove the massive black hole from its parent globular or nuclear cluster. We test whether it is possible for a massive merger-formed black hole in the pair-instability gap to be retained in its parent cluster and have low spin. Such a black hole would be indistinguishable from a primordial black hole. Methods. We employed results from numerical relativity calculations of black hole mergers to explore the range of gravitational-wave recoil velocities for various combinations of merging black hole masses and spins. We compared merger-formed massive black hole speeds with typical escape velocities from globular and nuclear clusters. Results. We show that a globular cluster is highly unlikely to form and retain a ∼100 M⊙ black hole if the spin of the black hole is low (a ≲ 0.3). Massive merger-formed black holes with low spins acquire high recoil speeds (≳ 200 km s−1) from gravitational-wave kick during formation that exceed typical escape speeds from globular clusters (∼ 50 km s−1). However, a very low-spinning (a ∼ 0.1) and massive (∼100 M⊙) black hole could be formed and retained in a galactic nuclear star cluster. Even though such massive merger-formed black holes with such low spins acquire high speeds during formation (∼ 400 km s−1), they may avoid ejection since massive nuclear clusters have high escape velocities (∼ 300−500 km s−1). A future detection of a massive black hole in the pair-instability mass gap with low spin would therefore not be proof of the existence of primordial black holes, which are sometimes claimed to have low spins and arbitrarily high masses.

We present our current best estimate of the plausible observing scenarios for the Advanced LIGO, Advanced Virgo and KAGRA gravitational-wave detectors over the next several years, with the intention of providing information to facilitate planning for multi-messenger astronomy with gravitational waves. We estimate the sensitivity of the network to transient gravitational-wave signals for the third (O3), fourth (O4) and fifth observing (O5) runs, including the planned upgrades of the Advanced LIGO and Advanced Virgo detectors. We study the capability of the network to determine the sky location of the source for gravitational-wave signals from the inspiral of binary systems of compact objects, that is binary neutron star, neutron star–black hole, and binary black hole systems. The ability to localize the sources is given as a sky-area probability, luminosity distance, and comoving volume. The median sky localization area (90% credible region) is expected to be a few hundreds of square degrees for all types of binary systems during O3 with the Advanced LIGO and Virgo (HLV) network. The median sky localization area will improve to a few tens of square degrees during O4 with the Advanced LIGO, Virgo, and KAGRA (HLVK) network. During O3, the median localization volume (90% credible region) is expected to be on the order of 10^{5}, 10^{6}, 10^{7}mathrm { Mpc}^3 for binary neutron star, neutron star–black hole, and binary black hole systems, respectively. The localization volume in O4 is expected to be about a factor two smaller than in O3. We predict a detection count of 1^{+12}_{-1}(10^{+52}_{-10}) for binary neutron star mergers, of 0^{+19}_{-0}(1^{+91}_{-1}) for neutron star–black hole mergers, and 17^{+22}_{-11}(79^{+89}_{-44}) for binary black hole mergers in a one-calendar-year observing run of the HLV network during O3 (HLVK network during O4). We evaluate sensitivity and localization expectations for unmodeled signal searches, including the search for intermediate mass black hole binary mergers.

The astrophysical origin of gravitational-wave (GW) events is one of the most timely problems in the wake of the LIGO/Virgo discoveries. In active galactic nuclei (AGNs), binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Previous studies have suggested that binary black hole (BBH) mergers in AGN disks can contribute significantly to BBH mergers observed by GW interferometers. Here we examine the distribution of the effective spin parameter χ eff of this GW source population. We extend our semi-analytical model of binary formation and evolution in AGN disks by following the evolution of the binary orbital angular momenta and black hole (BH) spins. BH spins change due to gas accretion and BH mergers, while the binary orbital angular momenta evolve due to gas accretion and binary–single interactions. We find that the distribution of χ eff predicted by our AGN model is similar to the distribution observed during LIGO/Virgo O1 and O2. On the other hand, if radial migration of BHs is inefficient, χ eff is skewed toward higher values than the observed distribution, because of the paucity of scattering events that would randomize spin directions relative to the orbital plane. We suggest that high binary masses and the positive correlation between binary mass and the standard deviation of χ eff for chirp masses up to can be possible signatures for mergers originating in AGN disks. Finally, hierarchical mergers in AGN disks naturally produce properties of the recent GW event GW190412, including a low mass ratio, a high primary BH spin, and a significant spin component in the orbital plane.

One of the crucial windows for distinguishing astrophysical black holes from primordial black holes is through the redshift evolution of their respective merger rates. The low redshift population of black holes of astrophysical origin is expected to follow the star formation rate. The corresponding peak in their merger rate peaks at a redshift smaller than that of the star formation rate peak (zp ≈ 2), depending on the time delay between the formation and mergers of black holes. Black holes of primordial origin are going to be present before the formation of the stars, and the merger rate of these sources at high redshift is going to be large. We propose a joint estimation of a hybrid merger rate from the stochastic gravitational wave background, which can use the cosmic history of merger rates to distinguish between the two populations of black holes. Using the latest bounds on the amplitude of the stochastic gravitational wave background amplitude from the third observation run of LIGO/Virgo, we obtain weak constraints at $68{{\ \rm per\ cent}}$ C.L. on the primordial black hole merger rate index $2.56_{-1.76}^{+1.64}$ and astrophysical black hole time delay $6.7_{-4.74}^{+4.22}$ Gyr. We should be able to distinguish between the different populations of black holes with the forthcoming O5 and A+ detector sensitivities.

A Triumph of Science−and Engineering